The Use and Limitations of Soil Testing for Predicting Foundation Settlement and Reducing Legal Exposure
by Benjamin T. Benumof, Ph.D., Esq.
“Unfortunately, soils are made by nature and not by man and the products of nature are always complex…”
Karl Terzaghi (1883-1963): one of the leading civil engineers of the 20th Century and widely known as the father of soil mechanics.
Soil, which underlies the majority of southern California's residential and commercial building sites, is quite possibly the most important of all building materials, and, accordingly, one of the most studied. Nevertheless, problems associated with the settlement and collapse of soils at building sites are common, especially in areas where soils contain high percentages of clay and silt (which naturally shrink and swell during changes in soil moisture), such as in many locations throughout Orange County and the rest of the western United States.
Although architects and builders are generally familiar with soil as it relates to the building of commercial and residential structures, unfavorable soil conditions have increased the cost of many buildings far beyond their original estimates, and, in many cases, led to considerable legal disputes. In fact, since the early 1970's, volume-changing clay soils have been deemed one the most costly natural hazards to buildings on shallow foundations in the United States. In large part, this is due to the fact that building sites, particularly in urban areas, are seldom selected on the basis of good soil properties, but rather the cost and availability of land within the area selected for the project. In addition, increased or unanticipated costs associated with unfavorable soil conditions often stem from the fact that foundation settlement is typically a protracted phenomenon, which occurs over extended time scales (i.e., years versus days or weeks).
Given substantial scientific advancements in understanding the physical properties of soil and the stresses that are created in soil building foundations, the purpose of this article is to briefly: (a) summarize information on the basic characteristics of soil; (b) differentiate the basic types of soil settlement; and (c) suggest the types of information that should be obtained by a proper soils investigation and included in a formal, yet readily understandable soils report.
The Basic Characteristics of Soil
The term soil is typically used by engineers and geologists to denote the unconsolidated surface and/or near-surface material that is composed of discrete solid particles with gases or liquids between the particles. The scientific use of soil, therefore, extends beyond its limited agricultural meaning, which is generally the thin layer of organic surface material that supports plant life, and includes material ranging in size from sand and gravel (particles that can be distinguished by the eye) to fine-grained silts and clays (particles that are often difficult to distinguish even when viewed through a low-powered microscope).
Regardless of the size of soil particles, however, all soils are: (1) derived from solid rock (i.e., are products of erosion, transport and deposition by geologic processes or in-place weathering of rock); and (2) contain voids between the individual soil particles. These voids can clearly be seen in coarse-grained materials such as sand or gravel, and occur on a much smaller scale in the case of silts or clays.
Importantly, when the voids between soil particles are filled with water, a soil is said to be saturated. Conversely, when the voids between soil particles are filled with air, the soil is said to be dry. This type of mixture of solid material, air and water is described technically as a “three phase system,” the interrelation of which can cause drastic changes in soil behavior from location to location, or even at a single building site. For example, a building site - such as many sites in Orange County - could be characterized by a mixture of sand, silt and clay, which when dry, is hard and rocklike. The very same mixture of sand, silt and clay, however, when exposed to water, could be prone to significant swelling and causing major construction problems. Consequently, the results of soil tests should be interpreted in light of local climate patterns, the geology of the site, and the investigator's experience and familiarity with soils of the subject area.
Types of Soil Settlement
(a) Settlement Due to Loading
There are three basic types of soil settlement that result from the compression of subsoil due to loading: (1) uniform settlement; (2) tilt; and (3) non-uniform, differential or disproportionate settlement. While uniform settlement and tilt may cause serious problems with building services and building appendages, such as water mains and connecting utility tunnels, uniform settlement and tilt (within reason) do not always significantly affect the safe use of a structure.
Non-uniform or differential settlement, on the other hand, is characterized by angular distortion of building foundations and may cause severe cracking or even structural failure. The degree of angular distortion is indicated by the ratio of differential settlement to the distance between building supports. In general, the amount of settlement that a building can tolerate (i.e., the “allowable” settlement) depends on its size, type and intended use.
(b) Settlement Due to Shrinking or Swelling of Clay
Although settlement is commonly caused by compression of soil, large movements can also occur from shrinking and swelling (i.e., “heaving”) of clay subsoil resulting from stresses unrelated to foundation pressure. For instance, fine-grained clay soils may be subjected to extremely high stresses due to cyclical drying and wetting, and can be highly influenced by the presence of vegetation (i.e., roots can be extremely efficient in extracting soil moisture). In addition, many soils will swell back to their original volume when they are rewetted. In fact, in southern California, some of the worst heaving conditions often develop in soils that have been subjected to greatly increased moisture conditions as a result of construction and irrigation practices.
What to Look For in a Soils Investigation Report
Although there are no invariable rules for soils investigations, guidelines for “minimum” boring, sampling and testing criteria, as well as the preparation of engineering geologic reports, are available from the American Society for Testing and Materials, the U.S. Department of Transportation Federal Highway Administration, as well as most state geological boards. While such guidelines provide information far too extensive to summarize here, one of the most important requirements is that soil tests be conducted on samples that are truly representative of the soil at the site. The significance of this requirement stems from the fact that the behavior of soil masses under imposed conditions can be predicted if certain soil properties can be measured. In general, this requires that: (a) the subject soil be tested at a representative number of locations and depths; and (b) the soil investigation include not only basic in-situ field tests, such probing of the ground with penetration devices to assess soil density, but a detailed examination of the subsoil by test pits or soil borings spaced according to the size and geological complexity of the site. In addition, soils investigations will often utilize a wide array of laboratory tests, as well as geophysical exploration tools (e.g., seismic refraction), which can be used to compute the elasticity, rigidity and strength of a material, to assess anticipated foundation conditions.
Conclusion
Given that the selection and design of building foundations is a vital part of the overall building design process, particularly where building foundations are to rest on soil, it is important to understand how a project site's soils will behave in response to various building-imposed and climatic stresses such as loading (compression), shear, wetting and drying. Landowners and developers should therefore always consult the appropriate experts to assess site-specific soil conditions, paying careful attention to the types, extent and quality of soils, prior to foundation design.
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Copyright © 2007, Manatt, Phelps & Phillips, LLP As published in The Orange County Business Journal, November 19, 2007.