Soil Testing Equipment

Referent and Standards Product for Soil Testing Equipment

Soil Testing Q & A

Soil testing... if you feel as though you have heard enough about soil sampling, you are in for a big surprise. With the increased focus on nutrient management planning of intensive livestock areas, and farm planning programs to manage fertilizer nutrients, we are on the verge of major soil sampling promotional campaigns in most provinces. For the average grower, fertilizer cost is also becoming a major challenge in budgetting for crop inputs. Soil testing is an excellent way to  minimize the ‘guesswork’ associated with seeding a crop for maximum  economic yield. This fertilization article reviews some of the common questions posed each year from crop advisers who take and interpret soil samples.

Collecting a good custom soil sample – why is it still a challenge? It has been said a thousand times,  soil sampling practices are where the vast majority of errors occur. If you  consider that the sample is subject to accepted North American handling protocols once it reaches the soil testing laboratory, any field sampling problem that affects the sample ahead of time will impact the results obtained and thus the recommendations made. For the sake of keeping a customer and  getting them used to the best science- based soil fertility planning tool, be sure you are getting good samples.

Why is it important to consult with, or take along, the farmer when custom soil sampling? The potential for sampling the ‘wrong’ location is very high in a field you have never been in before, trying to decide the best location to collect your soil cores. With the farmer riding along, or providing clear direction on paper ahead of time, the sample operator has clear guidance as to where not to sample in the field. This becomes critical,  considering the time and effort required to collect soil samples and the need for composite samples to be representative of the dominant production area in the field. Once these preferred sampling areas have been identified, marking them with GPS allows for future reference when sampling again.



Why do we insist that for a composite sample the operator collect 15 to 20 sample cores? When composite sampling, getting a representative sample is critical. While a composite sample will not provide any insight into the variability found in a particular field, picking the right  spots to represent the average production areas is critical. In collecting the sample, imagine what happens to the average value when one bad core (saline for instance) ends up mixed in with the rest. The more samples collected, the more dilution will occur from this one bad core.



For example, there are saline areas in many fields in semi-arid regions. However, when you get a soil sample back saying the field is saline, not just the three acres at the base of the hill you have always known about, you know that it is a poor representative sample for the field. One bad core mixed in with eight to 10 others is a far bigger problem than one bad core mixed in with 15 to 20 cores. If the differing areas of the field are large, they should be separated into a sub-region and a sample collected from that particular region. This will aid in fertilizer application rate management.  Sample number, or intensity in a field, should increase with the amount  of rainfall received, or where irrigation is used.

Sampling around last year’s fertilizer bands – how do you avoid high  residual nutrients? Selecting a sampling location to collect your soil cores also requires asking how fertilizer P and K were applied the year
before. While most N and S sources are mobile in the soil and move away from the band location in soil water, P and K additions will remain very close to their original band location. If applied in the seedrow, or in a side-band, these are areas to avoid sampling. Sometimes growers will apply P and K in the  mid-row band with their N and S, so these areas should also be avoided. A few simple questions can avoid excessively high soil nutrient levels which end up reducing confidence in the sample collected.

How does sampling to a uniform depth avoid disappointment? Nutrient concentration in the soil canvary significantly with soil depth. Care must be taken when collecting a  composite sample to ensure that the sampling depth is uniform, and if  different from the option provided on the sample bags, this is noted. For
example, if the laboratory suggests a sampling depth of zero to six inches and six to 24 inches, but you actually have samples that are zero to four inches and four to 16 inches, noting this on the sample bags will ensure that the laboratory uses the appropriate  conversion factors when estimating  the supply of available nutrients. A change in the depth you submit  to the laboratory is not a problem, it is just ensuring that you note on  the sample bag what depth the sample was collected.

Sample timing – Why is it important to sample at the same time each year?
One of the advantages that soil sampling provides is the opportunity to monitor soil nutrient changes and trends over time. However, to make a good comparison it requires sampling from the same areas from year to year, and sampling at the same time of year. Taking samples for spring seeded crops after fall harvest is ideal. Sampling in the fall prior to any tillage helps to  minimize field variability and ensures uniform sample core collection. Canola and pulse fields should be sampled later in the fall once soil temperatures drop below 50 degrees F (10 degrees C), while all cereal stubbles can be sampled after crop harvest. Where possible, avoid sampling frozen soils. Spring is also a good time to sample, with the laboratories offering rapid turn-aroundin sample handling.

Composite versus site-specific sampling, what are the advantages? One of the ways in which some farmers have changed their soil sampling  practices to minimize variability from year to year is to move to site-specific sampling. Using GPS, they establish a series of four or five co-ordinates in  their fields which they believe best  represent the field average. These are then provided to the sample operator, who drives to these locations using a truck-mounted GPS receiver. Once in these locations, the operator drives in a tight circle and collects four to five  samples for the composite of 15 to 20 total cores from the field. This  avoids sampling areas where problems are known to exist, such as salinity,  old yard sites and areas where manure was previously applied. With this site-specific information, the farmer can then make his own changes to application rates based on his past knowledge of crop response in specific areas of the field. 
Grid sampling – is it for your farm? Grid sampling has been proposed as a means of obtaining the most accurate picture of soil nutrient variability within fields. This variability can then be  managed with nutrient additions to increase field uniformity and uniformity in crop response. Grid sample cells vary in size, ranging from a low of one acre to a high of five acres. In many cases, grid sample results are used to manage less mobile nutrients like P and K. Areas low in P and K can receive more fertilizer, while those areas high in P and K will receive little or no fertilizer addition. From a cost of sampling perspective, grid sampling can be very expensive. While it is not conducted on an  annual basis, a five acre acre grid on a 160 acre field would require 32  separate analyses... cost prohibitive for many semi-arid cropping regions. While grid sampling has been shown to be  profitable in areas where building  soil P and K play a major role in increasing yield level and uniformity, it is not likely to be popular with  growers in areas where P and K are applied annually at rates similar  to crop removal.


*Dr. Adrian Johnston is Northern
Great Plains region director for
PPI/PPIC. Article reprinted with
permission of PPI/PPIC.
Reprinted in Canada from the November 2006 issue of Top Crop Manager (West) magazine with permission of Annex Publishing & Printing Inc


Selecting and Using a Soil Testing Laboratory

Taking a Soil Sample

Follow the specifc instructions provided by the soil testing lab you select.  The following guidelines will help ensure that a good sample is taken: Separate samples should be taken for distinct areas- front yard, back yard, vegetable garden, etc. The sample should represent  the soil in which the plants are or will be growing. Use a  spade or trowel to take 10-12 random samples across the area  of concern.  The samples are thin slices taken to a depth that  contains or will contain the bulk of the plant’s roots- 3 inches for turf; 6-8 inches for garden and landscape beds.  Mix together all of the slices in a clean bucket removing all rocks, debris, and
plant material.

Mailing in a Soil Sample

Don’t send wet soil; you should not be able to squeeze water  from the sample. Send a minimum of 1 cup and a maximum of  2 cups of soil per sample.  If no kit is provided, seal the soil in a  zip lock bag or use the special soil sample bag provided by the  Maryland Department of Agriculture and Maryland Cooperative  Extension.  Forms for most of the labs can be downloaded from  their websites. Be sure that all of your contact information is on  the form and mail it back to the lab with the sample, check for  the correct amount, and suffcient postage

Interpreting Test Results

Soil test reports from all of the labs will provide a graphical representation of results- the level of various nutrients from your soil (low medium, high, excessive).  “Optimal” and “excessive” levels mean that the nutrient concentration in the soil is more  than adequate for optimum plant growth.  Adding more of that  nutrient will not improve plant growth and may have undesirable  effects on the environment.

