土水特征曲线在滑坡预测中的应用性探讨

 通过模型试验与数值分析方法的结果对比,研究土水特征曲线中的主要增湿路径与主要减湿路径对降雨型滑坡预测的影响。研究结果表明,采用不同状态路径的土水特征曲线来进行降雨条件下边坡的有限元渗流分析,会得到不同的基质吸力和孔压变化趋势,进而影响强度折减有限元法的计算结果;而对于相同的基质吸力条件下,采用主要增湿路径所预测的非饱和渗透系数会低于主要减湿路径的预测值,使得主要减湿路径预测滑坡破坏时间较主要增湿路径快。此外,若边坡产生滑动型破坏,则实际滑坡发生时间在主要减湿路径与主要增湿路径的预测值之间。研究成果说明,采用作为界限的主要增湿和主要减湿路径作为土水特征曲线的滞后模型,对降雨型滑坡灾害的防灾预警具有一定的实用价值,可利用主要增湿与减湿路径预测滑坡发生的时间差,规划与建立合适的滑坡雨量预警基准。     

土体水力滞后对土石坝初次蓄水渗流场的影响

在土石坝的蓄水过程中,坝料的持水特性和渗透函数的不同会对坝体渗流场产生很大的影响。因此,对土坡的降雨入渗和土石坝的蓄水渗流场的分析应采用土-水特征吸湿曲线及相应的渗透函数。基于砾石土料的土-水特征曲线的实测结果,采用饱和-非饱和渗流有限元方法对不同程度各向异性的均质土坝初次蓄水过程中由脱湿和吸湿曲线计算所得的渗流场进行对比分析,探讨采用脱湿曲线代替吸湿曲线计算带来的影响。结果表明由脱湿曲线计算的孔隙水压力和体积含水率均明显高于由吸湿曲线计算的结果,渗透性的各向异性程度对坝体中的水力滞后性无明显影响。因此,在入渗分析时若采用脱湿曲线取代吸湿曲线会高估坝体中的孔隙水压力和含水率,低估非饱和区的吸力。     

Modelling the compaction curve of fine-grained soils

A theoretical model for the compaction curve of fine-grained soils at various compaction efforts for the entire range of water content is presented in this study. The prediction method is based on the assumption that the compaction curve represents the state surface at the yield state in an unsaturated condition. Thus, for each applied compaction effort, the compaction curve relates to one yielding point on the saturated normal consolidation line (NCL). For a given soil, the model requires the NCL, S_rc, and one point from any compaction curve to predict the compaction curves for different compaction efforts. Moreover, the lines of equal suctions on the compaction curves can be determined if the SWCC, the wetting path, is known. The model introduced here provides additional theoretical understanding of the soil׳s volume change behavior of the compaction curve. The model was verified in two ways: first it was verified quantitatively, by experimental results, and second it was verified qualitatively, by examining the relationships from other models in the literature. The model was further applied to experimental data reported in the literature on previous static and dynamic compaction tests. The results show that the model fits the experimental data very well. Finally, a simple chart, based on this model and using only liquid limits, is presented to estimate γ_dmax and OMC quickly.     

Measurements of Unsaturated Hydraulic Conductivity Functions of Two Fine-Grained Materials

Water unsaturated hydraulic conductivity (k_w) functions of two remolded fine-grained materials were measured over a wide range of degrees of saturation (S_r) with two methods. The instantaneous profile method (IPM) was used for S_r>50%. An original vapor equilibration method (also known as the vapor equilibrium technique, VET) was used for S_r<50%. Both materials compacted at the standard Proctor optimum water content and maximum density, have saturated hydraulic conductivities (k_sat)<10^-7 m/s. The VET couples the total soil suction (s) control from desiccators with saturated salt solutions with water mass measurements from a digital laboratory balance. The k_w measurements of the two techniques are consistent and complementary. The effect of hysteresis on the k_w functions at higher s values was also investigated. The experimental results suggest that the hysteretic effect on the k_w-S_r and k_w-s relationships cannot be neglected, and that the measured k_w are significantly dependent on the initial S_r. The VET tests on the specimens that were initially dried give the lowest values of k_w and the tests on the specimens that were initially saturated give the highest values of k_w. The relative hydraulic conductivity values are very small (k_rw<2×10^-5) in this saturation range (S_r<50%) for the tested materials.     

