Expansive Clay Soils and Vegetative Influence on Shallow Foundations

With clay soils and temperate climatic conditions, trees are the biggest cause of soil drying and subsidence, resulting in foundation movement and damage. Understanding how trees dry the soil, and how this drying can be controlled, is essential for decisions on methods of preventing or remedying this damage.

This paper presents a numerical model to predict the volume change in an expansive soil as a result of vegetation and environmental changes. The model formulation is based on the general theory of unsaturated soil behavior. Three typical volume change situations associated with water uptake by tree roots and water infiltration into soils due to watering at surface are presented.

There exists a need for a method of estimating soil swelling behavior for the design and practitioner community. Desirable method attributes include; low cost, reasonable accuracy and technical soundness. The method should be usable for a wide range of soil types. Practitioners should be able to use local soils testing services to get the data required by the method. The NRCS Soil Survey Laboratory (SSL) soils database is a large, quality controlled resource that provides the data for the development of such a method. The presented method allows for the estimation of the suction compression index using Atterberg limits, particle size classification, and the coefficient of linear extensibility (COLE) values as contained in the SSL database. Using a 6500 sample subset of the SSL database, a series of charts are presented yielding suction compression index values for various mineralogically based groups. The proposed method provides quick and stable prediction of an important soil property using low cost and commonly available soil test procedures.

The background and procedures embodied in the recent Texas Board for Professional Engineers Advisory Report on Residential Foundations Appendix A represents extensive geotechnical research and practice relating to unsaturated clay soils. The effect of local soil properties and environmental factors on new design and forensic analysis should be a part of geotechnical, structural and foundation engineer's tool kits. These procedures, as well as the effects of barriers, drainage and vegetation on foundation support conditions are discussed and illustrated. Procedures for determining soil support parameters (E_m) and (Y_m) values and examples of designs will be presented.

Characterization based on samples obtained in conventional field investigations is an important step in obtaining engineering properties for use in selecting foundation type and making design decisions. A practical method for classification and estimating expansive soils behavior is described through application to three sites. The method is based on suction testing expansive clay samples and is suitable for routine use in geotechnical testing laboratories. In this paper, three cases are reviewed and the experience discussed. The use of relatively simple suction testing of soils is a valuable addition to routine geotechnical investigations of expansive soils.

The term "active zone" generally refers to the zone of soil that either is contributing to or has the potential to produce heave. In order to predict heave at any particular time, it is necessary to define the zone of soil that has experienced an increase in water content and what the swell potential of that zone is. Different terms can be used for various zones that have in the past been used to define the "active zone". This paper discusses the usage of the term "active zone" and presents a definition that relates it to the movement of water in the soil profile. The nature of water movement in soil, the definition of important zones that influence heave, and the interaction of different foundation elements with the expansive soil are shown to be interrelated. The depth of the wetting front in the soil profile is an important factor in prediction of heave and pier movement. In order to compute the maximum heave that can occur, it is necessary to assume that the depth of wetting will eventually extend throughout the depth of potential heave. Therefore, this depth represents the maximum depth of the active zone.

Tree roots have been documented to withdraw volumes of water from expansive clay soils causing slab distress under certain circumstances. However, patterns of distress, areas of influence and potential distress magnitudes have not been systematically considered from a soil-structure interaction perspective. This Paper describes research into the soil-structure interaction between trees and heavy vegetation and at-grade structures including pavements and slab-on-grade foundation systems. Several areas of distressed pavements and slab-on-grade foundation systems in the North Texas area have been documented, with estimates and measurements of soil properties, tree geometry and proximity to the structure, tree concentration, relative elevation surveys and distress patterns to confirm the influence pattern of the root systems. Conclusions indicate concentrations of trees tend to have the most significant effects with influence patterns occurring radially to approximately the tree drip line. Confirmation of the horizontal and vertical influence pattern is provided by Geo-Electrical Moisture Material Imaging Resistivity (GMMIR) surveys using soil borings to help characterize the influence pattern of trees. The GMMIR plots indicate influences between about 1.2 and 3 meters in depth. Root influence models were constructed using unsaturated soil mechanics theory and the VOLFLO algorithm. Analysis of these models indicates that the influence of tree roots is a function of the soil type and that the influence at the extreme perimeter of the structure is as significant as the penetration of the tree roots significantly beneath the foundation with differential downward movements on the order of 127 to 152 millimeters (5 to 6 inches) at the perimeter moving from a wet condition (total soil suction pG = 4.3 (pF = 3.3)) to a desiccated tree condition (total soil suction pG = 5.6 (pF=4.6)) reducing to approximately 5.1 centimeters (2 inches) or less when the tree roots are at least 0.6 meters (2 feet) from the structure perimeter.

