Increase In Undrained Shear Strength Research Articles

A Case Study of Peat Ground Improvement by Vacuum Consolidation in Hokkaido, Japan

This paper presents the field performance of vacuum consolidation method on peat ground and some technical learning from the field trial. Peat is well known to be a soft soil which has particular characteristics, including extremely high compressibility and greatly low undrained shear strength. In case of an embankment building over peat ground, sliding failure and large settlement often occur due to the particular characteristics. For actual construction sites on peat ground, therefore, some kinds of ground improvement methods are commonly used. One of the methods is the vacuum consolidation method which can load vacuum pressure with the soft ground by vacuum pumps and prefabricated vertical drains to accelerate the consolidation and increases the strength of soft ground. Peat ground distributed widely in Hokkaido, the northernmost land of Japan. A full-scale trial construction of vacuum consolidation was conducted in a highway project over peat deposit in Hokkaido of Japan to reveal its performance. Although the undrained shear strength of peat ground at the trial construction site was approximately 10 kN/m2 and extremely low, a 10.7 m high embankment was successfully built in only 45 days. This experimental fact implies that the vacuum consolidation method has extremely high effects in improving the stability of peat ground. Based on a result of the trial construction, it also revealed that the increase of undrained shear strength of the peat ground using vacuum consolidation and the suitable spacing of prefabricated vertical drains for peat ground.

Dewatering and stabilization of Black cotton soil using Electrokinetic Geosynthetics

Electrokinetic Geosynthetics (EKG) is a platform technology used to dewater and stabilize highly saturated soft clayey soils, mine tailings, tunnelling slurry waste, sewage sludge etc. This laboratory study investigates the potential of EKG made of woven coir geotextile to stabilize highly expansive saturated black cotton soil collected from the Hubli-Dharward region of Karnataka state, India. An electoosmosis design was made in a tank setup where the soil was dewatered for a period of 7 days under three varying potential (15V, 30V & 60V) and different electrode configuration. The results demonstrated an increase in the rate of pore water dissipation by 500% at an optimum voltage gradient of 30V in various electrode spacing. The results of the EKG treatment at various voltages revealed that, as applied voltages increased up to 60 V, the liquid limits and plasticity indices rapidly dropped. An exponential increase in undrained shear strength of the soil was observed at an electrode configuration of 4A-1C.

Influence of Rate Effects, Temperature, and Moisture Content on the Geomechanical Behavior of Marine Clays

Abstract The study of the geomechanical behavior of marine clays is a basic requirement for project optimization of the oil and natural gas industry. As part of a large-scale project, this study explored the effects of moisture content, temperature, and rate effects using laboratory vane shear tests. A series of results has helped in the identification of the effects of ground freezing on the undrained shear strength. For temperatures below freezing (−2.0 °C) the undrained shear strength increases with increasing water content and decreases with increasing shear rates for applied angular velocities in the range of 0.0025– 0.015 rpm. Apparently, with the freezing of pore fluids, the shear strength is partially governed by the strength of the ice-soil particle bonds. The increase in shear rate appears to facilitate the breakage of the ice bonds and afterwards the ice crystals reducing the viscous effects on the mobilized shear strength. Conversely, samples tested at temperatures above freezing show an increase in undrained shear strength with the increase in the imposed angular velocity and decreases with increasing soil moisture. Based on these studies, it is concluded that rate effects should be coupled to the influence of temperature and moisture content in the design of offshore structures.

Impact of initial effective stress on the thermo-mechanical behavior of normally consolidated clay

This study aims to investigate the impact of initial mean effective stress on the thermo-mechanical behavior of saturated normally consolidated kaolinite clay. Specifically, a series of isotropic thermal triaxial tests in a cell equipped with image analysis for volume change tracking was performed to understand the impact of the initial mean effective stress on the drained thermal volume change response as well as the undrained shear strength before and after drained heating. Anisotropically consolidated clay specimens were recompressed isotropically to four different initial mean effective stresses corresponding to normally consolidated conditions before drained heating and undrained shearing. While contractive volumetric strains were observed during drained heating of all normally consolidated specimens, the thermal radial strains were greater than thermal axial strains due to the application of isotropic stresses after anisotropic consolidation from a slurry. The magnitude of thermal volumetric strain increased with increasing initial mean effective stress, which is a departure from expected trends from established constitutive models. A corresponding increase in undrained shear strength with both temperature and initial mean effective stress was observed. The results indicate the need for considering the impact of initial mean effective stress in geotechnical applications involving normally consolidated clay under non-isothermal conditions.

