International Journal of Civil Engineering
Volume:16 Issue: 4, Apr 2018

Soil–cement is a mixture of Portland cement, soil and water that sticks together due to the hydration of the cement and compression of its components to create a dense, durable compound, which has low permeability and is resistant to erosion. Unfortunately, these mixtures do not perform well under tensile load because soil–cement materials are brittle. In this study, three types of fibres were used to reinforce the materials to compensate for this flaw: jute (a natural fibre), polypropylene, and steel (a synthetic fibre) fibres. These fibres were randomly added to the soil–cement mixture in three percentages (1, 2, and 3%). Tests were then conducted on three different soil–cement gradations each with three fine contents of different mineral types (nine different gradations in total). First, sand equivalent and Atterberg limits were conducted on the soil samples. Then compaction, unconfined compression strength, indirect tensile strength and flexural tests were conducted on the soil–cement samples in two conditions: control (unreinforced) and reinforced soil–cement samples. Results showed an undeniable role of fibres in changing the behaviour of the soil–cement–fibre matrix from brittle to ductile producing post-peak behaviour. The results also show that compressive, tensile and flexural strengths of soil–cement materials improved dramatically by adding steel fibres to the matrix.

We conducted detailed fracture mapping of the soft sedimentary rocks (uniaxial compressive strength of 10–20 MPa) in shaft walls at the Horonobe Underground Research Laboratory to characterize fractures and to understand the influence of different excavation methods on rock mass damage. The mapping indicates that the fractures are numerous and can be divided into shear fractures and extension fractures. On the basis of orientation and frequency, the shear fractures are inferred to be pre-existing fractures, and the extension fractures are considered to be newly formed fractures (EDZ fractures) induced by the shaft excavation. The frequencies of pre-existing and newly formed fractures have a negative correlation, and we infer that stress relief leads to the formation of excavation damaged zone by the generation of the newly formed fractures in the parts of shaft that have intact rock, and by the reactivation of pre-existing fractures where such fractures are numerous. Although more newly formed fractures are formed by blasting excavation than by mechanical excavation, there is little difference in the comparative excavation rates. These results indicate that rock mass damage is caused by the mode of excavation rather than excavation rate. Therefore, the mechanical excavation is preferred to blasting excavation from the viewpoint of minimizing rock mass damage.

Triaxial tests were performed on artificially structured soils, including initially isotropic and initially stress-induced anisotropic soils, and remolded samples under consolidation drained conditions at confining pressures of 50, 100, 200 and 400 kPa to investigate their volumetric contraction behaviors upon axial unloading. The values of axial strain at which unloading begins are set as 3, 6, 9 and 12% to apply the axial unloading–reloading stress path. Factors, including confining pressure, axial strain at which unloading starts, structure and anisotropy, are investigated to analyze their influences on the behaviors of volumetric contraction due to axial unloading. The results demonstrate that: (1) for the three types of tested samples, including initially isotropic, initially stress-induced anisotropic and remolded samples, the volumetric strains are contractive during the process of axial unloading in unloading–reloading stress cycles under all of the confining pressures; (2) the higher the confining pressure and axial strain at which axial unloading starts, the greater the volumetric contraction due to axial unloading is; (3) in comparison with initially isotropic structured soils, initially stress-induced anisotropic samples have smaller volumetric contraction because of axial unloading at low confining pressure; and (4) stress-induced anisotropy can increase volumetric contraction due to axial unloading, but it has very weak influences on artificially structured soils at low confining pressure.

This study examined the effect of adding tyre powders and tyre shreds on the liquefaction potential of loose saturated sandy soil. Also, the dynamic properties of reinforced soil such as the damping ratio and shear modulus were investigated. To this end, a series of 1-g shaking table model tests were carried out at different percentages of sand–tyre powders and sand–tyre shreds mixtures. The results showed that the use of tyre powders and tyre shreds decreases pore-water pressure due to liquefaction. Maximum shear modulus of reinforced soil increased with the increase in tyre powder content in the mixture. However, an increased percentage of tyre shreds had no noticeable effect on maximum shear modulus. Furthermore, the mean damping ratio increases with the increase in tyre powder content in the specimens. Therefore, as the percentage of tyre shreds is increased up to 10%, the mean damping ratio experiences an increasing trend. Nevertheless, at values above 10%, the mean damping ratio reduces. In general, reinforcing soil with tyre powders and tyre shreds reduces the deformations caused by liquefaction.

