Journal of Geotechnical and Geoenvironmental Engineering

This paper aims to demystify the use of risk assessment as a decision management tool and present a methodology that places quantitative risk assessment within reach of every geotechnical engineer, even for routine engineering assignments. In particular, we propose using quantification of expert judgment (i.e., subjective probabilities) as a practical alternative for determining probability of slope failure. The writers present a semiempirical relationship between factor of safety and annual probability of failure that permits estimation of slope failure probabilities with relatively modest effort. The case study for a tailings dam shows that risk assessment based on quantification of expert judgment provides a framework to arrive at rational management and engineering decisions related to dam safety and other geotechnical problems. Using the semiempirical relationship presented here, practicing engineers can use this helpful tool by applying their current skills.

The reliability analysis of earth slopes is considered. For slope safety assessment, the first-order reliability method is employed for estimating the probability of failure or reliability index. Since the failure of any slip surfaces implies failure of the slope, the slope is considered as a series system. The system aspect of the slope in the reliability analysis is dealt with by defining a limit state of the system as a function of the minimum of the ratio of the shear strength to the mobilized shear strength for each of all potential slip surfaces. Such a ratio for a given slip surface is evaluated using the extended generalized method of slices. The reliability analysis procedure described is applied to example slopes to illustrate the impact of the probability distribution type, and the spatial variability of the soil properties on the probability of failure of the slopes.

A series of large-scale underwater shaking table tests was performed on a gravity type model caisson protected by a cushioning technique using tire chips (scrap tire derived recycled product). The function of the tire chips cushion is to reduce the load and restricting the permanent displacement of such waterfront retaining structures during earthquakes by exploiting the compressibility, the ductility and the energy absorbing capacity of tire chips. The seismic performance of such earthquake resistant techniques was evaluated by subjecting the soil-structure system into three different earthquake loadings (two actual earthquake records and one synthetic earthquake), and measuring the respective responses. The results demonstrated that the seismic load against the caisson quay wall could be substantially reduced using the proposed technique. In addition, the presence of the protective tire chips cushion could significantly reduce the earthquake-induced residual displacement of the caisson quay wall.

Detailed profiles of hydraulic conductivity are recovered from the deployment of direct-push permeameters at the Geohydrologic Experimental and Monitoring Site, Kansas. Measurements with thin tapered tips, and with standard cone penetration test (uCPT) tips, show only minor differences, suggesting that tip-local disturbance effects are small, and that routine uCPT measurements are therefore representative of pristine conditions. Permeameter measurements are correlated against closely deployed uCPT measurements, estimates of hydraulic conductivity from uCPT sounding correlations, and from grain size correlations derived from both vision CPT (VisCPT) and from cone metrics. On-the-fly evaluations of hydraulic conductivity require that the tip-local pressure field is both steady and partially drained. Continuous penetration is shown to yield pore pressures sufficiently close to steady to enable conductivities to be directly determined. Cone metrics of cone resistance, sleeve friction, and pore pressure ratio are shown to be sufficient to discriminate between partially drained and undrained behavior, and therefore to define the permissible regime where conductivities may be determined from uCPT sounding data. Estimates of hydraulic conductivities from uCPT sounding data are shown to correlate with independently measured magnitudes of hydraulic conductivity recovered using the permeameter tests. However, most of hydraulic conductivities from the permeameter tests ( $4.5\mathrm{cm}$ length screen) are underpredicted, suggesting that storage effects, the inability to reach a steady state, or the effects of dilation may influence the response. Profiles of hydraulic conductivities evaluated from the on-the-fly method also correlate well with the permeameter measurements. Predictions from soil classification and from VisCPT methods are also capable of estimating conductivities, with soil classifications giving the closest correlations of these two for this particular suite of data.