Centrifuge modeling of temperature effects on the pullout capacity of torpedo piles in soft clay

This study presents the results from centrifuge modeling experiments performed to understand the effects of temperature changes on the vertical pullout capacity of scale-model torpedo piles embedded in soft clay layers. The model torpedo pile is a pointed stainless-steel cylinder with fins at the top, installed by self-weight to the base of a clay layer using a stepper motor. An internal electrical resistance heater was used to control the pile temperature. The torpedo pile was first heated until the temperature and pore water pressure of the surrounding clay layer stabilized (drained conditions), after which the torpedo pile was cooled. Pullout tests performed on torpedo piles indicate that allowing drainage of excess pore water pressures induced by heating to different temperatures followed by cooling leads to an increase in axial pullout capacity with maximum temperature but does not affect the pullout stiffness. Push-pull T-bar penetration tests performed before and after pile heating indicate that an increase in undrained shear strength of the clay occurs near the torpedo pile, and post-test gravimetric water content measurements indicate a greater decrease in void ratio occurred in the soil layers heated to higher temperatures. The pullout capacity of the torpedo pile was found to follow a linear trend with maximum pile temperature change, but with a smaller slope than that observed for end-bearing energy piles tested in previous studies in the same clay.

Physical modeling of thermal improvement in normally consolidated and overconsolidated soil

Knowledge of the soil-temperature relation and its peculiarities have been widely discussed in Geotechnics. Previous studies show that heated fine-grained soil induces pore pressure, and as a result, the soil exhibits a consolidation effect similar to that of conventional consolidation when pore pressure is dissipated. The outcome of this process depends on soil characteristics such as the overconsolidation ratio and plasticity. Consolidated soil typically induces positive pore pressure while overconsolidated one induces negative pore pressure when heated. The phenomenon described was explored in this study aimed at the thermal consolidation product regarding the decrease in void ratio and consequent increase in undrained shear strength (Su) of a soil submitted to heating. These aspects were investigated using physical models at 1g gravity field, built on a laboratory scale. The heat source was placed inside a driven torpedo pile model in a medium consisting of a kaolin and metakaolin mixture. The undrained shear strength profile was defined by T-Bar tests involving different effective stresses and consolidation temperatures. The results show that for both mediums (normally consolidated and overconsolidated), undrained shear strength tends to grow proportionally to the temperature variation and permanent volume change is reached after a heating-cooling cycle. On the other hand, undrained shear strength indicates less significant variations for higher overconsolidation ratios.

Peripheral stability of circular tunnels in anisotropic undrained clay

In the present analysis, solutions have been obtained for evaluating the minimum support pressure to be offered by means of lining for maintaining peripheral stability of circular tunnels constructed in clay taking into account anisotropic undrained shear strength parameters. The finite element approach based on lower bound theorem of limit analysis was implemented for performing the computations and second order cone programming technique has been applied for optimizing the magnitude of support pressure. The solutions obtained in this analysis can be used for computing the support pressure for tunnels driven in anisotropic normally consolidated (NC) or lightly over consolidated (LOC) clay for linear increase in undrained shear strength with depth, and anisotropic heavily over consolidated (HOC) clay where undrained shear strength remains constant with depth. The anisotropic shear strength of clay substantially influences the required support pressure, that is, the support pressure computed using anisotropy strength parameters has been found to be higher for tunnel driven in NC/LOC clay and lower or higher for tunnel driven in HOC clay than that determined on the basis of isotropic strength parameters.

Stabilization of leachate contaminated soils using industrial by-products

This paper reports the results from an experimental program to examine the effects of adding fly ash and rice husk ash on select geotechnical characteristics of industrial leachate contaminated plastic soils obtained from multiple sites of two different cities in Pakistan. The analysis revealed that the addition of fly ash and rice husk ash in a textile leachate contaminated soil could significantly reduce the plasticity and compression indices while improving both undrained shear strength and compaction characteristics. Similarly, adding fly ash in a paper leachate contaminated soil resulted in marked decrease in compression and plasticity indices, accompanied with a significant increase in undrained shear strength. Based on comprehensive chemical analysis and the results of scanning electron microscopy imaging, the stabilization mechanism of contaminated soils has been presented that would enhance our understanding of this complex process. Furthermore, the practical implication of this study has been demonstrated with the help of a simple case study, whereby a leachate contaminated soil taken from a different site has been treated and results have been presented to demonstrate immediate practical significance of this work.

