Chadi S. El Mohtar

DYNAMIC RHEOLOGICAL PROPERTIES OF SODIUM PYROPHOSPHATE-MODIFIED BENTONITE SUSPENSIONS FOR LIQUEFACTION MITIGATION

The delivery of plastic fines such as bentonite into loose saturated granular soil deposits is an effective method for mitigating the liquefaction phenomenon. While the bentonite should be injected into the deposits in the form of a concentrated suspension, such application is limited in practice because of the low mobility of the suspension. The initial mobility of the bentonite suspension should be managed in order to increase the penetration depth. On the other hand, the suspension needs to maintain its thixotropic nature to improve the resistance of the treated soils under cyclic loading over time. The objective of the present study was to investigate the dynamic rheological properties of the bentonite suspensions modified with an ionic additive, sodium pyrophosphate (SPP), to evaluate a possible application of the modified suspensions in mitigation of liquefaction. In the present study, the storage and loss modulus of SPP-modified bentonite suspensions were measured using a strain-sweep (oscillatory shear) technique. Bentonite suspensions with clay contents of 5, 7.5, 10, and 12 wt.% (by total weight of suspension) were tested at various SPP concentrations (0 to 4 wt.% by weight of dry bentonite). The time-dependent behavior of the suspensions was evaluated with a critical storage modulus at various resting times (0 to 480 h). The results showed that the initial critical storage modulus decreased significantly with increasing SPP concentrations, but the reduced critical storage modulus increased gradually with resting times. This initial reduction in critical storage modulus is attributed to a reduction of the inter-aggregated 3-D networks due to the presence of SPP; the amount of 3-D network formed in a suspension governs the critical storage modulus. With time, the networks are formed gradually, resulting in recovery of critical storage modulus. The normalized modulus was degraded more slowly in the modified suspensions than in the unmodified suspensions, which is a desirable property of the suspensions for mitigation of liquefaction.

Combined Resonant Column and Cyclic Triaxial Tests for Measuring Undrained Shear Modulus Reduction of Sand With Plastic Fines

This paper investigates the undrained shear stiffness of sand-bentonite specimens (with 0 %, 3 %, and 5 % bentonite by dry mass of the sand) prepared at the same skeleton void ratio (Drsk ¼ 35 % to 40 %) using a dry pluviation technique. The experimental program consisted of (1) small strain tests using a resonant column apparatus and (2) large strain tests using a cyclic triaxial apparatus. The resonant column tests were performed at three confining stress levels (50, 100, and 193 kPa) under drained and undrained conditions. A comparison of the shear modulus reduction with shear strains for both drained and undrained conditions is presented; the effects of changes in effective stresses and the rate of mod- ulus reduction as a function of the effective stress are discussed to describe the discrepancy between the two sets of data. The results show a mar- ginal decrease in Gmax for specimens with bentonite, which is attributed to the presence of bentonite at the sand grain contacts. However, the presence of bentonite increases the linear elastic threshold, particularly in the case of undrained tests, in which a noticeable delay in excess pore pressure generation was measured. The strain level required in order to initiate excess pore pressure generation increased with increasing bentonite content. A similar trend was noted in cyclic triaxial tests, in which, for a given strain, specimens with bentonite generated lower excess pressure than sand specimens tested under similar conditions. Finally, a combined normalized G/Gmax curve from both tests is presented for specimens with 0 %, 3 %, and 5 % bentonite at 100 kPa.

Disturbance Effect on Time-Dependent Yield Stress Measurement of Bentonite Suspensions

Disturbance effect on time-dependent yield stress measurement of bentonite suspension was investigated using vane and cone and plate geometry. With cone and plate geometry, the disturbance occurs as a result of the large strains the suspensions are subjected to when placed on the lower plate and when the cone is lowered to the testing position before the actual shear stress is applied. As an alternative, vane ge- ometry has been used to measure yield stress in bentonite suspension to (1) minimize the placing disturbance, and (2) eliminate the loading disturb- ance. Even though a minimal amount of disturbance was still induced by vane geometry, it was assumed that the amount of disturbance was small enough to be ignored compared to that with cone and plate geometry. Yield stresses of bentonite suspensions at different weight fractions and vari- ous resting times were determined from stress ramp tests. Linear relationships were developed between yield stresses measured with the different geometries. Disturbance rapidly increased up to approximately 50 % with resting times, then converged at long resting times (60 % at 10 days). This indicates that cone and plate geometry significantly underestimated the actual yield stress of the tested material because of disturbance. Based on the quantified disturbance, a method to predict the undisturbed yield stress from disturbed one was proposed. The observations were interpreted with structural kinetics.

