Bentonite Grout Research Articles

Flowable Mixtures of Treated Soils for Repairing Damage Caused by Burrowing Animals

The adoption of flowable mixtures of treated soils is suggested as a sustainable solution for repairing the damage caused by burrowing animals in the Po River embankments. Soil from damaged sites is proposed to be mixed with lime or cement, other additives, and water in order to obtain mixtures with an adequate compromise between viscosity—which is crucial for flowing the mixture into the hole with site machines—and shrinkage that might occur over time according to the hole boundary conditions. Injections of soil-treated mixtures must guarantee hydraulic and mechanical continuity with the existing embankment and reduce shrinkage phenomena that frequently occur with cement–bentonite grout, which is commonly used for this purpose. This study presents an interdisciplinary experimental programme in the search for the best formulation among several recipes of mixtures of soils, collected from four embankments of Po River or its tributaries and treated with lime or cement, different percentages of water, and other additives. The mixtures, after soil classification and viscosity determinations, were then used to create samples for which shrinkage and erosion susceptibility were estimated, respectively, through a non-standard volume measurement procedure and crumb tests. An abacus will be then proposed in order to give, as a first estimation, applicative indications on the best recipe among the investigated mixtures.

Effect of tube-in-tube configuration on thermal performance of coaxial-type ground heat exchanger

Among various types of ground heat exchangers (GHEXs or GHEs), a coaxial-type GHEX exhibits outstanding thermal performance owing to the geometric characteristics of the concentric tube-in-tube configuration. In this study, the unique configuration of the coaxial-type GHEX was investigated through parametric studies, which provided the energy efficient configuration. A computational fluid dynamics (CFD) model was developed and verified by using the field test results for the coaxial-type GHEX constructed in a test bed. Then, the parametric studies were conducted using the developed CFD model for various influential factors (i.e., flow rate of a circulating fluid, hydraulic diameter, and thermal conductivities of inner pipe and bentonite grout), which were selected considering the tube-in-tube configuration. The thermal performance of the coaxial-type GHEX increased with an increase in the flow rate of a circulating fluid and the thermal conductivity of bentonite grout. In addition, the inner pipes of the coaxial-type GHEX should be composed of materials with a thermal conductivity lower than 0.025 W/(m⋅K) to relieve thermal interference between the outer and inner pipes. Notably, it was concluded that the coaxial-type GHEX should be designed to secure the sufficient heat exchangeable area and retention capacity of the circulating fluid.

Field performance of four vibrating-wire piezometer installation methods

Vibrating-wire piezometers provide a number of advantages over the traditional hydraulic piezometer design. There are currently many methods and configurations for installing vibrating-wire piezometers, the most common being: single piezometers in sand packs (SP), multilevel piezometers in sand packs (MLSP), and fully grouted multilevel piezometers using either bentonite (FGB) or cement−bentonite grout (FGCB). This study assesses the performance of these four different installation methods at a field site possessing complex stratigraphy, including glacial and marine sediments. To accomplish this objective, pore pressure data recorded between December 2017 and July 2019 were analyzed. Data indicate that SP, MLSP, and FGB piezometers performed most reliably, because piezometers installed at the same depth with these methods recorded similar pressure variations that were coherent with the hydrogeological setting. Of the two fully grouted installations using cement−bentonite grout, one installation failed completely due to a hydraulic short circuit, likely caused by preferential flow occurring along the wires of the embedded instruments. The lack of a standard method for mixing cement−bentonite grout at the time of construction likely contributed to the failure of the FGCB installations, as the grout mixture used in this study was likely too viscous to provide a suitable seal.

Effect of Fluid Chemistry on the Consolidation and Hydraulic Conductivity of Sand-Clay Liners

The clay swelling potential of sand-clay liners exposed to saline water or specific chemicals can influence their hydraulic conductivity and other consolidation properties. The effect of saline water or chemicals on the sand-clay liners was thus studied and evaluated. The consolidation characteristics of the sand-clay liner can be different when tested under different fluid media. Bentonite and cement grouts are chemicals that have a significant effect on the sand-clay liners. Cement and bentonite can be used to seal off the openings within the liner to repair a defect or a malfunction. In this study, Al-Qatif clay was used to form a sand-clay liner when mixed with fine-grained sand (clay is 20% by dry weight). Soil samples extracted from this liner were exposed to inorganic chemical solutions. NaCl and CaCl2 solutions with concentration ranges of 0.1%, 0.5%, and 1.5% were used. Acidic water with pH values of 4, 5, and 6 was similarly used as fluid media. The effects of NaCl, CaCl2, and water with different acidity on the consolidation characteristics and hydraulic conductivity were obtained and compared to those of the distilled water. The effects of grout materials containing bentonite (1%, 2%, and 3% by weight) and cement (2.5%, 5%, and 7.5% by weight) were also investigated. The addition of bentonite grout to the liner surface was found to improve its hydraulic conductivity. The cement effect on the compressibility was found to be very significant. The findings of this study can serve as a guide for selecting parameters in the design and assessment of sand-clay liners in semi-arid regions and coastal zones.