Assessment of tamping-based specimen preparation methods on static liquefaction of loose silty sand

Dry and moist tamping methods are widely used for the investigation of sandy soil liquefaction. However, the effect of these two tamping-based reconstitution methods on composite soil liquefaction behavior have been rarely assessed. In this study, the dry and moist tamping methods were assessed using loose silica sand matrix (HN31) containing various amounts of non-plastic silt particles (C500), and by performing a series of undrained triaxial compression tests. Microstructural observations using a high-resolution optical microscope were also performed on loose silty sand specimens that had the similar initial states to those for the undrained triaxial tests. Triaxial results show that for silty sand specimens prepared by dry tamping method, a global dilatant behavior was observed and the undrained shear strength decreased as the fraction of silt particles increased. By contrast, a total reversed phenomenon was observed for those prepared by moist tamping method: all specimens showed more or less limited liquefaction behavior and silt particles mitigated the contractive tendency, which provided beneficial contribution to the undrained shear strength. Microstructural observations show that in the dry tamping method, overall structure formed by the host sand and silt particles was regularly deposited, promoting a more stable strain-hardening response. Moreover, some silt particles were found to be located near sand-sand contact points, significantly loosening the sand matrix. On the contrary, by using moist tamping method, the sand was uniformly covered by bulked silt particles, and the aggregated sand formed the metastable structure, facilitating the occurrence of static liquefaction. Moreover, with the increase of fines fraction, this metastable structure was weakened, characterized by the filling of large inter-sand voids, leading to the increase of undrained shear strength. Thus, a critical value of total fines fraction FcTot equalling to 21% was determined, which theoretically verified the insignificant effect of specimen preparation methods on the undrained shear strength. In that case, the metastable structure in specimen prepared by moist tamping method was expected to disappear and the overall response of specimen changed from the dominance of host sand to that of fines.

Effects of geometry and soil properties on type and retrogression of landslides in sensitive clays

Flowslide and spread are two common types of landslides in sensitive clays. Empirical criteria, based on single or multiple soil properties and slope geometry, have been proposed for a rough assessment of potential landslide type and retrogression distance. A large variation has been found in the comparison of retrogression distance between empirical equations and field data. In the present study, flowslides and spreads are simulated using a Eulerian-based large-deformation finite-element (FE) method. In addition to strain-softening, a strain-rate-dependent undrained shear strength model that elevates the strain rate effects on the shear strength of liquefied clay flowing at high speed is used. In flowslides, a higher rate of increase in undrained shear strength with depth reduces the depth of subsequent slides, and thereby the retrogression distance. The maximum retrogression occurs for a uniform shear strength profile. The increase in the ratio of horizontal to vertical stress, resistance to downslope movement of the debris and decrease in soil brittleness and slope steepness change the failure pattern from a flowslide to a spread.

Probabilistic analysis of tunnel face stability considering spatially variable undrained shear strength with linearly increasing mean trend

The inherent spatial variability of soil properties has been considered as one of the main sources of uncertainties in geotechnical problems. The need for probabilistic analysis of the tunnel face stability that takes into account the variability of soil properties has been acknowledged. This article employed a probabilistic-based method, called random finite difference method, for evaluating the stability of tunnel face under the influence of the variability of undrained shear strength in clays. The two-dimensional spatial variation in soil undrained shear strength is modeled by random fields, which are discretized by the Covariance Matrix Decomposition method. The procedure for random finite difference method is presented. An illustrative example is employed to investigate the effect of soil variability. Particular attention has been paid to the situation that undrained shear strength increases with depth. The results demonstrate that ignoring the variability of undrained shear strength will result in overestimates of the tunnel face stability if the support pressure of the tunnel face exceeds the deterministic value, especially for higher coefficient of variation of soil undrained shear strength. Minor differences in the failure mechanism are observed in comparison to the deterministic case, considering only the global failure of the tunnel face is observed. In addition, ignoring the increase of undrained shear strength with depth will lead to conservative designs. The random finite difference method can provide a practical tool for evaluating the stability of a tunnel face in variable soils.