Hydromechanical behavior of unsaturated expansive clay under repetitive loading

Compacted layers of expansive soils are used in different engineering projects,such as subgrades,engineered clay barriers,and buffers for radioactive waste disposal.These layers are exposed to a variety of stresses and wetting conditions during field serviceability.Coupling between hydraulic and mechanical repeated loading provides insight understanding to the induced progressive deformation of expansive clay.This study was conducted to investigate the hydromechanical behavior of unsaturated compacted expansive clay under repeated loadingeunloading (RLU) conditions.Two series of onedimensional (1D) oedometer tests were conducted under controlled matric suction up to 1500 k Pa using the axis translation technique (Fredlund soil-water characteristic curve device,SWC-150).The first test series was carried out at different levels of controlled matric suction for non-repeated loading eunloading (NRLU) cycles.RLU cycles were applied in the second test series at different repetitivestress levels and under different levels of matric suction.The results indicated increasing axial wetting strain ε_a(s),axial swell pressure s_s(s),compression index C_c(s),and swell index C_s(s) with suction reduction.The estimated loadecollapse (LC) curves obtained from NRLU series (LCN) and RLU series (LCR)indicated increasing yield stress s_y(s) with increasing suction.This is attributed to the developed apparent cohesion between soil particles,which in turn rigidifies the material response.Applying repetitive loading induced a notable reduction of compression index C_c(s) at the same level of suction,whereas swell index C_s(s) seems to be independent of repetitive loading.Finally,repetitive loading exceeding initial yield stresses results in plastic hardening and,hence,enlargement of yield stress locus(i.e.LC_R curve).     

Automatic soil water retention test system with volume change measurement for sandy and silty soils

The determination of soil water retention curves requires the volume to be measured in order to calculate the void ratio and degree of saturation. The volume change of sample during drying and wetting cycles in soil water retention test is obvious and non-ignorable, especially for soils with deformability. The soil water retention curve is generally superimposed with the volume change of soil specimens. However, in general, many apparatus that are used for soil water retention testing cannot measure the volume change during the test process. In this study, a modified experimental system, which can measure and record volume change during test, and also can control the entire testing process via computer, is proposed to determine the soil water retention curve. The new system has several advantages over existing apparatus. Notable amongst them is that it can automatically determine both the wetting and the drying characteristics with high accuracy, and can measure volume change during test, using only one sample. This technical note presents the design detail and algorithm for control software. Water retention curves considering volume change are determined for four types of soil, ranging from sandy to silty. Then the effect of volume change on soil water retention curve is briefly discussed.     

Characteristics of Scanning Curves of Two Soils

Scanning curves of two different soils were obtained from three series of infiltration and drainage experiments on two physical models of soil slopes in the laboratory. The first slope model consisted of a fine sand layer overlying a gravelly sand layer, while the second slope model involved a silty sand layer overlying a gravelly sand layer. Each soil layer had a thickness of 200 mm and both slope models had an inclination angle of 30°. The slope models were subjected to artificial rainfalls of different intensities, followed by draining where no rainfall was applied. Various instruments were installed to continuously measure the changes in matric suction, volumetric water content and the water balance of the slope models during the experiment. Scanning curves were then constructed using the matric suction and water content data measured at the bottom, middle and top parts of the slope models and were compared with the primary drying and primary wetting soil-water characteristic curves that were measured separately. It was found that the scanning curves followed the primary wetting curve during the adsorption process and then followed the primary drying curve during the desorption process. During the transition period, over which the scanning curve moved from the primary drying curve to the primary wetting curve (or vice versa), the path of the scanning curve had a relatively flat slope as compared to the slope of the primary curves, and sometimes it was almost horizontal. However, the slope and the path of the scanning curves were found to be similar for the cases with similar initial conditions.     

