Unsaturated Residual Soil Research Articles

Infiltration Characteristics of Unsaturated Residual Soils of Various Weathering Grades

Kertas kerja ini memperihalkan kajian yang telah dibuat di lapangan mengenai ciri–ciri penyerapan air ke dalam tanah baki pelbagai gred yang tak tepu. Dua tapak ujian dikaji, iaitu tapak A dan tapak B. Tapak–tapak ini mewakili tanah baki daripada batuan yang lazimnya terdapat di Malaysia, iaitu tanah baki granit dan tanah baki sedimen (batu kapur). Kadar penyerapan air didapati bergantung kepada darjah luluhawa tanah. Untuk tanah baki granit, tanah bergred IV didapati mempunyai kadar penyerapan air yang tertinggi. Kadar penyerapan ini didapati bertambah daripada tanah bergred VI ke gred IV, yang kemudianya menurun daripada gred IV ke gred III. Kadar penyerapan air juga didapati bertambah dengan bertambahnya keliangan dan nisbah lompang tanah, dan menurun dengan meningkatnya ketumpatan tanah. Bagi tanah sedimen (batu kapur) pula, tanah bergred III didapati mempunyai kadar penyerapan yang tertinggi. Kadar penyerapan air di dapati meningkat daripada tanah bergred V ke gred III. This paper presents the results of a field study on the infiltration characteristics of unsaturated residual soils of various weathering grades. Two field sites were studied, namely site A and site B. These sites respectively represent two of the most commonly occurring residual soils or rocks in Malaysia, that is the granitic residual soil and sedimentary (sandstone) residual soil. The water infiltration rate is found to vary depending on the soil weathering grades. For the case of granitic residual soils, soil of weathering grade IV was found to have the highest infiltration rate. Water infiltration is found to increase from grade VI to grade IV, and decrease from grade IV to grade III. Water infiltration is also found to increase with the increase in the soil porosity and void ratio, and decrease with the increase in the soil density. While for case of sedimentary (sandstone) residual soil, the soil of weathering grade III was found to have the highest infiltration rate. Water infiltration is found to increase from grade V to grade III.

Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas

The apparent cohesion due to soil suction plays an important role in maintaining the stability of steep unsaturated soil slopes with deep ground water table. In this paper, a modified direct shear box is used to determine the relationships between the value of this additional cohesion and the associated soil suction. The apparatus incorporates a miniature tensiometer which allows for the simple and direct measurement of suction during shearing. The soil-water characteristic curves and shearing behavior of intact residual soils, being low-to-medium plasticity silts, as well as silty sand, taken from four landslide-prone areas in Thailand, have been investigated. The relatively low air-entry suctions (0–7 kPa) and bimodality of the soil-water characteristic curves gives an indication of the structured pore size distribution of the materials tested. Samples with higher suction tend to display stronger bonding at particle contacts and thus are more brittle. The shear strength is found to increase nonlinearly with suction, though linearization can be reasonably assumed for suction below around 30 kPa. Prediction of shear strength based on soil-water characteristic curves agrees better with ultimate than peak values. A simple equation is proposed for the minimum ultimate strength that can be expected in an unsaturated residual soil with a suction lower than about 30 kPa.

Critical-state parameters of an unsaturated residual clayey soil from Turkey

This paper deals with the evaluation of the critical-state parameters with respect to the matric suction for saturated and unsaturated undisturbed residual clayey soils from Turkey. In order to conduct the unsaturated triaxial compression testing procedures a conventional triaxial compression apparatus was redesigned. The data for critical-state conditions from these tests are presented with respect to matric suction, based on the critical-state parameters of M, q o, Γ, λ, which is commonly proposed by many authors. The critical state of the unsaturated samples is compared with that of the saturated samples. This experimental study has demonstrated that matric suction has no influence on parameters of M and λ. The parameters of M and λ are approximately 0.85 and 0.074 respectively for saturated and unsaturated conditions. The relationships between matric suction ( u a − u w) and the intercepts q o and Γ have been observed as nonlinear, and thus they can be defined as a function of matric suction ( u a − u w). Furthermore, a method is developed to predict the intercepts q 0 according to matric suction for unsaturated clayey soils.