Be aware that the specifc turfgrass fertilizer recommendations  you receive will not be identical to Maryland’s.  This is due to  differences in soils, climate, and and state water quality policies.   Always follow University of Maryland recommendations for applying the right amount of nitrogen for healthy lawns.  Go  to the “Publications” section of our website to download  the following fact sheets: HG 103, “Fertilizing Facts for
Home Lawns”; FS 702, “Lawns and the Chesapeake Bay” (has more detailed information on grasscycling, slow release  nitrogen fertilizers, and fertilizing responsibly); and HG 42,  “Soil Amendments and Fertilizers” (contains broad fertilizer guidelines for many garden and landscape plants).

Abbreviations and Terms Found in Soil Test
Reports

The labs listed in the chart provide defnitions and explanations  of soil sampling terms and concepts.  The list below will help you  better understand the soil test reports you receive. pH-  soil pH is a measure of a soil’s hydrogen ion concentration.  The greater the number of hydrogen ions the more acidic the soil.  The pH scale is 1-14. 

Soils with pH levels below 7.0 are considered acidic and soils with pH levels above 7.0 are  considered alkaline. Soil pH is a critical measurement for gardeners because it affects the availability of nutrients for uptake by plant roots.  Most garden and landscape plants grow best in soils with a pH of 5.5–7.0.  Certain plant nutrients can become  unavailable or excessively available outside this range, leading  to plant growth problems. But there are exceptions:  for example,  plants in the azalea and blueberry family grow best at pH 4.0-5.0.

Macronutrients: these are required in the greatest quantity by plants. Sulfur (S) is rarely tested because soils in Maryland are rarely defcient.  Nitrogen (N) is not usually tested because it is constantly changing.
P-  phosphorous
K-  potassium
Mg-  magnesium
Ca-  calcium

Micronutrients:  These important nutrients are required in  relatively small quantities.  Defciencies are rarely a problem in  Maryland soils, especially in the Central and Western regions.   Eastern Shore gardeners may want to be sure that boron is  included in the test they select.
Fe-  iron
Zn-  zinc
Cu-  copper
Mn-  manganese
B-  boron

Heavy metals:  excessive levels are a concern, especially in soils where food crops are grown and children play.  These elements can be a health hazard when 1)tracked into the house via shoes and tools, 2)ingested by young children, or 3)ingested from food crops grown in contaminated soil.  For more information on lead,
refer to Maryland Cooperative Extension fact sheet HG #18, ”Lead in Garden Soil.”
Pb-  lead
Ni-  nickel
Cd-  cadmium
Cr-  chromium

OM-  organic matter; includes living and decomposed plant and animal tissues (dead leaves, soil fungi, plant roots, etc.  Soil organic matter drives a soil’s biological and chemical processes.  OM test results are given on a weight basis. Usually a sample is weighed in the lab and then ignited to burn off the carbon compounds, leaving only the mineral soil.  The sample is reweighed to determine the OM%.   Gardeners who add lots of  organic matter to their soils may be surprised that the OM content is less than 5%.  This is because OM is lighter than mineral soil  and the measurement is based on weight, not volume. CEC-  cation exchange capacity measures the capacity of a soil  to hold and release nutrient ions.  Soils high in clay and organic
matter will have high CEC.  This measurement will vary across Maryland soils.  Adding organic matter is recommended where the CEC is less than 10.

Nitrogen

Plants need a relatively large amount of nitrogen for healthy growth.  Plant roots take up nitrogen in the
nitrate and ammonium forms.  Unlike most other nturients, nitrogen does not come from mineral soil.  Instead it comes “naturally” from organic matter, lightning and legumes (plants that convert nitrogen in air to nitrate nitrogen.)

Fertilizing Responsibly for a Healthy Chesapeake Bay

Nitrogen and phosphorous are the two key nutrient pollutants of  waterways in Maryland that contribute to the complex problem  known as eutrophication. These nutrients encourage blooms of  algae that cloud the water and block sunlight causing underwater grasses to die. This has negative affects on aquatic life and  birds. Huge numbers of microorganisms in the water then use up oxygen as they feed on and break down the algae once it has died. Dissolved oxygen in the water quickly declines, depriving fsh, crabs and other aquatic life forms of needed oxygen.It is estimated that up to 80% of the nitrogen entering groundwater and surface water comes from non-point sourcesfarms, public lands, and private landscapes.  About one-half of excessive or mis-applied nitrogen fertilizer enters surface water fairly quickly as run-off from hard surfaces, lawns and gardens.  The other half travels for at least 10 years through soil and underground water before it eventually enters the Chesapeake  Bay (for Marylanders who live east of the Eastern Continental
Divide, located in Garrett County).

So mistakes in measuring and applying fertilizers today can contribute to nutrient pollution  problems many years in the future.Farmers, municipalities, corporations, AND homeowners all  have a duty to reduce the fow of nitrogen and phosphorous into streams, rivers, and the Chesapeake Bay.    Marylanders with lawns should be careful to drastically reduce or eliminate phosphorous fertilization if their soil test results show phosphorous (phosphate) levels that are “adequate”, “optimum”, or “excessive”.  The following companies  offer lawn fertilizers that contain no phosphorous or a low percentage (3%-5%) of phosphorous: Lebanon-Seaboard (Greenview), Espoma, LESCO, Scotts.

10 Ways to Achieve a Healthy Home Landscape Without Harming the Chesapeake Bay

  1. Take a soil test every 3 to 4 years. Fertilize according tosoil test recommendations. Use less than the recommended amounts listed on fertilizer packages.
  2. Leave grass clippings on your lawn (grasscycling.) They are a source of nitrogen for your lawn and will not contribute to thatch build-up in fescue or bluegrass lawns.
  3. Home gardeners tend to over-fertilize fower and vegetable gardens.  Reduce or eliminate fertilizer applications in wellestablished beds if organic matter is being added each year.  
  4. Don’t fertilize trees and shrubs if they appear healthy  and are making adequate shoot and leaf growth.Compost plant residues or incorporate them directly into soil.  
  5. Discard plants with serious disease problems.
  6. When appropriate, substitute slow-release fertilizers for those that are highly soluble and substitute locally available organic fertilizers (well-decomposed farmyard manure, backyard compost and municipal leaf compost) for manufactured chemical fertilizers.
  7. Keep fertilizers off hard surfaces.  Rain water will carry fertilizer salts into storm drains and surface waters and contribute to nutrient pollution of our waterways. 
  8. Over time, rainfall causes bare soil to erode and become compacted. Keep bare soil covered with a mulch and plant ground covers in areas where turf won’t grow.  Plant winter cover crops in vegetable gardens - like oats, winter rye and crimson clover.  
  9. Avoid excessive foot or equipment traffc to prevent soil compaction, especially when the soil is wet.  Construct terraces for beds on sloped ground. Keep soil in raised beds framed with solid sides. 
  10. To melt winter ice, use calcium magnesium acetate (CMA), potassium chloride (KCl), sodium chloride (NaCl) or calcium chloride (CaCl2).  Do not use urea, potassium nitrate, or other chemical fertilizers containing nitrogen or phosphorous.  The salts in these fertilizers may burn the foliage and roots of adjacent plants and wash into and pollute waterways.
Feed The Soil First!
The surest way to improve soil quality and  plant growth is  the  regular  incorporation  of  organic matter  such  as composted  yard waste. Organic matter  improves  soil structure, slowly  releases nutrients,  increases benefcial microbial  activity,  and  reduces  the  need  for  purchased fertilizers

Authors : Jon Traunfeld, Regional Specialist, Home and Garden
Information Center, Maryland Cooperative Extension
Reviewers: Patricia Steinhilber, Ph.D., Nutrient Management
Coordinator, Maryland Cooperative Extension and Judy
McGowan, Nutrient Management Specialist, Maryland
Department of Agricultur

Three Types Of Soil Samples Can Be Recovered From Borings

Three Types Of Soil Samples Can Be Recovered From Borings :


FIGURE 6.1 Soil Samplers (no. 1 is the California Sampler in an open condition,
no. 2 is a Shelby Tube, and no. 3 is the Standard Penetration Test sampler.)


