Journal of Geotechnical and Geoenvironmental Engineering

Scour is the number one cause of bridge failures. Scour in coarse grained soils (sand, gravel) is relatively well known, but scour in fine grained soils (silt, clay) and weak rock is not. In coarse grained soils, scour takes place very rapidly and the scour rate is rarely an issue because one flood is likely to create the maximum scour depth. In fine grained soils, the scour process is much slower; as a result, even after a hundred years, a bridge may not experience the maximum depth of scour. Therefore, in fine grained soils it becomes necessary to predict the rate at which scour takes place. A new apparatus called the EFA (Erosion Function Apparatus; 〈http://tti.tamu.edu/geotech/scour〉) has been built and tested to measure the erosion rate of fine grained soils; the EFA can also be used to measure the erosion rate of coarse grained soils if necessary. The end of a Shelby tube sample from the bridge site is fitted through a tight opening at the bottom of a pipe with a rectangular cross section. Water flows through the pipe and erodes the soil sample, which protrudes 1 mm above the bottom of the pipe. The rate at which the sample erodes is measured, and the shear stress imposed by the water on the soil is calculated. The plot of erosion rate versus shear stress is the result of an EFA test. It indicates the critical shear stress at which erosion starts and the rate of erosion beyond that shear stress. This function can be used to predict the rate of scour at a bridge.

The SRICOS method was proposed in 1999 to predict the scour depth versus time curve at a cylindrical bridge pier for a constant velocity flow, a uniform soil, and a deep-water condition. In this article, the method is extended to include a random velocity-time history and a multilayer soil stratigraphy; it is called the Extended-SRICOS or E-SRICOS. The algorithms to accumulate the effects of different velocities and to sequence through a series of soil layers are described. The procedure followed by the computer program to step into time is outlined. A simplified version of E-SRICOS called S-SRICOS is also presented; calculations for the S-SRICOS method can easily be done by hand. Eight bridges in Texas are used as case histories to compare predictions by the two new methods (E-SRICOS and S-SRICOS) with measurements at the bridge sites.

Movement of volatile organic compounds (VOCs) through compacted clay liners was investigated using laboratory-scale column and tank tests. Hydraulic conductivity of the compacted clay was not significantly impacted by the introduction of VOCs in concentrations up to 20 mg/L. Soil-water partition coefficients of the seven VOCs tested had a strong logarithmic relationship with the octanol-water partition coefficient. Partition coefficients from batch tests were in good agreement with those measured directly on soil samples at the termination of the column/tank tests. The VOCs were degraded in the clay, with estimated half-lives ranging from 2 to 116 days. Mechanical dispersion was not significant in the range of the hydraulic conductivities of the test specimens (i.e., <10⁻⁷ cm/s). Effective molecular diffusion coefficients were mostly in 10⁻⁶ cm²/s and generally decreased with increasing aqueous solubility. Mass transport parameters of VOCs in clay liners can be estimated from laboratory batch tests and properly prepared small-scale column tests. However, accounting for degradation of VOCs and minimizing the number of transport parameters that are simultaneously estimated from a single response-time record are important considerations for accurate determination of transport parameters.

Classical limiting equilibrium analysis seeks the minimum factor of safety and its associated critical slip surface. This objective is mathematically convenient; however, it limits the analysis' practical usefulness. Introduced is a general framework for safety maps that allow for a physically meaningful extension of classical slope stability analysis. Safety maps are represented by a series of contour lines along which minimal safety factors are constant. Each contour line is determined using limit equilibrium analysis and thus represents a value of global safety factor. Since most problems of slope stability possess a flat minimum involving a large zone within which safety factors are practically the same, representation of the results as a safety map provides an instant diagnostic tool about the state of the stability of the slope. Such maps provide at a glance the spatial scope of remedial measures if such measures are required. That is, unlike the classical slope stability approach that identifies a single surface having the lowest factor of safety, the safety map displays zones within which safety factors may be smaller than an acceptable design value. The approach introduced results in a more meaningful application of limiting equilibrium concepts while preserving the simplicity and tangibility of limit equilibrium analysis. Culmann's method is used to demonstrate the principles and usefulness of the proposed approach because of its simplicity and ease of application. To further illustrate the practical implications of safety maps, a rather complex stability problem of a dam structure is analyzed. Spencer's method using generalized slip surfaces and an efficient search routine are used to yield the regions within the scope where the safety factors are below a certain value.