Effect of loading rate on the volume change behavior of unsaturated residual soil

Residual soils occur in most countries of the world but those that occur in greater area and depth are usually found in the tropical humid areas. This research examines the effects of loading rate of applied or net mean stress on the volume change behavior of an unsaturated granitic residual soil. Three loading rates were adopted: fast, slow and extra slow loading rate. It was observed that the loading rate of applied or net mean stress has a pronounced effect on the void ratio and degree of saturation but has an insignificant effect on the water content of residual soils subjected to constant matric suction.

On the evaluation of unsaturated flow in a natural slope in Rio de Janeiro, Brazil

Geotechnical engineers and geologists are frequently concerned with the complex phenomenon of rainstorm-induced landslides in mountainous tropical regions. This paper analyses the hydrological conditions that contributed to a huge deep-seated slide of a slope in Rio de Janeiro, Brazil. The slope failure occurred after a short period of intense rainfall, buried cars, and caused structural damages to an adjacent building. On the following day, the failure surface was completely saturated, despite the fact that no groundwater was observed within the slope. Understanding a rain-induced landslide is a considerable challenge. In the present case, there was an extra question to be answered: why were no slope movements observed a few months earlier, when a more intense rainfall reached the slope? Geological and geotechnical surveys were carried out aiming at determining complete soil profile characterization, piezometric levels and hydraulic parameters. Field investigations indicated a variable thickness unsaturated residual soil layer overlying a gneissic rock. The transition between the saprolitic soil and the sound rock consisted of a highly fractured and weathered rock. 3D flow modelling has shown that the amount of rain infiltration was insufficient to produce positive pore-water pressures. It was likely that development of preferential infiltration paths could explain the hydrological conditions of the slope during landslide. Water infiltration through the fractured soil–rock transition induced a rapid development of positive pore water within the soil mass. This favourable scenario might have been created by the lack of maintenance of the crest drainage channel, adjacent to the slide area. It is a common engineering practice to try to associate rainfall amounts to the risk of slope failure. The role that bedrock plays in generating pore pressure is seldom considered. The numerical analyses presented in this paper pointed out at the complexity of flow modeling of rain infiltration through unsaturated residual soil slopes. It essentially revealed that flow mechanisms other than rain infiltration over the slope surface might play a major role on hydrological pattern of slopes.

Stability Analysis and Stability Chart for Unsaturated Residual Soil Slope

In tropical residual soils most hill slope failures are caused by rainfall. It is therefore important to consider dynamic hydrological conditions when attempting to analyze the stability of residual soil slopes. This paper describes a coupled hydrology/stability model that has been developed to overcome the limitations of the standard method of analysis used to investigate stability of tropical soil slopes. A computational hydrology – limit equilibrium stability analysis model is outlined and examples are provided of the model output capabilities in terms of design charts. Although nowadays most realistic problem should be analyzed by computer and stability charts are mainly used to analyze simple slopes, they can be useful for preliminary analysis and enable the designer to quickly assess the sensitivity of a problem to changes in different input parameters.

Collapsibility and Volume Change Behavior of Unsaturated Residual Soil

Residual soils occur in most countries of the world but the greater areas and depths are normally found in tropical humid areas. In these places, the soil forming processes are still very active and the weathering development is much faster than the erosive factor. Most residual soil exhibit high suctions for most of the year. The absence of positive pore water pressure except immediately after rain, makes conventional soil mechanics for saturated soil not so relevant. Ignorance or lack of understanding of the geotechnical behavior of soil in the partially or unsaturated state has caused a lot of damages to infrastructures, buildings and other structures. For instance, the collapsibility and volume change of partially saturated soils in connection with the drying or wetting causes a lot of damage in foundation, roads and other structures. It is also observed that many shallow slope failures involve a slumping (collapse) type of failure. As such, the development of extended soil mechanics, which embraces the soil in the unsaturated state or subjected to soil suction, is essential. This study examines the collapsibility and volume change behavior specifically of an unsaturated residual soil under various levels of applied matric suction (ua-uw) and net mean stress (σ-ua) in a predetermined stress path. The volume change of the soil is found to be sensitive to both the applied matric suction and net mean stress. The soil is found to exhibit a collapsibility behavior upon a reduction in applied matric suction at constant net mean stress.

