Variation Of Matric Suction Research Articles

Numerical Modelling of Matric Suction in Unsaturated Soil under Shallow Foundation Under Varying Soil and Hydrological Conditions

In recent times, extreme hydrological events have disrupted the performance of various structures, particularly the structural foundations responsible for transferring the superstructure's weight to the natural ground. This disruption underscores the significance of matric suction and soil saturation, which are influenced by hydrological conditions like precipitation, soil shear strength, and foundation settlement. These factors are essential when designing structures in specific locations with distinct geotechnical parameters. To address these challenges, our research employs Plaxis 2D numerical modeling to investigate the dynamic changes in matric suction within soil beneath shallow foundations under varying rainfall conditions. Our approach involves a fully-coupled flow method, incorporating the Van Genuchten hydraulic model. In recognition of practical constraints, we utilize fundamental soil classification parameters for model configuration. Our findings reveal the substantial impact of matric suction fluctuations during rainfall on soil deformation, as indicated by displacement patterns. This highlights the critical importance of matric suction in comprehending soil behavior. Furthermore, we observe that higher initial water table levels correlate with reduced variations in matric suction and soil deformation during rainfall, emphasizing the regulatory role of water table depth. In conclusion, this study emphasizes the necessity of considering matric suction and water table depth in structural design and geotechnical analysis, particularly when faced with extreme hydrological events. By comprehending these factors, we can enhance our understanding of soil behavior, improve foundation stability, and develop more effective design strategies for structures in various environmental conditions.

Bearing capacity factor for strip foundations on unsaturated clay

ABSTRACT The matric suction in unsaturated soil depends on the type of soil and various flux conditions i.e., infiltration, evaporation and no flow. The shear strength of unsaturated soil changes with change in matric suction leading to varying resistance of soil under various flow conditions. In this study, the bearing capacity of strip foundations resting over unsaturated clay considering variation of matric suction with depth for different surface flux conditions and position of water table, has been obtained using finite element lower bound limit analysis. Modified Mohr-Coulomb yield criteria based on unified effective stress approach has been employed to incorporate the contribution of matric suction stress to the failure condition. A non-dimensional bearing capacity factor was introduced to estimate the bearing capacity of strip foundations and presented as a function of different influencing parameters such as foundation width, water table depth, soil properties, various flow conditions and their flow rate, and surcharge pressure. The influence of variation of unit weight of soil in the unsaturated zone, air entry and pore size distribution parameters, residual degree of saturation of soil, specific gravity of soil solids, and foundation-soil interface roughness on the bearing capacity has also been examined and found to be insignificant.

Study on erosion and stability of the ecological slope

Rainfall is the main influencing factor causing slope erosion, landslide, and instability in loess; thus, it is vital to comprehend the process of rainfall erosion on various slope surfaces and water penetration inside the slope. In this paper, the loess sample is from Heifangtai in Gansu Province, and triaxial shear tests were conducted on loess with roots under varying water contents to evaluate the slope-reinforcing impact of roots. The slope surface erosion process was analyzed using a soil moisture sensor and matric suction meter to monitor the variation of matric suction in the middle slope and slope foot in response to varying precipitation levels. The numerical simulation approach is utilized to analyze the fluctuation of slope stability under the effect of varying rainfall intensities and humid heat, and the analytical solution of the safety factor is compared to the model solution. The results indicate that the shortest generation time for bare slope runoff is 6 min, whereas the greatest generation time for the Bermuda grass slope is 12 min; the shorter the period, the less water penetration and the simpler it is to reach the slope erosion stage. The slope’s rise increases runoff velocity, strengthening water resistance on the slope surface. When the test slope is 30°, the maximum mass of scouring sediment on the bare slope is 15.2 g from 24 to 36 min, compared to 14.7 g from 24 to 36 min when the test slope is 60°. The amount of scouring reduces as the slope increases. The slope safety factor declined from 3.51 to 2.84 after 24 h of heavy rain, and the loss rate accelerated as the rainfall intensity increased.

