For the purposes of foundation design, earth materials are classified according to particle size, the presence of organic content, and, in the case of finer grained soils, sensitivity to moisture content:
- Rock is a continuous mass of solid mineral material, such as granite or limestone, which can only be removed by drilling and blasting. Rock is never completely monolithic, but is crossed by a system of joints (cracks) that vary in quantity and extent and that divide the rock into irregular blocks. Despite these joints, rock is generally the strongest and most stable material on which a building can be founded.
- Soil is a general term referring to any earth material that is particulate.
- If an individual particle of soil is too large to lift by hand or requires two hands to lift, it is a boulder.
- If it takes the whole hand to lift a particle, it is called a cobble.
- If a particle can be lifted easily between thumb and forefinger, the soil is gravel. In the Unified Soil Classification System , gravels are classified as having more than half their particles larger than 0.19 inch (4.75 mm) in diameter but none larger than 3 inches (76 mm).
- If individual soil particles can be seen but are too small to be picked up individually, the soil is sand. Sand particles range in size from about 0.19 to 0.003 inch (4.75– 0.075 mm). Both sand and gravel are referred to as coarse-grained soils.
- Individual silt particles are too small to be seen with the unaided eye and range in size from 0.003 to 0.0002 inch (0.075–0.005 mm). Like coarse-grained soil particles, silt particles are roughly spherical, or equidimensional, in shape.
- Clay particles are plate-shaped rather than spherical (Figure 1) and smaller than silt particles, less than 0.0002 inch (0.005 mm) in size. Both sands and silts are also referred to as fine-grained soils.
- Peat, topsoil, and other organic soils are not suitable for the support of building foundations. Because of their high organic matter content, they are spongy, they compress easily, and their properties can change over time due to changing water content or biological activity within the soil.
Properties of Soils
The ability of a coarse-grained soil (gravel or sand) to support the weight of a building depends primarily on the strength of the individual soil particles and the friction between them. Imagine holding a handful of spherical, smooth ball bearings: If you squeeze the bearings, they easily slide past one another in your hand. There is little friction between them.
However, if you squeeze a handful of crushed stone, whose particles have rough, angular facets, the frictional forces between the particles are large, and there will be little movement between them. This resistance to sliding, or shear resistance, of the crushed stone is also directly proportional to the confining force pushing the particles together.
Thus, sand confined by surrounding soil within the earth can support a heavy building, whereas a conical pile of sand deposited loosely on the surface of the ground can support very little, because there is little or no shear resistance between the unconfined particles. Soils that behave in this manner are termed frictional or cohesionless.
In fine-grained soils, particles are smaller, particle surface area is larger in relation to size and weight, and the spaces between particles, or soil pores, are smaller. As a consequence, surface forces also affect the properties of these soils. The properties of silts are more sensitive to the amount of water in the soil than are those of coarse-grained soils. With sufficient moisture content, capillary forces can reduce friction between particles and change the state of silt from solid to liquid. Clay particles, being extremely small and flatter in shape, have surface-area-to-volume ratios hundreds or thousands of times greater than even those of silt.
Electrostatic repulsive and attractive forces play an important role in clay soil’s properites, as do variations in the arrangement, or fabric, of the particles in sheets or other structures that are more complex than the simple close-packing typical of spherical particles in coarse-grained soils and silts. As a result, clays are generally cohesive; that is, even in the absence of confining force, they retain measurable shear strength.
Put simply, cohesive soils tend to stick together. It is often possible to dig vertical-walled excavations in clay soil (Figure 2). There is sufficient shear strength in the unconfined soil to prevent the soil wall from sliding into the excavation. In contrast, a cohesionless soil such as sand must be excavated at a much more shallow angle to avoid the collapse of the excavation wall. Cohesive soils also tend to be hard when dry and moldable, or plastic, when moist. Some silts also exhibit cohesive properties, though generally to a lesser extent than clays.
Soils for Building Foundations
Generally, soil groups listed toward the top are more desirable for supporting building foundations than those listed further down. The higher-listed soils tend to have better soil engineering properties, that is, they tend to have greater loadbearing capacity, to be more stable, and to react less to changes in moisture content.
Rock is generally the best material on which to found a building. When rock is too deep to be reached economically, the designer must choose from the strata of different soils that lie closer to the surface and design a foundation to perform satisfactorily in the selected soil.
Figure 3 gives some conservative values of loadbearing capacity for various types of soil. These values give only an approximate idea of the relative strengths of different soils; the strength of any particular soil is also dependent on factors such as the presence or absence of water, the depth at which the soil lies beneath the surface, and, to some extent, the manner in which the foundation acts upon it.
In practice, the designer may also choose to reduce the pressure of the foundations on the soil to well below these values in order to reduce the potential for building settlement. The stability of a soil is its ability to retain its structural properties under the varying conditions that may occur during the lifetime of the building. In general, rock, gravels, and sands tend to be the most stable soils, clays the least stable, and silts somewhere in between.
Many clays change size under changing subsurface moisture conditions, swelling considerably as they absorb water and shrinking as they dry. In the presence of highly expansive clay soils, a foundation may need to be designed with underlying void spaces into which the clay can expand to prevent structural damage to the foundation itself.