Cohesion of unsaturated residual soils as a function of volumetric water content

Soil cohesion changes with the moisture state of a soil. This paper presents an empirical equation to predict the cohesive component in the shear strength of unsaturated residual soils as an exponential function of volumetric water content. The formulation originated from a multiple linear-regression analysis for data sets obtained from shear tests using undisturbed soils with varying moisture contents. The empirical equations can realistically predict the reduction in soil cohesion due to wetting (R 2=0.88, 0.93). The methodology described in this paper provides a convenient alternative to the quantitative estimation of unsaturated shear strength, especially in an engineering practice such as a slope stability analysis as no matrix suction data are required.

Modified Shear Box Test Apparatus for Measuring Shear Strength of Unsaturated Residual Soil

Residual soils occur in most countries of the world but the greater areas and depths are normally found in tropical humid areas. Most of these soils exhibit high suctions for most of the year. The shear strength parameters, c’ and ?’, of soil can be obtained using conventional shear strength tests. However the conventional shear strength test equipments would not be able to measure ?b value (change of shear strength to change in suction) without certain modification to them. This study describes the modifications that have been made to a standard shear box test apparatus to enable it to test soil samples in unsaturated conditions. The modifications include fabrication of an air pressure chamber, modifications of the shear box assembly inside the air pressure chamber, modification to the normal loading system, as well as additions of data acquisition devices to enhance the performance and simplify the usage of the modified shear box test apparatus.

An experimental investigation of the mechanical behaviour of an unsaturated gneiss residual soil

An experimental investigation of the mechanical behaviour of an unsaturated gneiss residual soil

A Study on Suction-rainfall Response of a Cut Slope in Unsaturated Residual Soil Using a Field Rain Simulator

This research describes the design and construction of an artificial rain simulator system and the results obtained with regards to suction-rainfall response of a cut slope in unsaturated residual soil of weathered sandstone of various weathering grades. From the field measurements, the shallow tensiometers are found to record an immediate initial increase in matric suction within the first 20 min of the simulated rain before a gradual drop in suction takes place. However no such initial spikes in suction readings were recorded by the deeper tensiometers. With constant rainfall for the next 130 min, a clear drop in soil suction was observed in all cases illustrating the effect of net infiltration process. Positive pore water pressure was even recorded by some of the deeper tensiometers. The suction recorded by the shallow tensiometers was always higher than those of the deeper tensiometers, indicating a reduction in suction with depth below the ground surface. In term of weathering grades, residual soil of weathered sandstone of weathering grade III has the highest infiltration rate indicating the more porous nature of this soil material. Like wise there is a steeper drop in suction during the rain at the said location.

Field and Laboratory Suction- Soil Moisture Relationship of Unsaturated Residual Soils

Soils located above the groundwater table such as residual soils are generally unsaturated and possess negative pore-water pressures. A soil-water (moisture) characteristic curve (SWCC) that relates the water content of a soil to matric suction is an important relationship for the unsaturated soil mechanics. The SWCC essentially shows the ability of an unsaturated soil to retain water under various matric suctions. It has a similar role as the consolidation curve of a saturated soil that relates void ratio or water content to effective stress. This study describes a study that has been carried in the field and in the laboratory to examine the suction-soil moisture relationship of unsaturated residual soils of granite and sedimentary rocks origin. The field measurement shows a decreasing trend of suction with depth for both soils. The suction-soil moisture relationship shows two distinct curves, a wetting (sorption) curve and a drying (desorption) curve. While from the laboratory study, it is observed that there is a significant decrease in the soil moisture with increasing suction in the lower suction ranges, until a de-saturation or air entry point for both soils. Beyond this point, the magnitude of the decrease in soil moisture for the equal increment of applied suction is less. The de-saturation point of a particular soil appears to be dependent on the amount of clay content. Higher amount of fines in the soil constitute a more compact particle arrangement and a smaller pore size. Soils with smaller pore sizes de-saturate at higher matric suction.

