Journal of Geotechnical and Geoenvironmental Engineering

A new method called SRICOS is proposed to predict the scour depth z versus time t around a cylindrical bridge pier of diameter D founded in clay. The steps involved are: (1) taking samples at the bridge pier site; (2) testing them in an erosion function apparatus to obtain the scour rate ż versus the hydraulic shear stress applied τ; (3) predicting the maximum shear stress τ_max, which will be induced around the pier by the water flowing at v_o before the scour hole starts to develop; (4) using the measured ż versus τ curve to obtain the initial scour rate ż_i corresponding to τ_max; (5) predicting the maximum depth of scour z_max for the pier; (6) using ż_i and z_max to develop the hyperbolic function describing the scour depth z versus time t curve; and (7) reading the z versus t curve at a time corresponding to the duration of the flood to find the scour depth that will develop around that pier. A new apparatus is developed to measure the ż versus t curve of step 2, a series of advanced numerical simulations are performed to develop an equation for the τ_max value of step 3, and a series of flume tests are performed to develop an equation for the z_max value of step 5. The method is evaluated by comparing predictions and measurements in 42 flume experiments.

A multistep strategy for the prioritization of maintenance and repair funds for the monitoring of embankment dams is proposed wherein the most important device in the worst condition is preferred. The strategy involves determining the importance of each device (in terms of dam safety) and measuring its condition through field inspections. A condition definition for each device is developed in terms of its ability to function optimally in its diagnostic role. Relative importance of the monitoring devices is defined in terms of diagnostic value and proceeds by considering: (1) the failure modes and their conditional probabilities; (2) adverse conditions and their conditional probabilities; (3) indicators that provide information on these adverse conditions and their relative diagnostic value; and (4) monitoring devices that are used to assess the presence of the indicators and their relative diagnostic value. A panel of dam safety experts is used to answer the questions necessary to implement this methodology through a process of expert elicitation. An interactive DOS-based C++ computer program has been developed to assist in the expert elicitation necessary to implement this methodology.

The factors affecting the applicability of electrical conductivity (EC) breakthrough curves as an indicator of chemical equilibrium between effluent and influent solutions in compatibility tests are illustrated. The shapes of EC breakthrough curves are shown to be a function of the flow rate, the solute retardation factor, the species of cation and anion in the permeant liquid, and the cation initially occupying the exchange complex of the clay. Measured data show that the magnitude of the EC in the soil due to the existence of soluble salts relative to the EC of the influent solution (permeant liquid) affects significantly the observed shapes of the EC breakthrough curves. Comparison between theoretically predicted and measured EC breakthrough curves varies from good to excellent, depending on the initial conditions for the test. The results indicate that chemical equilibrium can not be attained before complete EC breakthrough is attained, regardless of the shape of the EC breakthrough curve. Thus, EC breakthrough curves offer a potentially simple, practical, and inexpensive method for determining chemical equilibrium in laboratory compatibility tests involving permeation with electrolyte solutions.

The in situ chemical and physical weathering of igneous and metamorphic rocks, indentified as the process of formation of Piedmont residual soils, is a fairly well understood phenomenon. However, the effect this weathering has on the physical, mechanical, and dynamic properties of the rock/soil is not understood fully. This study focuses on the dynamic shear modulus, G, and material damping ratio, D, of this soil formation for low- to mid-level amplitudes of vibration. The paper presents laboratory test results and correlations that demonstrate the effects that the degree of weathering has on these properties for various levels of confining pressure and shear strain amplitude. A total of 12 specimens of Piedmont residual soils from different depths were tested in a Resonant Column (RC) device. The specimens tested were SM and ML soils according to the USCS classification. The low-amplitude shear modulus and damping values were found to be similar to those reported in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolidation ratio exert a noticeable influence on the dynamic response as a result of variations in confining pressure. The understanding of these effects will allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on these soil deposits. The response in free field soil deposits compared with that of soils experiencing added confining stresses due to foundation loading appears to vary significantly in these geologic formations. Threshold strain and the variation of damping, D, with the normalized shear moduli, G/G_max, fall within the same range as those recently reported by other authors in similar soils.

The paper describes the heavy damages caused by blasting in the Peruća rockfill dam in Croatia in January 1993. Complete collapse of the dam by overtopping was prevented through quick action of the dam owner by dumping clayey gravel on the lowest sections of the dam crest and opening the bottom outlet of the reservoir, thus efficiently lowering the water level. After the damages were sufficiently established and alternatives for restoration of the dam were evaluated, it was decided to construct a diaphragm wall through the damaged core in the central dam part as the impermeable dam element and to rebuild the central clay core at the dam abutments. Reconstruction works are described.

