Showing papers in "International Journal of Geomechanics in 2008"

Numerical Modeling of Seismic Response of Rigid Foundation on Soft Soil

Abstract: A numerical model was developed to simulate the response of two instrumented, centrifuge model tests on soft clay and to investigate the factors that affect the seismic ground response. The centrifuge tests simulated the behavior of a rectangular building on 30 m uniform and layered soft soils. Each test model was subjected to several earthquakelike shaking events at a centrifugal acceleration level of 80g . The applied loading involved scaled versions of an artificial western Canada earthquake and the Port Island ground motion recorded during the 1995 Kobe Earthquake. The centrifuge model was simulated with the three-dimensional finite-difference-based fast Lagrangian analysis of continua program. The results predicted with the use of nonlinear elastic–plastic model for the soil are shown to be in good agreement with measured acceleration, soil response, and structural behavior. The validated model was used to study the effect of soil layering, depth, soil–structure interaction, and embedment effects on ...

Intrinsic Length Scales in Tool-Rock Interaction

Abstract: Indentation and cutting experiments in rocks reveal that the action of a tool can induce either ductile and/or brittle failure, with the ductile mode associated with damage of the rock and/or plastic flow, and the brittle mode with the propagation of cracks. In normal indentation, the development of a damaged zone precedes the initiation of tensile cracks; in cutting, the failure mechanism switches from a ductile to a brittle mode as the depth of cut is increased beyond a threshold value. In this paper, we first argue that these observations can be accounted for by introducing an intrinsic length scale lm ∼ ( KIc ∕ σc )2 in the rock description (with KIc denoting the toughness and σc the compressive strength). Next, we report the results of numerical simulation of indentation and cutting tests with the discrete element method. After showing that the internal length scale lm can be modified by the ratio of the shear to normal bond strength, we illustrate by numerical simulations that the selection of the f...

Effect of Arching on Active Earth Pressure for Rigid Retaining Walls Considering Translation Mode

Abstract: It has been established by the researchers that owing to the arching effect, the active earth pressure distribution on a horizontally translating rigid wall is not triangular but nonlinear. This is attributed to the arching behavior exhibited by soil. Also, the shape of the failure surface plays a critical role in determining the magnitude of lateral stresses and the height at which the resultant active earth force is centered from the base of the wall. In the present study, various combinations of shapes of critical failure surface and arch shapes were studied to estimate the coefficient of active earth pressure on the rigid retaining wall in cohesionless soil. The results were compared with field results and those predicted by other theories. A critical review has been made based on the comparison of results obtained from the present analyses with experimental observations. Design charts for modified active earth pressure coefficient and height of application of lateral force have also been suggested.

2D and 3D Numerical Modeling of Combined Surcharge and Vacuum Preloading with Vertical Drains

Abstract: This paper presents a three-dimensional (3D) and two-dimensional (2D) numerical analysis of a case study of a combined vacuum and surcharge preloading project for a storage yard at Tianjin Port, China. At this site, a vacuum pressure of 80 kPa and a fill surcharge of 50 kPa were applied on top of the 20-m-thick soft soil layer through prefabricated vertical drains (PVD) to achieve the desired settlements and to avoid embankment instability. In 3D analysis, the actual shape of PVDs and their installation pattern with the in situ soil parameters were simulated. In contrast, the validity of 2D plane strain analysis using equivalent permeability and transformed unit cell geometry was examined. In both cases, the vacuum pressure along the drain length was assumed to be constant as substantiated by the field observations. The finite-element code, ABAQUS, using the modified Cam-clay model was used in the numerical analysis. The predictions of settlement, pore-water pressure, and lateral displacement were compare...

Optimum Design of Cantilever Retaining Walls Using Target Reliability Approach

Abstract: In this paper, an approach for reliability-based design optimization of reinforced concrete cantilever retaining wall is described. A parametric study is conducted to assess the effect of uncertainties in design parameters on the probability of failure of cantilever retaining walls. In total, ten modes of failure are considered, viz. overturning of the wall about its toe, sliding of the wall on its base, eccentricity, bearing capacity failure below the base slab, and shear and moment failure in the toe slab, heel slab, and stem. The analysis is performed by treating backfill and foundation soil properties, geometric properties of wall, and reinforcement and concrete properties as random variables. These results are used to develop a set of reliability-based design charts for different coefficients of variation of friction angle of backfill soil (5 and 10%) and targeting reliability index ( βt ) in the range of 3–3.2 for all failure modes. A comparative study is also presented, which shows that optimized s...