Shear strength of a compacted residual soil from consolidated drained and constant water content triaxial tests

Rainfall-induced landslides in unsaturated residual soils can occur slowly under drained conditions or rapidly under undrained conditions. Consolidated drained (CD) and constant water content (CW) tests have been performed to simulate the stress paths followed by soil elements in a slope that fails under drained and undrained conditions. The study was carried out to investigate the shear strength characteristics of soils associated with rainfall-induced slope failures. The soil tested was residual soil from the Jurong sedimentary formation and was reconstituted using static compaction. The test results indicate that the shear strength of the compacted specimens obtained from the CW tests agrees well with the shear strength obtained from the CD tests for the specimens with initial matric suctions less than their air-entry values. The shear strength results from the CD and CW triaxial tests start to differ when the matric suction exceeds the air-entry value of the soil. The CD and CW triaxial tests also indicate that the compacted specimens behave as a normally consolidated soil at matric suctions below the air-entry value of the soil and as an overconsolidated soil at matric suctions above the air-entry value of the soil. Results of the CW triaxial tests show that the relationship between the response of pore-water pressure and the total volume change of the specimen is more complicated than that found in the saturated undrained triaxial tests. In other words, the change in pore-water pressure during shearing is not directly related to the overall volume change of the specimen.Key words: unsaturated soil, compacted soil, residual soil, consolidated drained triaxial test, constant water content triaxial test.

Unsaturated Constitutive Surfaces from Pressuremeter Tests

A methodology to identify the collapse potential of unsaturated soils is proposed in this paper on the basis of pressuremeter test results associated with independent measurements of the in situ matric suction. A solution combining the expansion of a cylindrical cavity to a modified Cam clay critical state model has been introduced and accommodated to the framework of unsaturated soil behavior. This accounts for changes in soil properties induced by suction changes. Interpretation of pressuremeter tests performed under unsaturated and soaked conditions links the amount of collapse to strength and stiffness changes and provides assessment to the constitutive soil parameters that are necessary to define the yield envelopes of the soil. A comprehensive site investigation program comprising field and laboratory tests carried out in two residual soil sites is discussed in order to validate the proposed methodology. Values of shear strength, in situ stress, and yield pressure derived from both field and laboratory data are used as input parameters of a constitutive model adopted for describing the yield envelopes of these unsaturated residual soil sites.

Effectiveness of horizontal drains for slope stability

Horizontal drains have been commonly used in stabilising unsaturated residual soil slopes. This study examines the effectiveness of horizontal drains in stabilising residual soil slopes against rainfall-induced slope failures under a tropical climate. The study includes field instrumentation at two residual soil slopes complemented with a parametric study relating to drain position. Field monitoring results indicate that rainfall infiltration is limited to a certain depth below which infiltration becomes insignificant. This zone tends to be unsuitable for horizontal drains. Horizontal drains were found to be most effective when located at the base of a slope. The parametric study indicated conditions under which horizontal drains are effective or ineffective in improving the stability of a slope. It was also found that horizontal drains have little role in minimising infiltration in an unsaturated residual soil slope. Benefits of using horizontal drains can be obtained through the lowering of the water table.

Collapse behaviour of a residual soil

Residually derived red soils occur in Bangalore District of Karnataka State, India. The porous and unsaturated nature of the red soils makes them susceptible to collapse on wetting under load. The present study analyses the collapse behaviour of an unsaturated bonded (undisturbed) red soil from Bangalore referenced to tests on samples in an unbonded (remoulded) state. A filter paper method was used to determine the matric suction of the bonded and unbonded specimens, and mercury intrusion porosimetry (MIP) was used to determine their soil structure. Analysis of the experimental results shows that bonding plays an important role in the collapse behaviour of the unsaturated residual soil. The results of the study also provide insight into the volume change behaviour of unsaturated bonded soils on wetting within and beyond the yield locus.