Development of a Laboratory Procedure to Evaluate the Consolidation Potential of Soft Contaminated Sediments

Consolidation settlement of non-aqueous phase liquid (NAPL) contaminated sediments may trigger NAPL mitigation. The consolidation potential and resulting NAPL mobilization of the sediments should be evaluated in the laboratory; however, due to the highly compressible and weak nature of riverbed sediments, it is usually not possible to conduct conventional consolidation tests on sediment specimens. In this study, a triaxial setup was modified to work effectively under low stresses. Kaolinite was used to represent the soil solid phase and Soltrol 130 (a type of mineral oil) was used to represent the NAPL. Both oil-wetted and water-wetted regimes were analyzed. Hexane Extraction and moisture content tests results confirmed the final fluid amounts in the specimen obtained by measuring the effluent volume during consolidation. The results of the tests show that approximately 0.1 g of NAPL per 1 g of soil solids is unlikely to be mobilized by consolidation. The developed procedure could also be employed to define the mobile and immobile fractions of NAPL and the expected compression of contaminated sediments. The volume of NAPL in excess of the retained residual can be used to design NAPL collection systems or to size layers of NAPL sorbent materials such as organo-clays.

Evaluation of Time-Dependent Yield Stress Using Dynamic Rheological Property of Bentonite Suspensions

Permeation grouting using weak grouts such as bentonite grout is one of the effective methods to mitigate liquefaction in loose saturated sand deposits. While a flow parameter of the grout such as yield stress determines its penetrability into the deposit and resistance to groundwater flow, a dynamic parameter such as critical storage modulus evaluates post-grouting performance of the treated soils under cyclic loading condition. However, the yield stress and critical storage modulus should be obtained through two different types of rheological tests: drag and oscillatory shear test. Although previous research has suggested a method to evaluate yield stress from an oscillatory shear test, the conventional method does not consider the time-dependent nature of bentonite grout, which is one of its crucial properties as a grout. In this study, flow and dynamic rheological properties of bentonite suspensions were measured using drag (stress ramp) and oscillatory shear (strain sweep) tests with a vane geometry for various weight fractions of bentonite suspensions (5, 7.5, 10, and 12 %) and resting times (0 to 480 h). At different resting times, elastic and crossover stresses from strain sweep tests were compared to yield stresses obtained from stress ramp tests. The results showed that both the elastic and crossover stresses from strain sweep tests were significantly lower (40 %–60 %) than the yield stresses measured by stress ramp tests. The comparison also showed a dependency on particle fractions. In order to evaluate yield stress from the oscillatory shear test, a time-independent relationship between yield stress and critical storage modulus was proposed. This study suggests an economical approach to evaluate an important design parameter (“undisturbed” yield stress) in permeation grouting using bentonite grout.

Comparison between Shear Strength of Dry Sand Measured in CSS Device Using Wire-Reinforced Membranes and Stacked Rings

The cyclic simple shear test (CSS) is widely regarded as the preferred element test for liquefaction because it best simulates vertically propagating shear waves. The CSS test provides reliable, high-quality laboratory data for model development and calibration in geotechnical earthquake engineering design. Direct simple shear (DSS) tests can be performed by using the same apparatus to measure the shear strength and define the failure envelope. The consolidation and shearing are performed under K0 conditions. The K0 conditions are maintained through lateral confinement that prevents lateral deformations. Wire-reinforced membranes (WR) and stacked rings (SR) have been the two major techniques used to achieve the no lateral strain condition. The motivation for this work was to study the effect of using a WR versus SR on the stress-strain behavior and shear strength of clean sand. The paper presents results from DSS tests performed on Monterey #0/30 and washed mortar sands. The tests were performed under a constant vertical stress instead of constant volume to monitor the effect of confinement systems on the contractive and dilative response of the sand samples. The results show that general agreement exists between the measured shear strength and volumetric response with SR and WR confinement systems.

New Three-Way Split Mold Design and Experimental Procedure for Testing Soft, Grouted Soils

