Clay Backfill Research Articles

Heat transfer characteristics and heat conductivity prediction model of waste steel slag–clay backfill material

The thermal performance of the backfill material used in ground source heat pump systems directly affects their heat transfer efficiency. To enhance the heat transfer efficiency, aiming at the low heat conductivity of existing backfill materials, this paper proposes a waste steel slag–calcareous clay mixture as the backfill material for buried pipes. First, the heat conductivities of 128 sets of waste steel slag–calcareous clay mixtures in different preparation states were tested by performing indoor thermal probe tests, and the influences of the steel slag mixture, moisture content, dry density, and particle gradation on the heat conductivity were investigated. Scanning electron microscopy and Charge coupled device imaging were employed to capture the microscopic morphology of the materials and to analyze their heat transfer mechanism. With the test results, a heat conductivity prediction model of the backfill material based on a back propagation neural network was developed. The incorporation of waste steel slag was found to significantly improve the heat conductivity of the mixed backfill materials, the heat conductivity of a mixture incorporated with 70% steel slag was the highest, with the value being 1.86 to 2.46 times higher than that of the clay material without steel slag. The established heat conductivity prediction model produced accurate results with an average relative error of 1.0007%. This study can provide reasonable thermal parameters for waste steel slag–clay mixtures as backfill materials in ground source pump systems, and the effective utilization of waste steel slag can improve its added value.

Research on the influence of clay shock on inert slurry in shield-tunneling construction

Shield-out synchronous grouting is a new technology used to fill temporarily shield-out spaces between the strata and the shield when creating subterraneous tunnels. The grouting materials are called ‘clay shock’, which is soft, plastic, sticky and easily dispersed by using a pressurized jet. This study analyzes the mixing process of clay shock and backfill grouting material (inert slurry) and investigates the influence of clay shock on the inert slurry through laboratory testing. The findings show that the fluidity and bleeding rate of the inert slurry continuously decrease with the clay shock volume. The unconfined compressive strength and flexural strength of the inert slurry increase initially before decreasing with the clay shock volume ratio. The X-ray diffraction and scanning electron microscopy results reveal that the alkalinity of the clay shock initiates an alkali excitation reaction, which generates more gelling products and increases the strength of the inert slurry. Excessive clay shock causes the inert slurry to retain a loose and porous structure, which then decreases its strength. To ensure the effect of the inert slurry on settlement control, the grouting volume of the clay shock should not exceed one-fourth of the inert slurry volume.

Upheaval Buckling Resistance of Pipelines Buried in Unsaturated Clay Backfills

Upheaval Buckling Resistance of Pipelines Buried in Unsaturated Clay Backfills

Temperature-dependent lateral earth pressures in partially saturated backfills

Stability analysis of retaining structures supporting variably saturated backfills is a topic of great significance in civil engineering. However, the backfill soil may be exposed to elevated temperature; this may result in changes of the saturation state, thus posing temperature-dependent effective stress, variable suction stress and different hydro-mechanical characteristics to partially saturated soils. In the present work, the significant impact of elevated temperature is incorporated into the soil-water retention curve and suction stress-based effective stress formulations. Accordingly, the lower bound theorems of limit analysis in conjunction with finite element and second-order cone programming are adopted so as to thoroughly examine the impact of elevated temperature on the coefficients of lateral earth pressure acting on a model retaining wall backfilled by two hypothetical partially saturated soils; a granular material (sand) and a cohesive material (clay). The effective stress-based formula for the strength of variably saturated soils is implemented to modify the conventional Mohr–Coulomb yield criterion. In general, it was shown that for unsaturated clay backfills, the active lateral earth pressure decreases while the passive lateral earth pressure increases with increasing the induced temperature; however these influences were observed to be small for the sand backfill.

Pullout testing and Particle Image Velocimetry (PIV) analysis of geogrid reinforcement embedded in granular drainage layers

The paper investigates the feasibility of using fine-grained soil as backfill material of geosynthetic-reinforced walls and slopes, through a laboratory study on pullout behavior of geogrids in granular layers. A series of pullout tests was carried out on an HDPE uniaxial geogrid in thin sand and gravel layers that were embedded in clay specimens.Aside from different soil arrangements, the influences of moisture content and overburden pressure on the geogrid pullout behavior is assessed and discussed. The tests were carried out at four different gravimetric water contents (GWC) on the dry and wet sides of the clay optimum moisture content (OMC), and overburden pressure values within the range σv = 25–100 kPa. Particle Image Velocimetry (PIV) was used to capture digital images during the tests, which were processed to help with the interpretation and improved understanding of the soil-geogrid interactions at different GWC values. Results show that embedding geogrid reinforcement in layers of sand or gravel can significantly increase the pullout resistance in an otherwise moist clay backfill, and this improved pullout efficiency is greater at higher overburden pressures. The improvement in pullout capacity was observed in clay specimens compacted at both the dry and wet sides of the OMC.

