Effect of wet-dry cycles on standard & polymer-amended GCLs in covers subjected to flow over the GCL

The performance of five different GCLs (two GCLs with standard sodium bentonite and three GCLs with polymer enhanced bentonite) subjected to three different climatic modes of wet-dry cycles simulating conditions to which a GCL might expose in cover systems over a prolonged time is reported. The wetting cycles lasted for 8 h, while the drying cycles varied between 16 h, seven days, and 14 days. It is shown that after around a year of accelerated aging, the hydraulic conductivity of the aged GCLs increased notably when permeated with tap water at an applied effective stress of 15 kPa for a range of heads (0.07, 0.14, 0.21, 0.49, and 1.2 m). The combined effects of the number and the duration of the wet-dry cycles, the GCL's mass per unit area, the carrier geotextile, the size and the number of the needle punch bundles, and the thermal treatment to bond the needle-punch bundles to the carrier geotextile are discussed. The poor hydraulic performance of the polymer-amended/modified bentonite GCLs is discussed.     

Combined influence of subsoil water content and mass per unit area on cation exchange behavior of geosynthetic clay liners

Hydration tests were performed on geosynthetic clay liners (GCL) to examine the influence of subsoil water content (w_subsoil) and mass per unit area (MPUA) on the cation exchange occurred during hydration. Increase in the w_subsoil increased the final water content of GCL (w_final). At MPUA of 4.0 kg/m², w_final increased from 70 to 109%. Such increase was less pronounced when MPUA was considered. Also, an inverse relation between MPUA and w_final existed where w_final increased as the MPUA decreased. The greatest w_final was determined when w_subsoil was ～17% and MPUA of GCL was ～3.0 kg/m² (112%). The exchange reaction was the most favorable when w_subsoil was ～8% and MPUA of GCL was ～3.0 kg/m² (mole fractions of monovalent cations, X_M, decreased from 0.81 to 0.65) and the least favorable when w_subsoil was ～17% and MPUA of GCL was ～3.0 kg/m² (X_M decreased from 0.81 to 0.80). Combined influence of MPUA and w_subsoil showed that although cation exchange depends on both factors, w_subsoil had significant role in this reaction more than MPUA. The findings of cation exchange analyses were supported with swell index tests. The hydrated GCLs had lower swell indices than the virgin GCL (17.5–19.0 mL/2g vs. 21.5 mL/2g), indicating cation exchange.     

Coupled solute transport through a polymer-enhanced bentonite

Polymer-enhanced bentonites for geoenvironmental containment barriers, such as bentonite-polyacrylic-acid composite (BPC), generally have low hydraulic conductivity (e.g., k < 10^?10 m/s) even when exposed to aggressive waste solutions. However, understanding of diffusion and membrane behavior properties of enhanced bentonites and associated impacts on coupled contaminant transport through the barrier remains limited. In this study, hydraulic conductivity (k), effective diffusion coefficients (D*), and membrane efficiencies (ω) were measured for BPC with 3.2 % polymer content (by mass; referred to as BPC-3.2). Tests were performed with potassium chloride (KCl) solutions ranging from dilute (2.5 mM) to aggressive (400 mM) concentrations. As concentration increased, D* increased by a factor of three, ω decreased by two orders of magnitude, and k remained relatively low (1.2 × 10^?11 to 2.9 × 10^?11 m/s). The experimental results were paired with an existing coupled solute transport model to evaluate the significance of membrane behavior and diffusion on predicted total solute flux through a geosynthetic clay liner (GCL) and a GCL overlying an attenuation layer. The predicted mass flux was diffusion dominated, with the diffusive flux greater than the advective flux by one to two orders of magnitude. Membrane behavior reduced predicted total solute flux through the GCL by 5.8 to 61 %. The results demonstrate the role of coupled solute transport in the long-term performance of bentonite barriers, and advance understanding of contaminant transport in BPC.     