1. Altered Soil.

During the boring operations, soil can be altered due to mixing or contamination. For example, if the boring is not cleaned out prior to sampling, a soil sample taken from the bottom of the borehole may actually consist of cuttings from the side of the borehole. These borehole cuttings, which have fallen to the bottom of the borehole, will not represent in-situ conditions at the depth sampled. In other cases, the soil sample may become contaminated with drilling fluid, which is used for wash-type borings. These types of soil samples that have been mixed or contaminated by the drilling process should not be used for laboratory tests because they will lead to incorrect conclusions regarding subsurface conditions. Soil that has a change in moisture content due to the drilling fluid or heat generated during the drilling operations should also be classified as altered soil. Soil that has been densified by over-pushing or over-driving the soil sampler should also be considered as altered because the process of over-pushing or over-driving could squeeze water from the soil.

2. Disturbed Samples.

Disturbed soil is defined as soil that has been remolded during the sampling process. For example, soil obtained from driven samplers, such as the Standard Penetration Test spilt spoon sampler, or chunks of intact soil brought to the surface in an auger bucket (i.e., bulk samples), are considered disturbed soil. Disturbed soil can be used for numerous types of laboratory tests

3. Undisturbed Sample.

It should be recognized that no soil sample can be taken from the ground in a perfectly undisturbed state. However, this terminology has been applied to those soil samples taken by certain sampling methods. Undisturbed samples are often defined as those samples obtained by slowly pushing thinwalled tubes, having sharp cutting ends and tip relief, into the soil. Two parameters, the inside clearance ratio and the area ratio, are often used to evaluate the disturbance potential of different samplers, and they are defined as follows:


where :
De =diameter at the sampler cutting tip
Di = inside diameter of the sampling tube
Do = outside diameter of the sampling tube

In general, a sampling tube for undisturbed soil specimens should have an inside clearance ratio of about 1% and an area ratio of about 10% or less. Having an inside clearance ratio of about 1% provides for tip relief of the soil and reduces the friction between the soil and inside of the sampling tube during the sampling process. A thin film of oil can be applied at the cutting edge to also reduce the friction between the soil and metal tube during sampling operations. The purpose of having a low area ratio and a sharp cutting end is to slice into the soil with as little disruption and displacement of the soil as possible. Shelby tubes are manufactured to meet these specifications and are considered to be undisturbed soil samplers. As a comparison, the California Sampler has an area ratio of 44% and is considered to be a thick-walled sampler.

It should be mentioned that using a thin-walled tube, such as a Shelby tube, will not guarantee an undisturbed soil specimen. Many other factors can cause soil disturbance, such as :

  • Pieces of hard gravel or shell fragments in the soil, which can cause voids to develop along the sides of the sampling tube during the sampling process
  • Soil adjustment caused by stress relief when making a borehole
  • Disruption of the soil structure due to hammering or pushing the sampling tube into the soil stratum
  • Expansion of gas during retrieval of the sampling tube
  • Jarring or banging the sampling tube during transportation to the laboratory
  • Roughly removing the soil from the sampling tube
  • Crudely cutting the soil specimen to a specific size for a laboratory test
he actions listed above cause a decrease in effective stress, a reduction in the interparticle bonds, and a rearrangement of the soil particles. An ‘‘undisturbed’’ soil specimen will have little rearrangement of the soil particles and perhaps no disturbance except that caused by stress relief where there is a change from the in-situ stress condition to an isotropic ‘‘perfect sample’’ stress condition. A disturbed soil specimen will have a disrupted soil structure with perhaps a total rearrangement of soil particles. When measuring the shear strength or deformation characteristics of the soil, the results of laboratory tests run on undisturbed specimens obviously better represent in-situ properties than laboratory tests run on disturbed specimens.

Soil samples recovered from the borehole should be kept within the sampling tube or sampling rings. The soil sampling tube should be tightly sealed with end caps or the sampling rings thoroughly sealed in containers to prevent a loss of moisture during transportation to the laboratory. The soil samples should be marked with the file or project number, date of sampling, name of engineer or geologist who performed the sampling, and boring number and depth.

Source : Robert W. Day Chief Engineer, American Geotechnical San Diego, California

Soil Mechanichs And Foundations Building Part.1

INTRODUCTION

1. Soil Mechanics

Soil mechanics is defined as the application of the laws and principles of mechanics and hydraulics to engineering problems dealing with soil as an engineering material. Soil has many different meanings, depending on the field of study. For example, in agronomy (application of science to farming), soil is defined as a surface deposit that contains mineral matter that originated from the original weathering of rock and also contains organic matter that has accumulated through the decomposition of plants and animals. To an agronomist, soil is that material that has been sufficiently altered and supplied with nutrients that it can support the growth of plant roots. But to a geotechnical engineer, soil has a much broader meaning and can include not only agronomic material, but also broken-up fragments of rock, volcanicash, alluvium, aeolian sand, glacial material, and any other residual or transported product of rock weathering. Difficulties naturally arise because there is not a distinct dividing line between rock and soil. For example, to a geologist a given material may be classified as a formational rock because it belongs to a definite geologic environment, but to a geotechnical engineer it may be sufficiently weathered or friable that it should be classified as a soil.

2. Rock Mechanics

Rock mechanics is defined as the application of the knowledge of the mechanical behavior of rock to engineering problems dealing with rock. To the geotechnical engineer, rock is a relatively solid mass that has permanent and strong bonds between the minerals. Rocks can be classified as being either sedimentary, igneous, or metamorphic. There are significant differences in the behavior of soil versus rock, and there is not much overlap between soil mechanics and rock mechanics.


3.Foundation Engineering

A foundation is defined as that part of the structure that supports the weight of the structure and transmits the load to underlying soil or rock. Foundation engineering applies the knowledge of soil mechanics, rock mechanics, geology, and structural engineering to the design and construction of foundations for buildings and other structures. The most basic aspect of foundation engineering deals with the selection of the type of foundation, such as using a shallow or deep foundation system. Another important aspect of foundation engineering involves the development of design parameters, such as the bearing capacity of the foundation. Foundation engineering could also include the actual foundation design, such as determining the type and spacing of steel reinforcement in concrete footings. As indicated in Table 6.2, foundations are commonly divided into two categories: shallow and deep foundations.


Table 6.1 presents a list of common soil and rock conditions that require special consideration by the geotechnical engineer.


TABLE 6.1 Problem Conditions Requiring Special Consideration


Source: ‘‘Standard Specifications for Highway Bridges,’’ 16th ed., American Association of State Highway and Transporation Officials, Washington, DC.