The crack zone simulation of clay backfill under seismic load

The lateral earth pressure on the earth retaining structure causes the crack on some part of the backfill. This cracked zone depends on the mechanical properties of soil and types of linear and nonlinear applied loads. The cracked zone nonlinearity is extending with unallowable applying loads and impact retaining wall seismic stability. The nonlinear seismic load significantly influences the load capacity and displacement of the backfill and retraining wall. In the present study, firstly the crack zone developing at the backfill zone predicted by analytical method, and I assume that the nonlinear load developing three deep cracks on the crack zone. The nonlinear finite element method (NFEM) was applying for realizing displacement of the backfill. The crack on the clayey soil become parameters for simulation and analyzing model displacement. The statistical model has verified the accuracy of this investigation. The results revealed the cracked zone is associated with the nonlinear displacement of the backfill model. The novelty of this study relates to introducing methodology for classify influence of backfill crack on displacement and vibration mechanism of the model in all stages of the numerical simulation. The outcome of this analytical and numerical study are illustrating that improve mechanical properties of the soil in cracking zone simulation, significantly is enhanced the economical construction of the backfill and minimizing the displacement of the model. The stability of the earth retaining structure depend on backfill stability which controlling load transmission.

Thermal Conductivity Mechanism Analysis and Neural Network Model of Clay Backfill Mixed with Different Amounts of Steel Slag

Thermal Conductivity Mechanism Analysis and Neural Network Model of Clay Backfill Mixed with Different Amounts of Steel Slag

Composite contiguous pile wall and deep mixing column wall as a dam –Design, construction and performance

This paper presents a case study on the application of a composite contiguous-pile wall and deep mixing (DM) column wall as a dam to develop a luxury lake in Lake Legend Chaengwatthana project, Bangkok, Thailand. This dam was constructed in a deep rainwater pond without providing the water removal, which could save on both time and cost construction. The design and construction method were challenging because the dam was located on the soft and thick Bangkok clay. The dam was designed by finite element analyses (FEA) using PLAXIS software. The 2 rows of contiguous-pile walls were first executed and then filled with soft clay. The DM columns were used to strengthen the clay backfill. The FEA results at normal and rain-shortage stages confirm the stability of the dam with accepted deformation. With insignificant water seepage over time, the dam acted as the waterproof barrier. The field performance of the dam during construction and at service state over 276 days were recorded and compared with FEA results. The numerical simulation slightly over-predicted the lateral displacement. The difference in the displacement between the measured and simulated data reduced significantly with depth. The rate of consolidation by the numerical simulation results were slightly slower than that by the field measurement. The outcome of this research will be fundamental to develop a design guideline of ground improvement in soft Bangkok clay and other similar soft clay deposits especially in Southeast Asian countries.

Gel Formation at the Front of Expanding Calcium Bentonites

The removal of potentially harmful radioactive waste from the anthroposphere will require disposal in geological repositories, the designs of which often favour the inclusion of a clay backfill or engineered barrier around the waste. Bentonite is often proposed as this engineered barrier and understanding its long-term performance and behaviour is vital in establishing the safety case for its usage. There are many different compositions of bentonite that exist and much research has focussed on the properties and behaviour of both sodium (Na) and calcium (Ca) bentonites. This study focusses on the results of a swelling test on Bulgarian Ca bentonite that showed an unusual gel formation at the expanding front, unobserved in previous tests of this type using the sodium bentonite MX80. The Bulgarian Ca bentonite was able to swell to completely fill an internal void space over the duration of the test, with a thin gel layer present on one end of the sample. The properties of the gel, along with the rest of the bulk sample, have been investigated using ESEM, EXDA and XRD analyses and the formation mechanism has been attributed to the migration of nanoparticulate smectite through a more silica-rich matrix of the bentonite substrate. The migration of smectite clay out of the bulk of the sample has important implications for bentonite erosion where this engineered barrier interacts with flowing groundwater in repository host rocks.