A unified compaction curve for raw earth material based on both static and dynamic compaction tests

This paper investigates the compaction characteristics of raw earth via a double-faced static compaction and the traditional Proctor test (dynamic process). The corresponding compaction energies are estimated for each individual sample, while the matric suctions are measured by a simple method using filter papers. The obtained results allow to draw the following observations and conclusions. Firstly, similar to classic Proctor tests, a family of iso-energy curves (i.e. tests with identical compaction energy) are identified for the set of static compaction tests. With increasing compaction energy, the iso-energy curve shifts leftwards and upwards, leading to a reduction of optimum moisture content and an increase of maximum dry density. Secondly, a new term called the optimum saturation degree (Sr)opt is introduced, defined as the degree of saturation Sr where (ρd)max is obtained for a given value of compaction energy and for a particular compaction method. For a specific earth type, and a given compaction energy, our experimental results indicate this (Sr)opt value is constant and that, a unique compression curve linking the degree of compaction Dc = ρd/(ρd)max to Sr−(Sr)opt exists regardless of the compaction method (static compaction, dynamic Proctor etc.) and compaction energy. Thirdly, the matric suction of specimen subject to static compaction tests is slightly higher than that in dynamic Proctor tests at the same moisture content, whereas the dry density has hardly any correlation with the variation of matric suction. Finally, a new method of earth compaction control is proposed here. This new protocol is organized to compact the earth to a target dry density ρd, by taking advantage of practically achievable compaction energies and meeting the design requirement, while keeping the Sr = (Sr)opt condition.

Effect of soil moisture content on the bearing capacity of small bored piles in the unsaturated soil of Maringá, Paraná, Brazil

In this study, the influence of soil moisture on the bearing capacity of piles founded in an unsaturated clay soil was investigated. The soil studied, composing the upper soil layer in Maringá, Brazil, is lateritic, has degree of saturation between 37% and 70% and has collapsible behaviour when wet. The bearing capacity was determined by full-scale load tests following the Brazilian Standard for Static Load Test. Two pile lengths, 4 m and 8 m, were considered. To analyse the influence of soil moisture, two tests were performed for each pile length: one in soil in its natural moisture content and another in pre-moistened soil. Results show that for both pile lengths, an increase in water content caused a significant reduction in bearing capacity, which is attributed to the decrease in the matric suction of the soil. This is confirmed by the results of the initial evaluation made on the variation of matric suction and its contribution to the bearing capacity with changes in water content. In summary, this study confirms that the pile bearing capacity in unsaturated soil is dependent on soil water content, highlighting the fact that the approach of assuming full saturation condition in the evaluation of the pile bearing capacity in such soil may give erroneous results. Moreover, this study demonstrate that the empirical methods most commonly used in Brazil for pile bearing capacity determination, the Décourt & Quaresma and Aoki & Velloso methods, are overly conservative when applied to the Maringá soil.

Variation of Matric Suction as a Function of Gypseous Soil Dry Density

Gypseous soil is one of the most problematic types of collapsible soils which is affected by many geotechnical factors. The most important factors are the effect of loading and wetting and their relation to soil density, especially when the soil at unsaturation condition. Suction pressure is the main criteria in determining the deformation behaviour of unsaturated collapsible soil when these soils distributed in arid or semi-arid region. In this study, disturbed sample of sandy soil of more than 70% gypsum content is taken from Al-Ramadi city western of Iraq. This study interested to investigate the variation of matric suction with the dry density and their effects on deformation of gypseous soil. For this purpose, a soil-model device provided with high accurate Tensiometers and Time Domain Reflectometry sensors in addition to data logger is designed and manufactured. Tensiometer sensor is used to monitor and measure the matric suction, while the Time Domain Reflectometry is used to monitor and measure the volumetric water content in the soil mass. The results of the tests showed that there is a significant effect of soil dry density on the relationship between the matric suction and water content.

A water retention model for compacted clays subjected to salinization and desalinization processes

Environmental actions are known to induce relevant effects on the fabric of compacted active clays, which are successfully described by adopting a double porosity framework. In particular, the role of aggregate deformation has been recognized as fundamental to interpret the water retention behavior and the transport properties. These aspects are particularly relevant in the context of clay liners, being the material cast in place in unsaturated conditions and subjected to wetting process by pore fluids characterized by a chemical composition that is different from the one of compaction. Experimental data evidence that the water retention properties of active clays evolve as a function of pore water chemistry, since for a given matric suction the mass of stored water changes with water salinity. In this paper, a double porosity water retention model is proposed, capable of reproducing the variation of matric suction with water content accounting for the salinity of pore fluid. The role of salinity changes is accounted for by a suitable evolution law for aggregate deformation, which in turn affects the inter-aggregate porosity and thus the storage properties of the material.

