Journal of Geotechnical and Geoenvironmental Engineering

Tunnels of 10 to 16 m span are frequently constructed for hydroelectric or transportation projects, and many of these tunnels are excavated in rock masses of very poor quality. When the ratio of rock mass strength to in situ stress falls below 0.2, squeezing of the rock mass becomes a problem that can cause instability of both the tunnel and the face. A method for predicting squeezing conditions is presented and the practical options for prereinforcing the face and supporting the tunnel to deal with these problems are discussed. Two case histories are included to illustrate how these prereinforcement and support measures can be incorporated into a tunnel design. Brief discussions are also given on water problems in tunneling, the use of tunnel boring machines (TBMs) in squeezing ground, and the construction costs for large tunnels in varying ground conditions.

Block-faced geosynthetic reinforced soil retaining walls, referred to as “segmental” retaining walls, have been extensively used in recent years as permanent civil engineering structures. The disjointed concrete facing blocks are held together through interface friction and concrete keys or mechanical connectors. Because of the disjointed nature of the facing blocks, the design of the segmental wall must consider the available shear resistance between these blocks. Connection capacity must also be considered. Of concern also is the permanent deformation of the segmental wall face following an earthquake. This paper describes a finite-element analysis of a model segmental wall subjected to earthquakelike loading generated by a shake table. The finite-element analysis used the computer program DYNA3D. Results from DYNA3D, using a simple model—Ramberg-Osgood model—to simulate the nonlinear hysteretic behavior of soil, are consistent with observed results from laboratory shake table tests on segmental walls.

This paper describes a finite-element analysis of two 6-m-high segmental walls, subjected to seismic loading, with special attention to connection performance and permanent deformation of the facing. The first segmental wall uses concrete facing blocks with pins, and the second wall uses the same concrete blocks but without pins. The analysis provides some insight into the behavior of segmental walls, particularly the load transfer mechanism between the geosynthetic reinforcement and the segmental facing units duiring earthquake loading.

This paper evaluates a simple approximate methodology for estimating the maximum internal forces of piles subjected to lateral seismic excitation. The method involves two main steps: computation of the free-field soil movements caused by the earthquake and the analysis of the response of the pile to the maximum free-field soil movements (considered as static movements) plus a static loading at the pile head, which depends on the computed spectral acceleration of the structure being supported. The applicability of this approach has been verified by an independent benchmark analysis developed by the writers. It is demonstrated that the proposed method yields reasonable estimates of the pile maximum moment and shear. The methodology is then used to obtain the response of the Ohba-Ohashi bridge in Japan to one of the earthquakes that occurred in the 1980s. Good agreement is found between the computed and measured pile moments.

The behavior of bored pile groups in cemented sands was examined by a field testing program at a site in South Surra, Kuwait. The program consisted of axial load tests on single bored piles in tension and compression and compression tests on two pile groups each consisting of five piles. The spacing of the piles in the groups was two- and three-pile diameters. Soil exploration included standard penetration tests, dynamic cone tests, and pressure meter tests. Laboratory tests included basic properties and drained triaxial compression tests. Test results on single piles indicated that 70% of the ultimate load was transmitted in side friction that was uniform along the pile shafts. The calculated pile group efficiencies were 1.22 and 1.93 for a pile spacing of two- and three-pile diameters, respectively. Since settlement usually controls the design of pile groups in sand, the group factor defined herein as the ratio of the settlement of the group to the settlement of a single pile at comparable loads in the elastic range was determined from test results. A comparison between the measured values and calculated values based on a simplified formula was made.

Unconfined compression tests, Brazilian tensile tests, and saturated drained triaxial compression tests with local strain measurement were carried out to evaluate the stress-strain behavior of a sandy soil improved through the addition of carbide lime and fly ash. The effects of initial and pozzolanic reactions were investigated. The addition of carbide lime to the soil-fly ash mixture caused short-term changes due to initial reactions, inducing increases in the friction angle, in the cohesive intercept, and in the average modulus. Such improvement might be of fundamental importance to allow site workability and speeding construction purposes. In addition, under the effect of initial reactions, the maximum triaxial stiffness occurred for specimens molded on the dry side of the optimum moisture content, while the maximum strength occurred at the optimum moisture content. After 28 days, pozzolanic reactions magnified brittleness and further increased triaxial peak strength and stiffness; the maximum triaxial strength and stiffness occurred on the dry side of the optimum moisture content.

The unique mechanical properties of Mexico City soils plot as extreme values on most geotechnical engineering charts. The purpose of this study is to analyze salient properties and to propose conceptual models to explain them. In particular, mechanical parameters synthesized from published laboratory data are systematically organized in terms of strain level. Three strain regions are identified. The threshold strains at the boundaries between these strain regions are examined in terms of microscale processes that participate at different strains.

The soil-water characteristic curves (SWCCs) for sand–pelletized diatomaceous earth mixtures are measured. The measured SWCCs are bimodal due to two distinct pore-size distributions associated with the microscopic (intrapellet) and macroscopic (interpellet) porosity regions of the pelletized diatomaceous earth. The measured data for the bimodal SWCCs are fit with modified forms of the Brooks-Corey, van Genuchten, and Fredlund-Xing SWCC functions, and the microscopic and macroscopic porosity portions of the mixtures are determined from the SWCC fits. Both the total and the microscopic porosity of the mixtures increase with increased percentage by dry weight of diatomaceous earth in the mixtures. However, the macroscopic porosity of the mixtures is essentially independent of the diatomaceous earth content due to the similarity in the particle-size distributions of the sand and the diatomaceous earth. The results suggest that sand–diatomaceous earth mixtures may be useful in applications where an increase in water-holding capacity (that is, relative to sand) is desired, such as in landfill covers. In such applications, a balance between the increase in cost and the increase in water-holding capacity due to the use of the diatomaceous earth must be achieved.

Measured barometric pressure fluctuations in a borehole drilled 600 m into the unsaturated zone at a site on the eastern slope of Yucca Mountain, Nevada, are used to compute pneumatic diffusivities and air permeabilities of layered tuffaceous rocks. Of particular interest is the occurrence of increasing pressure amplitude and decreasing phase lags observed at several deep stations. The pressure amplitude increases by 13% and the pressure peak arrives 65 to 72 h earlier as the depth of the unsaturated zone increases from 407 to 436 m. This observation is inconsistent with a conventional 1D downward pressure decay model, but can be explained by hypothesizing the existence of a lateral pneumatic flow path from a nearby fault. A 2D numerical model has been constructed to test this lateral flow hypothesis. The modeling results indicate that a leaky-fault hypothesis can reconcile the measured barometric data with other geologic mapping and permeability measurements at the site. The inferred fault permeability ranges from 0.6 × 10⁻¹² to 0.12 × 10⁻⁹ m², and that of the surrounding tuff layers ranges from 0.4 × 10⁻¹³ to 0.8 × 10⁻¹² m². Our analysis also demonstrates that although the larger-amplitude, longer-period synoptic signals lack coherence and periodicity, determination of air permeability in deep unsaturated zones must rely on these longer-period oscillations of barometric pressure whose periods are on the order of weeks to months because the shorter-period diurnal and semidiurnal barometric oscillations generally are not detectable at deep depths.