Model for Consolidation-Induced Solute Transport with Nonlinear and Nonequilibrium Sorption

Abstract: A numerical model, called CST2, is presented for coupled large strain consolidation and solute transport in saturated porous media. The consolidation and solute transport algorithms include the capabilities of a previous model, CST1, with the addition of a variable effective diffusion coefficient during consolidation and nonlinear nonequilibrium sorption. The model is based on a dual-Lagrangian framework that tracks separately the motions of fluid and solid phases. Verification checks of CST2 show excellent agreement with analytical and numerical solutions for solute transport in rigid porous media. A parametric study illustrates that, for the test cases considered, variation of effective diffusion coefficient during consolidation has an important effect on solute transport, whereas nonlinearity of the sorption isotherm has a less important effect. Additional simulations show that nonequilibrium sorption can have a strong effect on consolidation-induced solute transport and that this effect becomes more important as the rate of consolidation increases. The simulations also corroborate previous findings that consolidation can have a lasting effect on solute migration because transient advective flows change the distribution of solute mass which then becomes the initial condition for subsequent transport processes.

Evaluation of smear zone extent surrounding mandrel driven vertical drains using the cavity expansion theory

Abstract: In this study, an attempt is made to analyze the extent of the smear zone caused by mandrel driven vertical drains, employing the cavity expansion theory for soft clay obeying the modified Cam-clay model. The predictions are verified by large-scale laboratory tests, where the extent of the smear zone was estimated based on the indications, such as the pore pressure generated during mandrel driving, change in lateral permeability, and the water content reduction. This study reveals that the radius of smear zone is about 4 — 6 times the equivalent vertical drain radius, and the lateral permeability (inside the smear zone) is 61 — 92% of that of the outer undisturbed zone. Finally, the predicted size of the smear zone using the undrained cavity expansion solution is incorporated in the finite-element code PLAXIS to study the performance of a test embankment selected from the Sunshine Motorway, Queensland, Australia. A good agreement between the predicted values and field measurements was found.

Estimating Drilling Parameters for Diamond Bit Drilling Operations Using Artificial Neural Networks

Abstract: Diamond bit drilling is one of the most widely used and preferable drilling techniques because of its higher rate of penetration and core recovery in the hardest rocks, the ability to drill in any direction with less deviation, and the ability to drill with greater precision in coring and prospecting drilling. Conventional bit analysis techniques include mathematical methods such as specific energy and formation drillability. In this study, artificial neural network (ANN) analysis as opposed to conventional mathematical techniques is used to estimate major drilling parameters for diamond bit drilling, i.e., weight on bit, rotational speed, and bit type. The use of the proposed methodology is demonstrated using an ANN trained with information obtained from 45,000 m of diamond bit drilling operations conducted on several formations and locations in Turkey. The studied formations include shallow carbonates as well as sandstones in the Zonguldak hard coal basin. The neural network results are compared to thos...

Seismic Rotational Displacement of Gravity Walls by Pseudodynamic Method

Abstract: Prediction of the rotational displacements, induced by earthquake is a key aspect of the seismic design of retaining walls. In this paper, the pseudodynamic method is used to compute rotational displacements of the retaining wall supporting cohesionless backfill under seismic loading. The proposed method considers time, phase difference, and effect of amplification in shear and primary waves propagating through the backfill and the retaining wall. The influence of ground motion characteristics on rotational displacement of the wall is evaluated. Also the effects of variation of parameters like wall friction angle, soil friction angle, amplification factor, shear wave velocity, primary wave velocity, period of lateral shaking, horizontal, and vertical seismic accelerations on the rotational displacements are studied. Results are provided in graphical form with a comparison to the available pseudostatic result to validate the proposed theory. Present results give higher values of rotational displacements of...

