Effect of verification core hole on the point bearing capacity of drilled shafts

textFor many projects involving drilled shafts, cores are required to be taken below the shaft base for visual identification of the underlying material. For example, the Texas Department of Transportation (TxDOT) requires a core length of at least 1.5 m (5 ft) or equal to the shaft diameter, whichever is greater, at the shaft base. Although the verification cores are to be extracted at the shaft base, The Department of Transportation of many states do not provide guidance to eliminate the effect of the verification core on the point bearing capacity. A recent study shows that the verification core hole is either filled with concrete in dry condition or with sand-gravel mixture in wet pour (Raibagkar, 2008). This finding is crucial because the point bearing capacity of drilled shafts with an empty hole at the base should be significantly lower than that of drilled shafts without verification core. Although the materials that fill in the verification core remove the risk of losing large point bearing capacity, the exposure of the core holes to air-drying may have an adverse effect on the point bearing capacity tipped in clay shales, especially when the basal material is susceptible to weathering. Therefore, the effect of the verification core on the point bearing capacity has been thoroughly investigated with emphasis on changes in the material properties of four clay shales (Del Rio Clay, Eagle Ford Shale, Taylor Marl, and Navarro Shale) in central Texas. The effect of verification core on the point bearing capacity of drilled shafts was investigated using finite element method (FEM) software, PLAXIS. The results from laboratory tests were converted to input material parameters for Mohr-Coulomb failure criterion, and the thickness of degraded zone around the core was interpreted from fullscale condition degradation tests. The load-displacement curves at the shaft base were created from PLAXIS analyses, and the point bearing capacities were obtained at 5%D and 10%D displacement from load-displacement curves. These capacities were used to calculate reduction factors that relate the point bearing capacity of the reference model (without a verification core) with that of the “core models” (with a verification core). The reduction factors are good indicators to determine if verification core had a positive or negative effect on the point bearing capacity. It was found that the reduction in point bearing capacity of “core models” is typically within 10% capacity of the reference model, and a maximum reduction of 14% was found for the Taylor Marl that was dried for 48 hours.Civil, Architectural, and Environmental Engineerin

Microbially Induced Calcite Precipitation Employing Environmental Isolates

In this study, five microbes were employed to precipitate calcite in cohesionless soils. Four microbes were selected from calcite-precipitating microbes isolated from calcareous sand and limestone cave soils, with Sporosarcina pasteurii ATCC 11859 (standard strain) used as a control. Urease activities of the four microbes were higher than that of S. pasteurii. The microbes and urea–CaCl2 medium were injected at least four times into cohesionless soils of two different relative densities (60% and 80%), and the amount of calcite precipitation was measured. It was found that the relative density of cohesionless soils significantly affects the amount of calcite precipitation and that there is a weak correlation between urease activity and calcite precipitation

Effect of Temperature, pH, and Reaction Duration on Microbially Induced Calcite Precipitation

In this study, the amount of calcite precipitate resulting from microbially induced calcite precipitation (MICP) was estimated in order to determine the optimal conditions for precipitation. Two microbial species (Staphylococcus saprophyticus and Sporosarcina pasteurii) were tested by varying certain parameters such as (1) initial potential of hydrogen (pH) of urea-CaCl2 medium, (2) temperature during precipitation, and (3) the reaction duration. The pH values used for testing were 6, 7, 8, 9, and 10, the temperatures were 20, 30, 40, and 50 °C, and the reaction durations were 2, 3, and 4 days. Maximum calcite precipitation was observed at a pH of 7 and temperature of 30 °C. Most of the precipitation occurred within a reaction duration of 3 days. Under similar conditions, the amount of calcite precipitated by S. saprophyticus was estimated to be five times more than that by S. pasteurii. Both the species were sensitive to temperature; however, S. saprophyticus was less sensitive to pH and required a shorter reaction duration than S. pasteurii

Numerical Analysis of Laterally Loaded Piles Affected by Bedrock Depth

This study investigates the lateral behavior of pile foundations socketed into bedrocks using 3D finite difference analysis. The lateral load-displacement curve, pile deflection, and bending moment distribution were obtained for different bedrock depths between 3 and 20 m. It was discovered that bedrocks that have a depth of 7 m (7D) or less influence the lateral behavior of the pile. The p-y curves were collected at depths of 2.0–4.5 m, and the effect of the bedrock on the curves was evaluated. It was observed that the p-y curves were significantly affected by the material properties of the bedrock if the rock is located in close proximity (within 3D), but the effect is diminished if the p-y curves were 3.5 m (3.5D) or farther from the bedrock