Stability analysis of an unsaturated expansive soil slope subjected to rainfall infiltration

Shallow failures occur frequently in both engineered and natural slopes in expansive soils. Rainfall infiltration is the most predominant triggering factor that contributes to slope failures in both expansive soils and clayey soils. However, slope failures in expansive soils have some distinct characteristics in comparison to slopes in conventional clayey soils. They typically undergo shallow failures with gentle sliding retrogression characteristics. The shallow sliding mass near the slope surface is typically in a state of unsaturated condition and will exhibit significant volume changes with increasing water content during rainfall periods. Many other properties or characteristics change such as the shear strength, matric suction including stress distribution change with respect to depth and time. All these parameters have a significant contribution to the expansive soil slopes instability and are difficult to take into consideration in slope stability analysis using traditional slope stability analysis methods based on principles of saturated soil mechanics. In this paper, commercial software VADOSE/W that can account for climatic factors is used to predict variation of matric suction with respect to time for an expansive soil cut slope in China, which is reported in the literature. The variation of factor of safety with respect to time for this slope is computed using SLOPE/W by taking account of shear strength reduction associated with loss of matric suction extending state-of-the art understanding of the mechanics of unsaturated soils.

Clarifying the hydrological mechanisms and thresholds for rainfall-induced landslide: in situ monitoring of big data to unsaturated slope stability analysis

The development of early warning systems for landslide hazards has long been a challenge because the accuracy of such systems is limited by both the complicated underlying mechanisms of landslides and the lack of in situ data. In this study, we implemented a multivariate threshold criterion that integrates in situ monitoring data and data from unsaturated hydro-mechanical analyses as an early warning system for rainfall-induced landslides in the Wenchuan earthquake region of China. The results indicate that rainfall intensity is closely correlated with the probability of landslide occurrence. Variations in matric suction and suction stress were obtained from in situ measurements and used to quantify the soil water retention curve, which presented clear hysteresis characteristics. The impacts of rainfall infiltration on slope failure in post-earthquake landslide areas under transient rainfall conditions were quantified by hydro-mechanical modelling theories. Variations in the suction stress of unsaturated soil were used to calculate the safety factor. The influence of hydrological hysteresis processes on the slope failure mechanism was analysed. Multivariate threshold criteria that include the intensity-probability (I-P) threshold, soil moisture and matric suction based on in situ big data and unsaturated slope stability analysis benchmarks are proposed for use in an early warning system for rainfall-induced landslides.

Determination of Matric Suction and Saturation Degree for Unsaturated Soils, Comparative Study - Numerical Method versus Analytical Method

Matric suction is a soil parameter which influences the behaviour of unsaturated soils in both terms of shear strength and permeability. It is a necessary aspect to know the variation of matric suction in unsaturated soil zone for solving geotechnical issues like unsaturated soil slopes stability or bearing capacity for unsaturated foundation ground. Mathematical expression of the dependency between soil moisture content and it’s matric suction (soil water characteristic curve) has a powerful character of nonlinearity. This paper presents two methods to determine the variation of matric suction along the depth included between groundwater level and soil level. First method is an analytical approach to emphasize one direction steady state unsaturated infiltration phenomenon that occurs between the groundwater level and the soil level. There were simulated three different situations in terms of border conditions: precipitations (inflow conditions on ground surface), evaporation (outflow conditions on ground surface), and perfect equilibrium (no flow on ground surface). Numerical method is finite element method used for steady state, two-dimensional, unsaturated infiltration calculus. Regarding boundary conditions there were simulated identical situations as in analytical approach. For both methods, was adopted the equation proposed by van Genuchten-Mualen (1980) for mathematical expression of soil water characteristic curve. Also for the unsaturated soil permeability prediction model was adopted the equation proposed by van Genuchten-Mualen. The fitting parameters of these models were adopted according to RETC 6.02 software in function of soil type. The analyses were performed in both methods for three major soil types: clay, silt and sand. For each soil type were concluded analyses for three situations in terms of border conditions applied on soil surface: inflow, outflow, and no flow. The obtained results are presented in order to highlight the differences/similarities between the methods and the advantages / disadvantages of each one.