AVM nuclear glass/steel/claystone system altered by Callovo–Oxfordian poral water with and without cement–bentonite grout at 70°C

AbstractThe effect of a cement–bentonite grout material was studied through glass/steel interactions. Hence, two experimental mock‐ups, consisting of a “sandwich” of four materials in contact, stainless steel/AVM glass/AVM glass/P285NH steel, were leached by Callovo–Oxfordian poral water at 70°C for 1 year. CBG material was added for one experiment, whereas the second one was CBG‐free. Chemical and structural analyses (scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, Raman), performed mainly at the AVM glass/P285NH interface, evidenced a comparable alteration with and without the CBG. Indeed, its presence did not impact the gel formation by hydrolysis/condensation mechanism. For both experiments, the glass alteration rates corresponded to , highlighting a kinetic rate drop after a 1‐year leaching period. However, the CBG impacted the pH solution initially buffered by the claystone, promoting precipitation of Mg/Fe‐rich silicates on the gel surface. Regarding P285NH corrosion, no major difference was observed with the CBG. Steel corrosion layers in both experiments were Si‐free, and the corrosion rates were similar. Therefore, after 1 year at 70°C, the CBG had a limited effect on the glass/carbon steel interactions.

Experimental study on penetration of bentonite grout through granular soils

Permeation grouting using bentonite grouts is one of the effective methods to improve the engineering properties of granular soils. However, the low penetrability of bentonite grouts into soils limits their practical application in permeation grouting. This study presents a new approach to control the penetration length of bentonite grouts through granular soils using an ionic additive, sodium pyrophosphate (SPP). It is hypothesized that the chemical modification changes both rheological and physicochemical properties of the bentonite grout, and thus affects its penetration length through soils. The rheological properties (yield stress and apparent viscosity) of bentonite grouts with weight ratios (ratio of water to dry bentonite, W/B) of 19, 12.3, 9, and 7.3 were controlled by the addition of 1%–4% SPP by weight of dry bentonite. The bentonite grouts were also injected into sand columns prepared at various experimental conditions to evaluate the effect of each experimental parameter on their penetration lengths. The results show that the penetration length of bentonite grouts decreases with a decrease in W/B ratio and an increase in yield stress and apparent viscosity. Moreover, the penetration length increases with the increase of the normalized effective grain size and injection pressure, but the increase of fines content reduces the penetration length of the grouts. While the existing analytical equation produces good agreement with the measured penetration lengths for the grouts having high yield stress (>26 Pa), it significantly overestimates the penetration lengths of the SPP modified bentonite grouts due to filtration (especially, the grouts having low yield stress and low W/B ratios). Therefore, a new empirical correlation is proposed to predict the penetration length of the bentonite grouts based on filtration and rheological blocking.

The Kambarata 2 blast-fill dam, Kyrgyz Republic:blast event, geophysical monitoring and dam structure modelling