Soil grouting has become a popular method for soil improvement in recent years. Grouting is generally intended to increase a soil’s strength, increase its liquefaction resistance, or reduce its hydraulic conductivity. Soil grouting involves the injection, permeation, or mechanical mixing of cementitious, silica, or clay grout into a soil deposit. With the increase in popularity of these methods comes the issue of testing grouted soils to verify the expected soil improvement. While some of the methods and materials mentioned result in soils that are sufficiently cemented to produce trimmable specimens that can stand under their own weight, other methods produce softer materials that are very difficult to sample or even to recreate and test in the lab. Preparing such soft samples in the laboratory poses two challenges: 1) if the specimen is prepared in the triaxial cell directly, the grouting process might not be feasible because of the porous stones and small diameter tubing in the triaxial cell; and 2) if the specimens are prepared outside the triaxial cell, soft specimens might not be able to stand under their own weight without significant strains and damage to the soil structure. This paper will thoroughly describe a three-way split mold specifically designed to accommodate the permeation and testing of soils that are too soft or too weak to be easily sampled or tested in the lab. A simple procedure outlining the use of this three-way split mold will also be described. Finally, the results from a series of consolidated undrained, monotonically loaded triaxial tests will be presented as an example of the split mold application. These tests utilized the new three-way split mold for sample preparation of loose Ottawa sand permeated with a thixotropic bentonite suspension.

A review on sand sample reconstitution methods and procedures for undrained simple shear test

Abstract This paper discusses the common methods available for reconstituting sand samples that are used for soil strength characterization. Reconstituted samples are preferred to undisturbed samples because the current field sampling procedures cannot retrieve granular soil specimens at a reasonable cost. Water sedimentation is generally recognized as the better one than the other common methods such as moist tamping and air pluviation. Most of the existing literature about sand specimen reconstitution is designated for triaxial set-up, and there is no study specifically for simple shear set-up in this topic. Therefore, this paper discusses the differences in reconstituting sand specimens between triaxial and simple shear setups. Two reconstitution methods particularly designed for undrained simple shear tests were introduced and tested through a CSS testing programme. The CSS test data are also compared with the results from published literature, and the strain-based criteria for liquefaction initiation are reviewed.

Transferring innovative research into practical wisdom: the case of permeation grouting

For decades, permeation grouting has been used as a cost-effective low disturbance ground improvement technology that allowed geotechnical engineers to help strengthening the soils and provide proper control of water flow. While permeation grouting can be a very useful solution, the design of an appropriate grouting program can be the difference between a successful job and an expensive on-the-go repair. However, current grout design relies heavily on rules of thumb and outdated charts. The geotechnical investigation is rarely targeted towards grouting design and the final grouting decisions are made based on index measurements such as fines content or soil classification at most. The collective “past experience” is more commonly used for final grouting design than any other geotechnical design despite that flow of grouts through porous media is a very complex phenomenon owing to the diversity of possible flow stoppage mechanisms involved. For some cases, rheological blocking occurs due to viscous grouts; however, in most cases, permeation is controlled by filtration. Despite the complexity of the science behind permeation grouting, practitioners often reference in situ heterogeneity and variability to dismiss the need for a more systematic design approach. However, heterogeneity and variability never stopped geotechnical engineers from performing proper design based on measured engineering properties of the soil and this should be the case for grouting as well. Particularly, with all the recent advances to grouting materials and its accompanying additives, grouting is now possible with more concentrated suspensions (lower w/c ratios) and the grouts are more “flowable” than ever before. These advances are making most of existing charts and common rules of thumb obsolete and therefore, it is the optimal time to implement recent advances in grouting research into common practice. Recent research on the topic highlights the complexities of grouting and the potential for implementing advanced laboratory testing to properly characterize the grouts and design it for field specific in situ conditions and desired final improved properties. This paper discusses these limitations in practice, advances in research and provides a case study where these advances were implemented to design a site-specific grout.

Post-Grouting Stability of Bentonite Suspensions within Sand Pores

Permeation grouting can be an effective way to deliver bentonite into porous media to control ground water flow. Bentonite slurries can be permeated into the porous media to reduce the hydraulic conductivity of the original soil. While some work has been done on the change in hydraulic conductivity of the soils when introducing bentonite, the stability of the bentonite within the pore space has not been significantly investigated. The current paper presents a study on the post-grouting stability of bentonite grouts within a porous media. The performance of bentonite grout in permeated sands was investigated by studying the rheological properties of bentonite suspensions (particularly, yield stress) and stability of the ground under in-situ hydraulic gradients. The rheological parameters of bentonite suspensions were measured at various bentonite concentrations (ranging from 6 to 16% bentonite to total weight) with and without sodium pyrophosphate (SPP) modifications (SPP percentage ranged from 1 to 11% by dry weight of bentonite). SPP is an ionic additive used to temporary control the rheological properties of the bentonite grouts. The stability of bentonite grout was inspected by studying the hydraulic conductivity of sand permeated with bentonite suspensions under increasing hydraulic gradients; when washing out occurs, it results in an increase in hydraulic conductivity. The critical hydraulic gradient at which washing out of bentonite suspensions is initiated was examined. Accordingly, a relation between the yield stress of bentonite suspensions and the critical hydraulic conductivity is developed.