Jinchun Chai

Modelling the cutoff behavior of underground structure in multi-aquifer-aquitard groundwater system

The quaternary deposit of Shanghai is composed of an alternated multi-aquifer-aquitard system (MAAS) consisting of a sequence of aquitards laid over aquifers one by one. In the MAAS, any drawdown of groundwater head in an aquifer may cause consolidation of the overburden aquitard. When underground structures penetrate those aquifers, groundwater seepage path changes and drawdown occurs at the side characterized by the lower hydraulic potential along the flow direction (hereafter refers as to the lower side). This drawdown may cause additional subsidence at the lower side and unbalanced load between the two sides of the underground structure. In order to evaluate the cutoff effect of an underground structure on groundwater seepage in a MAAS representative of the underground of the city of Shanghai, a numerical analysis based on a groundwater flow model has been carried out. The simulated results have shown that underground structures which cut off groundwater flow locally change both magnitude and direction of the flow velocity field. The induced changes in the groundwater field are highly sensitive to the penetration depth and width of the underground structure. Design recommendations for underground structures in aquifers belonging to a MAAS are also presented, which has not yet been considered in the engineering practice of Shanghai.

Pullout force/displacement relationship of extensible grid reinforcements

Abstract A model for predicting the pullout resistance of polymer-grid reinforcement has been proposed. The influence of bearing member rigidity and spacing ratio (S/D) are explicitly expressed in the hyperbolic model. A new bearing capacity equation is incorporated for calculating the maximum pullout force. The displacement along the reinforcement is calculated by using the proposed pullout bearing resistance model together with the elongation of the grid longitudinal member. The validity of the method is confirmed by good agreement between calculated values and actual test data. The analytically determined effective reinforcement embedment lengths (i.e. the length of the reinforcement in tension) and pullout displacement to mobilize the desired pullout resistance of polymeric grids under different backfill conditions and under different applied normal pressures, provide useful information for the design of reinforced earth structures against pullout failure.

INTERACTION BETWEEN COHESIVE-FRICTIONAL SOIL AND VARIOUS GRID REINFORCEMENTS

Abstract A total of 52 large-scale laboratory pullout and 24 large-scale direct-shear tests were conducted to investigate the interaction behavior between the different reinforcements and cohesive-frictional soil. The reinforcements used were steel grids, bamboo grids, and polymer geogrids. The backfill material used was locally available weathered Bangkok clay. The test results show that the inextensible reinforcements, such as steel grids, move approximately as a rigid body during the pullout test, and the maximum pullout resistance was reached within a relatively small pullout displacement. For extensible reinforcements, such as Tensar geogrids, the degree of resistance mobilization along the reinforcement varies, and the pullout-resistance achieved in the tests was controlled by the stiffness of the reinforcement. For steel grids, the friction resistance from the longitudinal member contributed only to about 10% of the total pullout resistance of the grids. The pullout of the bamboo and Tensar geogrids without transverse members yields 80–90% of the pullout resistance of the corresponding grids with transverse members, attributed to the nodes or ribs on longitudinal members. The bond coefficient as calculated for steel and bamboo grids demonstrated that the steel grids yielded a higher bond coefficient than that of the bamboo grids with the same grid size. However, for a polymer geogrid, the bond coefficient cannot be calculated from a pullout test because of the complicated pullout-resistance-mobilization mechanism along the reinforcement. The large-scale direct-shear-test results showed that, for the soil/grid-reinforcement interfaces, shear resistance can exceed the direct-shear resistance of the soil itself owing to the influence of the apertures on the grids. Finally, for compacted weathered clay, the strength parameters obtained from large-scale direct-shear tests were found to be substantially smaller than the results of triaxial UU tests. This may be because the failure plane in the large-scale direct-shear test was formed progressively, and the peak soil strength along the predetermined shear plane may not have been mobilized simultaneously.