Self-healing of circular and slit defects in GCLs upon hydration from silty sand under applied stress

The self-healing of a GCL with artificial defects (circular holes and rectangular slits, both with and without the carrier geotextile preserved below the holes) upon hydration on a Godfrey silty sand (GSS) subgrade with w_fdn = 5, 10 and 16% under 2–100 kPa is examined. Circular holes with the carrier geotextile missing below holes with diameters up to 25.4 mm self-healed on the w_fdn = 5% and 10% GSS but not on 16% GSS, while none self-healed when carrier geotextile was preserved below the holes. When DI water was introduced to the surface under 100 kPa, circular holes with diameter up to 38.1 mm self-healed. Neither the single 15 mm-wide slit nor double 15 mm-wide parallel slits with 20 mm-wide strip of undamaged GCL between them resting on w_fdn = 10% GSS under 20 kPa fully self-healed. The introduction of simulated synthetic landfill leachate (SSL) to the GCL surface under 70 kPa did not result in self-healing. Post-hydration k tests found that GCL without a carrier geotextile below a hole up to 25.4 mm in diameter would not have a significant adverse effect on the hydraulic conductivity compared with an intact GCL provided the permeant was tap water rather than SSL.     

Effect of prehydration,permeant, and desiccation on GCL/Geomembrane interface transmissivity

A laboratory investigation was conducted on two different conventional GCLs (one with fine granular and another one with powdered bentonite) to explore the effect of prehydration and permeant fluid; GCL desiccation on the interface transmissivity, θ, between the interfaces of a 1.5 mm-thick high-density polyethylene (HDPE) geomembrane (GMB) and a GCL. The study also aimed to assess the self-healing capacity of desiccated GCLs for three different permeant solutions under a range of applied stresses (10–150 kPa). It was found that at stresses less than 70 kPa, θ was dominated by variability in the initial contact condition between the GMB-GCL interfaces. The effect of other factors was largely masked by the contact variability. At 100–150 kPa, the effects of initial variability were largely eliminated, but there was no notable effect of other factors on θ in the absence of desiccation. GCL desiccation increased θ by up to three orders of magnitude than an intact specimen at 10–100 kPa. Even at 150 kPa, desiccated specimens had a θ ≤ 8.0 × 10⁻⁹ m²/s for all specimens tested. The chemical composition of the permeant solutions, crack width, and nature of bentonite could play an important role in healing the cracks of desiccated GCLs.     

Influence of pore water chemistry on GCL self-healing with hydration from silica sand

The self-healing of a GCL with a circular hole is examined in experiments where the GCL, overlain by geomembrane, is hydrated from a silica sand subgrade (SSS) having three different pore water chemistries. Factors considered included: hole size, subgrade initial moisture content w_fdn, GCL mass per unit area, and overburden stress (20–100 kPa). GCL self-healing is better for w_fdn = 16% than for w_fdn = 10%, which is better than for 5%, when the SSS pore water has negligible cations (ionic strength, I < 0.1 mM). However, only the 14.3 mm-diameter hole fully self-healed and only when w_fdn = 16%. In contrast, when the GCL is hydrated from SSS with pore water having an ionic strength, I, of 20 and 30 mM, the self-healing for w_fdn = 5% is better than for w_fdn = 10%, which is better than for w_fdn = 16%, although none of the holes self-healed. When a ~0.5 m hydraulic head was applied above the GCL under σ_v = 20–100 kPa, a 38.1 mm-diameter hole self-healed with water having I < 0.1 mM, a 25.4 mm-diameter hole self-healed with pore water with I = 20 mM and 30 mM, but none self-healed with simulated synthetic landfill leachate (SSL). Post-hydration hydraulic conductivity (k) tests with SSL suggest that a hole up to 14.3 mm-diameter would not pose a significant adverse impact on the k compared to an intact GCL; however, this is not the case for the larger holes tested.     

Effect of added polymer on the desiccation and healing of a geosynthetic clay liner subject to thermal gradients

The desiccation and subsequent hydraulic conductivity of both a standard (GCL_A) and polymer-enhanced (GCL_B) Na-bentonite GCL hydrated from a well-graded sandy subsoil under 20 kPa, then subjected to a thermal gradient, and finally rehydrated and permeated with distilled water or 0.325 mol/L Na⁺ synthetic brine are reported.With moderate temperature of 40 °C applied to the top of the liner, GCL_B experienced less cracking than GCL_A, but this advantage disappeared when temperatures increased. Both desiccated specimens of GCL_A and B showed significant self-healing when permeated with distilled water and their hydraulic conductivities quickly reduced to around 10⁻¹¹ m/s at 20 kPa upon rehydration. However, when GCL_B desiccated specimens were permeated with the synthetic brine, their hydraulic conductivities were found to be one to two orders of magnitude higher than corresponding values obtained with distilled water. On the other hand, GCL_A (with no polymer treatment) maintained its hydraulic conductivities at the same level obtained with distilled water. It is concluded that caution should be exercised in using polymer-bentonite in applications in which GCLs are subjected to significant thermal gradients unless there is data to show they are resistant to thermal effects.     

