Geo-China 2016: Advances in Numerical and Experimental Analysis of Transportation Geomaterials and Geosystems for Sustainable Infrastructure

The processes of sinkhole development are strongly dominated by the geomechanical characteristics of rock and soil behavior that are complicated by the interaction with chemical reactive processes as well as hydraulic transport processes. Sinkholes formed in soils are generally of a sudden, catastrophic nature, and understanding of cover-collapse sinkholes presents probably the biggest challenge to research communities due to the difficulties in formulating an effective quantitative approach to assess a number of diverse mechanisms behind many intricate processes involved, which are further complicated by a variety of scenarios that can trigger these sinkholes. This paper presents some of the preliminary results of an ongoing research that is aimed to explore a geomechanical modelling approach to understand the process of sinkhole developments. The explicit finite difference program, FLAC is used to model the behavior of geomaterials around a cavity in a potential water drawdown scenario. A parametric study is conducted to examine the effects of different mechanical and hydraulic properties. The results show some key characteristics of sinkhole subsidence and collapse, and demonstrate the feasibility of a numerical modelling approach which must be complimented by theoretical and analytical developments to be able to explore many intricacies and complexities of the mechanisms involved in the mechanisms of cavity growth and propagation.

In many cases, rock mass slope stability is controlled by discontinuities, which determine the most likely mechanism of failure, depending on joints and slope geometry. Among these mechanisms, the wedge failure is one of the most common ways of failure in rock slopes. The stability of the rock wedge is controlled by: joint sets dip and dip direction, slope geometry and direction, external forces (including water pressure and earthquake) and rock and joints mechanical properties. However, due to the geological origin of rock masses, these inputs are uncertain. Thus, the stability assessment method has to be capable of coping with this uncertainty. Random sets arise as a powerful tool to deal with uncertainty, even when limited information of inputs is available. With this framework, this work presents a reliability assessment of wedge stability in rock slopes by random sets, considering the joints geometry and strength parameters as random uncertain variables.

Experimental tests and numerical analysis were undertaken to investigate the specific storage of sedimentary rocks for supercritical CO₂. New laboratory system of constant flow pump permeability test was developed in order to measure permeability and specific storage of sedimentary rock in similar physical condition of deep aquifer where CO₂ tends to be in supercritical phase. To analyze experimental results, numerical analysis was established by extending the theoretical model of constant flow permeability to deal with two phase flow drainage displacement. For the examination of its applicability, experimental test was undertaken in which Ainoura sandstone used as core sample, injected with supercritical CO₂ under laboratory condition of 35°C constant reservoir temperature, 10 MPa initial hydraulic pressure, and 20 MPa constant overburden pressure The experimental results show that, average specific storage of Ainoura sandstone is about 3.74×10^-4 1/Pa at a maximum CO₂ saturation of 0.55, and volumetric strain increases about 0.7%. The accuracy of specific storage measured was validated with ratio of the specific storage of the sandstone to the storage capacity of the pump used in the permeability test. This finding suggested that newly developed constant flow pump technique with its numerical analysis can be used to effectively obtain specific storage of sedimentary rock for CO₂ in supercritical phase.

A series of tri-axial and direct shear tests have been conducted for the unsaturated clayey sand which was collected from the subgrade in a trial section of the Guangzhou-Foshan Expressway. Drained tri-axial tests were carried out by controlling the matric suction and net room pressure. The direct shear tests were performed by controlling the matric suction and net vertical pressure. Through these tests, the stress-strain relation is obtained together with those deformation and strength parameters. Furthermore, the variations of deformation and strength parameters are explored as the matric suction and net confining pressure vary.

Rock-socketed piles is applied in the highway bridge more and more in China, because it has a wide range of advantages, including high bearing capacity, easy construction and low cost. As low strain method has the properties of easy-performing and quick-testing, it has been used widely in the detection of pile integrity. In this paper, the mechanism of low strain method is introduced firstly, on the basis of which the typical time domain curves of different rock-socketed piles are analyzed, and then the integrity of the highway bridge piles is evaluated.

The capacity of the Wuku-Yangmei section of Freeway No.1 is not sufficient to cater for developing the Taoyuan Aerotropolis. To provide more traffic lanes to the existing freeway, a series of pier foundations at its embankment are constructed for the Wuku-Yangmei Overpass which is to ease peak-hour congestion between Taipei and Taoyuan. Soil nail shoring system and an alternative shoring method with ring beam at slope portion and composite shoring system at foundation portion are applied to Piers B7 and C15, respectively, to compare their surface tension crack, lateral slope displacement, and lateral displacement-time curve. Field measurements indicate that the soil nail shoring system fails to limit slope lateral displacement leading to the extensive slope sliding incident. 3D finite element (FE) analyses using the parameters that are back-calculated by comparing the lateral displacement-depth curves to the inclinometer data have been conducted. The same parameters have been brought forward to the case of the alternative shoring. The modeling results are found to be in agreement with the field measurements and the shear strain profile may give a better understanding of the sliding surface position.