4. FIELD EXPLORATION

The purpose of the field exploration is to obtain the following (M. J. Tomlinson, ‘‘Foundation Design and Construction,’’ 5th ed., John Wiley & Sons, Inc., New York)

  1. Knowledge of the general topography of the site as it affects foundation design and construction, e.g., surface configuration, adjacent property, the presence of watercourses, ponds, hedges, trees, rock outcrops, etc., and the available access for construction vehicles and materials.
  2. The location of buried utilities such as electric power and telephone cables, water mains, and sewers.
  3. The general geology of the area, with particular reference to the main geologic formations underlying the site and the possibility of subsidence from mineral extraction or other causes.
  4. The previous history and use of the site, including information on any defects or failures of existing or former buildings attributable to foundation conditions.
  5. Any special features such as the possibility of earthquakes or climate factors such as flooding, seasonal swelling and shrinkage, permafrost, and soil erosion.
  6. The availability and quality of local construction materials such as concrete aggregates, building and road stone, and water for construction purposes.
  7. For maritime or river structures, information on tidal ranges and river levels, velocity of tidal and river currents, and other hydrographic and meteorological data.
  8. A detailed record of the soil and rock strata and groundwater conditions within the zones affected by foundation bearing pressures and construction operations, or of any deeper strata affecting the site conditions in any way.
  9. Results of laboratory tests on soil and rock samples appropriate to the particular foundation design or construction problems.
  10. Results of chemical analyses on soil or groundwater to determine possible deleterious effects of foundation structures.
5. Document Review

Some of the required information, such as the previous history and use of the site, can be obtained from a document review. For example, there may be old engineering reports indicating that the site contains deposits of fill, abandoned septic systems and leach fields, buried storage tanks, seepage pits, cisterns, mining shafts, tunnels, or other man-made surface and subsurface works that could impact the new proposed development. There may also be information concerning on-site utilities and underground pipelines, which may need to be capped or rerouted around the project.

TABLE 6.2 Common Types of Foundations


TABLE 6.2  Common Types of Foundations (Continued)


During the course of the work, it may be necessary to check reference materials, such as geologic and topographic maps. Geologic maps can be especially useful because they often indicate potential geologic hazards (e.g., faults, landslides) as well as the type of near-surface soil or rock at the site. Both old and recent topographic maps can also provide valuable site information. Topographic maps are usually to scale and show the locations of buildings, roads, freeways, train tracks, and other civil engineering works as well as natural features such as canyons, rivers, lagoons, sea cliffs, and beaches. The topographic maps can even show the locations of sewage disposal ponds and water tanks, and by using different colors and shading, they indicate older versus newer development. But the main purpose of the topographic map is to indicate ground surface elevations.

This information can be used to determine the major topographic features at the site and for the planning of subsurface exploration, such as available site access for drilling rigs. Another important source of information is aerial photographs, which are taken from an aircraft flying at a prescribed altitude along preestablished lines. Viewing a pair of aerial photographs, with the aid of a stereoscope, provides a three dimensional view of the land surface. This view may reveal important geologic information at the site, such as the presence of landslides, fault scarps, types of landforms (e.g., dunes, alluvial fans, glacial deposits such as moraines and eskers), erosional features, general type and approximate thickness of vegetation, and drain-
age patterns. By comparing older versus newer aerial photographs, the engineering geologist can also observe any man-made or natural changes that have occurred at the site.

6. Subsurface Exploration

In order for a detailed record of the soil and rock strata and groundwater conditions at the site to be determined, subsurface exploration is usually required. There are different types of subsurface exploration, such as borings, test pits, and trenches. Table 6.3 summarizes the boring, core drilling, sampling, and other exploratory techniques that can be used by the geotechnical engineer.

A boring is defined as a cylindrical hole drilled into the ground for the purposes of investigating subsurface conditions, performing field tests, and obtaining soil,rock, or groundwater specimens for testing. Borings can be excavated by hand (e.g., with a hand auger), although the usual procedure is to use mechanical equipment
to excavate the borings. Many different types of equipment are used to excavate borings. Typical types
of borings are listed in Table 6.3 and include:

  • Auger Boring. A mechanical auger is a very fast method of excavating a boring. The hole is excavated by rotating the auger while at the same time applying a downward pressure on the auger to help obtain penetration of the soil or rock. There are basically two types of augers: flight augers and bucket augers. Common available diameters of flight augers are 5 cm to 1.2 m (2 in to 4 ft) and of bucket augers are 0.3 m to 2.4 m (1 ft to 8 ft). The auger is periodically removed from the hole, and the soil lodged in the groves of the flight auger or contained in the bucket of the bucket auger is removed. A casing is generally not used for auger borings, and the hole may cave-in during the excavation of loose or soft
    soils or when the excavation is below the groundwater table. Augers are probably the most common type of equipment used to excavate borings.
  • Hollow-Stem Flight Auger. A hollow-stem flight auger has a circular hollow core which allows for sampling down the center of the auger. The hollow-stem auger acts like a casing and allows for sampling in loose or soft soils or when the excavation is below the groundwater table.
  • Wash-Type Borings. Wash-type borings use circulating drilling fluid, which removes cuttings from the borehole. The cuttings are created by the chopping, twisting, and jetting action of the drill bit, which breaks the soil or rock into small fragments. Casings are often used to prevent cave-in of the hole. Because drilling fluid is used during the excavation, it can be difficult to classify the soil and obtain uncontaminated soil samples. Rotary Coring. This type of boring equipment uses power rotation of the drilling bit as circulating fluid removes cuttings from the hole. Table 6.3 lists various types of rotary coring for soil and rock.
  • Percussion Drilling. This type of drilling equipment is often used to penetrate hard rock, for subsurface exploration or for the purpose of drilling wells. The drill bit works much like a jackhammer, rising and falling to break up and crush the rock material.
In addition to borings, other methods for performing subsurface exploration include test pits and trenches. Test pits are often square in plan view, with a typical dimension of 1.2 m by 1.2 m (4 ft by 4 ft). Trenches are long and narrow excavations usually made by a backhoe or bulldozer. Table 6.4 presents the uses, capabilities, and limitations of test pits and trenches. Test pits and trenches provide for a visual observation of subsurface conditions. They can also be used to obtain undisturbed block samples of soil.

The process consists of carving a block of soil from the side or bottom of the test pit or trench. Soil samples can also be obtained from the test pits or trenches by manually driving Shelby tubes, drive cylinders, or other types of sampling tubes into the ground. (See Art. 6.2.3.) Backhoe trenches are an economical means of performing subsurface exploration. The backhoe can quickly excavate the trench, which can then be used to observe and test the in-situ soil. In many subsurface explorations, backhoe trenches are used to evaluate near-surface and geologic conditions (i.e., up to 15 ft deep), with borings being used to investigate deeper subsurface conditions.

7. Soil Sampling

Many different types of samplers are used to retrieve soil and rock specimens from the borings. Common examples are indicated in Table 6.3. Figure 6.1 shows three types of samplers, the ‘‘California Sampler,’’ Shelby tube sampler, and Standard Penetration Test (SPT) sampler. The most common type of soil sampler used in the United States is the Shelby tube, which is a thin-walled sampling tube. It can be manufactured to different diameters and lengths, with a typical diameter varying from 5 to 7.6 cm (2 to 3 in) nd a length of 0.6 to 0.9 m (2 to 3 ft). The Shelby tube should be manufactured to meet exact specifications, such as those stated by ASTM D 1587-94 (1998).