Numerical Studies on Bit-Rock Fragmentation Mechanisms

Abstract: The rock fragmentation process induced by a single button-bit, two neighboring button-bits, and multiple button-bits are numerically studied using the rock and tool interaction code (R- T2D ) . Through this study, a better understanding of the bit-rock fragmentation mechanisms is gained. It is found that side crack is initiated from the crushed zone or bifurcated from Hertzian crack to propagate approximately parallel to the free rock surface but in a curvilinear path driven by the tensile stress associated with the expansion of the crushed zone during the loading process. In the crushed zone, the mechanism of side crack is mixed tensile and shear failure, but outside the crushed zone, the dominant mechanism of side crack is tensile failure. A semiempirical and semitheoretical relationship among the side crack length, the drilled rock property, and the drilling force is formulated to approximately predict the side crack length. In the simultaneous loading, the interaction and coalescence of side cracks in...

Reliability-Based Analysis of Strip Footings Using Response Surface Methodology

Abstract: A reliability-based analysis of a strip foundation subjected to a central vertical load is presented. Both the ultimate and the serviceability limit states are considered. Two deterministic models based on numerical simulations are used. The first one computes the ultimate bearing capacity of the foundation and the second one calculates the footing displacement due to an applied load. The response surface methodology is utilized for the assessment of the Hasofer-Lind reliability indexes. Only the soil shear strength parameters are considered as random variables while studying the ultimate limit state. Also, the randomness of only the soil elastic properties is taken into account in the serviceability limit state. The assumption of uncorrelated variables was found to be conservative in comparison to the one of negatively correlated variables. The failure probability of the ultimate limit state was highly influenced by the variability of the angle of internal friction. However, for the serviceability limit state, the accurate determination of the uncertainties of the Young's modulus was found to be very important in obtaining reliable probabilistic results. Finally, the computation of the system failure probability involving both ultimate and serviceability limit states was presented and discussed.

Effect of Footing Roughness on Lower Bound Nγ Values

Abstract: The bearing capacity factor Nγ was computed for a strip footing by considering the variation of footing-soil interface friction angle (δ) . The study follows a lower bound limit analysis in conjunction with finite elements and linear programming. The variation of Nγ with δ was obtained for different soil friction angles. The magnitude of Nγ increases significantly with the increase in δ . For δ=0 , the normal stress along the footing-soil interface was found to increase almost linearly with the increase in the distance from the footing edge. However, with the increase in δ , the location of the peak normal stress shifts gradually away from the center of the footing. The size of the plastic zone in relation to the footing width becomes gradually larger for greater values of δ . With respect to variation in δ , the obtained values of Nγ were found to be generally smaller than the results available in the literature.

Thermal Conductivity Evolution of Saturated Clay under Consolidation Process

Abstract: This paper presents the results of a study on the thermal conductivity of a soft saturated clay (Bangkok clay) carried out in relation to an investigation into thermal ground improvement using prefabricated vertical drains. The thermal conductivity of clay specimens was measured, at different porosities and temperature levels, using a simple nondestructive steady-state test method. In addition, a theoretical mixture model to simulate the evolution of thermal conductivity of saturated fine-grained soils has been introduced. It is formulated in terms of thermal conductivity and volume fraction of each soil phase (solid and water), and a morphological parameter controlled by the soil fabric condition. The proposed model has been validated against thermal conductivity results reported in the literature and results obtained from the present investigation. Reasonable agreement has been obtained between the predicted and measured thermal conductivity values.

Modeling of pipe-soil interaction and its application in numerical simulation

Abstract: This paper presents three plasticity models that can be applied to numerically simulate pipe–soil interaction. They can be applied individually to evaluate the force–displacement response of a small plane-strain pipe section or in combination to simulate a long pipeline system. In the latter, numerous pipe–soil elements are attached to structural finite elements, each simulating localized foundation restraint along the pipeline. The three models are increasing in sophistication, mainly due to the manner in which they account for the behavior within an allowable combined loading surface. The first is based on traditional strain-hardening plasticity theory and therefore assumes a purely elastic response inside a single expandable yield surface. The second allows some plasticity due to the use of a bounding surface, and the third accounts for kinematic hardening through the introduction of a second smaller surface. The models are detailed in this paper, allowing for simple numerical implementation. Important...