The effect of unsaturated soils on pavement performance is examined through a case history involving a narrow pavement constructed on an at-grade section. The pavement function was for bicycle and pedestrian traffic in a recreational project in Oklahoma City, Oklahoma. The pavement section consisted of asphalt concrete surface course underlain by aggregate base on a prepared subgrade. Following construction, and during a very dry weather cycle, the pavement experienced a total of 3237 m of predominately longitudinal cracking and surface undulation. A reconnaissance of the pavement extent (5.58 km) indicated that 68 percent of the cracking occurred in one pedological soil series. A geotechnical study was performed that included: pavement core and base course tests, hand auger and continuous Standard Penetration Test borings, index and engineering property tests and filter paper method soil suction testing. Examination of an average yearly water balance prior to and following pavement construction revealed that shrinkage rather than heave was the crack controlling mechanism. Estimates of field capacity were made from water balance calculations. The Thornthwaite Moisture Index (TMI) and the Soil Moisture Deficits (SMD) were estimated for the period prior to and following construction by comparisons of monthly rainfall and monthly potential evaporation resulting in indications of potential shrinkage and cracking. A characteristic curve was developed from the total suction measurements using the filter paper method. Analysis of available information indicates that the change in the total suction was responsible for soil shrinkage. Suction changes were determined through suction versus depth profiles over time to predict the initial and final suction input and using initial calculated suction from the characteristic curve(s). Finally it was shown that the change in suction, resulting in lateral shrinkage of the soil, caused lateral forces that easily exceeded the tensile strength of the asphalt concrete and aggregate base courses.

The central region of Oklahoma is underlain by sedimentary deposits of the Permian period, which include shales and related materials. The fine-grained materials weathered from these Permian shales are typically low to moderate plasticity clayey silts, silty clays, and clays. These fine-grained materials exhibit low to moderate shrink-swell potential with some local areas showing high shrink-swell potential. The general climate of the region is dry subhumid to semiarid with climatic extremes, as was the case during the summer of 2000, which was one of the driest on record. The influence of climate on the behavior of moderate to high shrink-swell potential soils can be more extreme, when the influence of vegetation is added. This paper looks at the occurrence of damage caused by shrinking and swelling soils and the role that vegetation had in causing the problem to be more severe. Some basic physiological properties of common species of trees in the region as well as local characteristics of the soils are reviewed. Case studies of shrinking and swelling soil damage to streets and residential construction are presented and discussed.

People in countries all over the world have to deal with houses built on expansive clays (clay soils that shrink and swell). The ways in which they tackle the problem vary considerably. Not only do solutions differ technically but they also depend on the property market, the legal framework of the country, insurance policies, the attitude of insurers, the history of the region, the degree of building regulation, the experience and training of construction professionals and perhaps above all the sensitivity of the house owner to the cracks in his house. Relatively minor cracking is seen in some places as an early sign of impending disaster. Rarely is the house owner's attitude in accordance with what technical research tells us is the structural significance of cracks. In the UK with the involvement of insurers for nearly three decades, there is high sensitivity to extremely small cracks. Costly procedures are used to deal with them. In Australia, South Africa and USA foundations consist mostly of reinforced concrete rafts which are cheap and quick to underpin. Where insurers are not involved house owners pay for their own remedial works, sometimes using additional mortgages. In general it appears that private financing and little local authority regulation of underpinning produces low cost and variable quality solutions.