The Shelby tube shown in Fig. 6.1 has an inside diameter of 6.35 cm (2.5 in). Many localities have developed samplers that have proven successful with local soil conditions. For example, in southern California, a common type of sampler is the California Sampler, which is a split-spoon type sampler that contains removable internal rings, 2.54 cm (1 in) in height. Figure 6.1 shows the California Sampler in an open condition, with the individual rings exposed. The California Sampler has a 7.6-cm (3.0 in) outside diameter and a 6.35-cm (2.50-in) inside diameter. This sturdy sampler, which is considered to be a thick-walled sampler, has proven successful in sampling hard and desiccated soil and soft sedimentary rock common
in southern California.

source : Robert W. Day Chief Engineer, American Geotechnical San Diego, California

USE OF CONE PENETRATION TEST IN PILE DESIGN

USE OF CONE PENETRATION TEST IN PILE DESIGN

Introduction

The prediction of pile capacity is complicated by the large variety of soil types and installation procedures. Many methods have been proposed to predict the axial capacity of single piles. These methods can be divided in three main groups:

  1. Full scale pile load test: This test exactly describes the piles behaviour with a load-settlement curve. At the moment this is the best method to predict the capacity of a single pile. The disadvantages of this method are: the costs of such a test are high, and it is rarely feasible in the stage of planning.
  2. Calculation methods based on results of laboratory tests: The low accuracyof these methods makes their economical use difficult.
  3. Calculation methods based on results of ‘in-situ’ tests: Among the numerous in-situ devices, the cone penetration test (CPT) and the piezocone (CPTu) represent the most versatile tools for geotechnical design. One of the earliest applications of this device was to predict a pile’s axial capacity. As a model pile it is pushed into the ground and measurements are made of the resistance to penetration of the cone. Using this test the pile capacity can be predicted time- and cost-efficiently even in the stage of planning. Nevertheless the accuracy of thismethod will not achieve that of full scale tests, but the reliability of predictions based on CPT is improving.
Test Site

Most of the tests were performed at the construction of highwayM3 in Hungary, but other tests were also performed at different sites in Hungary. The tested piles were CFA (continuous flight auger) piles. The length of the piles ranged from 6.20 m to 22.00 m, the diameter ranged from 0.60 m to 1.00 m. The pile load tests were performed in different soil conditions.

Interpretation of Pile Load Tests

The ultimate bearing capacity of the tested piles, determined by the method described in the Hungarian Code (MI-04-190), ranged from 830 kN to 3900 kN. The pile axial capacity consists of two components: end bearing load (base resistance) and side friction load (shaft resistance). At the pile load tests only total capacity of the piles is measured, so it was necessary to calculate the base resistance and the shaft resistance of the piles separately, based on the results of the performed tests. These values were determined based on detailed analysis of the test results and soil conditions.

Prediction of Pile Capacity Using CPT Data

The maximum bearing resistance of each single pile was predicted using the following methods:
• DIN 4014 (German Standard) method
• Bustamante and Giasenelli (1982) method (LCPC method)
• EUROCODE 7-3 method (process recommended by EC 7-3)
• ERTC3 method (De Cock, F. – Legrand C., 1997)

In all cases the pile capacity is derived from the known formula:
Fmax = Fmax, base + Fmax, shaft ,

where: 
  • Fmax is the maximum bearing resistance of the pile;
  • Fmax, base is the maximum base resistance;
  • Fmax, shaft is the maximum shaft resistance;
DIN 4014 Method

The German Standard provides different methods for cohesive and non-cohesive soils. In case of non-cohesive soils the unit base and shaft resistance of a bored pile can be calculated using the following tables (Table1, Table 2):





For the calculation of unit base resistance the average cone tip resistance is determined between the pile tip and depth of three times the pile diameter under the tip. In case of cohesive soils the unit base resistance and the unit skin friction can be determined with an indirect method, using the following tables (Table 3, Table 4):



Similarly to the non-cohesive case average c u is determined between the pile tip and depth of three times the pile diameter under the tip.

So the unit base and shaft resistance are calculated using the undrained shear strength (cu) of cohesive soils. There are numerous methods to predict c u based on CPT values, but the method used for this purpose has a great influence on the estimated pile capacity. So the final predicted pile capacity may be different using
different methods for undrained shear strength prediction.



Bustamante and Giasenelli (1982) Method

In this case the unit base and shaft resistances of a pile are calculated directly from the cone resistance (qc) using the following equation:



where:




EUROCODE-7-3 method

In this method the maximum unit base and shaft resistance can be derived form the following equations:


where :

qc,I, mean : is the mean of the qc,I values over the depth running from the pile base level to a level (critical depth) which is at least 0.7 times and at most 4 times the pile base diameter deeper. (Critical depth: where the calculated value of pmax, base becomes a minimum)



qc,II, mean : is the mean of the lowest qc,II values over the depth going upwards from the critical depth to the pile base




qc,III, mean : is themean value of the qc,III values over a depth interval running from the pile base level to a level of 8 times the pile base diameter higher. This procedure starts with the lowest qc,II value used for computation of qc,II, mean.



qc,z is qc at depth z
αp is the pile class factor
αs is a factor depending on the pile class and soil conditions (Table6).


ERTC3 Method

This method has the same process for calculation of unit base resistance as the EC 7-3 method, but the determination of the shaft resistance is modified. It can be calculated with the same process, but αs values are different (Table 7).

A. MAHLER



Fig. 1. DIN 4014 method


Fig. 2. LCPC method

Reliability of the Prediction Methods

In the following figures the results of the pile capacity predictions are shown. In each figure the predicted pile capacity values are plotted against the results of pile load tests (measured pile capacity). The continuous lines represent the perfect prediction ‘zone’.

Fig. 1 shows the results of the DIN 4014 method. As it was mentioned earlier it is an indirect method in case of cohesive soils, so the prediction method of undrained shear strength (cu ) has a serious effect on the calculated pile capacity. So it is obvious that the reliability of this method is varying with various c u prediction
methods.

LCPC (BUSTAMANTE and GIASENELLI, 1982) method provides a direct calculation process using solely the measured cone tip resistance (q c) values. The results of this method are shown in Fig. 2. By comparing the two figures we can say that this process gives similarly accurate pile capacity values. Fig. 3 shows the results of the EUROCODE 7-3 method. Among the investigated methods this one has the most complicated process for estimation of unit base resistance of a pile. This calculation process results in very accurate results, but the prediction of unit skin friction is less accurate in some cases. As it can be seen in Fig. 3 in these particular cases the pile capacities are extremely overestimated. These piles were of large diameter, long piles in stiff clays of high plasticity and in each case the skin friction of the piles were overestimated.

The ERTC 3 method has the same process for determination of unit base resistance, but the calculation process of unit skin friction is modified. In Fig.4 the results of this prediction method are shown. We can see that this method gives reliable results even in cases in which the pile capacities were extremely overestimated by EUROCODE 7-3 method.


Fig. 4. ERTC 3 method


Summary and Conclusion

Four pile capacity methods were evaluated using CPT data for thirteen full scale axial pile load tests. The test piles were CFA piles with various geometry and were measured in various soil conditions. DIN 4014, LCPC, EUROCODE 7-3 and ERTC 3 methods were used for pile capacity predictions.

Although the number of performed tests is not enough for statistical analysis, the results of performed tests are summarized in the following table (Table8):

In case of the EUROCODE 7-3 method the standard deviation and average estimated value are higher than the respective values of othermethods. Nevertheless these differences are caused by the overestimation of pile capacities of some piles. Without these results the EUROCODE 7-3 method gives similarly reliable results for pile capacity.


Table 8. Results of performed tests


It is necessary to note that the pile capacities were generally overestimated. To decide whether it is caused by the differing soil conditions or by differences in the pile installation process requires further studies. Nevertheless it is clear that these methods represent a useful tool in geotechnical design.