The blast- and earth-fill dam of the Kambarata 2 hydropower station is situated in the seismically active Central Tien Shan region of the Kyrgyz Republic. More than 70% of the dam volume was produced during a blast event on December 22, 2009. In 2010–2011, dam construction was completed after earth filling on top of the blasted material and installing concrete and clay screens together with bentonite grouts. A geophysical survey had been completed in 2012–2013, mainly to monitor the resistivities inside the dam. The geophysical survey completed on the Kambarata 2 dam site showed lower resistivity zones in the earth fill and relatively higher resistivities in the blast-fill material. Topographic, geophysical and piezometric inputs had been compiled within a 3D geomodel constructed with GOCAD software. This model was compared with the design structure of the dam in order to define the upper limits of the underlying alluvium, the deposited blast fill, earth fill and top gravel materials (represented by the dam surface). The central cross-section of this model was extrapolated over the full length of the main dam profile. On the basis of a calibrated hydrogeological model and known geomechanical properties of the materials, dam stability calculations were completed for different scenarios considering different reservoir levels and varying seismic conditions. Some of these scenarios indicated a critical vulnerability of the dam, e.g., if impacted by a horizontal seismic acceleration of Ah = 0.3 g and a vertical seismic acceleration Av = 0.15 g, with an estimated return period of 475 years. As a general conclusion, it was noted that this case study can be used as an example for surveys on much larger natural – landslide or moraine – dams. A series of geophysical methods (e.g., electrical and electro-magnetic techniques, seismic and microseismic measurements) can be applied to investigate even very deep dam structures. These methods have the advantage over classical direct prospecting techniques, such as drilling, of using equipment that is much lighter and thus more easily transportable and applicable in difficult terrain. Furthermore, they can provide continuous information over wider areas. This specific application to a blast-fill dam allows us to better outline the strengths and weaknesses of the exploration types and geomodels as a series of investigated parameters can be verified more easily than for natural dams.

A filtration model for evaluating maximum penetration distance of bentonite grout through granular soils

This paper presents a filtration model for evaluating the maximum penetration distance of chemically modified bentonite grouts. High-concentration bentonite grouts require chemical modifications to increase its penetration through soils; however, this modification complicates the evaluation of the maximum penetration distance due to the changes in the physicochemical and rheological properties of the bentonite grout. In this study, the yield stresses of 7.5%, 10%, and 12% bentonite grouts modified by 1–4% sodium pyrophosphate (SPP) (by dry weight of bentonite) were determined by matching a constitutive model (Herschel–Bulkley model) to the shear stress–shear rate curves obtained through rheological tests. The bentonite grouts were injected into sand columns with an injection pressure of 35 or 140kPa. The maximum penetration distances of the bentonite grouts through the sand columns were calculated using analytical equations (based on the yield stress of the grouts), and then compared to the measured maximum penetration distances. The results showed that the analytical equations could capture the maximum penetration distance at high yield stress (greater than 28Pa), but the equations significantly overestimated the maximum penetration distance at low yield stress due to filtration. A new filtration model is presented in this study to estimate the maximum penetration distance of the SPP modified bentonite grouts that can provide better prediction, particularly for grouts having low yield stresses. The model was calibrated based on the 1-D column tests and then independently validated with an additional set of column tests using a bentonite with different chemical composition.

Field measurement by fully grouted vibrating wire piezometers

Subsurface instrumentation was installed at a field monitoring site in Hyde Park and bordering Bayswater Road for measuring the ground responses to Crossrail tunnelling near Lancaster Gate. Prior to tunnel construction, pore-water pressures were measured, both in the ‘greenfield' ground and the ground in the vicinity of existing London Underground running tunnels, by three multi-level vibrating-wire piezometer boreholes fully backfilled with cement–bentonite grout. The pore-water pressures in the ‘greenfield' ground were at the same time measured by conventional standpipe piezometers and pushed-in spade cells with built-in vibrating-wire piezometers. This paper investigates the performance of the multi-level vibrating-wire piezometers by comparing their post-installation and steady-state pore-water pressure measurements with those from the other piezometer types. Generally they performed well, providing an efficient means of determining pore-water pressures at several depths within one borehole. One of the ‘greenfield' multi-level vibrating-wire piezometers indicated underdrainage within the London Clay while the other was influenced by inter-connectivity between individual piezometers within the borehole and also the presence of a claystone horizon. One borehole close to the existing tunnel indicated drainage of groundwater into it; this effect is compared with predictions made using a simplistic finite-difference model.

Lessons learnt from installation of field instrumentation

Imperial College in London, UK, as part of an Engineering and Physical Sciences Research Council (EPSRC) funded research project and in collaboration with Crossrail urban railway project, is performing field monitoring research to investigate how tunnelling affects existing tunnels. Comprehensive instrumentation was installed in Hyde Park and bordering Bayswater Road, beneath which the new Crossrail tunnels were constructed in London Clay below the existing London Underground Central Line tunnels. Surface and subsurface instruments were installed around the Crossrail tunnel alignments to monitor the ground response to the tunnel construction. Monitoring systems of sufficient resolution and accuracy were adopted to achieve high-quality data for assessing the tunnelling-induced ground response and mechanisms of movement from earth-pressure-balance machine tunnelling. The installation of surface and subsurface instrumentation took place in the summer of 2011. This paper describes and discusses the installation of rod extensometers, in-place inclinometers and multi-level vibrating-wire borehole piezometers. Selection of the appropriate cement–bentonite grout mixes for backfilling these borehole instruments is discussed, as this is critical for representative measurements of ground response. Some practical challenges arising during the installation process and how they were overcome are also described. Confidence in the instrument performance is demonstrated using example monitoring results from the piezometer and extensometer installations.