Analytical and Numerical Modeling of Consolidation by Vertical Drain beneath a Circular Embankment

Abstract: In the analysis of axisymmetric problems, it is often imperative that aspects of geometry, material properties, and loading characteristics are either maintained as constants or represented by continuous functions in the circumferential direction. In the case of radial consolidation beneath a circular embankment by vertical drains (i.e., circular oil tanks or silos), the discrete system of vertical drains can be substituted by continuous concentric rings of equivalent drain walls. An equivalent value for the coefficient of permeability of the soil is obtained by matching the degree of consolidation of a unit cell model. A rigorous solution to the continuity equation of radial drainage towards cylindrical drain walls is presented and verified by comparing its results with the existing unit cell model. The proposed model is then adopted to analyze the consolidation process by vertical drains at the Ska-Edeby circular test embankment (Area II). The calculated values of settlement, lateral displacement, and e...

Effect of Arching on Uplift Capacity of Pile Groups in Sand

Abstract: In this paper an analytical method has been proposed to predict the net ultimate uplift capacity of pile groups embedded in sand considering the arching effect. This method takes into consideration the embedded length (L) , diameter of the pile (d) , surface characteristics of pile, group configuration, spacing of the pile group ( 3d to 6d ), and the soil properties. Arching develops due to relative compressibility of sand relative to pile which activates the soil–pile friction. As piles/pile groups move up, the active state of soil is initiated. The modified value of active earth pressure coefficient considering arching effect has been derived. Typical charts for evaluation of net ultimate uplift capacity for pile groups are presented through the figures. The predicted values of ultimate uplift capacity of pile groups with different configuration and slenderness ratios are compared with the available experimental results. The predicted values considering arching effect are found to be in good agreement w...

Upper Bound for Cylinder Movement Using “Elastic” Fields and Its Possible Application to Pile Deformation Analysis

Abstract: A new upper bound failure mechanism for the problem of rigid cylinder motion is presented. The velocity field associated with the mechanism is derived from a known elastic solution by similitude of the deformation field. The obtained upper bound value is 21% higher than the exact solution. However, the failure mechanism is continuous, involving no discontinuity, not even on the cylinder perimeter. The solution has a certain advantage if one, for example, wishes to combine its mechanism with a strain path approach to investigate the T-bar penetration problem. The absence of discontinuities in the mechanism also allows evolution of deformation under serviceability conditions, by associating a mobilized strength as a function of an average strain. Based on this approach, a load transfer function for lateral loading of piles in an undrained clay is suggested. This load transfer function involves nonlinear scaling of a stress-strain curve obtained from a triaxial compression test. An analytical, closed form, solution is given for the case of a hyperbolic stress-strain curve.

Dynamic Modeling of Hydraulic Shovel Excavators for Geomaterials

Abstract: The hydraulic shovel excavator has found significant applications in surface mining, construction, and geotechnical operations due to its flexibility and mobility The key to high availability and utilization of this shovel is adequate understanding of machine dynamics and machine-formation interactions among other technical, operating, safety, and economic factors These shovels are capital intensive, complex in design and operation within severely constrained environments Detailed dynamic modeling and analysis are required to understand their effective utilization for achieving efficient operating performance and economic useful lives Previous attempts at solving these problems are limited because they do not provide knowledge on the resistive forces and moments for efficient excavation In this paper, the Newton-Euler techniques are used to develop hydraulic shovel dynamic models with numerical examples Detailed analysis of the results shows that: (1) the kinematics of the stick-bucket joint (joint

Equivalent Stress Equation for Unsaturated Soils. I: Equivalent Stress

Abstract: In 1959 Bishop stated his effective stress equation for unsaturated soils. However, the difficulties in estimating the value of its main parameter χ , made this equation useless and it was abandoned for some time. Only recently, it has been recognized that the use of Bishop’s stress equation can lead to simpler and more realistic constitutive models for unsaturated soils. However, up to now the most successful equations to quantify the value of parameter χ are empirical and not satisfactory for most soils. Based on the analysis of the equilibrium of the solid particles of a soil showing a bimodal structure and subject to certain suction, it was possible to establish an analytical expression for Bishop’s parameter χ . The resulting stress has been called equivalent stress (in contrast with effective stress) and can be used to predict the shear strength of unsaturated soils. The equivalent stress is written as a function of the net stress and suction and requires two parameters: the saturated fraction and t...