References

  1. BUSTAMANTE,M.–GIANESELLI, L., Pile Bearing Capacity Prediction by Means of Static Penetrometer CPT. Proceedings of the 2nd European Symposium on Penetration Testing, ESOPT-II, Amsterdam, 2 (1982), pp. 493–500.
  2. DE COCK,F.–LEGRAND, C. ed. Design of Axially Loaded Piles, European Practice, 1997.
  3. DE COCK,F.–LEGRAND,C.–LEHANE,B.ed. Survey Report on Design Methods for Axially Loaded Piles, European Practice, 1999.
  4. EUROCODE 7: Geotechnical Design – Part 3: Design Assisted by Fieldtesting; European Committee for Standardization, 1999.
  5. DIN 4014 Bohrpfähle – Herstellung, Bemessung und Tragverhalten; Deutsche Institut für Normen, 1990.

Source : PERIODICA POLYTECHNICA SER. CIV. ENG. VOL. 47, NO. 2, PP. 189–197 (2003)

NOTES on the CONE PENETROMETER TEST

Introduction

The standardized cone-penetrometer test  (CPT) involves pushing a 1.41-inch diameter 55o to 60o cone (Figs. 1 thru 3) through the underlying ground at a rate of 1 to 2 cm/sec.  CPT soundings can be very effective in site characterization, especially sites with discrete stratigraphic horizons or discontinuous lenses.  Cone penetrometer testing, or CPT (ASTM  D-3441, adopted in 1974) is a valuable method of assessing subsurface stratigraphy associated with soft materials, discontinuous lenses, organic materials (peat), potentially liquefiable   materials (silt, sands and granule gravel) and landslides.

Cone rigs can usually penetrate normally consolidated soils and colluvium, but have also been employed to characterize d weathered Quaternary and Tertiary-age strata.  Cemented or unweathered horizons, such as sandstone, conglomerate or massive volcanic rock can impede advancement of the probe, but the author has always been able to advance CPT cones in materials of Tertiary-age sedimentary rocks.  The cone is able to delineate even the smallest (0.64 mm/1/4-inch thick) low strength horizons, easily missed in conventional (small-diameter) sampling programs.  Some examples of CPT electronic logs are attached, along with hand-drawn lithologic interpretations. 

Most of the commercially-available CPT rigs operate electronic friction cone and piezocone penetrometers, whose testing procedures are outlined in  ASTM D-5778, adopted in 1995.  These devices produce a computerized log of tip and sleeve resistance, the ratio between the two, induced pore pressure just behind the cone tip, pore pressure ratio (change in pore pressure divided by measured pressure) and lithologic interpretation of each 2 cm interval are continuously logged and printed out.

Tip Resistance

The tip resistance is measured by load cells located just behind the tapered cone (Figure 4).  The tip resistance is theoretically related to undrained  shear strength of a saturated cohesive material, while the sleeve friction is theoretically related to the friction of the horizon being penetrated (Robinson and Campanella, 1986,  Guidelines for Use and Interpretation of the Electric Cone Penetration Test, 3rd Ed.: Hogentogler & Co.,   Gaithersburg, MD, 196 p.).   The tapered cone head forces failure of the soil about 15 inches ahead of the tip and  the resistance is measured with an embedded load cell in tons/ft2 (tsf).


Local Friction


The local friction is measured by tension load cells embedded in the sleeve for a distance of 4 inches behind the tip (Figure 4).  They measure the average skin friction as the probes advanced through the soil.  If cohesive soils are partially saturated, they may exert appreciable skin friction, negating the interpretive program.  







Figure 1 (left) – Cone tip exposed beneath truck, just before being advanced into the ground at a rate between 1 and 2 cm/sec.  The cone has an area of either 10 or 15 cm and is typically advanced in 1 m increments because the rods are of that length.

Figure 2 (right) – Hogentogler CPT rig operated by Ertec in Long Beach, CA.  Cone rigs weigh about 18 tons, so are capable of exerting considerable normal force on the advancing rod.   The cone tip includes a plumb meter to warn of out-of-vertical penetration.  


 

 
Figure 3 – Manufacturing and operating tolerances of cones, taken from ASTM D5778.


Figure 4 – Schematic section through an electric friction-cone penetrometer tip, taken from ASTM D3441.

Friction ratio

The friction ratio is given in percent.  It is the ratio of skin friction divided by the tip resistance (both in tsf).  It is used to classify the soil, by its behavior, or reaction to the cone being forced through the soil.   High ratios generally indicate clayey materials (high c, low Ø) while lower ratios are typical of  sandy materials (or dry desiccated clays).  Typical skin friction to tip friction ratios  are 1 % to 10%.  The ratio seldom, if ever, exceeds 15%.  Sands are generally identified by exhibiting a ratio < 1%. 

Piezocones also measure insitu pore pressure (in psi), in either dynamic (while advancing the cone) or static (holding the cone stationary) modes.  Piezocones employ a porous plastic insert just behind the tapered head that is made of hydrophilic polypropylene, with a nominal particle size of 120 microns (Figure 5).  The piezocell must be saturated with glycerin prior to its employment.  The filter permeability is about 0.01 cm/sec (1 x 10-2 cm/sec).

When using the cone to penetrate dense layers, such as cemented siltstone, sandstone or conglomerate, the piezo filter element can become compressed, thereby inducing high positive pore pressures.  But, the plastic filters do not exhibit this tendency, though they do become brittle with time and may need to be  replaced periodically.  In stiff over-consolidated clays  the pore pressure gradient around the cone may be quite
high.  This pore pressure gradient often results in dissipations recorded behind the CPT tip that initially increase before decreasing to the equilibrium value.


Figure 5- Schematic section  through a piezocone head, showing  the piezo-element and friction sleeve.Taken  from ASTM D5778.

Differential Pore Pressure

The Differential Pore Pressure Ratio is used to aid in soil classification according to the Unified Soil Classification System (USCS).   When the cone penetrates dense materials like sand, the sand dilates and the pore pressure drops.  In clayey materials high pore pressures may be induced by the driving of the cone head.  If transient pore pressures are being recorded that seem non-hydrostatic, most experienced operators will ask that the penetration be halted and allowed at least 5 minutes to equilibrate, so a quasi-static pore pressure reading can be recorded.  Sometimes equilibration can take 10 to 30 minutes, depending on the soil.  In practice experienced operators try to stop the advance and take pore pressure measurements in recognized aquifers and just above or adjacent to indicated aquacludes.


Temperature sensor

One great advantage of the electric cone is the temperature sensor.  This has been found to be very useful in assessing the precise position of the zone, or  zones, of saturation, which is of great import in slope stability and consolidation studies. A temperature shift of about 6o F is common at the groundwater interface, even perched horizons within landslides. 

Corrected Logs

Most CPT rigs are equipped  with one or several automated interpretation programs, which classify 1 cm horizons according to the Unified Soil Classification System.  The most widely employed routine has been that originally developed by Robinson and Campanella, available from Hogentogler & Co., of Gaithersburg, MD or from the Natural Sciences and Engineering Research Council of Canada.  An alternative interpretation  program was developed by Dr. Richard Olsen (available through www.liquefaction.com).

The interpretation programs evaluates all of  the measured properties and classifies the horizon according to its behavior (in lieu of petrology).  For instance, when classifying a clayey material the interpretive programs consider undrained shear strength, tip resistance and differential pore pressure.  A high differential pore pressure is assumed diagnostic of more clayey materials.