Use of geosynthetic clay liner as a remedial measure of claystone degradation in Lam Ta Khong hydropower plant

In this study, the downstream slope of a dam impounding the upper reservoir of Lam Ta Khong (LTK) hydroelectric energy storage in Thailand was found to slide at a higher pace during the rainy season. After a thorough site investigation, laboratory tests, and numerical modeling to identify the main cause of the movement, it was found that as rainfall infiltrated the upper soil layer, the claystone of the downstream slope deteriorated when in contact with the water. As a remedial measure, 174,750 m² of geosynthetic clay liner (GCL) was used to cover the entire downstream slope of the dam, and proved to be an effective and economical solution for reducing the ongoing movement. The GCL included a textured high-density polyethylene (HDPE) layer for improving the resistance and minimizing the slippage at the interface between the GCL and underlying rock. Before the dam's remediation, the settlement point demonstrated a movement of nearly 0.1 m/year. In contrast, after the placement of the GCL, almost all settlement points moved less than 0.1 m for a recorded period of more than 4 years.     

Effect of geosynthetic component characteristics on the potential for GCL internal erosion

Experiments quantifying GCL permittivity and the ultimate water head the GCLs can sustain before the initiation of internal erosion when underlain by a 50 mm angular to subangular gravel subgrade are conducted. The influence of different geotextiles over the subgrade, water heads, hydration periods before testing, masses per unit area of bentonite within the GCL, and ionic strengths of the solution (cation exchange) are considered. Test results show that GCL with the scrim-reinforced nonwoven geotextile over the subgrade has the best hydraulic performance against internal erosion, followed by the woven geotextile coated with a 110 g/m² polypropylene film. A woven or nonwoven is the least useful for preventing internal erosion, with the corresponding threshold water head initiating internal erosion >39 m for scrim-reinforced nonwoven, 21 m for lightly coated woven, 4–5 m for woven and nonwoven alone, respectively. Cation exchange, length of hydration, and mass per unit area of bentonite do not notably affect the threshold water head for the subgrade examined. Once internal erosion occurs, there is a 3-order of magnitude increase in permittivity. The practical implications are discussed.     

Experimental investigation on methane advection and diffusion in geosynthetic clay liners

Geosynthetic clay liners (GCLs) are widely used in landfill and heap-leach facility cover system for mitigating rainfall infiltration and gas migration into atmosphere. Laboratory tests were conducted to investigate methane diffusion and advection through GCLs. Gas permeability coefficient of GCL for the case with moisture content = 47.5% is one and two orders of magnitude greater than the cases with moisture content = 68.5% and 80.9%, respectively, when 20 kPa vertical stress was applied. The batch adsorption tests indicated that adsorption of methane onto bentonite is negligible. The concentration variation for the adsorption of methane onto bentonite can be neglected. However, methane concentration decreased by 14.2% for the test of methane adsorption onto GCL during the first 2–3 days. This is because methane was adsorbed by the geotextiles rather than by the bentonite in GCL. The large porosity and surface area of geotextiles provide lots of micropores for methane adsorption. Analytical model was then developed to analyze the performance of GCL-based liners system with respect to methane transport. The results indicate that methane emission fluxes for the case with SL + GCL are 7.8 and 5.1 times less than the cases with SL + CCL when the moisture contents were 25.9% and 35.1%, respectively. The methane emission fluxes for both of the SL + GCL and SL + CCL can be neglected when they are fully saturated. GCL is recommended to be used in arid and semi-arid regions rather than CCL. GCL is recommended to be used in arid and semi-arid areas rather than CCL. Advection plays a more important role in methane migration through SL + GCL and SL + CCL than that of diffusion. With moisture contents = 25.9% and 32%, methane emission flux attributed to advection accounts for more than 90% of the total emission flux for both cases of SL + GCL and SL + CCL. With the increase of moisture content of SL, the effectiveness of SL in reducing methane emission increases. The saved space for using GCL + SL composite cover compared with using a single SL cover is 0.7 m when the moisture content equals 25.9%, which is 0.5 m greater than the case when moisture content equals 32%. GMB plays a dominant role in inhibiting methane migration and reducing methane emission flux. When moisture content equals 25.9%, the methane emission fluxes for SL + GMB + GCL and SL + GMB + CCL are 343 times and 2643 times less than the cases with SL + GCL and SL + CCL, respectively.     