Damage to structures constructed on expansive clay soils occurs throughout the United States each year. The extent to which expansive clay soils can damage an individual foundation will depend largely on the amount and location of moisture change in the soil beneath that foundation as well as the foundation's stiffness. Significant changes in moisture levels typically occur from poor drainage and/or from large vegetation causing heave or shrinkage respectively. Once a structure has experienced damaging differential movements and repair is needed, an engineer must consider sources and magnitudes of previous and future movements in designing repairs. This paper presents a tested technique, which involves using an aqueous solution of potassium and ammonium ions to treat expansive clay soils in remediation of damaged structures. The technique relies on reactions between the solution ions and the clay soil to adjust rheological properties of the clay. Mechanisms by which potassium and ammonium treatment reduce swelling motions and pressures are described herein. A case history of a forty-five year old building damaged by both heave and shrinkage movements is presented. Identification of movement sources, reactivity laboratory testing of site soils with the aqueous solution, predicted injection and long-term movements, description of the injection modification technique, verification quality assurance testing and foundation performance are presented.

Monitoring drought is a task made difficult by the fact that droughts vary by intensity, duration, spatial extent, and the impacts they cause. Initially, scientists developed a series of drought indices designed to monitor drought for specific purposes. Recently, a consensus is forming that a better drought monitoring approach is one that puts together an ensemble of indicators rather than relying on one indicator alone. The appropriate analogy for this technique is the collection of the appropriate tools to place into a drought monitoring tool box. Improvements in technology, such as the Internet and GIS, have made this approach possible. This article provides an overview of the drought monitoring tools available for a tool box, and ends with some suggestions for equipping a specific tool box for monitoring droughts in regions with expansive soils.

Movements caused by expansive clay soils continue to contribute to major foundation concerns for residential homes and commercial buildings. The original observed effects of expansive clay formations were discovered a relatively short time ago, in the 1920's and 1930's. Though many studies have been done, the profession is still in its youth and more information is needed to provide improved preventative designs. An important inquiry to be addressed for foundation investigations in expansive clay is "What is the future expected natural moisture content under the slab sometime after construction?" The amount of local precipitation and the average rate of evapotranspiration control soil moisture and groundwater levels in underdeveloped areas. A balance of precipitation and evapotranspiration will result in a quasi-static moisture and groundwater level in the soil. However, a slab-on-grade foundation interferes with this natural balance. Soils beneath a slab are normally at a somewhat higher moisture content than soils at the same depth away from the slab. The purpose of this paper is to provide information related to the boundaries of moisture content and volumetric condition for clay formations in the San Antonio and Houston areas. This information can eventually be used to develop improved shrink/swell estimation procedures. This paper presents data from laboratory tests performed on over 10,000 samples obtained from foundation investigations related to in-situ soils beneath existing commercial and residential slab-on-grade foundations. The investigations were performed in and around the San Antonio and Houston areas.

This paper reports on an evaluation of wetting and drying filter paper suction calibration and soil total and matric suction measurement techniques of filter paper method. Calibration of the method was investigated by constructing two calibration curves; one by using the process of wetting the filter papers through vapor flow and the other by using the method of drying the filter papers through fluid flow. The wetting curve was constructed using sodium chloride (NaCl) salt solutions and Schleicher & Schuell No. 589-WH filter papers. It was found that the change in the wetting suction curve is very sensitive to minor changes in filter paper water content below about 1.5 log kPa (2.5 pF) suction. The drying curve was established by employing both pressure plate and pressure membrane devices and the same filter papers. In developing the filter paper calibration curves, the capabilities, pitfalls, and limitations of the method are also discussed.