광산폐기물 적치사면의 불포화 특성변화 모니터링 시스템 구축

본 연구에서는 부산 임기광산 광산폐기물 사면을 대상으로 강우 침투에 따른 지반내 불포화 특성변화를 살펴보기 위하여 불포화토 특성변화 측정유닛 및 시스템을 현장에 설치하였다. 불포화토 특성변화 측정시스템은 각종 측정유닛으로부터 측정된 자료를 저장, 관리 및 전달하는 시스템으로 Data Acquisition System(DAS), Solar System 및 불포화토 특성변화 측정유닛으로 구성되어 있다. 현장에 설치된 각종 측정유닛으로부터 측정된 강우, 체적함수비, 모관흡수력 등의 자료를 정리하여 분석하였다. 이들 결과를 살펴보면 강우시 체적함수비와 모관흡수력의 변화는 강우강도에 의해 크게 영향을 받으며, 강우시와 건기시 지표면 부근에서 가장 크게 증가하였다가 감소하는 것으로 나타났다. 그리고 현장에서 측정된 모관흡수력은 5-35kPa, 체적함수비는 0.12-0.24 범위에 존재하는 것으로 나타났다. 그러나 지하수위는 강우발생 직후 바로 증가하는 것이 아니라 2-3일의 시간이 경과한 뒤 증가함을 알 수 있다. Field measurement units and a system were constructed and installed in a waste-dump slope at the Imgi mine to investigate and analyze the variations in the unsaturated characteristics of the soil. The field instrumentation system was composed of a data acquisition system (DAS), a solar system, and measuring sensors. The rainfall, matric suction, and volumetric water contents were continuously measured from the units in the instrumented site. The variations in matric suction and volumetric water content were primarily affected by the rainfall intensity. At the surface of the slope, the largest increase and decrease in the changes in matric suction and volumetric water content were observed during the wetting and drying processes, respectively. Also, the matric suction and volumetric water content were 5-35 kPa and 0.12-0.24, respectively. However, the ground water level was not suddenly increased just after rainfall but gradually increased after 2 or 3 days later.

Analyses on variations in the unsaturated characteristics of a mine waste-dump slope during rainfall

Field measurement units and a system were constructed and installed in a waste-dump slope at the Imgi mine to investigate and analyze the variations in the unsaturated characteristics of the soil. The field instrumentation system was composed of a data acquisition system, a solar system, and measuring sensors. The rainfall, matric suction, and volumetric water contents were continuously measured from the units in the instrumented site and analyzed with the soil water characteristic curve (SWCC) estimated from laboratory experiments. The variations in matric suction and volumetric water content were primarily affected by the rainfall intensity. At the surface of the slope, the largest increase and decrease in the changes in matric suction and volumetric water content were observed during the wetting and drying processes, respectively. The matric suction and volumetric water content measured from the instrumented area were compared with the SWCCs obtained from the laboratory test, and the measured data were located between the drying and wetting paths. The drying and wetting paths obtained from the laboratory test are regarded as primary drying and wetting curves, respectively. Therefore, the measured data can be defined as scanning curves, which are located between primary drying and wetting curves. The variations in matric suction according to volumetric water content were dependent on depth, and a wide range of variations was observed at shallow depths. It was confirmed that the unsaturated soil near the ground surface has the sensitive reaction induced by the infiltration and evaporation in nature.