Deformation Analysis in Soft Ground Improvement

Preface Notation 1 Introduction 1.1 What is ground improvement and when and why is it necessary? 1.2 Techniques of ground improvement 1.3 Why do we need to estimate ground deformations? 1.4 What is this book all about? 1.5 References 2 Modelling Soft Clay Behaviour 2.1 Introduction 2.2 Initial stiffness and undrained shear strength 2.3 Modelling the embankment construction process 2.4 Effect of large deformations on embankment stability 2.5 Buoyancy effects due to large deformation 2.6 Summary 2.7 References 3 Vertical Drains 3.1 Consolidation theory for prefabricated vertical drains 3.2 Parameter determination 3.3 Optimum PVD installation depth 3.4 Two-dimensional modelling of PVD-improved soil 3.5 Modelling a large scale laboratory test 3.6 Application to a case history 3.7 Summary 3.8 References 4 Vacuum Consolidation 4.1 Introduction 4.2 Field methods for vacuum consolidation 4.3 Theory of consolidation due to vacuum pressure 4.4 Characteristics of vacuum consolidation 4.5 Optimum PVD penetration depth 4.6 Estimating deformations induced by vacuum pressure 4.7 Deformations associated with the vacuum-drain method 4.8 Summary 4.9 References 5 Soil-cement Columns 5.1 Introduction 5.2 Settlement predictions 5.3 Degree of consolidation 5.4 Settlement - time curve 5.5 Deformations induced by column installation 5.6 Summary 5.7 References 6 Concluding Remarks 6.1 What else needs to be done? 6.2 Hybrid soft ground improvement techniques 6.3 References Index

FE analysis of grid reinforced embankment system on soft Bangkok clay

Abstract The behavior of a reinforced embankment on soft Bangkok clay has been analyzed by plane strain finite element method. The finite element analysis considers the selection of proper soil/reinforcement properties according to the relative displacement pattern of upper and lower interface elements. The large deformation phenomenon is simulated by updating the node coordinates, including those of the embankment elements above the current construction level, which ensures that the applied fill thickness simulates the actual field value. A full scale test reinforced embankment with a vertical face (wall) on Bangkok clay has been analyzed by the proposed finite element method, and the numerical results are compared with the field data. The response of a reinforced embankment on soft ground is principally controlled by the interaction between the reinforced soil mass and soft ground and the interaction between the grid reinforcement and the backfill soil. The tension in reinforcement and lateral displacement of the wall face varied during consolidation of foundation soil. The maximum tension force occurred in the reinforcement layer placed at the base of reinforced mass, due to bending of the reinforced mass resulting from differential settlements. It is considered necessary to account for the permeability variation of the soft ground foundation in the finite element analysis.

Cr(VI) concentration from batch contact/tank leaching and column percolation test using fly ash with additives

Batch contact, tank leaching and column percolation tests were conducted to investigate the Cr(VI) concentration in the solution/leachate from two fly ashes (fly ash A and B) with additives. The additives used were cement, low alkalinity additive and Ariake clay. There are several factors influencing Cr(VI) concentration in solution/leachate, namely (1) properties of solid/liquid mixture (chemical composition, pH value, etc.), (2) cementation effect, (3) amount of water in contact with the solid mass (solid/liquid ratio in case of batch contact test), and (4) adsorption characteristics of the solid particles to Cr ions. The test results indicate that fly ash A has less cementation component (CaO of 1.92%) and the amount of water in contact with the fly ash played an important role. As a result, Cr(VI) concentration from the column percolation test was much higher than that of the batch contact test. Adding Ariake clay had more effect on reducing Cr(VI) concentration for fly ash A than B because the pH value of the solution from fly ash A was lower, which provided a favorable condition for Cr(VI) ions to be reduced to Cr(III) and possibly to be adsorbed by clay particles. Fly ash B has more cementation component (7.15%) and for column percolation test, curing the sample for 1 week reduced Cr(VI) concentration significantly. The test results indicate that in engineering practice, a method which closely simulates the field condition should be selected to assess possible environmental effects and corresponding countermeasure methods.

