Subgrade Modeling

Engineers are often faced with construction of pavement structures over soft and highly compressible subgrade. Such conditions render the structures unable to withstand the required design loads and susceptible to high settlements with associated excessive distress, leading to pavement damage. Because using imported quality fill to improve the load-bearing capacity of the subgrade has limited benefits, an alternative construction approach is needed to ensure the required strength of the soil structure is reached. The use of geosynthetics in soil to improve bearing capacity and stability offers a better alternative to imported fill. This research was conducted to determine the degree of improvement of the load-bearing capacity and reduction in settlement as a result of the geosynthetic reinforcement of soft clay overlain by granular material. Compression tests on the unreinforced clay subgrade and geosynthetic-reinforced two-layered soil composite were conducted at bench-scale using a Universal Compression Machine at the University of Cape Town Geotechnical Engineering Laboratory. The geosynthetic products used to reinforce the soil structure in this study included geogrids and geotextiles. The reinforcement layer was placed alternatively within the layer of granular material and at the interface of the soil and a range of fill thicknesses and depths of placement were tested to determine their influence on mobilized strength. The use of these composites could potentially lead to reduced construction costs as a result of less aggregate material used, an extension in the design life of the pavement structure and, consequently, a reduced necessity for maintenance work.

Contemporary seismic design codes have become more stringent with respect to the requirements for design forces and deformations in building design. This paper demonstrates that it could be worthwhile to consider the introduction of soil-structure-interaction provisions into local design codes. This is partly to be able to offset the costs incurred by the high magnitude of base shear demand in most buildings attributed to site amplifications due to soft soil sites, as per the requirements of current codes, including the recent South African seismic design code. This beneficial effect of site soils is as a result of lengthening of the fundamental period and of the increased effective damping of the overall system due to soil-structure interaction, which in most cases lead to reduced design spectral values. The paper shows that, if pertinent provisions in some international codes are properly adapted, a substantial reduction in the base shear force can be achieved, in many cases resulting in structural-work cost saving. With this, the paper attempts to address the legitimate concern of design engineers regarding the potential escalation of construction costs associated with the introduction of stringent
requirements of contemporary seismic design spectra, especially for soft soil sites.

Contemporary seismic design codes have become more stringent with respect to the requirements for design forces and deformations in building design. This paper demonstrates that it could be worthwhile to consider the introduction of soil-structure-interaction provisions into local design codes. This is partly to be able to offset the costs incurred by the high magnitude of base shear demand in most buildings attributed to site amplifications due to soft soil sites, as per the requirements of current codes, including the recent South African seismic design code. This beneficial effect of site soils is as a result of lengthening of the fundamental period and of the increased effective damping of the overall system due to soil-structure interaction, which in most cases lead to reduced design spectral values. The paper shows that, if pertinent provisions in some international codes are properly adapted, a substantial reduction in the base shear force can be achieved, in many cases resul...