Estimation of the Variation of Matric Suction with Respect to Depth in a Vertical Unsaturated Soil Trench Associated with Rainfall Infiltration

Soil trenching is extensively used in geotechnical, mining, tunneling and geo-environmental infrastructures. Safe height and stand-up time are two key factors that are required for the rational design of soil trenches. Rainfall infiltration has a significant influence on the safe height and stand-up time of unsaturated soil trenches since it can significantly alter the shear strength of soils by influencing the matric suction. In other words, predicting the variation of matric suction of soils associated with rainfall infiltration is vital to the design of unsaturated soil trenches. In this paper, finite element analysis is carried out to reproduce the variation of matric suction profile in unsaturated soil trenches associated with rainfall infiltration using the published results of a full scale instrumented test trench at the site of BBRI at Limelette, Belgium. The analysis results showed that the variation of matric suction in unsaturated soil trenches can be reliably estimated using the information of environmental factors such as the rainfall measurements.

Seismic response of earth dams considering dynamic properties of unsaturated zone

It is conventionally assumed in the analysis and design of earth dams that the soil located above the phreatic line, i.e. the uppermost seepage flow line, is completely dry. However, there is often an unsaturated flow of water through an unsaturated zone above this borderline and variation in moisture content in this zone results in variation of matric suction throughout this region. Variation of matric suction, in turn, results in variation of effective stresses in this zone. In this research, the seismic response of earth dams in terms of the displacement and acceleration at the crown of the dam as well as the stress distribution in the dam body is investigated. Taking into account the effect of unsaturated zone, a comparison is made to investigate the effect of conventional simplification in ignoring the dynamic characteristics of the unsaturated zone above the phreatic line and the more complicated analysis which includes the unsaturated zone. A function for the soil-water retention curve (SWRC) was assigned to the soil in the unsaturated zone to determine the variation of matric suction in this zone and analyses were made using finite difference software (FLAC). Results are then compared to the conventional method for homogeneous dams. In these analyzes the soil shear modulus was assumed to vary with the mean effective stress both for saturated and unsaturated zones. Among various results, it was notable that the history of crest x-displacement, and acceleration show higher values in models accounting for the unsaturated region. It was attributed to the considerably lower values of damping ratio in the crest region in the unsaturated models.

불포화 사면의 포화 부근 침투 특성을 고려한 수리전도도 모델

불포화 수리전도도는 Mualem 모세관 모델에 의하여 이론적으로 함수특성곡선으로부터 적분된다. 하지만 예측된 수리전도도는 포화 부근에서 모관흡수력의 미소한 변화에도 극도로 민감하다. 원활한 형태의 함수특성곡선에 의한 Mualem 수리전도도는 포화 부근에서 급격하게 감소하며, 수리적 거동을 신뢰할 수 있게 모델하지 못하거나 수치해의 안정성을 저해 한다. van Genuchten-Mualem(VGM) 수리전도도를 개선하기 위하여, 낮은 모관흡수력 수준에 있는 임의의 공기함입치이내에서 van Genuchten 함수특성곡선을 수정하였다. 수정 VG 곡선은 대수축에서 임의의 공기함입치에서 포화상태까지 선형화된다. 수정 VG 함수특성곡선은 실제 함수특성거동의 회귀분석에 영향을 끼치지 않으며 원래의 VG 함수특성곡선의 계수를 그대로 사용한다. 수정 VG 곡선을 이용하여, VGM 수리전도도는 임의의 공기합입치를 기준으로 구간별 적분하여 수정되었다. 수정 VGM 수리전도도가 해석적 해로 제안되었으며, 포화부근 영역에서 수리전도도가 급격하게 감소하는 현상이 제거되었다. 실제 사면의 2차원 침투해석을 통하여 VGM 수리전도도와 제안된 모델에 따른 수리거동을 비교하였다. 제안된 모델은 여러 강우조건에 따른 해의 수렴성을 확보하였지만, VGM 수리전도도를 적용하면, 포화부근의 수리전도도가 급격하게 감소하여 강우량이 많은 경우 해가 수렴하지 않았다. 특히 선행강우에 의한 초기 안전율과 집중강우 후 최종 안전율을 크게 평가할 수 있었다. 제안된 수리전도도 모델은 침투해석과 안정해석을 통하여 실제 사면의 붕괴를 재현할 수 있었다. Unsaturated hydraulic conductivity (HC) is integrated theoretically from soil water retention curves (SWRC) by Mualem capillary model, but the prediction of HC is extremely sensitive to small variation of matric suction near saturation. Near saturation, the Mualem HC based on smooth SWRC decreases abruptly and has problems in the reliability of hydraulic behavior and the stability of numerical solutions. To improve van Genuchten-Mualem (VGM) HC, the van Genuchten SWRC model is modified within range of low matric suction (arbitrary air entry pressure). At an arbitrary air entry pressure, the VG SWRC is linearized in log scale until full saturation. The modified VG SWRC does not affect the fit of actual retention behavior and either the parameters of original VG SWRC fit. Using the modified VG SWRC, the VGM HC is modified to integrate for each interval decomposed by arbitrary air entry pressure. An analytical solution on modified VGM HC is proposed each interval, to protect the rapid change in HC near saturation. For silty soils, VGM models of HC function underestimate the unsaturated permeability characteristics and especially show rapid reduction near saturation. The modified VGM model predicts more accurate HC functions for Korean weathered soils. Furthermore, near saturation, the saturated HC is conserved by the modified VGM model. After 2-D infiltration analysis of an actual slope, the hydraulic behaviors are compared for VGM and the modified models. The prediction by the proposed model conserved the convergence of solutions on various rainfall conditions. However, the solution by VGM model did not converge since the conductivity near saturation reduced abruptly for heavy rainfall condition. Using VGM model, the factor of safety is overestimated in both initial and final stage during heavy rainfall. Stability analysis based on infiltration analysis could simulate the actual slope failure by the proposed model on HC.