In this paper, localized damage and failure process of brittle rock in compression is investigated using rock failure process analysis (RFPA) model. The model accounts for material heterogeneity to capture the cooperative interaction between microcracks in the transition from distributed to localized damage and the temporal and spatial evolution of acoustic emission during the progressive damage process. Correspondingly, a series of numerical compressive tests on rock specimens are performed to investigate the damage evolution and localized fracturing of brittle rock. The study shows that heterogeneity and scale of specimen have significant effect on damage localization and failure mode of rock specimens. In addition, post-peak response of stress-strain curve is due to the deformation of the structural elements, and localization means that the descending branch of the stress-strain curve of specimen is size dependent and the stress-strain curve can therefore not be regarded as a pure material property.

The maximum tensile stress calculation formula in the normal plane at 3D crack front local coordinates is derived from the coordinate and vector transformation. The maximal circumferential tensile stress criterion is adopted to determine the crack propagation load, orientations and steps. In the impact domain of node in 3D element free Galerkin (EFG) method, the appropriate node density is selected to ensure the calculation accuracy, and built-in oblique circular 3D crack propagation under uniaxial compression is numerically simulated. According to the numerical calculation results, cracking loads and crack propagation angles and expansion steps are analyzed and the rupture process of built-in oblique 3D crack is revealed. Besides, the rupture shape of build-in oblique 3D crack is clearly shown by interpolation method. Built-in oblique circular 3D crack begins to rupture from the point of 80° to two sides but not the tip of crack. The closer the distance from the crack tip is, the longer the propagation steps are and the smaller the propagation angles are. The longer the distance from the crack tip is, the smaller the propagation steps are and the larger the propagation angles are. The crack does not rupture within [-30°, 30°]. This is a new phenomenon found in the rupture process of built-in 3D crack.

Shaking table tests were conducted on two model soil slopes to understand the seismic behaviour and deformation characteristics of both unreinforced and reinforced (using nails) soil slopes. The shaking table tests were conducted on two slopes i.e., with and without nails and both having slope angle of 45° and slope height of 0.4m using an uni-axial shake table. The reinforcement of the soil slope has been done with nail length of 0.3m and nail inclination of 0°. The seismic excitation of the model slopes was carried out using the input time history i.e., 1989 Loma Prieta earthquake having the peak ground acceleration (PGA) of 0.57g recorded at Los Gatos Presentation Center. The experimental investigation on the model slopes using shaking table was carried out with proper instrumentation to examine various parameters of the slopes. Additionally, finite element analyses were also performed on both the slopes to examine their response to seismic loading. The numerical models developed have been used to examine the shaking table test results of both the slopes. The seismic resistance and failure mechanism of both unreinforced and reinforced soil slopes are studied in details. A reasonable agreement between the finite element and shaking table test results are obtained.

Cantilever retaining wall with pressure relief shelves is considered one of the special types of retaining walls. The concept of providing pressure relief shelves towards the backfill side of a retaining wall reduces the total earth pressure on the wall, which results in reducing the thickness of the wall and ultimately to get an economic design. Limited solutions were presented for this type of walls in some researches and references. This paper presents a set of finite element analyses for this type of wall using PLAXIS2D-AE. The reduced total active earth pressure due to provision of shelf has been depicted. It was found that the shelves have a significant effect on the resulted distribution of the earth pressure. It follows approximately the distribution of the Klein’s 1964 solution. Parametric study is conducted to discuss the effect of the number of shelves, shelf rigidity and shelf position on the resulted lateral earth pressure distribution, top wall movement and the maximum acting bending moment on the wall. For high retaining walls and some repairing systems of constructed walls having stability problems; it is much recommended to provide the cantilever wall by one shelf at one third of the wall height from wall top or more shelves at different levels.

This paper presents the effect of the base soil stiffness for sandy soils, under a retaining wall on the resulted earth pressure and the wall deformation. The most common method that is widely used in the calculation of the lateral earth pressure on the retaining wall is Rankine theory. This method didn’t take the effect of the base soil stiffness on the value of the earth pressure into consideration. Earth pressure also depends on the value of the wall displacement, which in turns; depends on the wall rigidity. In this study, a set of 2D finite element models were prepared to simulate the effect of retaining wall’s rigidity, different base soil stiffness and the embedded width of the footing inside the backfill on the lateral earth pressure distribution and the maximum bending moment acting on the wall. It is found that the soil base and the wall rigidity are significantly affecting the movement of the wall. The different FE models showed that the wall’s base is rotated and displaced horizontally and vertically due to the backfill weight and the base soil stiffness. Also, the wall rigidity and the rotation of the wall base are affecting the maximum deflection the wall. Due to this movement, the resulted earth pressure is affected by the base soil rigidity and the wall rigidity. In general, Rankine theory is still having a good agreement with the results and has a satisfactory safety factor in most cases.