Soil O2 profile affected by gas diffusivity and water retention in a drained peat layer

Peatlands consist of hundreds of gigatons of soil carbon storage. Once a peatland is drained, however, it is transformed into a carbon source, since better aerated conditions promote the aerobic microbial decomposition of the peat soil layers. Since the aerobic depth should depend on the balance between the capability of the peat layers to transport the O₂ and the intensity of the O₂ consumption by aerobic microbes, our aim in the present study was to characterize the soil gas diffusivity (D_s/D₀) and the soil water retention curves (SWRCs) of two types of peat soils found in sub-boreal and tropical peatlands. We also numerically simulated the effects of D_s/D₀ and SWRCs on the aerobic depth into which the atmospheric O₂ can penetrate in unsaturated peat layers with various groundwater table levels. For the numerical simulations, we modified the Millington-Quirk-type D_s/D₀ model, while the O₂ consumptive behavior in the peat layers was described by the Monod-type equation. The D_s/D₀, as a function of the air-filled porosity, was consistently larger for the tropical-peat samples than for the sub-boreal ones, implying that the tropical-peat samples had less tortuous pore geometry than the sub-boreal ones. However, the features of the SWRCs indicated that the tropical-peat samples showed higher water retentivity than the sub-boreal ones; and therefore, lowering the groundwater table would not raise the air-filled porosity of the tropical-peat samples more than that of the sub-boreal ones. Through numerical simulations contrasting the features of the D_s/D₀ and the SWRC curves for the tropical-peat samples with those of the sub-boreal ones, unsaturated anaerobic layers were seen to form more easily in the simulations representing the tropical-peat samples than in those representing the sub-boreal ones, suggesting high water retentivity. In turn, the difficulty in aeration inhibited the gaseous diffusion in the tropical-peat soils even though the D_s/D₀ values for the tropical-peat samples were higher than those for the sub-boreal ones for a series of air-filled porosity. Furthermore, the development of unsaturated anaerobic layers caused a “ceiling” for the increase in the surface CO₂ emission rate associated with the fall in the groundwater table.     

Shrinkage and desaturation properties during desiccation of reconstituted cohesive soil

A vacuum evaporation method, proposed by the authors to reduce the water content more quickly than by air drying, was applied to six saturated reconstituted cohesive soil samples to investigate shrinkage and desaturation properties during desiccation. The test conditions were a vacuum pressure of p_v=?93.9 to ?97.5 kPa, a consolidation pressure of σ_v=68.6–392 kPa, an initial water content of w₀=0.59–0.92 w_L, and an initial surface area of the specimen of A_s0=20–205 cm², where w_L is the liquid limit. The results obtained for these restricted conditions are as follows. The vacuum evaporation of pore water from the soil occurs at a vacuum pressure higher than about ?93 kPa (|p_v|>93 kPa), but the evaporation process is very slow. The minimum void ratio, e_min, at the no-shrinkage phase of the soil subjected to the vacuum pressure, becomes a constant value. The relations e_min≈1.15 e_s and w_s≈87(e_min/G_s) are obtained, where e_s is the void ratio corresponding to the shrinkage limit, w_s, and G_s is the specific gravity of the soil particles. Using the vacuum evaporation method, the continuous relations for w?e, w?V/V₀, and w?S_r are more easily and more rapidly obtainable than with the conventional method by air drying. These three relations were formulated using two parameters, namely, an experimental parameter that is simply obtained using vacuum evaporation tests and a parameter that can be assumed and determined easily. The three formulated relations show a good agreement with the experimentally obtained results. Furthermore, if the basic physical parameter, w_s, has already been obtained, then the three relations can be estimated roughly without the performance of any tests.     