During installation of prefabricated piles in saturated clayey soils, excess pore water pressure (EPWP) is generated around the pile shaft and tip. The developed excess pore pressure is extremely important for evaluation of changes of stress state in clay and subsequent pile bearing capacity. Therefore, the main objective of this paper is to present a numerical finite-element model (FEM) to simulate a mini-pile installation and subsequent pore water pressure dissipation over time. The mini-pile is inserted from the ground surface into the soil to the desired depth. The numerical model is validated using the results of a physical model in which a piezocone is penetrated into a consolidation chamber containing saturated clay. The effects of Over-Consolidation Ratio (OCR), coefficient of lateral soil pressure (K0), penetration rate (PR), and soil hydraulic conductivity (K i ) on soil stress state are investigated. The verification results have shown that the numerical model has successfully simulated the penetration of mini-pile, generation, and subsequently dissipation of the excess pore water pressure (EPWP). The parametric study revealed that increasing the OCR and K0 has resulted in increase of the effective radial stresses on the pile skin compared to its initial value, after dissipation of EPWP. The results indicate that the variations in radial stresses at the end of dissipation are independent from the penetration rate. The dissipation of EPWP is more correlated with horizontal hydraulic conductivity of the soil, because the dissipation rate is accelerated in the radial direction.

Preloading method has been a widely used alternative to improve soft soil in coastal areas in China. In this paper, a field test of PVD-reinforced soft soil with surcharge preloading and vacuum preloading was introduced. Based on the field test, three-dimensional finite-element analyses were conducted to evaluate the performance of reinforced soft soil. The subsoils were simulated as linearly elastic–perfectly plastic materials with Mohr–Coulomb failure criteria. The PVDs were modeled individually as solid elements with the cross section of real PVD. The computed settlements, layered settlements, lateral displacements and excess pore water pressure were compared with the field data. The results show that the differential settlement on the ground can be minimized in the vacuum preloading. However, the environmental influence area of vacuum preloading was greater than that of surcharge preloading outside the reinforced area. The influence depth of vacuum preloading under PVD base was time dependent. And the cut-off wall had a significant effect on mitigating vacuum loss at the boundary.

In this paper, a new approach for back analysis of a geogrid reinforced soil (GRS) wall failure is presented. A new zero-thickness cohesive fracture element is utilized to simulate the slip surface behind the GRS wall. This element can simulate displacement discontinuity as well as tractions across the shear band effectively. The numerical results are compared with the measured values from the physical test as well as the obtained values from the typical finite element method. This paper demonstrates that the proposed finite element algorithm via discrete modeling of the shear band can effectively improve the quality of numerical back analysis of the soil failure which explains its necessity.

This study investigates the microstructure of lime-treated clayey soils using mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM) analyses. Parameters that were varied include lime percent (3, 6, 9%), curing duration (7, 28 days and 1 year), soil pulverization level and mellowing period (1 and 24 h). All samples were compacted at optimum water contents using standard Proctor compaction energy. The 34 MIP and several ESEM analyses conducted on these samples showed that lime content and curing duration had significant impact on the resulting microstructure. MIP results, presented as mercury intrusion curves, total porosity values and pore size distribution histograms revealed that lime stabilization changed the microfabric of clayey soils through a dynamic pore refinement process. Although increases in pore sizes and porosities were observed in the short term (up to 28 days), after a curing period of 1 year, considerable decreases in pore sizes and porosities were noted. A novel “Pore Size Amplification Factor”, (PSAF) was calculated to determine the amplification and/or deamplification of different pore size ranges compared to the untreated soil. ESEM analyses confirmed that while the addition of lime to clayey soils initially increased pore size within the microstructure, over time, as the pores became partially or even completely blocked, the pore sizes reduced. Pores of different sizes and cementation within and on the particles were visible. ESEM findings also showed that pore shapes were not always circular as is assumed in MIP analyses. The results of this study add valuable insight into the time related changes in the microfabric of lime-treated soils.