The paper pertains to the slope reliability analysis under a probabilistic framework in strain-softening cohesive soils, using the first order reliability method (FORM). The performance function is based on the Bishop simplified method modified to take strain-softening into account in terms of the residual factor R_F over a potential slip surface, estimated based on a progressive failure model proposed in the literature. The reliability analysis is performed on the surface of minimum factor of safety determined by using the sequential quadratic programming (SQP). The random shear strength parameters are assumed to follow normal distribution while the residual factor has been considered both as a deterministic parameter and a beta-distributed random variable. The results obtained for an illustrative example shows substantial reduction (21%) in the value of reliability index when R_F is considered as a random variable with an assumed COV of 0.3. Results of FORM-based sensitivity analyses also reveal that R_F has the most dominating influence on reliability and thus justifies its inclusion as one of the random variables. A parametric study, varying the assumed correlation coefficient between the random shear strength parameters from 0 to 1, shows that there is a maximum reduction of 16% in reliability index.

The paper presents a new insight into the assessment of unsaturated slope stability under rainfall using the probabilistic method. The rainfall infiltration, correlation length of soil properties, and the cross-correlation between different shear strength parameters are taken into consideration to examine its influences on the stability status of spatially variable soil slope. The factor of safety and the failure probability are simultaneously used to estimate the slope. Results show that coarse grained soil slope is influenced most by the short term rainfall with high rainfall intensity, while the fine grained soil slope influenced most by the long term rainfall. There exists a threshold value of rainfall intensity for different type of soil. The slope would suffer the minimum danger as its constructed soil possesses strength parameters with small correlation length and variation coefficient. And in addition, the failure probability grows bigger when the correlation coefficient between the cohesion strength and friction angle increases.

The aims of this study are to investigate rainfall-induced failure of volcanic embankments subjected to both freeze-thaw action and cyclic loadings such as seismic loadings, and to elucidate its mechanical behavior. A series of model tests was performed on model embankments using both a shaking table and a model test apparatus which is capable of simulation of rainfall. In the model tests on embankments subjected to freeze-thaw action after cyclic loadings, the surface was frozen with dry ices over a period of eight hours, and was thawed; thereafter, rainfall tests were performed until failure. During rainfall tests, pore water pressure behavior, deformation behavior and variation in saturation degree were monitored. In particular, the effect of freeze-thaw action and cyclic loadings such as seismic loadings on mechanical behavior at failure was clarified. Based on the test results, an evaluation method of slope stability on volcanic embankments was proposed.

Seismic stability analysis is an important aspect for design of safe retaining walls in earthquake prone areas. In this study, limit equilibrium method is used to compute the rotational displacement of the gravity retaining wall on rigid foundation supporting cohesionless backfill along with pseudo-dynamic seismic forces. In the computation of rotational displacement, the location of the rotating wall and the shift in the point of application of the seismic earth pressure after each time step is included which is ignored in the rotating block method. From the present analysis, it is found that the rotational displacement depends on the characteristics of the input motion, properties of the backfill and the wall material.

This paper presents a statistical study on the correlations between standard penetration test (SPT) blow count N-value and pressuremeter (PMT) parameters: the pressuremeter modulus (E_PMT) and limit pressure (P_L) for glacial tills in the Greater Toronto Area (GTA). This study is based on a comprehensive geotechnical investigation program for a light rail transit project in the City of Toronto. Based on this statistical analysis, two linear relationships between uncorrected SPT-N value and PMT parameters (E_PMT and P_L) are proposed for glacial tills in the GTA.

The task of predicting the performance of deep excavations is challenging due to the existence of many influencing factors. Strength and deformation parameters of soil, type, stiffness of the support system, and sequence of excavation and installation of support elements are very important factors in studying the performance of deep excavations. Performance of deep excavation support systems is related to both stability and deformation. Ground deformation around the excavations can damage surrounding buildings, streets and utilities. Therefore understanding the factors affecting the performance of deep excavations and the ability to predict the behavior of the support system and the associated ground deformations is an important issue for geotechnical engineers. In this paper, two case studies with well documented data of field measurements are analyzed numerically. The analyses are performed using the finite element program “PLAXIS-2D”. Two material soil models are implemented in the analyses: the hardening soil model and Mohr Coulomb model. In both analyzed case studies, good agreement between the field measured displacements and the calculated displacement using the hardening soil model is obtained. Results of analysis showed also that the hardening soil model is superior to Mohr Coulomb model in predicting the displacements in both case studies. After calibrating the numerical model, a parametric study is performed to investigate the effect of anchor-pretension-force on the resulting deformation for case study (A) and the effect of strut stiffness on the resulting deformation for case study (B). Results of these analyses showed that both factors have important effects on the resulting excavation-support-system deformation.