Performance Analysis of Geosynthetic Reinforced Earth Wall by Analytical Experimental and Finite Element Methods

Ease of construction and performance under earthquake loads causes interest in construction of reinforced earth walls worldwide. In this study, reinforced earth walls with sand and clay backfill at a height of 450 mm are constructed and tested under bridge abutment loading conditions. A finite element analysis is performed and results are compared with experiment results. In the parametric study made by conducting finite element method, effect of reinforcement stiffness and reinforcement density on the horizontal displacements along the retaining wall face, the maximum tensile force, total maximum tensile force, and slope stability are investigated.

Probabilistic analysis of the uplift resistance of buried pipelines in clay

Uplift resistance provided by soil cover is a key aspect in the design of a buried offshore pipeline. The capacity must be sufficient to avoid upheaval buckling but prediction of the uplift resistance is complicated by disturbance of the soil structure during installation. Previous numerical investigations of pipeline uplift capacity in undrained clays have employed simple elasto-plastic models and consider a homogeneous clay. These simplifications neglect critical features of soil-pipeline interaction and may not describe the real mechanical behaviour. In this paper, an advanced kinematic hardening model implemented in a finite element code is used to capture the degradation of structure as a pipeline buried in a natural clay is lifted upwards. The spatial variability of clay structure is represented by a random field and Monte Carlo simulation used to characterise the response. This novel framework shows that clay structure has a significant effect on the failure mechanism and uplift capacity of a buried pipeline. The probabilistic approach accounts for the uncertainty in the condition of the clay backfill and reveals that the spatial distribution of intact and remoulded material can change the mode of failure, emphasising the importance of considering clay disturbance in design.

Apparent salt diffusion coefficients for soil–bentonite backfills

Apparent diffusion coefficients, Da, were measured for two soil–bentonite (SB) backfills characteristic of those used in SB vertical cutoff walls for subsurface control of contaminant migration. The base soils for the backfills comprised either a natural lean clay or sand–bentonite mixtures. The base soils were mixed with 5% bentonite–water slurry to obtain a slump of 125 mm, resulting in total bentonite contents ranging from 4.76% to 7.31%. Values of Dafor sodium chloride were measured using a recently developed dialysis-leaching test method. The Davalues for the clay–bentonite and sand–bentonite backfills ranged from 2.5 × 10−10to 5.3 × 10−10m2/s and from 1.4 × 10−10to 8.1 × 10−10m2/s, respectively. Values of Dafor both backfills increased with increasing average salt concentration in the specimen (Cave). Values of Dadecreased by ≤50% with increasing backfill bentonite content. For all Cavevalues, the clay backfills exhibited lower Dathan the sand–bentonite backfills, likely due to additional fines from the lean clay. Results of this study enhance understanding of solute diffusion through SB cutoff walls, as well as support future use of the dialysis-leaching test method to measure diffusion properties of SB backfills.

CHARACTERISTICS OF SOILS IMPROVED WITH POLYETHYLENE TEREPHTHALATE WASTES

In this study we investigated the possibilities of using recyclable fine and coarse grain waste obtained by grinding Polyethylene Terephthalate (PET) containers, as light filling materials for different applications (clay dams, highway embankments, and backfills behind retaining structures. Various geotechnical tests were performed on clayey soil and soil-PET mixtures using recycled fine and coarse granules of Polyethylene Terephthalate bottles. The results indicated that the use of the recycled polymer mixed with clayey soils as a filling material is favorable for soil properties. The compaction test results indicate that the optimum dry densities of PET-soil mixtures are less than the optimum dry density of typical soils including the clayey soil used in this study (varying between 13.83 kN/m3 and 12.55 kN/m3). P-wave velocities values were correlated for PET mixtures. The results indicate that data concerning the dependence of P-wave velocity values and PET mixtures percentage can be correlated with a coefficient of determination between R2=0.93 to R2=0.73. Finally, the results show a good potential for using the PET waste as lightweight fill material.

TO A QUESTION OF DETERMINATION OF HYDRAULIC RADIUS OF A BACKFILL FLOW FOR THE EXCENTRATIVE PLACEMENT OF PIPES IN DRILLING BORE

The processes of displacement of drilling clay backfill flow from the intertubular space formed by internal surface of external and external surfaces of inner tubes are considered. In case of eccentric placement of pipes it is proposed in the form of a cross-section of the flow of a backfill to take the area limited from the outside by the Pascal's snail, and from the inside - the outer surface of the inner pipe at the first time.
 The formulas for determining areas of the cross section of the flow and the stagnant zone, the perimeter and the hydraulic radius of the section ofthe flow for the eccentric placement of the pipes in the bore are proposed. The dimensionless parameter α is introduced as the ratio of the centripetal distance to difference between internal radius of outer pipe and outer radius of inner tube. It was found that with increasing of α cross-sectional area and their hydraulic radii are increasing, and the area of stagnant zones decreases according to parabolic laws.It is established that with the growth of α velocity of the backfill flow decreases according to parabolic laws and their values are in good agreement with the experimental data.

