Design of drilled shaft and tied-back retaining walls in high plasticity clays
The current design procedure adopted by Texas Department of Transportation for designing simple cut type retaining walls such as Tied-back, Drilled shaft and Soil nail is based on the lateral pressure calculated from the classical Rankine's or Coulomb's methods considering drained shear strength parameters and it does not include any lateral pressure due to swelling of the expansive soil as a result of change in moisture content. The lateral pressure due to the expansion of soil, as predicted by the consultants is 10 to 20 times higher than the normal lateral pressure on the walls, which resulted in increase in the thickness of the walls in order to accommodate such large pressures due to expansion, without knowing the actual pressure pattern due to expansion.
The focus of this study is to provide TxDOT in particular and the field of geotechnical engineering in general, the guidelines and charts for the design of cut-type retaining walls in high plasticity clay considering the pressure due to swelling of soil and moreover provide a profile of change in water content of soil with time for a site in San Antonio, Texas (intersection of Walter's Street and I-35).
This study includes three major parts: 1. Measurement of variation of moisture content with time. 2. Laboratory testing on the undisturbed expansive soil samples (conducted by Texas A&M University in College Station) and 3. Coupled finite element modeling (FEM) of retaining structures in expansive soil, with varying suction and loading using commercially available software (SV Office and Geo-Studio).
The findings from this study can be summarized as: 1. Wetting envelope for high plasticity soil located at the intersection of I-35 and Walters Street in San Antonio, Texas. 2. Relation between water content and pore-water pressure (i.e., Soil Water Characteristic Curve), for the site located at the intersection of I-35 and Walters Street in San Antonio, Texas. 3. Horizontal pressure patterns due to swelling of high plasticity clays, on the drilled shaft and tie-back retaining structures. 4. Guidelines for the design of retaining structures in high plasticity clays.