Compacted soil-cement has been used worldwide for subgrade stabilization. A case study is presented herein to show the beneficial of using this technique to restore a slope failure in a conjected residential area. Such approach is not common, however, the experience described herein has shown that the compacted soil-cement toe berm together with soil nailing successfully meets all the site criteria. Based on field monitoring, the slope is stable and safe to date. It is a valuable and sustainable solution particularly for those sites that have a major concern in disposal of the sloughing silty debris.

2D numerical models are used for resolving plane strain problems and plane stress problems while many geotechnical problems are actually three-dimensional. An approach is developed in the paper to handle some three-dimensional geotechnical problems with 2D numerical models based on some assumptions of stress spreading with depth. The approach is verified with FLAC3D model and the results based on the approach agree well with the 3D model results.

This paper developed a study of risk assessment for landslide disaster from oil and gas pipeline construction by systematically analyzing the characteristics of landslide bodies during the construction phase. Typical cases of slope landslides caused by pipeline construction are introduced and combined with the landslide disaster that occurred at the China-Burma oil and gas pipeline, supporting the following conclusions: the concept of risk is introduced by proposing adoption of semi-quantitative risk assessment methods. Summarizing the causal factors for the landslide disasters, during the China-Burma pipeline construction, and establishing a landslide risk evaluation indicator system. By evaluating the risk level at three typical geological disaster-prone zones, we confirm that evaluation results coincide with actual situation. Therefore, the proposed semi-quantitative evaluation methods have reference value for risk management of pipeline construction at geological disaster points (segments).

Conventional slope stability analyses have commonly been based on a deterministic approach. Various deterministic-based analysis methods developed to date can assess the stability of a given slope using the factor of safety. However, it has been strongly debated that the use of only the factor of safety does not explicitly account for the uncertainties in soil parameters. In light of this, this paper uses the finite element limit analysis methods and conducts a probabilistic-based analysis of fill slope for the specific case of two-layered undrained clay. Results obtained show that slopes with large variations in soil properties may present an extremely high risk of a slope failure and this cannot be known if only a deterministic-based analysis is performed. Thus, this shows that more soil investigations can be performed to reduce the variation of the soil properties thereby reducing the risk of a slope failure. Different probabilistic charts based on different coefficients of variation in soil properties are provided in this paper. This study demonstrates that the finite element limit analysis methods can be applied in a probabilistic analysis.

Over the past decade, major advances have occurred in both understanding and practice with regard to assessment and mitigation of hazard associated with seismically induced soil liquefaction. Soil liquefaction engineering has evolved into a sub-field in its own right, and engineering assessment and mitigation of seismic soil liquefaction hazard is increasingly well addressed in both research and practice. In this research a study of the effect of Sabkha formation at the southwest part of Saudi Arabia on the liquefaction resistance probability. The Sabkha formation is different in its formations, properties, densification and location from ground surface. This paper studies the effect of the different earthquake magnitudes on the soil liquefaction resistance probability. It is found that the soil property, fine percentages and its densification has a significant effect on the soil liquefaction resistance. Recommendations for different methods to increase the factor of safety against liquefaction hazards are presented.

Municipal solid waste (MSW) landfills or dump sites located in earthquake prone regions of moderate to high intensity require seismic studies to be conducted for ensuring the stability and integrity of waste mass and containment system. This paper present the results of one-dimensional site specific seismic response analyses performed for an old and tall MSW dump site in Delhi, India. Unit weight profile and dynamic properties of MSW evaluated from field and large scale laboratory tests for this site were utilized in seismic analysis. Synthetically generated strong ground motions using finite source model and specific barrier model, with appropriate seismological parameters for Delhi region, were used in the analyses. Results indicated that the ground motion amplification for low intensity events in the present study compare well with that reported for Operating Industries Inc. landfill in California instrumented with strong ground motion recording stations. Amplification observed in the present study can be useful for the design of cover system. Also, results in terms of maximum horizontal equivalent acceleration for base-sliding case are in agreement with those reported in the literature and can be used for preliminary estimates for the design of linear system of new engineered MSW landfills in Delhi.