부분포화토의 침투와 흙의 거동에 따른 유효응력 계수 분석

열역학적 가정을 토대로 평형방정식을 이용하여 지반의 침투와 흙의 거동을 지배하는 불포화토 유한요소해석을 위한 수학적인 알고리즘을 개발하고, 여러 가지 유효응력 개념들을 수치해석을 통해 비교 분석하였다. 불포화토의 유효응력은 지반의 강도를 나타내며 지반이 함유하고 있는 수분과 모관흡수력의 중요한 구성관계를 표현한다. 특히, 불포화 지반내에서 물의 침투가 흙의 알갱이 거동을 예측하기 위해 여러 문헌에서 제안하고 있는 유효응력계수 <TEX>${\chi}$</TEX>의 특성을 지반의 잔류포화도와 변위에 관련된 구성방정식 조건을 이해하기 위해 분석하였다. 그 결과, 불포화토 유효응력계수 <TEX>${\chi}$</TEX>는 외부하중과 침투가 있는 지반내에서 모관흡수력의 분포를 변화시키지만, 지반의 잔류포화도에 따라 유효응력계수의 차이점을 보이며, 외부하중에 의한 지반 변위가 작아지면 유효응력계수 <TEX>${\chi}$</TEX>는 지반내의 모관흡수력에 대한 영향력이 작아진다는 것을 확인할 수 있었다. Based on thermodynamics, the mathematical framework governing the hydro-mechanical behavior of partially saturated soil is derived by using balance equations, and the numerical analysis through implementation of various effective stress definitions is performed. Effective stress on partially saturated soil describes the soil strength which is presented by the relationship between water content and soil suction. For the estimation of hydro-mechanical behavior on partially saturated soil, effective stress parameter <TEX>${\chi}$</TEX> defined from various literatures is especially analyzed to understand the conditions of constitutive equations regarding residual saturation and displacement of soil. As a result, effective stress parameter <TEX>${\chi}$</TEX> has an influence on the variation of matric suction in soil with an external load and seepage. However it was found that the effect of each parameter <TEX>${\chi}$</TEX> varies with residual degree of saturation, and that of each parameter <TEX>${\chi}$</TEX> decreased with decrease in displacement of soil caused by an external load.

Monitored and simulated variations in matric suction during rainfall in a residual soil slope

This research combines field, laboratory and numerical investigations to estimate the development of a wetting front within a 1.2 m residual soil mantle on a steep forested slope during rainfall events. The field-monitored variations in matric suction due to rain-water infiltration during various events revealed that the maximum infiltration rate was much higher when the wetting front resided in the upper 20 cm of soil compared to the case when the wetting front advanced to depths > 20 cm. Laboratory investigations on soil hydraulic properties (i.e., soil water characteristic curve, and hydraulic conductivity) were useful to establish the parameters of a multilayer finite-element model for one-dimensional vertical infiltration. These parameters were subsequently calibrated by matching the predicted and field measured transient pore water pressure responses during actual rainstorms with irregular rainfall patterns. The calibrated simulation model was used to assess the migration of the wetting front under uniform rainfall with different intensities. Based on the numerical results, a hyperbolic equation was developed to predict the duration of uniform rainfall required for the propagation of wetting front to a certain depth for a given rainfall intensity. The proposed equation was subsequently tested against field-monitored advancements of the wetting front during real rainstorms with variable rainfall intensity.

