Master of Science / Department of Architectural Engineering and Construction Science / Darren D. Reynolds / Highly expansive soils can severely damage the foundations which they support. These damages create unnecessary maintenance cost to the owner and can be detrimental to the building superstructure. Post-tensioned ribbed mat foundations are commonly used in light commercial construction in areas in the United States that have highly expansive soils. Mild reinforced ribbed mat foundations are rarely used in these areas. This report investigates why post-tensioned ribbed mat foundations are more common in these areas than mild-reinforced ribbed mat foundations. The approach to this investigation is a design example which designs and compares the two foundation types. The design example is a typical 2-story office building located in Dallas, Texas, which is an area that has highly expansive soils. First, a post-tensioned ribbed mat foundation is designed for the office building. Next, a mild-reinforced ribbed mat foundation is designed for the same building. A comparison is done between the two foundations based on serviceability, strength requirements and construction costs. The findings in the comparison is that post-tensioning is a more economical and constructible method. Using mild-reinforcement requires the use of shear reinforcement in the ribs which is not typical in foundation design and construction and is less economical, and additional reinforcement in the slab is needed to resist bending stresses which is also less economical. The finding of the report is that of the two foundation types, the post-tensioned ribbed mat foundation is the better design based on the three areas of interest listed above. The use of a mild-reinforced mat foundation would require construction procedures that are not typical and would be less economical.

Wood construction platforms are a common method for inexpensive, temporary soil stabilization under heavy machinery; however, platforms are not typically thought of as an engineered product. Review of literature has shown that only one design method is currently available and is specific to one type of platform configuration. The purpose of this thesis is to develop a design method that is simple, versatile and accurate. The proposed design method was intentionally developed so that the designer would have input in multiple areas of the design. Instrumentation allowed for increased insight into the mechanical behavior of the platforms. The objective of this research is to use measured strain, load, and deflection in conjunction with fundamental engineering mechanics principles to predict a single platform’s mechanical behavior on the ground. Results from this method compare favorably with the only other design guide available and improves the knowledge base by developing design guidance for any type of wood construction platform.

Occurrence of displacements of shallow mat foundations resting on saturated silt-clay mixtures were reported in Mexico City during 1985 Mexico Earthquake, and in Adapazari during 1999 Kocaeli (izmit) Earthquake. Soft surface soils, shallow ground water, limited foundation embedments and deep alluvial deposits were the common features pertaining to such foundation displacements in either case. Experience shows, while uniform foundation settlements, even when excessive, do not limit post earthquake serviceability of building structures, tilting is particularly problematic. In this study, a simplified methodology is developed to estimate the seismically induced irrecoverable tilting potential of shallow mats on fine saturated soils. The undrained shear and deformation behavior of silt-clay mixtures encountered at the Adapazari sites with significant foundation displacements are investigated through a series of standard and rapid monotonic, and stress-controlled cyclic triaxial tests conducted over anisotropically consolidated natural soil samples. Test results show that, while the shear strength of these soils do not significantly degrade under means of loading comparable to that of Kocaeli earthquake, their plastic strain accumulation characteristics critically depend on the mode of loading as well as the relative levels of applied load with regard to the monotonic strength. Based on the results of laboratory tests, the response of nonlinear soil-foundation-structure system is reduced to a single-degree-of-freedom oscillator with elastic-perfectly plastic behavior. The natural period of the system is expressed by simplified soil-structure-interaction equations. Pseudo-static yield acceleration, which is required to initiate the foundation bearing capacity failure when applied to the structural mass, is estimated by the finite-element method. Eventually, the tilting potential of the foundations is estimated utilizing inelastic response of the nonlinear oscillator. Response of the deep alluvium sites, which involves velocity pulses with periods consistent with the fundamental site period, is significant in determination of inelastic response of low bearing capacity systems. Predictive capability of the methodology developed is tested with actual case data. The methodology is observed to predict irrecoverable tilting potential of foundations consistent with the observations, except for the cases with low seismic bearing capacity. Deviations are explained considering the sensitivity of low-strength systems to asymmetrical behavior and uncertainties involved in seismic demand.