Numerical evaluation of reinforced slopes with various backfill-reinforcement-drainage systems subject to rainfall infiltration

This paper presents a numerical study investigating the hydraulic response and stability of geosynthetic-reinforced soil slopes subject to rainfall. A series of numerical simulations of unsaturated slopes with various backfill–reinforcement–drainage systems subject to rainfall infiltration was performed by comprehensively considering the combined effect of backfill (i.e., sand, silt, and silty clay), reinforcement type (i.e., geogrid or nonwoven geotextile), and rainfall intensity (350 and 500 mm/day). The backfills were modeled using three soil–water characteristic curves (SWCCs) representing the general suction range associated with sand, silt, and clay. The influence of sand cushions in improving the stability of reinforced clay slopes was also assessed. The numerical results reveal that the loss of matric suction and development of a capillary barrier effect within clay backfills could have adverse impacts on both the global and local stabilities of the reinforced clay slopes. The contribution of matric suction in enhancing slope stability was initially high for reinforced clay slopes; however, the global stability of the reinforced clay slope substantially decreased due to the loss of matric suction as the rainfall infiltration proceeded. The local instability of the geotextile-reinforced clay slope occurred due to the capillary barrier effect at the geotextile–clay interface. The reinforced marginal soil slopes cannot effectively drain the infiltrating water under torrential rainfall. Free drainage conditions may not be assumed for these slopes if the drainage is not properly considered. Both the global and local factors of safety (FS) of the reinforced sand slope were minimally influenced by the loss of matric suction induced by rainfall infiltration. The required reinforcement tensile strengths for the reinforced silt and clay slopes to maintain FS = 1.3 were, respectively, approximately 3 and 4 times larger than that for reinforced sand slopes. Numerical results also indicated that the inclusion of sand cushions, which provide both strength and drainage functions, can effectively enhance the slope stability. An optimal sand cushion thickness of 15 cm (replacing 20% of marginal backfill with sand) was determined in this study.

Performance of the bearing reinforcement earth wall as a retaining structure in the Mae Moh mine, Thailand

In this research, a Bearing Reinforcement Earth (BRE) wall with a residual clay stone backfill was successfully implemented as an alternative truck ramp support for an on-site crusher plant in the Mae Moh mine, Thailand. The performance of the BRE wall during and after the end of construction as well as during the service state was evaluated in terms of, settlement, bearing stress, lateral movement, lateral earth pressure and tension force in the reinforcements. Bearing reinforcement is a cost-effective inextensible earth reinforcement, which is composed of a longitudinal member and transverse members. The maximum settlement at the end of construction (20 days) was about 5 mm. The installation of the truck ramp (10 days after the end of construction) resulted in an immediate settlement of about 2 mm. The final settlement due to the backfill, truck ramp and truck load after 270 days was found to be uniform due to the contribution of bearing reinforcement and was approximately 25 mm. The bearing stress which was uniformly distributed was found to increase rapidly with construction time, which was in agreement with the relatively uniform settlements. The lateral wall movement at the front and lateral sides at the end of construction was very small with the maximum movement (at the top of the wall) found to be less than 10 mm. As such, the ratio of lateral movement to height (δ/H) was found to be approximately 0.12%, which was lower than the allowable value of 0.4%. With this low δ/H and the insignificant change in the measured settlement and lateral movement during service, the BRE wall was considered to have a very high stability. The coefficients of lateral earth pressure, K and depth relationship were proposed based on the analysis of measured maximum tensile force in the reinforcements. The maximum tension plane of the BRE wall could be represented by the coherent gravity hypothesis. Using the proposed K and maximum tension plane, the internal stability of the BRE wall was furthermore examined. A proposed method of designing the BRE wall with claystone backfill was also proposed.