Engineering behaviour of MICP treated marine clays

In the present scenario, with much focus on sustainable development worldwide, Microbially Induced Calcite Precipitation (MICP) is a promising biological soil improvement technology. However, only very limited research is reported on the effectiveness of this technique in marine clays. This paper presents the salient features of an experimental study conducted on two typical marine clays stabilised by MICP. Effectiveness of the technique was evaluated through a series of one-dimensional consolidation tests, unconfined compression tests, and index property determinations. It is found that biostimulation approach is not effective in marine clay; bio-augmentation is needed for soil improvement. Bio-augmentation results in the reduction of liquid limit and plasticity index to about 29% and 47%, respectively for the marine clays. A comparable improvement in volume change behaviour is also observed. There is a marked increase in undrained shear strength, upto about 148%, of MICP treated marine clays at toughness limit water content. Curing is also found to have a significant role in soil improvement. The observed transition in the nature of the tested marine clays from that of fat clay to elastic silt suggests the potential of the proposed approach. An empirical equation is also proposed to predict compression index of MICP treated marine clays.

Combined Load Capacity of a Preloaded Skirted Circular Foundation in Clay

AbstractPreloaded foundation capacity has recently been a topic of major interest in offshore foundation engineering. The increase in undrained shear strength due to preloading and consolidation en...

Influence of fibers on the shear strength and dewatering performance of geotextile tubes

Geotextile tubes have been widely used for the dewatering of high water content fine sediments. Fine sediments include lake sediments, organic soils and coal combustion products, which generally have low shear strength. Several researchers have emphasized the effect of the retained sediment (filter cake) properties on dewatering performance and stability of the geotextile tubes. Maintaining retained sediment properties, in particular permeability and shear strength, allows for faster dewatering time, better retention of sediments and improved stability of the geotextile tubes during multiple filling stages and during the tube stacking process. Several studies have shown that the use of synthetic fibers can increase the shear strength of sand, silt and clays. However, the use of fibers to improve the strength of retained sediments in geotextile tubes has not been explored. In this study, three synthetic fibers varying in length and diameter were used to evaluate the effect of fibers on the dewatering performance and shear strength of the fine sediments. A cationic polyacrylamide flocculant was used as an optimum conditioner for the selected sediment. The fibers were mixed with the flocculant conditioned slurry at 0.25, 0.5, 1 and 2% fiber concentration by mass of the dry solids. The effect of fibers on the undrained shear strength of the dewatered sediments was evaluated using a lab vane test, fall cone test and undrained unconsolidated triaxial test. Jar tests and pressure filtration tests were also conducted to evaluate the effect of fiber on the required flocculant dosage and on the dewatering performance. Based on the results of this study, 0.5% of F1 fiber type was found to be the optimum fiber concentration, yielding an approximately 100% increase in undrained shear strength. It was also found that the diameter and length of the fibers play a significant role in altering the undrained shear strength of the tested sediments. The relatively short (6 mm) and thick (38 μm) fiber did not yield an increase in shear strength for all the tested fiber concentrations, whereas thin fibers (9 μm) of similar length increased the shear strength by 100%. Furthermore, it was found that the use of fibers at any concentration decreases the required optimum doses of flocculant by about 15%, and increases the dewatering rate of the effluents by about 50%.