Performance of multicomponent GCLs in high salinity impoundment applications

The interface transmissivity (θ) of two multicomponent geosynthetic clay liners (GCLs) is investigated upon hydration and permeation with a highly saline solution (TDS ≈ 260,000 mg/l; Na⁺ ~ 95,000 mg/l; K⁺~12,000 mg/l) at two stress levels (10 kPa and 150 kPa). One GCL had a smooth 0.2 mm-thick coating whereas the second GCL had a textured 1 mm-thick coating. For both GCLs, the interface transmissivity after 2-weeks is shown to be higher than at steady-state. The lower the geomembrane's (GMB) stiffness, the lower interface transmissivity. However, the effect is generally diminished at steady state and higher stress. The effect of GMB stiffness at 10 kPa is shown to be 1.6-times that at 150 kPa. Similarly, the 2-week and steady state interface transmissivity for the textured GMB was higher at 10 kPa than at 150 kPa. Coating texture and coating orientation are shown to have a significant effect on GMB/multicomponent GCL interface transmissivity. A hole in the coating aligned with GMB hole creates an additional flow path at the coating/GCL interface (θ_Geofilm/GCL), however most of the flow occurs at the coating/GMB interface (θ_Geofilm/GMB).     

Barrier properties of a geosynthetic clay liner using polymerized sodium bentonite

The hydraulic and swelling properties of a polymerized bentonite (PB), and the self-healing capacity of a geosynthetic clay liner (GCL) using the PB as the core material (PB-GCL) were investigated experimentally. Five different test liquids included of deionized water, NaCl solutions (0.1 M and 0.6 M) and CaCl₂ solutions (0.1 M and 0.6 M) were used in this study. The PB exhibited a higher free swelling index (FSI) than that of the untreated bentonite (UB) for all test liquids. For permeability test, under a given void ratio (e), the value of k of the PB is much lower than that of the UB in NaCl and CaCl₂ solutions. The PB-GCL specimens demonstrated a higher self-healing capacity than that of the corresponding GCL specimens using the UB (UB-GCL). Specifically, when using a 0.6 M CaCl₂ solution for a 20-mm-diameter damage hole, the UB-GCL specimen provided a zero healing ratio (healed damage area/total damage area), but the PB-GCL specimen demonstrated an approximately 76% healing ratio. The results from this study indicate that the PB-GCL provided better barrier performance against cationic liquids with higher cation valence and concentrations compared to that of the UB-GCL.     

Hydrodynamic assessment of bentonite granule size and granule swelling on hydraulic conductivity of geosynthetic clay liners

Flow in an idealized geosynthetic clay liner (GCL) containing bentonite comprised of equisized and equispaced square granules was simulated using a hydrodynamic model to quantitatively evaluate the premise that the hydraulic conductivity of GCLs diminishes as the bentonite granules hydrate and swell into adjacent intergranular pores, creating smaller and tortuous intergranular flow paths. Predictions with the model indicate that hydraulic conductivity decreases as granules swell and intergranular pores become smaller, and that greater granule swelling during hydration is required to achieve low hydraulic conductivity when the bentonite is comprised of larger granules, or the bentonite density is lower (lower bentonite mass per unit area). Predictions made with the model indicate that intergranular pores become extremely small (<1 μm) as the hydraulic conductivity approaches 10⁻¹¹ m/s. These outcomes are consistent with experimental data showing that GCLs are more permeable when hydrated and permeated with solutions that suppress swelling of the bentonite granules, and that the hydraulic conductivity of GCLs with bentonite having smaller intergranular pores (e.g., GCLs with smaller bentonite granules, more broadly graded particles, or higher bentonite density) is less sensitive to solutions that suppress swelling.     

