Chapter 3
Soils and Foundations

Soil conditions must be investigated to determine the suitability of construction materials. Field exploration provides soil samples from various strata. The bearing capacity and settlement characteristics of the foundation and the stability of slopes and earth pressures on supporting structures are determined from soil samples. Subsurface strata and groundwater conditions indicate potential problems in excavating and/or removing groundwater during construction. Residential construction precautions are necessary in some geographic areas because of sinkholes, caves and mine tunnels.

Soil characteristics vary widely from one geographic region to another. Soil on steep slopes is generally less productive than soil on gentle slopes. Soil developed from sandstone is more sandy and less fertile than soil formed from shale. Soil properties are quite different when developed under tropical climates compared with temperate or arctic conditions. Scientists have recognized soil variations and have established classification systems of characteristics for identifying sandy, clay, loam and other soils. Soil variations dictate the residential foundations viable in different geographic locations. Most foundation problems are diagnosed from analysis of soil composition.

SOILS
Soil is produced by material deposits and geologic forces. Soil characteristics are determined by physical and mineral composition of the parent material, climate under which the soil material has existed since accumulation, plant and animal life on and in the soil, topography and length of time forces have acted on the material. All determining factors influence soil characteristics, and the significance of each factor varies from one geographic location to another. In one area a certain factor may dominate soil formation, whereas in another area the same factor may be less important. The interrelationship of these factors is complex, and the effects of a single factor cannot be isolated and completely evaluated.

Properties
Information relating to soil properties is collected during a soil survey. Evaluation of soil properties is based on field and laboratory tests of samples from the survey area and on laboratory tests of samples of similar soils in nearby areas.

During a soil survey, shallow borings are made and examined to identify and classify soils. Samples are tested in a laboratory to determine grain-size distribution, plasticity, compaction characteristics and mechanical properties. General soil property information can be found for a specific geographic area by consulting the local county soil conservation service.

Survey
A soil survey is an inventory and evaluation of the soils to determine the suitabilities and limitations of land use. Residential construction should not be started without reliable information on the soil-bearing values of the site. Investigation of underlying materials is essential for foundation design. The location and approximate depth of rock, abnormal soil conditions and groundwater level must be considered before residential construction begins.

Soil-bearing values for structures subject to earthquake forces in combination with other vertical loads are increased by approximately 33 percent when the foundation is supported on natural, dense, relatively incompressible bearing strata, such as hardpan rock and cemented or compacted sand and gravel. For tall, flexible structures, sites having firm soil or rock at shallow depth are preferred over sites having weaker soils, even if the sites with weaker soils are a greater distance from a fault. Short, stiff structures are most susceptible to damage when the site is near an active fault.

Ideal sites are relatively rare in seismic areas. Weak soil sites in seismic areas must be made suitable for construction by improving the sites. There is no standard test that will permit prediction of the response of the foundation soil to earthquake shocks. However, information can be obtained in the field and laboratory to assist in the evaluation of the probable behavior of certain soils.

In preparing a soil survey, soil scientists, conservationists and engineers collect extensive field information on erosion, droughts, flooding and other factors that affect various soil use and management. Field experience and collected information on soil properties and performance are used as a basis for predicting soil behavior.

Planners using soil survey information can evaluate the effect of specific land use on productivity and on the environment. Planners use the soil survey to create and maintain a land-use pattern in harmony with the natural soil. Contractors use the soil survey to locate sources of sand, gravel, roadfill and topsoil. Soil surveys also identify areas where bedrock, water or firm soil layers are located.

Soil scientists conduct a soil survey by observing the length and shape of slopes, size of streams and pattern of drainage, native plants or crops, rocks and soil profile. A soil profile is the sequence of natural layers in the soil from the surface to the parent material that has not been changed by leaching or by plant roots.

Soil characteristics are determined from test results, records, field experience and state and local specialists.

Classifications
Soils are classified as gravel, sand, silt and clay based on texture size. Most soil consists of loam. Loam is a mixture of two or more soil ingredients. A soil classification depends on the ingredient that has the most influence on the soil's behavior. A silty clay loam is primarily a clay, but contains silt. An organic silt is composed primarily of silt, but contains organic material. Most organic soils are weaker and more compressible than soils without organic material. (See Figure 3.2.)

Coarse-Grained Soils
Gravel and sand are coarse-grained soils. If individual particles can be seen by the naked eye, the soil is considered coarse-grained. Compacted coarse-grained soils are dense with few voids. Soil compressibility is the reduction of voids under a load and is the major cause of foundation settlement. Gravel and sand-based soils do not compress unless the grains are rearranged by erosion caused by poor drainage or vibration.

Soil shearing is a function that affects foundation performance. The weight of a structure may create stress forces that cause the soil to slide away from the foundation footing. The resistance in coarse-grained soils is due to internal friction. When the foundation footing has a sufficient bearing surface and proper soil compaction, the shearing strength of coarse-grained soils is very high.

Fine-Grained Soils
Fine-grained soils are silts and clays. Silts are finer than sands, but coarser than clays. Clays consist of microscopic flake-shaped crystalline minerals. Silts and clays are cohesive in nature and tend to compress, deform and creep under a load. Generally, silts are more stable than clays. Clays tend to cause major problems in foundation stability because clays expand with moisture.

Three clays that cause foundation problems are kaolinite, montmorillonite and illite. Montmorillonite clay is flakelike and is held together by a weak oxygen-bond linkage. Montmorillonite clay can swell 10 to 15 times its size and exert a force of 3 to 16 tons per square foot. The expanding clay can move a typical house that weighs 300 to 400 pounds per square feet.

A soil engineer rates the potential change of volume in a soil by the plasticity index (PI). The PI is the range of moisture content in which the soil remains plastic. The higher the PI rating, the greater the shrink-swell potential. The foundation design should be based on a report generated from a soil test. Color does not affect the expansiveness of clay; clay can be black, gray, orange, etc. Silts or sands obtain bearing strength by friction, and clays obtain bearing strength by cohesion.

It is important that an inspector be familiar with the areas that he or she inspects because of the local soil conditions. A prudent inspector needs to be knowledgeable about the various requirements of the local and national codes for good construction practice. However, a real estate inspector does not inspect to codes and is not a code inspector, but having the knowledge will help when inspecting the property.

There are factors other than the type of soil that influence the construction of a foundation. The topography is very important, as is whether the property is situated on a lake, creek or river; the depth of the frost line for the area; the depth of the water table or underground springs (although underground springs may not be active year-round)-all help determine the recommended depth and type of footing required.

Sometimes it is difficult because of the landscaping to visualize the topography of the property as it was before the structure was built, but in most cases the inspector is able to determine if (1) the property is located in an area of hills, (2) the property is cut out of a hillside or (3) the property is constructed on the side of a hill where it could be subject to erosion and/or mud slides. Other adverse conditions that the inspector should be aware of are whether the area is subject to earthquakes, wind loads or snow loads. Also, is the property located in an area where there are no codes, such as out in the country or in a small village?