Observations on Return Mapping Algorithms for Piecewise Linear Yield Criteria

Abstract: This paper shows that for perfect plasticity, the closest point projection method (CPPM) and the cutting plane algorithm (CPA) for return mapping are exactly equivalent for piecewise linear yield criteria under both associated and nonassociated plastic flow. The paper demonstrates this by presenting closed-form expressions for returned stresses in terms of predicted stresses. A consequence of this exact approach is that the final stresses can be obtained in a single iteration. The equivalence of CPPM and CPA is further demonstrated numerically by comparing five previously published algorithms for return mapping to Mohr–Coulomb in a finite-element analysis of bearing capacity. The analyses also highlight issues relating to singularities that occur at the corners of the Mohr–Coulomb surface. It is shown that many of these problems can be avoided if the return mapping is performed in principal stress space as opposed to general stress space.

Three-Dimensional Joint/Interface Element for Rough Undulating Major Discontinuities in Rock Masses

Abstract: Major civil engineering structures are being constructed now a days in complex geological environment with faults, shear zones, and other major discontinuities. These major discontinuities can cause a variety of problems in both surface and underground constructions. Unfavorably dipping major discontinuities may create unstable conditions in underground openings and contribute to the deformations of a rock mass under external static loading. Hence, rock–structure interaction analysis should simulate arbitrarily oriented rough and undulating major discontinuities within the rock mass, as well as the undulating interface along the structure and the rock mass such as dam foundations and underground excavations intersected by fault/shear zones. Realistic simulation of the mechanical behavior of rock joints is a prerequisite for successful numerical modeling of discontinuous rocks. When joint modeling is designed to include different degrees of joint roughness, dilation, and aperture, then realistic response d...

Undrained Shear Strength of K 0 Consolidated Soft Soils

Abstract: On the basis of critical state soil mechanics, this study derives theoretical formulas for predicting the undrained shear strength of K0 consolidated soft soils in triaxial compression and extension. Although the modified Cam-clay model is often utilized to predict the undrained shear strength of soft clays, it is applicable mainly to isotropically consolidated soils. Because of the anisotropy under K0 consolidation, an inclined elliptical yield surface is chosen, which is different from those methods based on the original Cam-clay model. The inclined elliptical yield surface is testified to be appropriate to the K0 consolidated soft soil and results in a better prediction of undrained strength, especially for the triaxial extension test. It is concluded that the analytical solutions obtained in this paper are in good agreement with the available test results and back-analysis of slope failures. On the basis of the investigation of soil properties, a simple formula is proposed for calculating the mean und...

Composite Element Analysis of Gravity Dam on a Complicated Rock Foundation

Abstract: This paper presents the formulation and application of a composite element method, which is intended for numerical modeling of discontinuous rock masses. This method allows analysis of fractured rock masses using regular meshes that do not need to rigorously respect the orientations and positions of discontinuities. It can be incorporated in conventional finite-element programs. The performance of this method are illustrated through its use for the analysis of the mechanical behavior of the Baozhusi gravity dam which is constructed on a complex rock foundation.

Simple Formulas for the Response of Shallow Foundations on Compressible Sands

Abstract: The engineering design of shallow foundations on sand is almost universally based on one of the variants of the classical bearing capacity formula. However, this formula is suitable only where the sand exhibits dilative behavior and a clear rupture mechanism forms at failure. The main challenge then is choosing a suitable friction angle, taking into account the soil density and the high stresses beneath the footing. When other conditions apply, in particular when the footing is large or founded on compressible materials, alternative approaches need more focus on soil compressibility. Two simple semianalytical formulas are proposed and explored in this paper: (1) an analysis using a one-dimensional (1D) compression equation; and (2) an analysis using the concept of “bearing modulus.” It is argued that the bearing modulus approach may be used for conditions that reflect moderate design parameters (i.e., moderate foundation size and sand compressibility), but for very large foundations or highly compressible...

Immediate Settlement of Shallow Foundations Bearing on Clay

Abstract: Immediate and long-term settlement checks are an integral part of foundation design. Therefore, reasonably accurate estimates of the immediate settlement of shallow foundations bearing on clay are necessary, particularly for highly plastic clays or organic soils, for which the immediate settlement may be significant. This immediate settlement is due entirely to the distortion of the clay underneath the shallow foundations because, in the short term, there is no opportunity for change in the clay volume. Since soil stress-strain response is nonlinear even at small strains, design procedures based on linear elasticity cannot accurately predict soil deformations. Hence, an immediate settlement analysis that takes soil nonlinearity into account is needed. In this paper, finite-element analysis is used to develop design charts that can be used to estimate the immediate settlement of axially loaded square, rectangular, and strip footings bearing on clay. The clay is modeled with a simple nonlinear constitutive ...