Modelling Sulfide Fluxes and Cu Canister Corrosion Rates in the Engineered Barrier System of a Spent Fuel Repository

Sulfide is a corrodant for the copper canister in the planned spent fuel repository in Finland and Sweden. Sulfide fluxes and canister corrosion rates in the clay barrier have been assessed by a reactive transport model using the geochemical simulator PFLOTRAN. These rates are linked to the dissolution of gypsum, present in the clay backfill, and the fluxes of sulfate, organic carbon and Fe(II) into interface zones where microbial activity occurs. The permeability of the rock adjacent to the clay backfill exerts a strong control on the biogeochemical processes and the resulting copper corrosion rates.

Strength reduction for upheaval buckling of buried pipes in blocky clay backfill

Offshore pipelines are often buried to protect the pipeline from external loads and upheaval buckling. Models for pipe uplift resistance in clay soils are based predominantly on homogenous backfill conditions. In practice, however, there will be significant soil disturbance during installation. With certain trenching techniques this may produce a backfill more akin to a matrix of lumps of intact soil connected by weaker remoulded interfaces. This research uses centrifuge modeling to assess the resistance provided by a representative lumpy clay backfill that has experienced self-weight consolidation. A model pipe is then uplifted through this model backfill in order to assess the soil uplift resistance. Results show that the uplift resistance in this material is governed strongly by the size of the lumps and, to a lesser extent, by the rate at which displacement occurs. When interpreted in terms of the strength reduction η, that may be used to correct between theoretical and measured uplifts, lower values were derived than those currently used based on intact soils. The value of η is seen to be controlled by a non-dimensional drainage parameter, but may be practically estimated based on an estimate of the size of lumps relative to the pipe diameter.

Uplift tests on full-scale pipe segment in lumpy soft clay backfill

Upheaval buckling of pipelines caused by thermal- and pressure-induced loading is an important issue in pipeline design. The uplift capacity of pipelines is determined by the pipe–soil interaction during pipeline upheaval in soil. Pipelines to be installed in soft clay are usually placed into trenches and then backfilled. In this paper, a set of test devices were developed and a series of full-scale model tests were carried out on a pipe segment buried in lumpy soft clay backfill, including backfilling tests, load-controlled uplift tests, and a displacement-controlled test. Eight total pressure transducers were embedded in the wall of the pipe segment to measure soil pressures on the pipe segment, and five linear variable differential displacement transducers (LVDTs) were arranged to record the vertical displacement of the pipe segment and the surface of the soft clay ground. The stabilizing force keeping the pipe segment in place during the backfilling process was found to fit a nearly linear relationship with the dimensionless undrained shear strength of soft clay. The variation of soil pressures on the pipe segment during uplift loading was significantly affected by the buried depth of the pipe segment and the undrained shear strength of the soil. For all present load-controlled tests in lumpy soft clay backfill, the test ultimate uplift resistances were only about 19%–81% of the results calculated by the Det Norske Veritas approach. Mainly due to the voids’ compression, shearing and strain softening of lumpy soft clay backfill, the difference between initial and stable displacements in a loading step for a load-controlled test or initial and stable loads in a displacement step for a displacement-controlled test is remarkable. The limits of uplift resistances are recommended for the instant and sustaining behaviors of the pipe segment, respectively.

Characterization of Fe(III)-saturated montmorillonite and evaluation its sorption behavior for U(VI)

Abstract Radioactive waste is usually sealed in steel canisters surrounded by a layer of compacted clay back-fill, and permanent buried in a deep geological repository. Unavoidably, the radionuclide contaminants can be released from repository and then sorbed onto the waste container corrosion products or the Fe-rich minerals. Herein, we characterized the Fe(III)-saturated montmorillonite (Fe(III)-MMT) by using Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET), and found the changes of the surface incorporating Fe(III) and surface micropores. The sorption of U(VI) on Fe(III)-MMT and Na-montmorillonite (Na-MMT) was investigated by batch experiments. The larger surface area and cation exchange capacity, and the existence of Fe(III) (hydr)oxide phases in Fe(III)-MMT contributed greatly to its sorption capacity. In the whole pH range, the sorption of U(VI) on Fe(III)-MMT was higher than on Na-MMT, and the sorption was strongly depended on pH and ionic strength. The sorption isotherms were simulated well by the Langmuir and Freundlich models. The thermodynamic parameters (ΔH, ΔS and ΔG) calculated from the temperature dependent sorption isotherms indicated that the sorption of U(VI) on Fe(III)-MMT was an endothermic and spontaneous process. The observations suggest that the interactions between U(VI) and Fe(III)-MMT are important in controlling U(VI) retention. The phenomena need to be considered in risk assessment and reactive transport modeling.