The combination of in situ geotechnical testing and continuously measured geophysical data can be a powerful tool in geotechnical site investigation. In two cases from Sweden and one case from Denmark electric resistivity surveys are used successfully in geotechnical site investigations. The main contribution of resistivity results is the possibility to interpret continuous geological models. An improved methodology combines two-dimensional (2D) smooth inversion and 2D laterally constrained inversion (2D-LCI) to significantly increase interpretability. The 2D smooth inversion has high horizontal resolution and 2D-LCI high vertical resolution. The possibility to add a priori information from, for example, drill log data to constrain the 2D-LCI increases the confidence in the inverted model and limits ambiguity. In a site investigation for a railway trench in southern Sweden a geotechnical data set is used as a priori data to increase the reliability of the inversion of the resistivity data. From this combined survey a complex Quaternary geology is described in detail. A slope stability study from south of Stockholm, Sweden, employed resistivity data together with refraction seismic and geotechnical drill log data. The result gives necessary geometrical information for the important geological units, for example for stability calculations. Both these surveys were performed with a multielectrode system. In the third case a pulled array resistivity survey was used to map the uppermost $15–20\mathrm{m}$ to estimate the distribution of the geological formations for freeway construction in Denmark. The result enables a more accurate estimate of the total freeway construction costs.

The bender element test has shown to be a reliable tool to determine the shear wave velocity. This paper aims to extend the bender element technique to also measure the material damping ratio in a triaxial cell. Three different interpretation techniques are developed for this purpose. The first technique uses a modal test of a cylindrical soil specimen in a triaxial cell, the second approach uses multiple arrivals of a bender element signal, and third, a self-correcting method using different travel paths is presented. These methods are applied on test results on undisturbed samples of a silt and a heavily overconsolidated clay, and show promising results for the assessment of the material damping ratio.

Determination of strain in resonant column and torsional shear (RC/TS) tests is complicated due to nonuniform stress–strain variation occurring linearly with the radius in a soil specimen in torsion. The equivalent radius approach is adequate when calculating strain at low to intermediate levels, however, the approach is less accurate when performing the tests at higher strains. The stress integration approach involving integration of an assumed soil stress–strain model was developed to account for this problem more precisely. This approach was used to generate the plots of equivalent radius ratio versus strain developed based upon shear modulus and damping. Results showed that the equivalent radius ratio curves converge to a value of approximately 0.8 at low strains and decrease as strain increases. The equivalent radius ratio curves based upon damping decrease to significantly lower values at high strain than curves based upon shear modulus. This study suggests that using the same values of equivalent radius ratio to calculate strains for both shear modulus and damping is not appropriate. The stress integration approach provides an accurate analysis technique for evaluating both modulus and damping behavior of soil, over any range of strains in RC/TS testing.

To evaluate compaction meter value and machine drive power roller-integrated compaction technologies, a field study was conducted with $30\text{-}\mathrm{m}$ test strips using five granular materials. The test strips were compacted using a prototype CS-533E vibratory smooth drum roller and tested for various compaction parameters using in situ test methods (e.g., nuclear moisture density, dynamic cone penetrometer, plate load tests, etc.). To characterize the roller machine-ground interaction, soil testing focused on measuring soil compaction parameters of the compaction layer, to a depth not exceeding $300\mathrm{mm}$ . The experimental testing of five test strips provided roller data and in situ measurements for several stages of compaction that were used in performing statistical regression analyses. The relationships between data from the roller-integrated compaction technologies were investigated with special consideration for the relative variation that was observed for each measurement system. Statistical averaging mitigated measurement variability and revealed statistically significant $(R^2>0.9)$ relationships between in situ and roller-integrated compaction measurements. This research demonstrates statistical analysis techniques for which calibration procedures using roller-integrated compaction technologies may be developed.

To evaluate roller-integrated machine drive power (MDP) technology for predicting the compaction parameters of cohesive soils considering the influences of soil type, moisture content, and lift thickness on machine power response, a field study was conducted with $15\text{-}\mathrm{m}$ test strips using three cohesive soils and several nominal moisture contents. Test strips were compacted using a prototype CP-533 static padfoot roller with integrated MDP technology and tested using various in situ compaction measurement devices. To characterize the roller machine-soil interaction, soil testing focused on measuring compaction parameters for the compaction layer. Variation in both MDP and in situ measurements was observed and attributed to inherent variability of the compaction layer and measurement errors. Considering the controlled operations to create relatively uniform conditions of the test strips, measurement variability observed in this study establishes a baseline for acceptable variation in production operations using MDP technology in cohesive soils. Predictions of in situ compaction measurements from MDP were found to be highly correlated when moisture content and MDP-moisture interaction terms were incorporated into regression models.