Information Feedback Analysis in Deep Excavations

Abstract: Deep excavation often causes displacement of adjacent structures. Hence, necessary construction measures must be taken in order to minimize such disturbances. Appropriate construction measures depend on effective and reliable estimation of the induced ground movement during an excavation. This paper presents a systematic procedure, referred to as “information feedback analysis,” which is used to predict excavation-induced deformation by collecting field information, such as displacements. With the use of optimization algorithms, the analyses result in a “best set” of soil parameters. These back-calculated soil parameters are then used to predict the deformation in the subsequent stages, one stage at a time, until the end of the excavation, which result in additional updated information continuously entered into the system, and hence, the prediction becomes progressively more and more accurate. This study has shown that the proposed approach exhibits at least two advantages over the conventional analysis. ...

Equivalent Stress Equation for Unsaturated Soils. II: Solid-Porous Model

Abstract: Based on the study of the equilibrium of the particles of a soil showing a bimodal structure and subject to certain suction, it was possible to establish an analytical expression for the value of Bishop’s parameter χ (see the companion paper). This parameter can be written as a function of the saturated fraction and the degree of saturation of the unsaturated fraction of the soil. However, the determination of these last two parameters cannot be made from current experimental procedures. Therefore, a solid-porous model simulating the structure of the soil is proposed herein and used to determine these parameters. The data required for the solid-porous model are obtained from the grain and pore size distributions, void ratios, and secondary soil–water retention curves of the soil. The plots of the deviator stress versus equivalent stress shows a unique failure line for a series of tests performed at different confining net stresses and suctions, confirming that the proposed equivalent stress equation is ad...

Implicit Algorithm for Modeling Unsaturated Soil Response in Three-Invariant Stress Space

Abstract: An implicit integration algorithm has been refined to predict the stress–strain–strength response of unsaturated soil under suction-controlled, multiaxial stress paths that are not achievable in a conventional cylindrical cell. The algorithm supports numerical analyses in a deviatoric plane by using a mixed control constitutive driver, in conjunction with a generalized Cam-Clay model that also incorporates the influence of a third stress invariant, or Lode-angle θ , within a constant-suction scheme. True triaxial data from a previously accomplished series of suction-controlled triaxial compression, triaxial extension, and simple shear tests on 10-cm cubical specimens of silty sand, were used for the tuning and validation of the refined algorithm. The elliptical Willam–Warnke surface was adopted for simulation of unsaturated soil response in three-invariant stress space. Reasonably satisfactory agreement was observed between experimental and predicted deviatoric stress versus principal strain response for ...

Numerical Analysis of a Tunnel Support Design in Conjunction with Empirical Methods

Abstract: In this paper, preliminary support design of a tunnel was analyzed by numerical and empirical approaches. The case study for this analysis is a tunnel to be constructed on the Bilecik-Istanbul roadway in Turkey. The rock mass properties of the tunnel route and design support recommendations were obtained by using an empirical approach. The rock mass properties obtained from the empirical method were used as input parameters for the numerical analysis. The empirical and numerical results, in terms of support design, were evaluated. It was seen that the numerical analysis results supported by empirical values were logical and reliable.

Intelligent Cable Shovel Excavation Modeling and Simulation

Abstract: Cable shovel excavators are used for primary production of geomaterials in many surface mining operations. A major problem in excavation is the variability of material diggability, resulting in varying mechanical energy input and stress loading of shovel dipper-and-tooth assembly across the working bench. This variability impacts the shovel dipper and tooth assembly in hard formations. In addition, the geometrical constraints within the working environment impose production limitations resulting in low production efficiency and high operating costs. An intelligent shovel excavation (ISE) technology has been proposed as a potential solution to these problems. This paper addresses the requirements of the dynamic models of the cable shovel underlying the ISE technology. The dynamic equations are developed using the Newton–Euler techniques. These models are validated with real-world data and simulated in a virtual prototype environment. The results provide the path trajectories, dynamic velocity and accelerat...