Importance of “ground-truthing”

Like geophysical techniques, CPT soundings are most meaningful when “ground-truthed” with established lithologic horizons. The easiest method to “ground truthing” CPT data is to advance a sounding next to a bucket auger or conventional boring, from which subsurface samples are collected.  In  this way the electronic “signature” of the sounding can be compared with the various lithologies already identified in the substory.  This comparison can prove especially valuable in identifying potentially liquefiable materials and old landslide slip surfaces.   Once the CPT sounding is “ground-truthed”, the rig can traverse the job site, commonly advancing 10 or 12 soundings in a single day. This allows for an expanded data set,  which allows superior three-dimensional characterization of the site under evaluation, and allows construction of reliable geologic cross sections through the area.

Notes of Caution

Some notes of caution are advised when applying the CPT method to evaluating discrete low-strength horizons or partings, such as landslide slip surfaces.  The 60o tip of the cone forces a passive failure of the ground in front of the  advancing tip.  The instrumented tip senses soil resistance about 21cm (8.4 in)  ahead of the advancing tip. 

This means that the tip resistance reported as “undrained shear strength” is actually an average value, taken over the zone within 21 cm of the cone tip.  If the tip penetrates low strength horizons less than 21 cm thick, such as a landslide slip surface, the tip resistance reported on the CPT log may be much higher than actually exists on the discrete plane of slippage, which maybe only a fraction of an inch thick.

Another problem with the CPT method is that cone soundings advanced through desiccated clay will often be interpreted as sand or silt mixtures (by the computerized lithologic interpretation routine) because of  recorded sleeve friction.  The opposite  problem occurs when reporting Standard Penetration Test (SPT) blow counts after advancing drive samples through clayey horizons!  The SPT test is only intended for granular materials, and blow counts in such materials must be regarded with some degree of skepticism as they may shift dramatically upon later absorption of moisture.

Sample CPT logs

The attached logs are representative of the features common to electronic friction cones.  They include raw data sensed by the cone as it is pushed through the ground.  This data includes: Friction Ratio, Local Friction, Tip Resistance, Pore Pressure, Differential Pore Pressure Ratio and an interpreted lithologic profile (often printed out on a separate sheet, depending on which interpretation program is being utilized). 








Electric Cone Penetration Test (CPT)

Cone penetration tests (CPT)

An Electric Cone Penetration Test (CPT) is a  geomechanical probing technique for shallow  subsurface exploration. Probing through weak  ground to locate firmer strata at depth has been  practised since 1917, but CPT developed into its  final form in the Netherlands during the 1930’s  (Lunne et al. 1997). CPT combines rapid and  cheap insight in the mechanical composition of  the subsurface in the upper tens of meters. The  widest application is currently found in  geomechanical applications, i.e. surveys for road  and railway constructions and the foundation of  buildings and houses in areas with weak  subsurface. The principles of CPT are published in  Lunne et al. (1997) and Coerts (1996).

Principles of cone penetration tests

Cone resistance, sleeve friction and friction ratio
CPT surveying involves the penetration of a metal electrical cone with a surface of 10 cm2 into the  subsurface (Fig. 4.9.1). From beneath a heavy  truck, the cone is penetrated at a constant rate of  1 cm/s. During penetration, a number of variables  are recorded at the cone head or along the  sleeve. At the cone head the cone resistance (qc)  is recorded (in MPa), which expresses the  resistance of the sediments to penetration. Along  the cone the sleeve friction (fs) is recorded (also in  MPa); indicative for the adhesive strength of the  material.

Terminology for cone penetrometers
(from Lunne et al. 1997).

From the cone resistance and the sleeve friction  the friction ratio (Rf) can be calculated according: 

Rf = [(fs /qc)*100]

Numerous analyses of data have lead to an  empirical relationship between Rf and inferred  lithology (Table Below). The friction ratio is, in  combination with cone resistance, broadly used in geomechanical applications.

Empirical relation between the dimensionless friction ratio and inferred lithology in CPT.


Pore water pressure

Another useful parameter that can be recorded in CPTU surveying (the so-called piezocone test) is
the pore pressure u (Fig. 4.9.1). In the saturated or vadose zone increasing values occur with increasing depth, expressed in MPa. Also perched ground water tables can be detected using this technique.

Data interpretation

CPT interpretation mainly involves pattern analysis of the cone resistance and friction ratio
curves. In common practice it is possible to define CPT „facies“ for certain sedimentary deposits. In buried valley environments for example, the friction ratio curve characteristics of “pot clay“ (or  Lauenburger Ton) are well known. Similar typical CPT facies units can be defined for cover sands, boulder clay (till), several fluvial deposits and so forth. Figure 4.9.2 demonstrates an example of a CPT plot in which typical “pot clay“
patterns can be recognized between 22 and 29 m depth. Less distinct are the clayey deposits between 0 and 22 m depth.


Fig 2

 Fig.2 - CPT log showing sedimentary units associated to buried valley infill, including “pot clay“ deposits (PENI). PE=Peelo Formation; PENI = Nieuwolda Member of the Peelo Formation, BX = Boxtel Formation (Weichselian deposits).

Ideally, site-specific CPT „facies“ have to be verified with borehole data or observations from exposures to ascertain relationships with actual lithofacies units.


Application of CPT in the study of buried valleys

CPT is a useful, fast and cost-effective technique that can be used for the following applications related to the characterisation of buried valleys;
1. the establishment of the occurrence, extenand upper boundary of “pot clay“ bodies
2. the establishment of protecting impermeable beds above buried valley aquifer systems (such as “pot clay“, boulder clay, etc.)
3. characterisation of the upper sedimentary records outside buried valleys.

Figure 5.6.8 presents a CPT transect, combined with lithological columns of boreholes, of the
Groningen Burval project area. Particularly between 2 and 4 km along the profile, a large clayey body is identified. The presence and rough outline of this unit was also demonstrated by Helicopter Electromagnetics . Below this unit other sediments associated with the Peelo Formation occur, while near the surface Weichselian deposits are present (Boxtel Formation). Figure 4.9.3 shows an enlarged part of Figure 5.6.8 to demonstrate the correlation more clearly.

The CPT characteristics of the “pot clay“ (PENI) are clearly defined. The deeper undifferentiated  deposits of the Peelo Formations are characterised by a strong lateral and vertical heterogeneity.





Fig. 4.9.3:  Enlarged part of Figure 5.6.8. CPT transect with lithological columns of boreholes across unit of low-resistivity in HEM data. PE=Peelo Formation; PENI=Nieuwolda Member of the Peelo Formation, consisting of “pot clay“. BX =Boxtel Formation (Weichselian deposits).

Some remarks on the application of CPT

CPT can be broadly used in unconsolidated sediments; however there are certain limitations that have to be kept in mind:

  1. The empirical relationship presented in Table 4.9.1 is based on observations below the ground water table. Above the ground water table a clayey bed (for example) can be partly dried out, leading to higher cone resistance and lower sleeve friction, hence lowering the friction ratio number. However in a climate with excess rainfall like that in northwestern Europe, hanging water is likely in the  unsaturated zone. Hence it should be  possible to discriminate clayey beds from  their sand or gravely counterparts in the  unsaturated zone.
  2. In buried valley environments glaciotectonized sediments can occur. The same is true for overconsolidated sediments due to glacial loading. In both situations geomechanical properties of the sedimentary record is potentially modified: sediments could respond different to CPT than expected in in situ sedimentary sequences following the stratigraphical rule (Bakker 2004). In situations with overconsolidated clay adhesive strength is relatively high, but possibly reduced due to mechanical expel of pore water. Cone resistance is enhanced (higher compaction), leading to a reduced Rf. Hence, Rf values can differ from normal sedimentaryconditions, with normal setting and compaction due to overburden.
The examples demonstrated above, attest that CPT is a powerful technique for the identification and mapping of large sedimentary units to a maximum depth of about 60 m below the surface. Combined with the low-costs CPT is a technique that is highly recommendable in environments with unconsolidated sediments such as buried valley systems.

