INTRODUCTION
Geosynthetics include exclusively manmade polymeric products such as geotextiles, geogrids, geonets, geomembranes, geosynthetic clay liners, and geocomposites. The term “geosynthetic” is used in favor of
geotextiles and geofabrics because
- additional polymeric products are being developed and used with soils and
- the application is becoming more diversified.
Polypropylene, polyester, polyethylene, polyamide, polyvinyl choride, and polystyrene are the major polymers used to manufacture geosynthetics. It is not the properties of the polymers, but the properties of the final polymeric products that are of interest to civil and environmental engineering applications. Geosynthetics are used as part of the geotechnical, transportation, and environmental facilities. Geosynthetic products perform
five main functions: separation, reinforcement, filtration, drainage, and containment (hydraulic barrier). However, in most applications, geosynthetics typically perform more than one major function.
TESTING STANDARDS AND DESIGN
A. Testing Standards
Some basic standards used for geotextiles are adopted from the textile industries. However, geotechnical engineers realized the deficiencies and started to develop the standards relevant to their applications. The American Society for Testing and Materials (ASTM) is one developer of standardized testing procedures for
different geosynthetics. Most testing standards adopted or developed in other countries are outgrowths of ASTM standards. The Geosynthetic Research Institute (GRI) also provides testing standards to serve industrial needs, especially when related ASTM standards have not been developed. GRI standards are usually removed as related ASTM standards become available. ASTM standards, developed under Committee D35 for Geosynthetics, are listed in the Appendix. These standards were developed under several
subcommittees: terminology, mechanical properties, endurance properties, permeability and filtration, geosynthetic clay liners, geosynthetic erosion control, and geomembranes. New standards are constantly being developed. Details of test standards are published in ASTM (2000).
While ASTM standards are index tests, many civil engineering designs and applications require the geosynthetic materials to be tested with site-specific soils, with the testing conditions representing those in the field. These kinds of tests are known as performance tests.
B. Design by Function
Different organizations, agencies, and manufacturers provide design guidelines for geosynthetic applications. These design methods are determined by cost, specification (design by specification), or function (design by function). Public agencies have widely adopted the design-by-specification method. The minimum required value of the geosynthetic properties used in a particular application is specified. In the design-by-function method, the primary function of the geosynthetic material is identified. The available and required value of the particular property for that function is assessed to give a factor of safety (FS):
FS = allowable property from testing / required property for design
Designs require a factor of safety greater than unity to account for various uncertainties.
source : Marcel Dekker Inc (Reinforced Soil Engineering)
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
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
- Take a soil test every 3 to 4 years. Fertilize according tosoil test recommendations. Use less than the recommended amounts listed on fertilizer packages.
- 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.
- 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.
- 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.
- Discard plants with serious disease problems.
- 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.
- 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.
- 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.
- 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.
- 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 :
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
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)
- 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.
- The location of buried utilities such as electric power and telephone cables, water mains, and sewers.
- 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.
- The previous history and use of the site, including information on any defects or failures of existing or former buildings attributable to foundation conditions.
- Any special features such as the possibility of earthquakes or climate factors such as flooding, seasonal swelling and shrinkage, permafrost, and soil erosion.
- The availability and quality of local construction materials such as concrete aggregates, building and road stone, and water for construction purposes.
- For maritime or river structures, information on tidal ranges and river levels, velocity of tidal and river currents, and other hydrographic and meteorological data.
- 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.
- Results of laboratory tests on soil and rock samples appropriate to the particular foundation design or construction problems.
- 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