A theoretical approach for calculating pyrite oxidation in acid sulfate soil with a macropore/matrix structure is described. This approach accounts for vertical oxygen transport through soil macropores and the subsequent lateral diffusion of oxygen into the soil matrix. As oxygen is supplied into the matrix, it is consumed by pyrite and other oxygen-consuming processes. A numerical solution to the theoretical model was developed and used in the computer simulation model ACID3D. The numerical approach is based on a linear relationship between oxygen consumption and dissolved oxygen concentration. The numerical scheme is shown to be in good agreement with the analytical solutions. ACID3D was used in conjunction with a commercially available saturated/unsaturated water flow model to assess the effectiveness of a ground-water management strategy to minimize acid generation caused by pyrite oxidation currently being carried out on a trial site on the south coast of New South Wales, Australia.

As the use of the cone penetration test (CPT) has increased for geotechnical site characterization, significant research has been performed to identify and control the factors that affect the tip (q_c), sleeve (f_s), and pore pressure (u) measurements. However, a number of factors that affect the friction sleeve have yet to be understood, appreciated, and accounted for in penetrometer designs. This paper highlights a number of these issues, with specific attention centered on the effect of surface texturing on the friction sleeve measurement. An understanding of the role of surface roughness on soil-geomaterial interfaces provides a framework for analyzing the effect on the friction sleeve measurement and could provide a basis to improve its design. A series of CPT soundings were performed in the southeast United States with conventional smooth and textured friction sleeves. Results indicate that friction measurements with a textured sleeve are 70% greater on average than the value obtained with a conventional smooth friction sleeve in sand and provide a basis for developing new design procedures where interface values are required.

Fiber-reinforced polymer (FRP) composites represent an alternative construction material without many of the performance disadvantages of traditional materials. Although composite materials present a number of difficulties related to pile driving, including low stiffness, high damping, and low strength, the use of FRP as a pile material can eliminate deterioration problems of conventional piling materials in waterfront environments and aggressive soils. This paper is a theoretical parametric study of the effect of various pile properties and soil conditions on the driveability of FRP composite piling in a typical waterfront site. All analyses performed show that composite piling could be driven to reasonable capacities for load-bearing piles. The parameters studied include the effect of pile modulus, damping ratio, unit weight, residual stresses, and hammer type on the efficiency of driving of FRP piling relative to conventional piling materials.

This paper describes the performance of geosynthetic-reinforced asphalt pavement under monotonic, cyclic, and dynamic loading conditions. The study differed from current practice where geosynthetics are typically used as separators or to improve the bearing capacity of the subgrade. A geogrid layer was installed at the bottom of the asphalt concrete layer, along the asphalt-subgrade interface, to function as tensile reinforcement. The load was applied to the surface of the asphalt concrete layer using a rigid rectangular footing under plane strain conditions. The strains that developed along the geogrid over time and at different load levels were monitored. Two different types of geogrid reinforcements were used, and their restraining effects on the layered system were compared. The study showed that geosynthetic reinforcement increased the stiffness and bearing capacity of the asphalt concrete pavement. Under dynamic loading, the life of the asphalt concrete layer was prolonged in the presence of geosynthetic reinforcement. The stiffness of the geogrid and its interlocking with the asphalt concrete contributed to the restraining effect.

Although pile caps have considerable ability to resist lateral loads, this resistance is often neglected in design. Published cases involving a variety of pile and cap sizes, soil conditions, and loading conditions indicate that the lateral-load resistance of pile caps can be significant, but it is difficult to generalize on the basis of these results because of the variations in conditions involved in the tests. To develop a more systematic basis for evaluating cap resistance, a field test facility was constructed to perform full-scale lateral-load tests on single piles and pile groups, with the pile caps embedded in the stiff natural soil at the site and with the pile caps backfilled with granular soil. Thirty-one tests were conducted to evaluate the lateral-load resistance of pile caps by comparing the response of pile groups with caps fully embedded and with soil removed from around the caps. The results of the tests show that pile caps provide significant resistance to lateral load. This resistance depends primarily on the stiffness and strength of the soil in front of the cap and the depth of cap embedment.