Role of Matric Suction in Collapse of Compacted Clay Soil

This paper examines the influence of variations in matric suction on the collapse behavior of compacted Bangalore clay soil. The ASTM filter paper method measured the matric suctions of the compacted soil specimens. The matric suction of the compacted clay soil specimens ranged between 50 and 8,000 kPa at the as-compacted $S_r$ values of 90 and 35%. Comparison of the matric suction-gravimetric water content relations of various compacted soils showed that the soil with a higher liquid limit has a higher matric suction at a given gravimetric water content. Variations in as-compacted degrees of saturation at a constant relative compaction or variations in relative compaction at a constant as-compacted degree of saturation notably affected the matric suction of the Bangalore clay soil. Experimental results also showed that the influence of matric suction on the collapse behavior of this compacted clay soil greatly depended on the relative compaction of the specimens and the pressure at which they were inundated.

Pore water pressure and streambank stability: results from a monitoring site on the Sieve River, Italy

To investigate the role of pore water pressures in the stability of a streambank, a series of tensiometers and piezometers was installed in a bank of the Sieve River, Tuscany, Italy. Fluvial entrainment at the bank toe was monitored by repeated cross-profiling, erosion pins and marked pebbles. Fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage, and variations in water table. An expression was derived for the safety factor with respect to mass movement of the upper bank, incorporating the failure criterion for unsaturated soils and the normal Mohr–Coulomb criterion for saturated conditions. Variations in matric suction have important effects on the stability of the streambank. During low-flow periods, the shear strength term due to the matric suction allows the bank to remain stable at a steep angle. However, during rainfall and flow events, reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure, without development of significant positive pore water pressures. During the rising limb of high-flow events, the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river, despite a reduction in matric suction. During drawdown in the river, when the suction values are still low and the confining pressure in the river decreases to zero, the factor of safety falls to lower values than those experienced prior to the runoff event. Measurements of fluvial entrainment reveal that, although the processes, mechanisms and the frequency of retreat of basal and upper bank zones differ significantly, the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure. Copyright © 1999 John Wiley & Sons, Ltd.

Stability of streambanks formed in partially saturated soils and effects of negative pore water pressures: the Sieve River (Italy)

Streambanks of alluvial channels are usually composed of loose materials, which are unsaturated in ambient conditions. Unsaturated soils are subject to negative pore water pressures, which cause an apparent cohesion. The latter is the main factor in allowing the stability of near-vertical banks. Even during moderate in-bank flow events, the apparent cohesion can be strongly reduced as the material approaches full saturation; therefore, during the drawdown phase, as the confining pressure of the water in the channel disappears, a bank failure is likely to occur. Channel bed-level lowering along the Sieve River, Central Italy, has caused widespread bank instability. A geomorphological reconnaissance of forms and processes was followed by in situ tests to determine the shear strength of the banks. Interpretation of the tests and a streambank stability analysis were based on concepts of soil mechanics for unsaturated soils, in order to obtain relations between bank angle and height in limit equilibrium conditions. A stability chart was obtained with curves for different apparent cohesion values, and a stability analysis was performed taking into account the effects of flow events. In order to investigate the pore pressure effects, a series of piezo-tensiometers were installed in a streambank of the Sieve River. Data from a 1 year monitoring period show variations in pore water pressure and matric suction as a consequence of rainfall, evapotranspiration, and water stage variations. A planar failure with a tension crack occurred in the upper cohesive part of the bank during December 1996. The safety factor has been expressed as a function of the geometry of the bank and of the shear strength of the material. Safety factor variations through time are therefore shown as a function of seasonal variations in matric suction.