Rate effects on the undrained shear strength of compacted clay

Unconsolidated-undrained (UU) triaxial compression tests were performed on low-plasticity clay specimens compacted to the same void ratio but different initial degrees of saturation to evaluate the impact of axial strain rates ranging from 0.1 to 150%/min on the undrained shear strength. Although an effective stress analysis cannot be performed on the results, they are useful to evaluate the relative roles of initial hydraulic conditions (i.e., matric suction and degree of saturation) and compaction effects (i.e., potential changes in soil structure with compaction water content). This evaluation is relevant due to difficulty in measuring shear-induced pore water and air pressures in consolidated-undrained (CU) compression tests on unsaturated clay. In all tests, the undrained shear strength quantified as the maximum principal stress difference increased log-linearly with axial strain rate, with rates of increase ranging from 4.1 to 9.7% per log cycle of axial strain rate for specimens having initial degrees of saturation ranging from 0.99 to 0.59. The undrained shear strength, rate of increase in undrained shear strength with axial strain rate, and secant moduli all increased nonlinearly with decreasing initial degree of saturation, although compaction effects played an important role in these trends. The increase in undrained shear strength with axial strain rate can be attributed to a reduction in the magnitude of excess pore water pressure, with similar reductions in magnitude for all the degrees of saturation considered. A comparison between the measured undrained shear strength values and the drained shear strength values estimated using the suction stress concept was useful in delineating the impacts of initial hydraulic conditions and compaction effects on the trends in measured undrained shear strength.

Bearing capacity of strip footing in reinforced granular bed over soft non-homogeneous ground stabilized with granular trench

This paper presents a method to estimate the bearing capacity of a strip footing embedded in a geosynthetic reinforced granular bed over soft non-homogeneous ground, i.e., increase of undrained shear strength with depth, stabilized with granular trench. Madhav and Vitkar’s solution for bearing capacity of granular trench-supported strip footing in soft ground together with Meyerhof’s punching failure mechanism for dense sand overlying soft clay, extended to include the effect of axial tension in the reinforcement, form the basis of the analysis. A parametric study is performed and charts are developed to quantify the contributions of various parameters on the degree of bearing capacity improvement. The results of the present study are compared with experimental results from literature.

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

Effect of sand compaction pile installation on strength of soft clay

In the design of sand compaction piles, the increase in undrained shear strength of the soft clay around the sand piles is often ignored. This paper examines the long-term gain in strength in the soft clay using centrifuge modelling and finite-element analysis. Stress, pore pressure and strength measurements in the centrifuge models are used to benchmark the finite-element results. Parametric studies are then conducted using finite-element analyses to explore the effects of the friction angle and modulus of the soft clay on the trend and quantum of strength increase. The findings show that the ratio of improved strength to original shear strength decays as a function of the logarithm of radial distance. A method of estimating strength increase is proposed, based on two parameters that are related to the friction angle and modulus of the soil. For multiple piles driven with almost no time lag for pore dissipation, strength gain does not accumulate with successive piles. On the other hand, if there is sufficient time for pore pressure dissipation between the driving of successive piles, the strength gains from the successive piles can be accumulated.

Observed Behaviour of Laterally Expanded Stone Column in Soft Soil

The paper describes the behavior of remolded kaolin clay reinforced by stone column as investigated in laboratory. The installation of stone column was simulated by performing lateral expansion at different rates within hollow cylindrical remolded kaolin specimens initially subjected to K0 consolidation path. These specimens were, then, subjected to classical consolidated undrained triaxial tests while recording excess pore pressure values. The objective of the experimental programme was to quantify the effects of consolidation stress and stone column on the undrained Young’s modulus and shear strength of kaolin clay. Obtained results showed a significant improvement in Young’s modulus when the cavity expansion ratio and the consolidation stress increase. It was also found that the increase in undrained shear strength of improved kaolin clay mainly occurred at lower consolidation stress. Another important finding of this study is that the ratio of undrained Young’s modulus to undrained shear stress increases when the consolidation stress decreases. Finally, the paper presents a model developed for the design of stone columns.

Influence of ethanol/water mixture on the undrained shear strength of pure clays

The mechanical properties of clays are highly dependent not only on the stress/strain ratio to which the material is subjected but also on the chemistry of the pore fluids which in turn affects the intergranular or the effective stresses. Atterberg limits and vane shear tests were performed with different pore fluids in order to observe how the fine-grained material mechanically responded. The diffuse double layer theory has been used to interpret the data of vane shear tests in order to explain the variation of geotechnical responses with the different clays. Van der Waals forces and double layer forces were obtained and capillary forces calculated. The results show that while for kaolinite and illite the chemistry of the pore fluids has no influence on the water content and hence on the mechanical behaviour of the material, Na-smectite shows a strong correlation between the dielectric constant of the pore fluids and an increase in undrained shear strength. The data obtained extends an understanding of the influence of the dielectric constant (e) of the pore fluids on the geotechnical properties of fine-grained materials.