Viscosity and salinity effect on thermal performance of bentonite-based grouts for ground heat exchanger

Due to its high swelling potential and low hydraulic conductivity, the bentonite-based grout has been popularly used to backfill a borehole constructed for a closed-loop vertical ground heat exchanger in a geothermal heat pump system. In this study, three types of bentonites were compared in terms of viscosity and thermal conductivity. With the consideration of the groundwater inflow, the viscosity and thermal conductivity of the bentonite grouts having bentonite contents of 5%, 10%, 15%, 20% and 25% by weight were also examined. To evaluate the effect of salinity in the coastal area on the swelling potential of the bentonite, volume reduction tests were performed. And in order to reproduce the particle segregation of bentonite–sand mixture when the bentonite-based grout has relatively low viscosity, the segregation was observed using a ring-type separable test apparatus.

Groutability of Granular Soils Using Bentonite Grout Based on Filtration Model

This paper describes an experimental and a numerical study on the groutability of granular soils using sodium pyrophosphate (SPP)—modified bentonite grout to investigate its possible application in permeation grouting. While the groutability criteria based on the relative grain size of the soil and the grout have been widely used for particulate grouts, such criteria do not account for the change in groutability of soils due to chemical modifications of the grouts. Moreover, the bentonite grout has different physicochemical properties such as particle size and pH from those in the cement-based grout, resulting in different filtration character, and thus different groutability of soils. Therefore, it is difficult to apply the existing criteria developed for the cement-based grout to the bentonite grout. In this study, the apparent viscosities of various bentonite grouts (5, 7.5, 10, and 12 %, wt/wt %) with 0–4 % SPP were measured using an advanced rheometer. The grouts were injected through sand columns having different effective particle sizes (0.2, 0.28, and 0.31 mm) under two constant pressures (35 and 140 kPa). A 1D filtration model was utilized to simulate the grout flow through the sand columns. A lumped parameter that accounts for physicochemical properties of bentonite grouts was evaluated by calibrating the model with the experimental data. The results showed that the apparent viscosity of 7.5, 10, and 12 % bentonite suspensions decreased approximately 50 % with the SPP concentrations of 1, 3, and 4 %, respectively. Moreover, the lumped parameter increased with the decrease in the effective grain size of the sand and decreased with the decrease in the apparent viscosity of the grout. This implies that filtration during the grout flow was reduced with the SPP modification. The parameter also decreased with the increase in injection pressure and relative density. A new groutability criterion for bentonite grout was proposed based on the relationship between the injected pore volume and the lumped parameter. The criterion was compared to the existing criteria, leading to more accurate prediction of groutability in the tested sands. The proposed groutability criterion will be beneficial to design grouting works by more accurately predicting the groutability of granular soils for SPP modified bentonite grouts than the existing criteria.

Thermal Conductivity of Bentonite Grout Containing Graphite or Chopped Carbon Fibers

The effectiveness of chopped carbon fibers to enhance the thermal conductivity of bentonite-based grout was examined. Fibers of 3 mm and 150 μm in length were added to sodium bentonite and silica sand mixtures at different volumetric concentrations. The thermal conductivity of the resulting composite material was then measured using a non-steady-state needle probe technique. The conductivities of the fibrous materials were compared to bentonite and sand mixtures containing natural flake graphite and milled, compressed exfoliated graphite at corresponding volumetric concentrations. The resulting conductivities for all tested materials increased with the volumetric fraction of additive. However, the 3-mm carbon fibers were more effective as compared to either granular graphite; at a volume fraction of 0.68% the 3-mm fibers were approximately twice as effective as either form of graphite.

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

Abstract 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.