Consolidation enhanced membrane behavior of a geosynthetic clay liner

Semipermeable membrane behavior in clays refers to the ability of clays to restrict the migration of solutes. Thus, membrane behavior represents a potential benefit to the containment function of clay barriers used for hydraulic containment applications. In this regard, the potential influence of consolidation effective stress, σ′, on the membrane behavior of a geosynthetic clay liner (GCL) containing sodium bentonite was evaluated in the laboratory by establishing differences in salt (KCl) concentrations ranging from 3.9 to 47 mM across specimens of the GCL in a flexible-wall cell under closed-system boundary conditions. The membrane behavior exhibited by the GCL was enhanced via consolidation such that an increase in σ′ from 34.5 kPa (5 psi) to 241 kPa (35 psi) correlated with an increase in membrane efficiency from 0.015 (1.5%) to 0.784 (78.4%), respectively. The membrane efficiencies measured in this study at σ′ of 172 kPa (25 psi) and 241 kPa (35 psi) were similar to those previously reported for the same GCL using a rigid-wall cell but at unknown states of stress. The practical significance of the results is illustrated in the form of an analysis showing a reduction in liquid flux across the GCL with increasing membrane efficiency.     

Household and behavioral determinants of indoor PM2.5 in a rural solid fuel burning Native American community

Indoor and outdoor concentrations of PM_2.5 were measured for 24 h during heating and non-heating seasons in a rural solid fuel burning Native American community. Household building characteristics were collected during the initial home sampling visit using technician walkthrough questionnaires, and behavioral factors were collected through questionnaires by interviewers. To identify seasonal behavioral factors and household characteristics associated with indoor PM_2.5, data were analyzed separately by heating and non-heating seasons using multivariable regression. Concentrations of PM_2.5 were significantly higher during the heating season (indoor: 36.2 μg/m³; outdoor: 22.1 μg/m³) compared with the non-heating season (indoor: 14.6 μg/m³; outdoor: 9.3 μg/m³). Heating season indoor PM_2.5 was strongly associated with heating fuel type, housing type, indoor pests, use of a climate control unit, number of interior doors, and indoor relative humidity. During the non-heating season, different behavioral and household characteristics were associated with indoor PM_2.5 concentrations (indoor smoking and/or burning incense, opening doors and windows, area of surrounding environment, building size and height, and outdoor PM_2.5). Homes heated with coal and/or wood, or a combination of coal and/or wood with electricity and/or natural gas had elevated indoor PM_2.5 concentrations that exceeded both the EPA ambient standard (35 μg/m³) and the WHO guideline (25 μg/m³).     

Sulphate resistance of fibre reinforced cement-based foams

This paper describes the results of an investigation on the resistance of plain and fibre reinforced cement-based foams to sulphate exposure. A synthetic foaming agent was used to produce foamed cementitious composites with essentially a closed cellular structure at 1200 kg/m³, 750 kg/m³, and 475 kg/m³. Polymeric microfibres were introduced at 0% and 0.2% volume fraction to result in 6 mixes. Prismatic specimens were immersed in a sodium sulphate solution to be tested in flexure, after specific intervals of exposure, according to ASTM C1609. A comparison with the response of unexposed specimens reveals that the heavier cement-based foams are more susceptible to sulphate attack and perform poorly with an increase in the duration of exposure. On the other hand, the lightest of the mixes—at 475 kg/m³—registered higher flexural strength and toughness factors up to 30 days of exposure before succumbing to sulphate attack. This self-healing response was attributed to the space available in such highly porous composites that allows for the unhindered growth of ettringite without attendant cracking. The presence of microfibres facilitated self-healing, as evident from the flexural toughness factor.     

Application of the gas tracer method for measuring oxygen transfer rates in subsurface flow constructed wetlands

The oxygen transfer rate (OTR) has a significant impact on the design, optimal operation and modelling of constructed wetlands treating wastewater. Oxygen consumption is very fast in wetlands and the OTR cannot be determined using an oxygen mass balance. This problem is circumvented in this study by applying the gas tracer method. Experiments were conducted in an unplanted gravel bed (dimensions L × W × d 125 × 50 × 35 cm filled with a 30-cm layer of 10-11-mm gravel) and a planted horizontal subsurface flow constructed wetland (HSSFCW) (L × W × d 110 × 70 × 38 cm filled with a 30-cm layer of 3.5-mm gravel with Phragmites australis). Tap water saturated with propane as gas tracer (pure or commercial cooking gas, depending on the test) was used. The mass transfer ratio between oxygen and commercial propane gas was quite constant and averaged R = 1.03, which is slightly lower than the value of R = 1.39 that is usually reported for pure propane. The OTR ranged from 0.31 to 5.04 g O₂ m⁻² d⁻¹ in the unplanted gravel bed and from 0.3 to 3.2 g O₂ m⁻² d⁻¹ in the HSSFCW, depending on the hydraulic retention time (HRT). The results of this study suggest that the OTR in HSSFCW is very low for the oxygen demand of standard wastewater and the OTR calculations based on mass balances and theoretical stoichiometric considerations overestimate OTR values by a factor that ranges from 10 to 100. The gas tracer method is a promising tool for determining OTR in constructed wetlands, with commercial gas proving to be a viable low-cost alternative for determining OTR.     