Collapse behaviour of a residual soil

Residually derived red soils occur in Bangalore District of Karnataka State, India. The porous and unsaturated nature of the red soils makes them susceptible to collapse on wetting under load. The present study analyses the collapse behaviour of an unsaturated bonded (undisturbed) red soil from Bangalore referenced to tests on samples in an unbonded (remoulded) state. A filter paper method was used to determine the matric suction of the bonded and unbonded specimens, and mercury intrusion porosimetry (MIP) was used to determine their soil structure. Analysis of the experimental results shows that bonding plays an important role in the collapse behaviour of the unsaturated residual soil. The results of the study also provide insight into the volume change behaviour of unsaturated bonded soils on wetting within and beyond the yield locus.

Relationship between porosimetry measurement and soil—water characteristic curve for an unsaturated residual soil

Soil–water characteristic curve (SWCC) is an important tool for determining the engineering properties of unsaturated soil. This depends on the size and distribution of pore structures which control the permeability and amount of volume change. Mercury Intrusion Porosimetry tests can be used to determine the size, amount and distribution of pore spaces of the soil in a shorter time period compared to the pressure plate test. A soil-air characteristic curve (SACC) can be determined using the volumetric air content as measured in the Mercury Intrusion Porosimetry test. In this paper, SACC parameters are introduced. There appears to be a relationship between the soil-air characteristic curve and the soil-water characteristic curve of a soil. Relations between the SACC and the SWCC parameters are analysed in the light of unsaturated soil mechanics. The results indicate that the pore size distribution of a residual soil varies with depth due to differing degrees of weathering.

Instrumentation of an Unsaturated Residual Soil Slope

Abstract The stability of slopes has become a major concern in areas that experience frequent periods of heavy rainfall such as the tropical areas of Southeast Asia. Previous research has indicated that rainfall has a detrimental effect on the stability of residual soil slopes. The reason for this is that the additional shear strength that exists in unsaturated soils due to negative pore-water pressures is lost as a result of rainwater infiltration into the soil. To study the infiltration characteristics in slopes, a residual soil slope in Singapore was instrumented. The instruments were used to determine the changes in the pore-water pressure distribution throughout the slope in response to evaporation and infiltration. Tensiometers were used to measure negative pore-water pressures, piezometers were used to measure the groundwater level, and a rain gage was used to measure rainfall intensities on the slope. An automated data acquisition system was used to collect the data from the slope. This paper describes the instrumentation program and the procedures associated with the installation of the instruments into the slope. Typical measurement data collected from the slope are presented, and their interpretations are discussed.

A numerical investigation of the stability of unsaturated soil slopes subjected to transient seepage

Abstract Slope instability in unsaturated residual soils and loose fills has attracted increasing attention in recent years around the world such as Brazil, South Africa, Japan and in the Far East. Rain-induced failures are the most common ones. Rainfall leads to the development of perched water table, rising the main groundwater level and soil erosion (due to concentrated water flow), resulting in an increase in pore water pressure or a reduction in soil matric suction. This, in turn, results in a decrease in shear strength on the potential failure surface to a point where equilibrium can no longer be sustained in the slope and then failures occur. However, the present understanding of the influence of transient seepage in unsaturated soils, due to water infiltration under various boundary and ground conditions, and hydrogeological regimes on slope stability is still relatively poor compared with other elements of geomechanics. To investigate the influence of various rainfall events and initial ground conditions on transient seepage and hence slope stability, a parametric study has been conducted using the finite element method. A typical steep unsaturated cut slope in Hong Kong has been adopted for the parametric study. Variables considered in the parametric study include isotropic and anisotropic soil permeability, initial water table at upslope boundary, rainfall intensity and duration. Pore water pressures or suctions predicted during the transient seepage analyses are then used as input ground water conditions for subsequent limit equilibrium analyses of the stability of the slope. Factor of safety is calculated using Bishop’s simplified method, with modified Mohr–Coulomb failure criterion to allow for shear strength variation due to the presence of matric suction. Infiltration due to rain water causes a reduction of matrix suction, but an increase in moisture content and soil permeability in unsaturated soils. Perched water table is developed above the main water table. The factor of safety is not only governed by the intensity of rainfall, initial ground water table and the anisotropic permeability ratio, but it also depends on antecedent rainfall duration. A critical rainfall duration can be identified, at which the factor of safety is the lowest.