지열히트펌프 지중열교환기용 벤토나이트계 그라우트재의 열전도특성 및 기본성능

In this study, the thermal conductive characteristics and basic properties of the nine commercial products of bentonite grouts were studied. Six of the nine products for ground heat exchanger systems are imported and others for civil engineering are domestic. The thermal conductivities of all bentonite products are nearly similar among products. The free swell indexes, viscosities and filter losses of the ground heat exchanger grouts are lower than those of the civil engineering ones. These characteristics seem to increase of the fluidity to fill the bentonite slurry to bore-hall perfectly, rather than to prevent underground water penetration. Thus, the mixtures of bentonites and sands are recommended for high thermal conduction grouts.

Performance evaluation of closed-loop vertical ground heat exchangers by conducting in-situ thermal response tests

The effective thermal conductivity of six vertical closed-loop ground heat exchangers (GHEXs), which were installed in a test bed located in Wonju, South Korea, has been experimentally evaluated by performing in-situ thermal response tests (TRTs). To compare the thermal efficiency of the GHEXs in field, various installation conditions are considered such as different grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite) and shapes of the circulating pipe-section (conventional U-loop type vs. 3-pipe type). From the test results, it can be concluded that the cement grout has higher effective thermal conductivity than the bentonite grout by 7.4–10.1%, and the graphite outperforms the silica sand by 6.7–9.1% as a thermally-enhancing additive. In addition, the new 3-pipe type heat exchange pipe that yields less thermal interference between the inlet and outlet pipes shows better thermal performance over the conventional U-loop type heat exchange pipe by 14.1–14.5%. Based on the results from the in-situ thermal response tests, a series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe section. For the same condition, the cement grouting can reduce the construction cost of GHEXs by around 40% in the given cost analysis scenario. In addition, an addition of graphite and use the new 3-pipe heat exchange pipe lead to about 8% and 6% cost reduction, respectively.

Enhancement of geothermal borehole heat exchangers performances by improvement of bentonite grouts conductivity

Abstract Conventional boreholes used as vertical ground heat exchangers are significantly limited in thermal performances by the bentonite grouts thermal conductivity. Bentonite-graphite composites have been elaborated using Graphite Flakes, Expanded Natural Graphite (ENG) and ground Compressed Expanded Natural Graphite CENGg particles. Using graphite flakes, the grouts thermal conductivity enhancements are similar to those already published in previous studies while using CENGg particles, the expected effective thermal conductivity of 5 W m −1 K −1 was obtained at reduced graphite content (5% wt). Partial drying of the grouts induces significant decrease in thermal conductivity in the range of 1 W m −1 K −1 for 10% wt of water content reduction. This inhibiting effect has to be taken into account for geothermal heat storage using borehole based systems.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat exchanger.

Applicability of cement-based grout for ground heat exchanger considering heating-cooling cycles

The applicability of cement grout (or cement-based grout) has been considered as an alternative to bentonite grout commonly used to backfill closed-loop vertical ground heat exchangers. In a geothermal heat pump system, repeated heating-cooling cycles may cause adverse effects on the integrity of cement grout in the ground heat exchanger. To account for the temperature cycling effect, the strength degradation of cement grout due to temperature cycling has been examined by measuring the unconfined compression strength of cured specimens in a humidity-temperature controlling chamber with applying temperature cycles between −5°C and 50°C. There is a tendency that the unconfined compression strength decreases with an increase in the number of temperature cycles. On the other hand, an equivalent hydraulic conductivity of a pipe-embedded cement grout specimen was evaluated by carrying out a modified flexible wall permeameter test equipped with a water circulating system to control temperature inside the pipe section. The applied operating temperature range was from 5 to 35°C. After three cycles of heating-cooling circulation, the equivalent hydraulic conductivity becomes asymptotic to a constant value, which implies there is no severe detachment of the pipe from the cement grout.

Characteristics of thermally-enhanced bentonite grouts for geothermal heat exchanger in South Korea

The thermal conductivity and viscosity of bentonite grouts have been evaluated and compared each other to determine the suitability of these materials for backfilling vertical boreholes of ground heat exchangers. Seven bentonite grouts from different product sources were considered in this paper. Two additives, silica sand and graphite were added in bentonite grouts to enhance thermal performance. The bentonite grouts indicate that both the thermal conductivity and the viscosity increase with the content of silica sand and graphite. Therefore, it is recommended to select cautiously the amount of silica sand and graphite considering not only thermal conductivity but also viscosity for the optimum condition of backfilling. Finally, the effect of salinity in the pore water on the change of swelling potential of the bentonite-based grouts has been quantitatively evaluated to show the feasibility of bentonite grouts in the coastal area.