Hydraulic and volume change behaviors of compacted highly expansive soil under cyclic wetting and drying

The wide engineered application of compacted expansive soils necessitates understanding their behavior under field conditions. The results of this study demonstrate how seasonal climatic variation and stress and boundary conditions individually or collectively influence the hydraulic and volume change behavior of compacted highly expansive soils. The cyclic wetting and drying (CWD) process was applied for two boundary conditions, i.e. constant stress (CS) and constant volume (CV), and for a wide range of axial stress states. The adopted CWD process affected the hydraulic and volume change behaviors of expansive soils, with the first cycle of wetting and drying being the most effective. The CWD process under CS conditions resulted in shrinkage accumulation and reduction in saturated hydraulic conductivity (k_sat). On the other hand, CWD under CV conditions caused a reduction of swell pressure while has almost no impact on k_sat. An elastic response to CWD was achieved after the third cycle for saturated hydraulic conductivity (k_sat), the third to fourth cycle for the volume change potential under the CV conditions, and the fourth to fifth cycle for the volume change potential under the CS conditions. Finally, both swell pressure (σ_s) and saturated hydraulic conductivity (k_sat) are not fundamental parameters of the expansive soil but rather depend on stress, boundary and wetting conditions.     

Full scale consolidation test on ultra-soft soil improved by prefabricated vertical drains in MAE MOH mine,Thailand

The reclamation of slurry pond with ultra-soft soil deposit using the prefabricated vertical drains (PVDs) with preloading technique in the Mae Moh mine, Lampang, Thailand is a challenging work and is illustrated in this paper. Geotextile reinforcement was used to strengthen bearing capacity of the soil foundation prior to the installation of sand platform. The delay of excess pore water pressure dissipation at the early loading stage occurred despite the occurrence of large settlements was a distinct behavior of the ultra-soft soil. Within the delayed time, the calculated average degree of consolidation based on measured settlements, U_s increased while the calculated average degree of consolidation based on measured excess pore water pressures, U_e and the undrained shear strength, S_u remained unchanged. Beyond the delayed time, both U_e and S_u increased significantly with time and when U_s > 90%, the difference between U_e and U_s was observed to be small. It was suggested to use U_s for approximation of S_u when U_s > 90% based on the SHANSEP's method. The successful installation and application of PVD to improve ultra-soft soil in this research is applicable for the ground improvement of similar problematic ultra-soft soils in international land reclamation projects.     

An Approach for Assessment of Compaction Curves of Fine Grained Soils at Various Energies Using a One Point Test

Compaction curves of soils are essential for establishing practical and reliable criteria for an effective control of field compaction. This paper deals with the development of a practical method of assessing laboratory compaction curves of fine-grained soils. It is found that for a given fine-grained soil compacted at a particular compaction energy, the relationships between water content (w) and degree of saturation (S) are represented by power function, which are w=A_dS^B_d and w=A_wS^B_w for the dry and the wet sides of optimum, respectively (where A_d, A_w, B_d and B_w are constant). The B_d and B_w values and optimum degree of saturation (ODS) are mainly dependent upon soil type irrespective of compaction energy. The A_d and A_w values decrease with the logarithm of compaction energy and the decrease rates are practically the same for any compacted fine-grained soil. This leads to a simple and rational method to assess the compaction curve wherein the compaction energy varies over a wide range using a one point test (a single test). Assuming that fine-grained soils compacted under standard Proctor energy behave in agreement with Ohio's curves, the modified Ohio's curves for the other three compaction energy levels (296.3, 1346.6 and 2693.3 kJ/m³) are developed based on the proposed method. These curves can be used to assess the entire compaction curves at the required compaction energy based on a single set data of dry unit weight and water content.     

Sorption-desorption column tests to evaluate the attenuation layer using soil amended with a stabilising agent

