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.
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.
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:
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.
in southern California.
source : Robert W. Day Chief Engineer, American Geotechnical San Diego, California
4 comments:
Words are technically laid down but colloquially delivered and explained. With the help of my Engineer friend, he explained to me some words I just seldom hear on science books and news breaks. I am in deep research for our industrial design project, and our professor pin-points about our building design's relation to the target location's geology and topography. We then toggled to soil stabilization, methods of proper excavation and placing a structure beyond its surface. And then we need to come up with non-subjective ideas to repair cracked foundation and all possible downfall if ever our proposed design exists in the real world.
With these, our long way to finishing our student thesis will go smoothly, just like how words are explained here.
And now, even the question of how to repair foundations and other problems are all answerable.
Thanks to this article! I now fully understand the importance of soil in laying foundation.
I am actually about to attempt building my own home within the next few years, so this information will hopefully be of great use to me. What's the best way to prevent problems happening with the foundation for as long as possible? I know that eventually I will probably need San Antonio foundation repair, but I want to postpone that for as long as I can. Thanks again for the great advice.
This is really a nice information.Must necessary to share this blog . This will help to get full knowledge and help also for ours coming future and u also.
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