A large number of centrifuge model tests for simulating the shield construction process in dry sand was carried out by using a newly developed 100 mm diameter miniature shield tunnel. Different construction processes were modeled by three series of tests. A “buried tube test” was conducted to measure the lining stress under a centrifugal acceleration, a “tail void test” simulated the process of tail void formation only, and a “shield test” dealt with the complete process of shield construction. Lining stresses, transverse and longitudinal surface settlements, and earth pressures around the tunnel were simultaneously measured during advancement of the shield at a centrifugal acceleration of 25g. The lining stress at the crown elevation was well predicted by Terzaghi's loosening earth pressure. From the comparison with the recent field measurements and the comparative results of three series of tests, it was confirmed that the shield construction process was successfully simulated in a centrifuge. An experimental formula for estimating the surface settlement above the shield was deduced by a function of tail void thickness and cover-to-diameter ratio.

In probabilistic slope stability analysis, the similarity and differences of the surface of minimum factor of safety (FS) and the surface of minimum reliability index (β) have not been investigated fully. Probabilistic analyses have often been performed on the surface of minimum FS, and sometimes on an arbitrary surface. The surface of minimum FS and the surface of minimum β do not generally coincide. In this paper, an expedient technique is presented to search for the critical probabilistic surface utilizing existing deterministic slope stability programs. This technique is based on searching the critical surfaces obtained using specific combinations of soil parameter values. The critical probabilistic surface was found to generally coincide with that obtained by setting one dominant parameter to a low value. The technique is examined by a parametric study and applied to some case studies. It proved to provide a practical and efficient method to locate the critical probabilistic surface.

Differential settlements between a bridge and the backfill behind the abutment have been a major problem in the construction of expressway embankments over a soft clay foundation. Deep cement mixing (DCM) columns with varying lengths were used to reduce such differential settlements along the Fu-Xia Expressway, Fujian Province, China. The performance and the feasibility of the DCM method were investigated in a trial embankment constructed prior to the actual construction. This paper presents the results of the instrumentation including total settlements, multipoint settlements, soil pressures on both the DCM columns and soil surface, pore-water pressures, and lateral movements obtained from in situ monitoring. The strength of the soil-cement from laboratory mix tests and from in situ quality control tests on DCM columns is presented in the paper. Study results indicate that DCM columns with varying lengths were a simple and effective method to reduce the total and differential settlements from a soft clay foundation at a bridge approach.

Important geotechnical structures constructed on compacted cohesive soils often involve compaction either around or on the wet side of optimum water content. In general, at these water content values, water voids are continuous and air voids are occluded, and the soil may be assumed to be in a state termed as “quasi-saturated.” This paper evaluates the effective stress behavior of such quasi-saturated compacted specimens of Gangetic silt and Canyon dam clay in the broad framework of the conventional modified Cam-clay model. The initial state of quasi-saturated compacted specimens is shown to lie on the recompression line in w versus ln(p′) space. The actual recompression line on which the specimen state would lie, and the corresponding equivalent past maximum pressure, are found to depend only on the amount of compaction energy and the soil structure, and are independent of the molding water content or initial dry density. It is observed that, at low effective confining stresses, quasi-saturated compacted soils behave like overconsolidated soils and the effective stress paths during undrained shear lie on the Hvorslev surface. However, at confining stresses greater than the past maximum pressure, these soils behave like normally consolidated soils and the effective stress paths move practically along the Roscoe surface toward the critical state line.

Rapid degradation of aqueous trichloroethylene (TCE) was observed in batch experiments conducted with soil/bentonite slurry wall materials amended with the addition of zero-valent iron. The first-order TCE decay constants for soil/bentonite/iron mixtures, when normalized to the available iron surface area, were approximately 1–2 orders of magnitude higher than observed in batch experiments with pure iron systems. Permeability tests indicated an increase in SB hydraulic conductivity roughly proportional to the amount of iron added. Based on the observed reaction rates and the assumption of sustained long-term performance, significantly less than one percent added iron would be required to reduce the diffusive flux of TCE across an installed slurry wall by over 10 orders of magnitude. However, the release of hydrogen gas was noted as a potential problem for low permeability systems containing zero-valent iron.