The effect of primary treatment and flow regime on clogging development in horizontal subsurface flow constructed wetlands: An experimental evaluation

The effect of both the type of primary treatment (hydrolitic up-flow sludge blanket (HUSB) reactor and conventional settling) and the flow regime (batch and continuous) on clogging development in subsurface flow constructed wetlands (SSF CWs) was studied. Clogging indicators (such as accumulated solids, hydraulic conductivity and drainable porosity) were determined in an experimental plant with three treatment lines. Correlations were encountered between the solids accumulated and both saturated hydraulic conductivity and drainable porosity reduction over time (74.5% and 89.2% of correlation, respectively). SSF CW implemented with a HUSB reactor accumulated ca. 30% lower sludge (1.9 kg DM/m²) than a system with a settler (2.5-2.8 kg DM/m²). However, no significant differences were recorded among treatment lines concerning hydraulic parameters (such as hydraulic conductivity or porosity). Root system development contributed to clogging. Accordingly, planted wetlands showed between 30% and 40% and 10% lower hydraulic conductivity and porosity reduction, respectively, than non-planted wetlands.     

Geosynthetic clay liners: Perceptions and misconceptions

The behaviour of geosynthetic clay liners (GCLs) as part of a physical-environmental system is examined. Consideration is given to: (a) both the physical and hydraulic interactions with the materials, and the chemical interactions with the fluids, above and below the liner, (b) time-dependent changes in the materials, (c) heat generated from the material to be contained, as well as (d) the climatic conditions both during construction and during service. This paper explores some common perceptions about GCL behaviour and then examines the misconceptions that can arise and their implications. It demonstrates how what may first appear obvious is not always as one expects and that more is not always better. It discusses: (i) the pore structure of a GCL, (ii) the dependency of the water retention curve of the GCL on its structure, bentonite particle sizes and applied stress, (iii) the effect of the subgrade pore water chemistry, (iv) the mineralogy of the subgrade, and (v) thermal effects. The desirability of a GCL being reasonably well-hydrated before being permeated is examined. The critical size of needle-punch bundles at which preferential flow can increase hydraulic conductivity by orders of magnitude is illustrated. The dependency of self-healing of holes on the interaction between GCL and subgrade is discussed. Finally, the transmissivity of the geomembrane/GCL interface is shown to be a function of GCL and geomembrane characteristics and to be poorly correlated with GCL hydraulic conductivity.     

Investigation of mechanisms of bentonite extrusion from GCL and related effects on the shear strength of GCL/GM interfaces

Two groups of laboratory tests were carried out to investigate the effect of bentonite extrusion from a hydrated GCL on the shear strength of GCL/GM interfaces. All tests were performed with the woven geotextile side of GCL against the GM. The first group of tests were one-dimensional loading tests in which the GCL/GM specimens were subjected to hydration and vertical loading involving different sequences and loading rates. The second group of tests were large direct shear tests that studied the effect of shearing on bentonite extrusion and hence on shear strength reduction. It was found that bentonite extrusion occurs more readily from GCL/GM interfaces subjected to a swelling-loading sequence than those subjected to a loading-swelling sequence. The quantity of extruded bentonite during the normal loading showed an increasing trend with an increase in loading rate. The total mass/area of extruded bentonite during the normal loading ranged from 0 to 21.9 g/m², which was less than the quantity of bentonite extruded during the subsequent shearing (i.e., 10.7 – 81.1 g/m²). It was found that the volume of bentonite extruded at the large shear displacement caused a significant strength loss equivalent with 8° in terms of interface friction angle. The influence of bentonite extrusion on the peak shear strength showed a magnitude of 3.5° in terms of interface friction angle. The relatively insignificant bentonite extrusion during hydration and normal loading was observed to have a minor effect on the strength loss. Observations from the experimental results provide further insight into the mechanisms of bentonite extrusion.