Improved Prediction of Lateral Deformations due to Installation of Soil-Cement Columns

A modified method is proposed for predicting the lateral displacements of the ground caused by installation of soil-cement columns. The method is a combination of the original method derived on the basis of the theory of cylindrical cavity expansion in an infinite medium and a correction function introduced to consider the effect of the limited length of the columns. The correction function has been developed by comparing the solutions obtained using the spherical and the cylindrical cavity expansion theories for a single column installation. Both the original and the modified methods have been applied to a case history reported in the literature, which involves clay soils, and the predictions are compared with field measurements. The advantage of the modified method over the original method is demonstrated. Finally, the modified method has also been applied to a case history involving loose sandy ground, and the calculations show that the method can also be used for this type of soil provided that appropriate consideration is given to the volumetric strain occurring in the plastic zone of soil surrounding the soil-cement columns.

Anisotropic Consolidation Behavior of Ariake Clay from Three Different CRS Tests

Three types of constant rate of strain (CRS) consolidation tests were conducted on samples of undisturbed Ariake clay, using a newly developed consolidometer, to investigate the anisotropic consolidation behavior of the clay. CRS tests conducted using vertically cut specimens (with respect to the in situ condition) tested with vertical (or end) drainage (with respect to test condition) were designated as CRS-V-V tests. Specimens cut vertically but with radial drainage were designated CRS-V-R, whereas those cut horizontally and tested with vertical drainage were designated CRS-H-V. The test results show that the ratio of the consolidation yield stress of a horizontally cut specimen (pch) to that of a vertically cut specimen (pcv) is in a range from 0.5 to 1.0. Both pcv and pch increased about 15 % with a tenfold increase in strain rate, but there was no clear difference in the degree of strain-rate dependency for pcv and pch. Values of the coefficient of consolidation obtained from CRS-H-V (chh) and CRS-V-R (chv) test are larger than those measured in CRS-V-V (cv) tests, and it has been identified that these differences arise mainly from the anisotropy of hydraulic conductivity (k). The ratio of k in the horizontal direction (kh) measured in a CRS-V-R test to that in the vertical direction (kv) from a CRS-V-V test is about 1.65, and the ratio of chv/cv is about 1.54. The value of kh from a CRS-H-V test is generally smaller than that from a CRS-V-R test.

Stress Distribution in Composite Ground of Column-Slab System Under Road Pavement

ABSTRACT In Column Approach and/or Column System methods, soil-cement columns are installed in a pillar-shaped condition with varying lengths. In the calculation of settlement and bearing capacity of composite ground, the stress ratio between column and surrounding soil is required. This paper is aimed to elucidate the stress transferring mechanism for a column-slab improved composite ground under road pavement based on finite element analysis. The results show for a column-slab system, the floating columns are more economical than end bearing column. Analytical results also show that the stress ratio decreases exponentially with the increase of the thickness of slab and improvement ratio, however, it increases with the increase of length-diameter ratio of column.

Reliability-based analysis of embankment on soft Bangkok clay☆

Abstract Six slope failures occurred at random locations along a 10 km embankment adjacent to an irrigation canal. The slope failures occurred when the embankment was raised to 2.05 m above MSL from an average elevation of 1.7 m above MSL coinciding with the lowering of the canal water level at the end of the dry season. Slope stability analysis was carried out using both conventional and reliability-based procedures. The spatial variability of undrained strength, the actual variation in embankment geometry, and the varying water level in the canal were considered in the analysis. Both idealized and empirical autocorrelation functions (ACF) of the undrained shear strengths were used in the analyses. An analysis using a factor of safety based on the deterministic soil profile defined by the mean undrained strength resulted in a prediction favoring a reverse failure pattern along the embankment. Using the probability of failure which incorporates spatial variation of undrained strength and uncertainties associated with stability prediction yielded a result conforming to the actual failure pattern along the embankment. The use of empirical autocorrelation function (ACF) seems to confirm and explain better the occurrence of the failure zones than utilizing the idealized ACF.