Sorption-desorption column tests using acrylic columns (? 5 cm × h 10 cm) were employed to evaluate the sorption performance of an attenuation layer against geogenic contamination. The attenuation layer material was silica sand amended with 1, 5, or 10% of a stabilising agent. The main component of the agent was magnesium oxide. The sorption behaviour of the materials was determined by a fluoride solution (C₀ = 80 mg/L F^-), while the desorption behaviour was determined by distilled water. Breakthroughs (C/C₀ > 0.05) occurred after approximately 1, 20, and 50 PVF for stabilising agent contents of 1, 5, and 10%, respectively. The one-dimensional advection-dispersion equation modelled the breakthrough curves obtained from the tests. The predictions gave unrealistic estimates, especially for the breakthrough point where C/C₀ = 0.05. For the 1% agent content, approximately 20% of the sorbed mass, S_s, was desorbed, but the percentage of desorbed mass, S_d, was much smaller for the higher agent contents. The difference between the sorbed and desorbed masses was defined as the immobilised fraction, S_s - S_d. For the 5% agent content, S_s - S_d = 4.0 mg/g. The results suggest that when silica sand is amended with magnesium oxide as an agent, the mixture can immobilise the fluoride in the attenuation layer.     

Suction variations and soil fabric of swelling compacted soils

This study addresses firstly the soil fabric variations of loose and dense compacted soil samples during a single wetting/drying cycle at suctions between 0 and 287.9 MPa using mainly the mercury intrusion porosimetry（MIP） tests.Two suction techniques were employed to apply this wide suction range：the osmotic technique for suctions less than 8.5 MPa,and the vapor equilibrium or salt solution technique for suctions higher than 8.5 MPa.Secondly,the soil water retention curves（SWRCs） were predicted by the MIP test results for both loose and dense soil samples.A reasonable correspondence between MIP results and SWRCs was found on the wetting path at lower suctions close to saturation and on drying path at higher suctions.     

Investigation of correlations between shear wave velocities and CPT data: a case study at Eskisehir in Turkey

Seismic waves result from fault movement during earthquakes. Depending on the features of the physical environment through which they pass, there are variations in the velocity and amplitude of body waves, which occur underground, and surface waves, which occur on the Earth’s surface. The ratio of shear wave velocity (V _s) to near-surface velocity is a parameter used widely in land use planning to predict the potential for amplified seismic shaking, especially in urban areas. The main objective of this study was to estimate V _s by using cone resistance (q _c) and lateral friction (f _s) for a study area at Eskisehir Graben, to help mitigate geotechnical earthquake engineering problems in civil engineering and land use planning. In geotechnical shallow soil research, certain geophysical methods are used for measuring V _s —a major form of seismic energy propagation—at the near surface. In this study, cone penetrometer data collected from seismic cone penetration tests (SCPT) includes q _c, f _s, and downhole V _s. S-type seismic energy waveforms, which are produced on the surface, were measured at different depths using an S-type geophone in the city center of Eskisehir via SCPT. With SCPT, q _c, friction ratios (R _f), and V _s values were measured at 42 different test points. R _f properties are associated with soil thickness, and these were compared with dynamic soil properties (V _s) using a standard statistical method; we calculated correlations amongst V _s, q _c, and R _f measured from cone penetration tests.     

A Hysteresis Model for the Soil-Water Characteristic Curves of Simple Granular Soils

A theoretical model for predicting the soil-water characteristic curves of simple granular soils during different wetting and drying processes is proposed. In this model, the irregular pores in a simple granular soil are assumed to be regular symmetrical cone-shaped capillary pores. The wetting and drying processes of the soil are considered to be the filling and draining processes of the symmetrical cone-shaped capillary pores. The degree of saturation of the soil is determined by estimating the amount of water in the cone-shaped capillary pores. The suction in the soil is estimated by applying the so-called capillary law to the cone-shaped capillary pores. The influence of the closed air bubbles in pore water on the soil-water characteristic curves is also simulated in the theoretical model. All model parameters in the model have definite physical base. The model is verified using experimental data.     

Consolidation behavior of an unsaturated silty soil during drying and wetting

In this work, the effect of hysteresis phenomenon on the consolidation behavior of an unsaturated silty soil was investigated through a program of experimental tests. Compacted samples were prepared by the slurry method and experimental tests were carried out in a double-walled triaxial cell. Consolidation tests were conducted by the ramping method at suctions of 0, 100, 200, 250 and 300 kPa on drying and wetting paths of the soil water characteristic curve. The results show that the paths of specific water volume and specific volume are not consistent during stabilization in either condition (drying or wetting). In addition, the yield stress for the wetting path is higher than that for drying. The trend of variations of the specific water volume during loading is similar to the consolidation curves for different suction. For both conditions of drying and wetting, the slope and intercept of the virgin line due to variations of specific volume and specific water volume are a function of suction. While their values decrease with increasing suction, these values are higher for the dry path than wetting.     