References :

  1. Bakker MAJ (2004): The internal structure of  Pleistocene push moraines. A multidisciplinary approach with emphasis on ground- penetrating radar. – PhD thesis, Queen Mary,  University of London, 177 pp.
  2. Coerts A (1996): Analysis of static cone penetration test data for subsurface modelling. A methodology. – PhD thesis, Netherlands Geographical Studies 210, 263 pp.
  3. Lunne T, Robertson PK, Powell JJM (1997): Cone Penetration Testing in Geotechnical Practice. – Blackie Academic & Professional, London, 312 pp.
Source : Burval

Cone Penetration Test

Description

Cone Penetrometer Technology (CPT) is a method of providing real-time data for use in characterizing the subsurface, as opposed to older methods of analyzing subsurface conditions in the laboratory. It consists of a steel cone that is hydraulically pushed into the ground at up to 40,000 pounds of pressure. Sensors on the tip of the cone collect data. Standard cone penetrometers collect information to classify soil type by using sensors that measure cone-tip pressure and friction. CPT is often used in conjunction with Hydropunch tests, which use the CPT holes to extract groundwater for laboratory analysis. An innovation of the CPT (i.e., the wireline CPT) allows multiple CPT tools to be interchanged during a single penetration, without withdrawing the CPT rod string from the ground.

Initially developed to collect information about soil characteristics, as sensor technology was developed CPT also became a platform for collecting information about a variety of contaminants. Recent advances in sensor technology have expanded cone penetrometer capabilities to detect the presence of petroleum hydrocarbons. Sensors are being tested or demonstrated for the detection of other organics, compounds, metals, radioactivity, explosives, and soil moisture.

 Cone penetration testing (CPT) is the most versatile device for in situ soil testing. Without disturbing the ground, it provides information about soil type, geotechnical parameters like shear strength, density, elastic modulus, rates of consolidation and environmental properties. Further, as it can be seen as a small scale test pile, it is the best and most cost-effective device to design piled foundations and sheet piles.


Limitations and Concerns

CPT cannot be used at some sites due to high soil density. Most sensors are now used as screening tools that provide initial site characterization data. The data is confirmed by collecting samples that are analyzed in the laboratory. This is due to limitations in sensor technology, and it will likely diminish in importance as the technology improves.

CPT is useful on sites that contain unconsolidated sediments (e.g., soil and clay that are not cemented together). On the other hand, sites with large boulders, rock or cemented layers are difficult to penetrate.
CPT sensors, such as lasers, that require a lens may be hampered by fouling of the lens due to a reaction to dust. Decontamination may be necessary if the CPT comes into contact with contaminated material.


Cone Penetration Tests are conducted to obtain the cone resistance, the side friction and, if there is a piezocone, the pore pressure. The soil type can be determined by analysing these results, the values can also be used in the design of shallow foundations through the estimation of stiffness and shear strength of cohesive soils.
A 60o cone with face area 10cm2 and 150cm2  'friction sleeve' is hydraulically pushed into the ground at a constant speed (ranging form 1.5 to 2.5 cm/s). The force required to maintain this penetration rate, and the shear force acting on the friction sleeve are recorded. The friction ratio (cone resistance / side friction) gives an indication of the soil type.
Cone Resistance qc = Fc / Ac
Side Friction fs = Fs / As
Friction Ratio Rf = fs / qc
Where  Fc = pushing force, Ac = cone plan area, Fs = shear force on friction sleeve, As = area of friction sleeve.


 source : civcal.media.hku.hk, www.conepenetration.com, www.cpeo.org

California Bearing Ratio C.B.R





California Bearing Ratio  C.B.R

The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%).  It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.) (AASHTO, 2000).  It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states, counties, U.S. federal agencies and internationally.  As a result, most agency and commercial geotechnical laboratories in the U.S. are equipped to perform CBR tests.

The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05") per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.).  Figure Below is a sketch of a typical CBR sample.
CBR Sample

Values obtained are inserted into the following equation to obtain a CBR value:

where: x = material resistance or the unit load on the piston (pressure)
for 2.54 mm (0.1") or 5.08 mm (0.2") of penetration

y = standard unit load (pressure) for well graded crushed stone


= for 2.54 mm (0.1") penetration = 6.9 MPa (1000 psi)


= for 5.08 mm (0.2") penetration = 10.3 MPa (1500 psi)

Table below shows some typical CBR ranges.


Typical CBR Ranges

Typical CBR Ranges
General Soil Type
USC Soil Type
CBR Range
Coarse-grained soils
GW
40 - 80
GP
30 - 60
GM
20 - 60
GC
20 - 40
SW
20 - 40
SP
10 - 40
SM
10 - 40
SC
5 - 20
Fine-grained soils
ML
15 or less
CL LL < 50%
15 or less
OL
5 or less
MH
10 or less
CH LL > 50%
15 or less
OH
5 or less

Standard CBR test methods are:
AASHTO T 193: The California Bearing Ratio
ASTM D 1883: Bearing Ratio of Laboratory Compacted Soils
Source : training.ce.washington.edu

Other source :

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation.
The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO T193.
The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High quality crushed rock has a CBR over 80. The standard material for this test is crushed California limestone which has a value of 100.

CBR=\frac {p}{p_s} \cdot 100 \quad

CBR \quad = CBR [%]
p \quad = measured pressure for site soils [N/mm²]
p_s \quad = pressure to achieve equal penetration on standard soil [N/mm²]

 source : wikipedia

CBR Component :


  1. Swell Plate : Contact end of the stem is easily locked in place with a knurled nut. 
  2. Swell Tripod Attachment : Metal tripod supports dial gauge for measuring the amount of swell during soaking. Attachment is used with swell plate. Order dial indicator separately.
  3. Dial Indicator : Dial indicator has 1.000" operating range, graduated in 0.001" divisions, clockwise movement and revolution counter.
  4. Cutting Edge : Machined from seamless tubing with a sharpened edge to enable undisturbed samples to be taken in the field, cutting edge is plated for rust resistance. Cutting edge has 6" (152mm) ID and is 2" (51mm) high. Recess in upper section allows edge to be mounted at either end of the Compaction Mold or CBR Mold with Perforated Base mold to facilitate sample removal in the field.
  5. Filter Screen :
  6. Filter Paper : used to separate spacer disc and soil in the CBR mold during compaction operation or over the top surface of the soil when the compaction operation is completed.
  7. Surcharge Weight : Used in the application of surcharged loads on the soil‘s surface during soaking and penetration. Rust-resistant, plated annular disc weighs 5 lbs. (2.3kg), 5-7/8" (149mm) OD with a 2-1/8" (54mm) ID hole in center.
  8. Slotted Surcharge Weight 
  9. Spacer Disc : Disc is used as a false bottom in a soil mold during the compaction process.
  10. Penetration Piston : CBR Penetration Piston has 3 sq. in. (19.35cm2) base area and is about 7-1/2" (191mm) long. Designed for use in conjunction with weights Surcharge Weight  and Slotted Surcharge Weight to apply penetration surcharge loads.
  11. Dial Indicator Bracket : Bracket used to attach a dial indicator to the penetration piston.
  12. Swivel Base : for mechanical jack
 source : humbolt

TYPE OF CALIFORNIA BEARING RATIO 

1. 




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