Advances in Shallow Foundations

The most common foundation for light structures founded on shrink-swell soils is the stiffened slab on grade. The beam depth is an intrinsic design parameter for this type of foundation, it usually ranges between 0.6 and 1.2 m with common beam spacing of 3 to 6 m in both directions. The following methods are among the most common methods used to calculate the beam depth, spacing, and reinforcement needed for a safe and serviceable foundation: 1) BRAB Method; 2) WRI Method; 3) PTI Method; 4) AS 2870; 5) PTI Method. In 2002, the Texas section of ASCE provided recommended practice for the design of residential foundations that impacts the beam depth using WRI and BRAB. To examine the influence of this recommended practice in the resulting beam depths, 27 cases are designed. They include three simple rectangular stiffened slabs subjected to three different weather patterns, built on three different shrink-swell soils, and loaded with a uniform pressure and perimeter line load. The resulting beam depths using the WRI and BRAB methods with and without the recommendations of the Texas section of ASCE are presented and discussed for different soil and weather conditions. The PTI and Australian method are also presented for additional comparison purposes.

Most residential development in Florida is done without any testing for foundation design, as the builders rely on traditional foundation design methods with assumed bearing capacity and no settlement control. As a result, few "step" cracks appear within months of completion of the residential structure. Lack of understanding the soil behavior and the lack of adequate quality control tests result in the structure suffering differential settlements. Design of shallow foundations in sandy soils using elastic theory is adequate unless loose soils were present within the zone of influence. Some form of soil stabilization would be required to improve the relative density of loose soils and to control settlements. Surficial compaction methods do not achieve the required densification beyond a depth of about 1 m below the surface. Traditionally, stone columns have been used for commercial buildings but this is not an affordable option in residential construction. A method of soil stabilization called CSV soil stabilization introduced recently has proven effective, fast and easy to install, and is reasonably economical. This paper discusses the use of the CSV method for one to three-story residential structures on shallow spread footings. Case histories are presented to show that the CSV stabilization is a viable and economical alternative to vibro-compaction and stone columns, which are currently the most frequently used techniques. The CSV automatically records certain parameters that provide a reasonable quantitative measurement of the effectiveness of the soil densification.

The near surface, highly weathered limestones of the state of Qatar have complex lithologies attributable, mainly, to post depositional changes that have altered some relatively hard rocks into soils and some carbonate soils into a rock matrix. The matrix, often clayey, sometimes cemented (with gypsum, anhydrite, and/or calcite grains) is not a rock nor could be considered as a soil per se. These "altered" deposits are: (i) difficult to sample in the undisturbed state, (ii) extremely variable in composition and properties, and (iii) are susceptible to degradation, particularly when wet. The paper presents typical geotechnical information from three selected sites where these deposits (diagenetic limestone) have been encountered. Plate load test data, carried out to predict settlements, are shown. Arrival at appropriate foundation geometries and allowable bearing capacity values, consistent with field conditions, are noted. For the present, design of shallow foundations in Qatar and nearby areas has remained largely empirical and dependent upon site-specific information aided by: judgment, local experience, and plate load test results.

The bearing capacity of a shallow foundation is usually assessed by applying various correction factors to the classical three-term bearing capacity equation. Correction factors are typically included to account for foundation shape and embedment, load inclination and eccentricity, and the inclination of the foundation base. However, the bearing capacity depends on the evaluation of the bearing capacity factors N_γ, N_q and N_c, which differ depending on the source consulted (e.g., Terzaghi, Meyerhof, Hansen or Vesic). Unfortunately, there have been very few full scale tests that have documented the accuracy of such equations causing considerable uncertainty in the mind of the practicing engineer. In addition, complicating factors lead to confusion regarding the meaning and application of the assessed bearing capacity. Such factors include the choice of total or effective stress strength parameters, the use of direct shear, triaxial or plane strain friction angle for cohesionless material, the effect of confining pressure variation in different zones of the failure mass on the friction angle, and the effect of apparent cohesion, particularly in the interpretation of plate load test results. The following paper provides discussion and illustration of these various matters, including the introduction of a simple bearing capacity equation that can also a yield load-settlement response.

The aim of this paper is the computation of the failure loads of a rough rigid strip footing subjected to a vertical, inclined or eccentric loading using the finite difference code FLAC^3D. For the case of vertical load, controlled downward vertical velocities are applied to the footing nodes until a steady state of plastic flow is obtained in the soil. For the construction of the (H,V) failure envelope of an inclined load, a uniform normal stress distribution is first applied to the base of the footing and the system is solved until it reaches an equilibrium state. Then, controlled horizontal velocities are applied to the nodes of the footing bottom until a steady state of plastic flow is obtained in the soil. For the (M,V) failure envelope of an eccentric load, a vertical downward velocity is applied at various eccentricities. For each eccentricity, damping of the system is performed until a steady state of plastic flow is developed in the soil. Results of failure loads are presented and compared with those of other authors.