Tunnel excavation in cohesionless soils implies the use of a temporary supporting structure prior to lining installation. This temporary structure has to couple safety and economy, and can be conveniently realized using ground improvement techniques (for instance, by creating an arch of partially overlapped subhorizontal jet grouted columns). The adoption of ground improvement techniques results in structures far from having a perfect shape because they are intrinsically affected by defects (in both geometrical and mechanical characteristics), and therefore their design may hide unforeseen risks. As a consequence, this is the typical case in which sophisticated numerical analyses may just give the illusion of being refined, if possible defects are not correctly taken into account. In this paper a simple yet rational analytical method for the design of a nonclosed tunnel supporting structure that may be of some help to this aim is presented. It is done with reference to a simple two-dimensional scheme. In the first part of the paper, a design chart of optimal shape and minimum structural thickness of the cross section of the supporting structure is shown. In the second part, an iterative procedure to verify the stability or to design the minimum structural thickness of an existing supporting structure with a predefined shape is described. This method, coupled with the analysis of structural demand, allows one in principle to plot design charts. This approach can easily take into account structural defects with a semiprobabilistic approach and therefore with a chosen risk level, which is of great help to the designer at least in a preliminary design stage. The proposed semiprobabilistic procedure is applied to the case of a temporary supporting structure realized by partially overlapped subhorizontal jet grouted columns, intrinsically affected by defects in diameter and position. The variability of these geometrical parameters was considered based on the large quantity of experimental evidence collected in field trials by the writers and published elsewhere.

Column tests were conducted to evaluate two treatment strategies for reducing and stabilizing hexavalent chromium, Cr(VI), in chromium ore processing residue (COPR): permeation with a $\mathrm{Fe}\mathrm{S}{\mathrm{O}}_4–{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ solution and blending with a cationic polysulfide reagent $\left(\mathrm{Ca}{\mathrm{S}}_X\right)$ . Cr(VI) leached at concentrations exceeding $50\mathrm{mg}∕\mathrm{L}$ from untreated COPR permeated with synthetic groundwater for $>20$ pore volumes of flow (PVF), and concentrations of Cr(VI) in the solid phase remained high $(\mathrm{6,600}\mathrm{mg}∕\mathrm{kg})$ . Permeation with solutions containing $\mathrm{Fe}\mathrm{S}{\mathrm{O}}_4–{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ eliminated Cr(VI) from the effluent after initial, elevated leaching of Cr(VI) $(100–\mathrm{1,500}\mathrm{mg}∕\mathrm{kg})$ ; however, high solid-phase concentrations of Cr(VI) remained in the column residuals $(>\mathrm{1,300}\mathrm{mg}∕\mathrm{kg})$ . COPR treated with $\mathrm{Ca}{\mathrm{S}}_X$ leached Cr at $<0.33\mathrm{mg}∕\mathrm{L}$ for 23.5 PVF and had solid-phase concentrations of Cr(VI) $<10\mathrm{mg}∕\mathrm{kg}$ , although mineralogical analyses of treated solids showed potential chromate-containing mineral phases. Mineralogical analyses showed that precipitation and cementation occurred in the pore space of the COPR permeated with $\mathrm{Fe}\mathrm{S}{\mathrm{O}}_4–{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ , initially lowering the hydraulic conductivity > two orders of magnitude. However, acid dissolution channels eventually formed, resulting in preferential flow. COPR permeated with $\mathrm{Fe}\mathrm{S}{\mathrm{O}}_4–{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ contained less brownmillerite and Cr(VI)-bearing hydrocalumite and hydrogarnet relative to untreated COPR. For COPR treated with $\mathrm{Ca}{\mathrm{S}}_x$ , S encapsulated the subparticles of COPR with some micropore penetration, suggesting permanence of excess reductant after leaching with 23.5 PVF of synthetic rainwater.

