Journal of Geotechnical and Geoenvironmental Engineering

In the design and construction of embankment dams, our current capability for precise mathematical analysis and modeling of induced stresses and deformations, or of potential seepage patterns, far exceeds our capability to make judgments of comparable accuracy concerning, for example, the site and geology or how the soil properties may be affected by the weather or by the contractor’s methods. In addition, there is often a lack of adequate communication between the design and the supervision of construction. These uncertainties or doubts about the actual performance of the dam when constructed are discussed in the paper and illustrated by case history examples, with particular reference to the uncertain effects of cold weather, to the use of broadly graded soils (tills) as core and to problems in the placement, and segregation of tills and filter materials.

This paper presents the reliability-based design (RBD) initiative sponsored by the Electric Power Research Institute for transmission line structure foundations. The role of RBD is presented within the context of geotechnical limit state design. Design parameters that are amenable to statistical description can be propagated systematically to a consistent measure of design risk using reliability techniques such as the first-order reliability method (FORM). Less quantifiable factors are incorporated approximately into RBD by judicious selection of the target reliability level. A simplified RBD approach based on the load and resistance factor design (LRFD) and multiple resistance factor design (MRFD) formats is proposed for practical implementation. The resistance factors for the LRFD and MRFD formats are calibrated rigorously using FORM to produce designs that can achieve a known level of reliability consistently.

This paper presents the development of simplified reliability-based design (RBD) equations that are suitable for spread foundations subjected to uplift. Emphasis is placed on the loading and foundation characteristics relevant to the electric utility industry. A general reliability calibration procedure is used to derive robust resistance/deformation factors for load and resistance factor design (LRFD) and multiple resistance factor design (MRFD) formats. Two target reliability indices of 3.2 and 2.6 are proposed based on an extensive study of existing designs for ultimate and serviceability limit state, respectively. The main advantage of using these RBD factors is that a known level of reliability can be consistently achieved over a wide range of design conditions. Simple design calculations using the MRFD format are shown to demonstrate their ability to account for parametric uncertainties in a rational manner.

Nail head strength is one of the parameters used in design of soil-nailed structures. It determines the stability of the nailed structure against failure involving the facing element. The nail head strength is a function of a number of factors, primarily the material strengths, vertical and horizontal nail spacing, and the nail head connection details. The variations in nail head strength as a result of changes in the vertical and horizontal soil-nail spacing and material strength properties are examined in this paper. Both temporary and permanent facing design is considered. For equal vertical to horizontal spacing, the nominal nail head strength for a temporary facing decreases continuously as the spacing is increased. The nominal nail head strength for permanent facing decreases initially and then remains nearly constant. For a fixed vertical spacing $\left(S_v\right)$ the nail head strength for temporary facing decreases linearly as the horizontal spacing $\left(S_h\right)$ increases. For permanent facing the nail head strength decreases very slowly for vertical to horizontal nail spacing ratio between 0.8 and 1.0 and then declines rapidly. The nail head strength increases most rapidly with increase in facing thickness. Higher strength reinforcement and concrete have a relatively smaller influence in raising the nail head strength. Charts are presented that can help in optimizing the design of soil-nailed structures.

A pure fixed-head (zero-rotation) condition at the top of a group of laterally loaded piles is seldom achievable in the field, even when piles are installed in a group that is “rigidly” constrained by a stiff concrete pile cap. Assuming complete fixity during design (zero rotation at the pile head) can result in underestimated values of pile-head deflection, and incorrect estimates of the magnitude and the location of maximum bending moments. A simple and practical approach is presented for estimating the moment restraint that is provided by the pile cap at the top of a pile group. The moment restraint, represented by the rotational restraint coefficient $\left(K_{M\theta }\right)\text{,}$ serves as a boundary condition for analyzing groups of laterally loaded piles. Full-scale field tests performed on two pile groups with concrete pile caps show that the proposed method for estimating rotational restraint provides results that are in good agreement with measured field performance.

Consolidation of inhomogeneous clay with a system of horizontal and vertical drains is considered. Horizontal drains in the system are made of multiple thin pervious layers such as sand layers and geotextile sheets, while vertical drains are vertical cylindrical drains. Consolidation behavior with the drain system is formulated using the transfer matrix method. Special care is given to formulation of thin pervious layers for efficient computation. The developed formulation is verified using available numerical and field information. Parametric studies are conducted to study the consolidation characteristics of clay with the drain system. Based on the findings, a design method for an optimum system of horizontal and vertical drains is proposed and design charts are presented for such a design.

Based on the distinct element method, a numerical procedure is presented for simulation of creep behavior of jointed rock slopes due to excavation unloading. The Kelvin model is used to simulate viscous deformation of joints. A numerical scheme is introduced to create incremental contact forces, which are equivalent to producing creep deformation of a rock-joint system. The corresponding displacement of discrete blocks due to creep deformation of contact joints can be calculated by equilibrium iteration. Comparisons of results between the numerical model and theoretical solutions of a benchmark example show that the presented model has excellent accuracy for analysis of creep deformation of rock-joint structures. As an application of the model, residual deformations of the high rock slopes of the Three Gorges shiplock due to excavation unloading and creep behavior are investigated. By simulating the actual excavation process, the deformation history of a shiplock slope is studied. Good agreement has been achieved between numerical prediction and field measurements. It demonstrates the effectiveness of the presented model in analysis of the creep deformation due to excavation unloading of high rock slopes.

Prefabricated vertical drains (PVDs) normally consist of a core and a filter (sleeve) made with polymeric materials. Modern installation rigs are capable of installing them at considerable speed in soft soil deposits. This note reports the tension developed in a PVD filter sleeve during the installation process, which was monitored with strain gauges. A tension of about 1,000 N was measured near the shoe shortly after the commencement of mandrel withdrawal. The need to watch the stresses in PVD in relation to the material properties of the components is highlighted.

The classic approach for static analysis and design for the effect of shear forces transmitted by shallow foundations is to modify the bearing capacity by introducing inclination factors to reduce the shear strength provided by cohesion, surcharge and self-weight in the standard bearing capacity equation. These static inclination factors are usually based on empirically derived relationships determined by laboratory and field tests. However, it is possible to derive them from limit analysis of a simple Coulomb-type mechanism and thereby extend the concept of inclination factors to the dynamic case giving values for seismic design.