William S. Kaggwa

Measuring the Risk of Geotechnical Site Investigations

The site investigation phase of any geotechnical design plays a vital role, where inadequate characterization of the subsurface conditions may contribute to either a significantly over designed solution that is not cost-effective, or an under design, which may lead to potential failures. Although it is intuitive to expect that the financial risk of a design will reduce as the site investigation scope increases (i.e. additional sampling), it is not known to what degree the risk is reduced, nor whether other uncertainties have an impact on this relationship. As such, this paper discusses research to measure the impact of varying the scope of a site investigation, on the financial risk of a foundation design project. The financial risk is defined as the total cost, which includes costs associated with undertaking the site investigation, constructing the foundation and superstructure, and any works required to rehabilitate a foundation failure. The analysis is numerically based, where a foundation design simulation model is incorporated into a Monte Carlo framework, in order to generate GSP 170 Probabilistic Applications in Geotechnical Engineering Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 2 expected costs, and a measure of the financial risk. Results indicate that the risk of a foundation design is considerably reduced as the scope of a site investigation increases. However, results also indicate that there is an optimal site investigation expenditure, which leads to the least financial risk, and where additional sampling becomes redundant.

Effect of sample location on the reliability based design of pad foundations

Site investigations that aim to sufficiently characterize a soil profile for foundation design, typically consist of a combination of in situ and laboratory tests. The number of tests and/or soil samples is generally determined by the budget and time considerations placed upon the investigation. Therefore, it is necessary to plan the locations of such tests to provide the most suitable information for use in design. This is considered the sampling strategy. However, the spatial variability of soil properties increases the complexity of this exercise. Results presented in this paper identify the errors associated with using soil properties from a single sample location on a pad foundation designed for settlement. Sample locations are distributed around the site to identify the most appropriate sample location and the relative benefits of taking soil samples closer to the center of the proposed footing. The variability of the underlying soil profile is also shown to a have a significant effect on the errors due to sampling location. Such effects have been shown in terms of the statistical properties of the soil profile. The performance of several common settlement relationships to design a foundation based on the results of a single sample location have also been examined.

Assessing the influence of root reinforcement on slope stability by finite elements

This paper aims to investigate the effect of root reinforcement on slope stability using finite element methods. It is well recognised that plant roots can improve the shear strength of soils by their high tensile strength and closely spaced root matrix system. The increase in soil shear strength due to root reinforcement is considered as an increase in apparent soil cohesion, called root cohesion, cr. In this paper, a freely available (http://www.inside.mines.edu/~vgriffit/slope64) finite element code called slope64 described by Griffiths and Lane (Géotechnique 49(3):387–403, 1999) is used to model the effect of root reinforcement on slope stability. The root cohesion is added directly to the soil cohesion for the soil elements that are reinforced by plant roots. The results from the finite element analyses demonstrate that the factor of safety of a slope increases when the effect of root reinforcement is taken into consideration. A series of stability charts are developed which can be used for assessing the influence of root reinforcement on slope stability.

AN IMPROVED STATISTICALLY BASED TECHNIQUE FOR EVALUATING THE CPT FRICTION RATIO

This paper examines a statistical technique known as the cross-correlation function (CCF) for determining the shift distance associated with the cone penetration test (CPT). When evaluating the friction ratio, FR ( = fr/qc), for soil classification purposes, it is essential that the measured values of ), for soil classification purposes, it is essential that the measured values of qc and fs are shifted relative to one another because of the physical offset between the cone and the friction sleeve. Generally, the shift distance is estimated by means of empirical and subjective methods, a value of 75 to 100 mm is adopted, or it is ignored all together. Using a series of case studies, this paper demonstrates that the CCF is a useful and objective technique for estimating the shift distance. In addition, a phenomenon associated with sleeve friction measurements related to elastic rebound of clay soils is discussed.