A frequency domain method is presented to compute the impulsive seismic response of circular surface mounted steel and concrete liquid storage tanks incorporating soil-structure interaction (SSI) for layered sites. The method introduces the concept of a near field region in close proximity to the mat foundation and a far field at distance. The near field is modeled as a region of nonlinear soil response with strain compatible shear stiffness and viscous material damping. The shear strain in a representative soil element is used as the basis for strain compatibility in the near field. In the far field, radiation damping using low strain soil response is used. Frequency dependent complex dynamic impedance functions are used in a model that incorporates horizontal displacement and rotation of the foundation. The focus of the paper is on the computation of the horizontal shear force and moment on the tank foundation to enable foundation design. Significant SSI effects are shown to occur for tanks sited on soft soil, especially tanks of a tall slender nature. SSI effects take the form of period elongation and energy loss by radiation damping and foundation soil damping. The effects of SSI for tanks are shown to reverse the trend of force and moment reduction under earthquake loading as is usually assumed by designers. The reasons for this important effect in tank design are given in the paper and relate to the very short period of most tanks, hence, period lengthening may result in load increase. A comparison is made with SSI effects evaluated using the code SEI/ASCE 7-02. Period elongation is found to be similar for relatively stiff soils when assessed by the code compared with the results of the dynamic analysis. For soft soils, the agreement is not as good. Code values of system damping are found to agree reasonably well with an assessment based on the dynamic analyses for the range of periods covered by the code. Energy loss by material damping and radiation damping is discussed. It is shown that energy loss may be computed using the complex dynamic impedance function associated with the viscous dashpot in the analytical model. The proportion of energy loss in the translation mode compared to that dissipated in the rotational mode is addressed as a function of the slenderness of the tank. Energy loss increases substantially with the volume of liquid being stored.

This technical note defines an “effective soil density” that controls the velocity of small strain shear waves in saturated soil. Biot theory indicates that the ratio of effective density to saturated density will generally range from 0.75 to 1.0 and is a function of specific gravity of solids, porosity, hydraulic conductivity, and shear wave frequency. For many geotechnical applications, effective density will be equal to saturated density for low hydraulic conductivity soils (clays and silts) and may be less than saturated density for high hydraulic conductivity soils (clean sands and gravels). The findings are relevant to applications involving the propagation of small strain shear waves through saturated soil, and in particular for laboratory and field tests in which shear modulus is back-calculated from measured shear-wave velocity.

Experiments were conducted to evaluate the role of mineral composition on the direct simple shear response (i.e., volume change during shear) of uniform sands composed of “strong” quartz grains and “weak” calcareous grains. The dry specimens were loaded in a custom-made direct simple shear apparatus and separated after testing with respect to grain size by sieving and with respect to mineral composition by dissolving the calcareous grains retained on each sieve. The results indicate that the presence of weak grains affects the direct simple shear response of predominately strong-grained sands: at low stresses, the response is controlled by particle shape; at high stresses, the response is controlled by weak particle breakage.

In this technical note, an evaluation of the robustness and predictive ability of a constitutive model for sands is performed. The model is shown to capture the main features of sand behavior under both drained and undrained monotonic loadings for a wide range of relative densities and stress paths. The main contribution of this technical note is to evaluate a robust, yet simple, constitutive framework based on a solid theoretical basis that fulfils the most fundamental requirement of any useful constitutive law: accurate predictions.

A new method of estimating the permeability of soils using an acoustic technique is proposed in this technical note. Biot’s coupled theory of mixtures from the mid-1950s, which addresses the relationship between the permeability of saturated soils and the characteristic frequency of elastic waves, is used. The characteristic frequency is determined from the attenuation versus frequency curves obtained by acoustic sweep tests. This study presents the experimental technique for measuring the characteristic frequency of soils and calculation procedure for estimating the permeability of sandy soils. This technical note holds discussion for silty or clayey soils.