Low Vertical Stress Research Articles

Field and laboratory time-dependent behaviors of geotextiles in reinforced soil walls

Laboratory and field investigation of time-dependent behavior of geotextiles reinforcing a fine-grained soil are evaluated herein. The field assessment consisted of analyses of two instrumented sections of a geotextile reinforced soil wall with identical layouts, including one section with a woven and another with a nonwoven geotextile. In addition, in-soil and in-isolation laboratory creep tests were conducted using the same geosynthetics and soil used in the Geosynthetic-Reinforced Soil (GRS) wall sections to better assess time-dependent behaviors in the field. Construction and time-dependent behavior of the full-scale GRS wall proved to be satisfactory over the 4 years of monitoring of both woven and nonwoven geotextile structures. Soil confinement, due to vertical earth pressure on the reinforcement, was found to greatly affect the deformability of the nonwoven geotextile and to have a minor effect on the woven geotextile. Time-dependent strain rates were higher when the failure stress state of the soil was reached. Under lower vertical stresses, the time-dependent behavior of both geosynthetics was similar. Time-dependent behavior was underestimated from in-soil laboratory creep tests and better estimated using in-isolation laboratory tests. However, the GRS wall data did not account for installation damage and wetting-drying processes, which were found to influence time-dependent strain predictions.

Laboratory investigation on the compressibility of Singapore marine clays

ABSTRACTThe compressibility characteristics of Singapore marine clay in reconstituted and undisturbed states were studied using oedometer, constant rate of strain, Rowe cell, and isotropic consolidation tests. The intrinsic compression curve of the reconstituted clay was found to be similar to that proposed earlier with some minor deviations at low vertical stresses of less than 100 kPa. The field and laboratory compression behaviors were found to be similar, hence the laboratory curve could be used as a reference for interpreting the field behavior. Factors affecting the measurements of compression index and yield stress were discussed. As the coefficient of lateral earth pressure at the top upper clay was close to 1, the compression curves of vertically and horizontally trimmed samples were almost similar. The yield stress was mainly controlled by the strain rate; i.e., the higher strain rate resulted in the higher yield stress. Constrained modulus as derived from cone penetration tests and flat dilatometer tests were also examined and compared with laboratory test results. The in situ tests showed the decrease in constrained modulus with depth and generally the dilatometer test was found to register a higher modulus value.

Numerical Investigation into the Influence of Bedding Plane on Hydraulic Fracture Network Propagation in Shale Formations

Shale formations are often characterized by low matrix permeability and contain numerous bedding planes (BPs) and natural fractures (NFs). Massive hydraulic fracturing is an important technology for the economic development of shale formations in which a large-scale hydraulic fracture network (HFN) is generated for hydrocarbon flow. In this study, HFN propagation is numerically investigated in a horizontally layered and naturally fractured shale formation by using a newly developed complex fracturing model based on the 3D discrete element method. In this model, a succession of continuous horizontal BP interfaces and vertical NFs is explicitly represented and a shale matrix block is considered impermeable, transversely isotropic, and linearly elastic. A series of simulations is performed to illustrate the influence of anisotropy, associated with the presence of BPs, on the HFN propagation geometry in shale formations. Modeling results reveal that the presence of BP interfaces increases the injection pressure during fracturing. HF deflection into a BP interface tends to occur under high strength and elastic anisotropy as well as in low vertical stress anisotropy conditions, which generate a T-shaped or horizontal fracture. Opened BP interfaces may limit the growth of the fracture upward and downward, resulting in a very low stimulated thickness. However, the opened BP interfaces favor fracture complexity because of the improved connection between HFs and NFs horizontally under moderate vertical stress anisotropy. This study may help predict the HF growth geometry and optimize the fracturing treatment designs in shale formations with complex depositional heterogeneity.