When wet clay is put under pressure, water can be slowly squeezed out of it, with a corresponding gradual reduction in volume. In this circumstance, long-term settlement of a foundation bearing on such soil is a risk that must be considered. Taken together, these properties make many clays the least predictable soils for supporting buildings.
The fine-grained soil groups having a liquid limit greater than 50 are generally the ones most affected by water content, exhibiting higher plasticity (moldability) and greater expansion when wet and lower strength when dry. In regions of significant earthquake risk, stability of soils during seismic events is also a concern.
Sands and silts with high water content are particularly susceptible to liquefaction, that is, a temporary change from solid to liquid state during cyclic shaking. Soil liquefaction can lead to loss of support for a building foundation or excessive pressure on foundation walls. The drainage characteristics of a soil are important in predicting how water will fl ow on and under building sites and around building substructures.
Where a coarse-grained soil is composed of particles mostly of the same size, it has the greatest possible volume of void space between particles, and water will pass through it most readily. Where coarse-grained soils are composed of particles with a diverse range of sizes, the volume of void space between particles is reduced, and such soils drain water less efficiently.
Coarsegrained soils consisting of particles of all sizes are termed well graded or poorly sorted, those with a smaller range of particle sizes are termed poorly graded or well sorted, and those with particles mostly of one size are termed uniformly graded (Figure 4).
Because of their smaller particle size, fine-grained soils also tend to drain water less efficiently: Water passes slowly through very fine sands and silts and almost not at all through many clays. A building site with clayey or silty soils near the surface drains poorly and is likely to be muddy and covered with puddles during rainy periods, whereas a gravelly site is likely to remain dry.
Underground, water passes quickly through strata of gravel and sand but tends to accumulate above layers of clay and fine silt. An excellent way to keep a basement dry is to surround it with a layer of uniformly graded gravel or crushed stone. Water passing through the soil toward the building cannot reach the basement without first falling to the bottom of this porous layer, from where it can be drawn off in perforated pipes before it accumulates.
It does little good to place perforated drainage pipes directly in clay or silt because water cannot fl ow through the impervious soil toward the pipes. Rarely is the soil beneath a building site composed of a single type. Beneath most buildings, soils of various types are arranged in superimposed layers (strata) that were formed by various past geologic processes.
Frequently, soils in any one layer are themselves also mixtures of different soil groups, bearing descriptions such as well-graded gravel with silty clay and sand, poorly graded sand with clay, lean clay with gravel, and so on. Determining the suitability of any particular site’s soils for support of a building foundation, then, depends on the behaviors of the various soils types and how they interact with each other and with the building foundation.
Subsurface Exploration and Soils Testing
Prior to designing a foundation for any building larger than a single-family house (and even for some single-family houses), it is necessary to determine the soil and water conditions beneath the site. This can be done by digging test pits or by making test borings. Test pits are useful when the foundation is not expected to extend deeper than roughly about 16 feet (3 m), the maximum practical reach of small excavating machines. The strata of the soil can be observed in the pit, and soil samples can be taken for laboratory testing.
The level of the water table (the elevation at which the soil is saturated and the pressure of the groundwater is atmospheric) will be readily apparent in coarse-grained soils if it falls within the depth of the pit because water will seep through the walls of the pit up to the level of the water table.
On the other hand, if a test pit is excavated below the water table in clay, free water will not seep into the pit because the clay is relatively impermeable. In this case, the level of the water table must be determined by means of an observation well or special devices that are installed to measure water pressure. If desired, a load test can be performed on the soil in the bottom of a test pit to determine the stress the soil can safely carry and the amount of settlement that should be anticipated under load.
If a pit is not dug, borings with standard penetration tests can give an indication of the bearing capacity of the soil by the number of blows of a standard driving hammer required to advance a sampling tube into the soil by a fixed amount. Laboratory-quality soil samples can also be recovered for testing.
Test boring extends the possible range of exploration much deeper into the earth than test pits and returns information on the thickness and locations of the soil strata and the depth of the water table. Usually, a number of holes are drilled across the site; the information from the holes is coordinated and interpolated in the preparation of drawings that document the subsurface conditions for the use of the engineer who will design the foundation (Figure 2.8).
Laboratory testing of soil samples is important for foundation design. By passing a dried sample of coarsegrained soil through a set of sieves with graduated mesh sizes, the particle size distribution in the soil can be determined. Further tests on fine-grained soils assist in their identification and provide information on their engineering properties.
Important among these are tests that establish the liquid limit, the water content at which the soil passes from a plastic state to a liquid state, and the plastic limit, the water content at which the soil loses its plasticity and begins to behave as a solid.
Additional tests can determine the water content of the soil, its permeability to water, its shrinkage when dried, its shear and compressive strengths, the amount by which the soil can be expected to consolidate under the load, and the rate at which consolidation will take place. The last two qualities are helpful in predicting the rate and magnitude of foundation settlement in a building.
The information gained through subsurface exploration and soil testing is summarized in a written geotechnical report. This report includes the results of both the field tests and the laboratory tests, recommended types of foundations for the site, recommended depths and bearing stresses for the foundations, and an estimate of the expected rate of foundation settlement.
This information can be used directly by foundation and structural engineers in the design of the excavations, dewatering and slope support systems, foundations, and substructure. It is also used by contractors in the planning and execution of sitework.