Searching for critical slip surfaces of slopes using stress fields by numerical manifold method

This study first reviews the numerical manifold method (NMM) which possesses some advantages over the traditional limit equilibrium methods (LEMs) in calculating the factors of safety (F_s) of the slopes. Then, with regard to a trial slip surface (TSS), associated stress fields reproduced by NMM as well as the enhanced limit equilibrium method are combined to compute F_s. In order to search for the potential critical slip surface (CSS), the MAX-MIN ant colony optimization algorithm (MMACOA), one of the best performing algorithms for some optimization problems, is adopted. Procedures to obtain F_s in conjunction with the potential CSS are described. Finally, the proposed numerical model and traditional methods are compared with stability analysis of three typical slopes. The numerical results show that F_s and CSSs of the slopes can be accurately calculated with the proposed model.     

Effect of different types of wetting fluids on the behaviour of expansive soil during wetting and drying

This paper presents the results of an experimental investigation into the mechanical behaviour of an expansive soil during wetting and drying cycles. The experimental tests were conducted in a modified oedometer under two different surcharge pressures (10 and 20 kPa). During the tests, the samples were inundated with different types of wetting fluids (distilled water, saline water and acidic water). The volumetric deformation, void ratio and water content of the samples were determined during cycles of wetting and drying. The results show that the swelling potential increases with an increasing number of wetting and drying cycles. The effect of the distilled water on the swelling potential is not the same as that of the saline water or the acidic water, particularly for different surcharge pressures. The variations in void ratio and water content show that, at the equilibrium condition, the wetting and drying paths converge to nearly an S-shaped curve. This curve consists of a linear portion and two curved portions, and the majority of the deformation is located between the saturation curves of 90% and 40%.     

Accelerated carbonation of alkaline construction sludge by paper sludge ash-based stabilizer and carbon dioxide

Construction sludge frequently has high alkalinity after its generation or during the intermediate treatment process. The aim of this study is to experimentally investigate the potential of combining accelerated carbonation and a paper sludge ash-based stabilizer (PSAS) to neutralize the alkalinity of construction sludge in a short period and to improve its strength for use as a recycled material. The experimental results indicate that the addition of a PSAS significantly granulated the alkaline sludge, and once granulated, the PSAS successfully accelerated the pH neutralization of the alkaline sludge. It was also found that the decrease in dry density ρ_d and the degree of saturation S_r of the PSAS-treated sludge was able to reduce the period required for the pH neutralization, t_N. The decrease in ρ_d is thought to allow fresh CO₂ gas to penetrate the specimen more easily. However, if S_r is below a certain limit, it does not strongly facilitate the reduction of t_N. This implies that pH neutralization cannot be accelerated when the amount of water in the sludge is below a certain level. Moreover, it was found that mean particle diameter D₅₀ also affected t_N. The strength development of the PSAS-treated sludge was evaluated using a series of cone index tests. It was found that the strength of the alkaline sludge without the PSAS was significantly decreased by accelerated carbonation, but was significantly increased even after accelerated carbonation when the PSAS was present. Due to the porosities of the remaining PS ash particles, most of the contribution of the water absorption and retention performance of the PSAS to the strength development of the PSAS-treated sludge was secured after accelerated carbonation. In addition, the granulated particles of the PSAS-treated sludge retained their granular shape to some extent. Therefore, it is presumed that the friction and interlocking of the particles did not decrease significantly. It was also found that, after carbonation, the q_c of the PSAS-treated sludge increased more rapidly than that of the alkaline sludge without the PSAS. A further detailed examination of the test results showed that under air-curing conditions, the q_c of the treated sludge with accelerated carbonation increased relatively gradually compared to that of the treated sludge without accelerated carbonation.