International Concrete Abstracts Portal

Subgrade response is the most important parameter in analyzing and designing mat foundations. Rational design of a mat foundation requires consideration of immediate and long term subgrade response. The soil response determines mat behavior, and differential movement exacerbates moments. Often, long term movement provides the most severe mat behavior characteristics. The popular use of a single modulus of subgrade reaction k, to model subgrade response is wrong and will lead to wrong designs. Mat analysis and design should be performed using the discrete area method, in which subgrade responses can be properly modeled because of the use of varying moduli of subgrade reaction. The geotechnical engineer and the structural engineer must form a solid bond to cope with mat foundation design from early planning through construction; both must work together to assure successful performance. Often, construction details and procedures will govern performance and can ruin any analysis. For this reason, the geotechnical engineer should be on site accessing conditions.

Reviews methods used in preliminary design of mat foundations, as well as procedures used in final analysis and design. Emphasis is placed on the effect that different structural and soil properties have on mat design bending moments and pressure distribution. Using results of parametric studies on two actual mat foundations, the sensitivity of mat moments and pressure distribution to changes in design parameters is investigated. The soil-dishing phenomena and its effect on mat structural design is discussed. The effect that superstructure stiffness has on mat behavior and the effect that differential settlement within the mat has in the superstructure are also presented and discussed. Finally, specific recommendations are provided regarding selection of proper structural and soil properties to be used in analysis and design of foundation mats.

Describes three innovative mat foundation designs and the close interaction required between the structural engineer and the geotechnical engineer. The significance of load deformation prediction reliability in the three different soil profiles is illustrated. The cases reviewed include a three-story office building with single basement build on a mat over peat; a 26-story apartment building with basement built on a modified mat in a thin dense sand stratum over soft clay; and a 19-story hotel with two basements built on a mat in a sand layer over medium clay. The mat of the 19-story hotel was supplemented with selective high capacity piles at the column locations designed to ultimate soil capacity at working loads and utilized to reduce both mat settlement and design mat thickness. The instrumentation used to confirm design assumptions in the three cases is briefly described.

The St. Luke's Medical Tower is the Texas Medical Center's tallest building and Houston's tallest building to open the 1990s. The combination of the following unique foundation features reduced development costs by over $1,000,000: (1) tallest soil-supported building on shallow foundations in the Southwest; (2) temporary dewatering system designed to function as the permanent system; (3) excavation-bracing system designed to form the permanent basement wall and only individual braces were temporary; (4) basement walls esigned to accept loads from future contiguous towers; (5) drilled pier soldier piles installed with polymer drilling fluid (the first use on a major Texas project); and (6) drill pier soldier piles installed in accordance with the new American Concrete Institute Standard Specification for the Construction of Drilled Piers (ACI 336.1-89), the first known use of the specification. The factored load condition was considered fictional in foundation, and basement wall design in that factored load-concrete-subgrade compatibility was not achieved. Significant cost savings was achieved by allowing the geotechnical engineer to be part of the design team beginning with project concept studies and extending throughout underground construction. The geotechnical engineer and the team developed feasible foundation schemes that could be integrated into construction needs, instead of relying only on specialty design builders.

Describes the design of a load-compensated mat foundation on highly compressible soil. The mat was used to support over 800,000 square feet of variable height building. While the design of the mat was mostly routine, the behavior at the mat edges was difficult to determine. The deformations at mat edges were the major concern since they were influenced by the need to raise grades around the building perimeter. The design procedure incorporated soil-structure interaction analysis to determine the extent of lightweight fill zones required to control edge deformations. Settlement monitoring over a period of two years has confirmed the design approach.