Effects of solution concentration and vertical stress on the swelling behavior of compacted GMZ01 bentonite

Compacted bentonite has been considered as buffer/backfill material in radioactive waste disposal. This study deals with the effects of the concentration of infiltration saturation and vertical stress on the swelling behavior of compacted GMZ bentonite. One-dimensional swelling tests were performed on specimens at an initial dry density of 1.7Mg/m3. NaCl solutions with concentrations of 0 (de-ionized water), 0.1, 0.5 and 1.0mol/L were used to saturate the specimens while three vertical stresses of 0.06, 0.062 and 1.85MPa were applied. Results show that, for all the infiltration solutions, the swelling strain decreases with increasing vertical stress; the impact of the vertical stress is more significant for low vertical loads while it is less patent when the high salinity solution is infiltrated. For all the vertical stresses applied, the swelling strain decreases with the increase in concentration of the infiltration solution; the impact of the concentration of infiltration solution is not only related to the vertical stress, but also to the soil permeability. Furthermore, the coefficient of primary swelling decreases with the increase in concentration and vertical stress. The concentration of infiltration solution has significant restricting effect on the secondary swelling deformation in case of low vertical stress, while it has no or few effects on the secondary swelling deformation in case of higher vertical stress.

Heat and moisture migration in a geomembrane–GCL composite liner subjected to high temperatures and low vertical stresses

This paper presents the results of an experimental and numerical modelling of heat and moisture migration conducted on a composite liner comprised of a geomembrane (GMB) and a geosynthetic clay liner (GCL), over a compacted subgrade and subjected to prolonged elevated temperatures at low overburden stresses typical of brine storage ponds or solar evaporation ponds. Results are presented for a GMB sitting on a fully hydrated GCL. Heating the top of the composite liner caused a measurable increase in subgrade temperature to at least to 250 mm below the GCL. However, the presence of an air gap, simulating the presence of a wrinkle in the geomembrane, at the interface between the GMB and the GCL reduced the impact of the high temperatures on the subgrade temperature profile with depth. The change in temperature profile was accompanied by moisture migration from the GCL to the subgrade material. However no desiccation cracks were observed in the GCL and the bentonite was still in a gel form at the end of the time period investigated. Numerical modelling using finite element method (FEM) was performed to simulate the results obtained experimentally. It was found to predict accurately the temperature changes that have occurred in the subgrade material and moisture changes that occurred in both the GCL and subgrade materials.

Experimental study on vibration reduction by using soilbags

The effectiveness of soilbags in reducing mechanical vibration is investigated, and the influential factors are examined through a series of laboratory tests, including cyclic lateral shear tests, vertical excitation tests and small shaking table tests as well as a soilbag-filled trench field test. The test results illustrate that soilbags have a relatively high damping ratio and variable horizontal stiffness, which are greatly influenced by the materials inside the bags at low vertical stress but become nearly independent of the materials inside the bags at high vertical stress. We find that vertical and horizontal vibrations can be effectively reduced through the use of soilbags.

Investigation of the hydro-mechanical behaviour of compacted expansive clay

The hydro-mechanical behaviour of compacted expansive Romainville clay was investigated. The soil was air-dried, crushed, and passed through a 2 mm sieve before being statically compacted to a dry density of 1.35 Mg/m3. The mechanical behaviour was investigated by tests in oedometer with controlled suction using the vapor equilibrium technique (suction s = 0, 9, 39, and 110 MPa). The vertical stress was applied in the range of 0–800 kPa. The experimental results are shown as follows: 1) wetting-induced swelling was higher at lower vertical stresses; 2) the vertical stress under which no swelling occurred during water flooding was estimated at 60 kPa, which can be considered as the swelling pressure of the soil tested; 3) the soil compressibility (changes of volume upon stress increases) was strongly influenced by the soil suction: the lower the suction, the higher the compressibility. The hydraulic behaviour was investigated using a large-scale inltration chamber (800 mm × 1000 mm in section and 1000 mm high). The large size of the soil column allowed burying the volumetric water content sensors (ThetaProbe) without signicantly affecting the water transfer and the soil swelling during inltration. The soil suction was monitored along the soil height (every 100 mm) using various relative humidity sensors and psychrometers. In the inltration test, water was kept on the soil surface and changes in suction and volumetric water content were monitored for 338 d. The wetting front has reached the bottom of the soil column at the end of the test. The data from the simultaneous monitoring of suction and water content were used to determine the water retention curve and the unsaturated hydraulic conductivity using the instantaneous prole method. It has been observed that the soil water retention curve depends on the soil depth; that is to be related to the soil depth-dependent swelling. The unsaturated hydraulic conductivity was found to be quite low, comprised between 3 × 10−11 m/s (at saturated state) and 10−14 m/s (at about 100 MPa suction).