Muawia Dafalla

Effects of Clay and Moisture Content on Direct Shear Tests for Clay-Sand Mixtures

The direct shear test using shear box is commonly recommended by practicing geotechnical engineers to obtain the cohesion and angle of internal friction for granular soils. The clay liners involve sand as a main constituent with added clay of variable proportions. This research aims at investigating the reliability of using the direct shear test for different clay contents and different moisture contents using an adequate shearing strain. These factors were found to affect the bilinear trends of shear force versus horizontal displacement profile as well as vertical displacement versus horizontal displacement curves. The cohesion of the mixture was found to increase consistently with the increase of clay content. Increase in moisture content was found to cause a drop in both cohesion and angle of internal friction. These changes are not independent of the density state of clay-sand mixtures. Standard compaction properties for a range of clay-sand mixtures were investigated. This work provides the general trends expected in direct shear tests for clay-sand mixtures of variable clay and moisture contents.

Evaluation of Chemical Stabilization of a Highly Expansive Clayey Soil

Lime and cement are widely used in stabilization and improvement of the expansive subsoils that support pavement infrastructure. In this research, an attempt is made to stabilize a highly expansive soil rich with smectite minerals with an aim to understand the mechanisms that result in soil property improvements. Influences of both the type of stabilizer and their dosages were studied for modifying expansive clay from the north-central region of Saudi Arabia. Al-Qatif soil is known for its highly expansive nature, and it is known to induce major damage to low-volume roads built on untreated to unbound sandy bases. This research was performed by first characterizing and classifying Al-Qatif clayey soil by means of routine index properties, X-ray diffraction studies, and swell and shrinkage tests. Next, lime and cement were used to stabilize control clayey soils, and the treated soils were then subjected to the same soil tests. Effects of lime and cement treatments on Atterberg limits, swell, and linear shrinkage strain properties are addressed. Major conclusions and findings from experimental investigations are presented.

Characterization of problematic expansive soils from mineralogical and swell characterization studies

Expansive soils are those that expand when hydrated, and shrink when they were drying. These soil movements cause extensive damage to buildings, roads, pipelines, and other structures and the repair costs are estimated to be billion dollars annually. Expansive soils cover across most of the continents in the world, including several regions in the Saudi Arabia. For example, the regions including Tabuk, Al-Hafuf, Al-Qatif and Al-Ghatt have expansive soils and several infrastructure problems were reported in these regions. A research study was conducted at the King Saud University to characterize the severity of problem natures of these soils. Soil samples from these regions were collected and cataloged and these soils were subjected to a multitude of geotechnical related swell and shrink characterization studies. Three dimensional swell and linear shrinkage bar tests were conducted on these soil samples at targeted moisture contents to measure their volume change potentials. Severity nature of these expansive soils is then characterized based on an established swell/shrinkage ranking method practiced in the expansive soils literature. This paper presents a summary of these results and ranking assessments on the four expansive soils.

LIME LEACHABILITY STUDIES ON STABILIZED EXPANSIVE SEMI-ARID SOIL

The present study is aimed at examining the role of lime and the nature of leaching solution on the lime leaching characteristics of an expansive soil originating from the Kingdom of Saudi Arabia. Lime leachability studies were carried out in specially fabricated molds subjected to continuous leaching conditions. The specimens were compacted directly into specially fabricated perspex molds and cured for 7, 14 and 28 days under constant humidity conditions. In order to study the rate at which the calcium ions leach out from the lime treated clay matrix, water (leaching solution) was allowed to flow through the compacted specimen continuously for seven days and the resultant calcium concentration in the leachate was determined. The effects of various parameters like lime content, curing period and the pH of leaching solution on the lime leachability values has been studied. It is observed that, at a given lime content, the calcium concentration in the leachate reduces with curing period. Relatively higher amounts of lime leached under acidic conditions and the flow period does not affect the lime leaching patterns.

Evaluation of Soil Water Characteristic Curves of Sand-Clay Mixtures

This paper presents an experimental investigation performed to evaluate the soil water characteristic curves of sand/Al-Qatif clay mixtures. Al-Qatif clay is natural expansive clay widely spread in the eastern province of Saudi Arabia. Detailed mineralogical and physical tests were performed on Al-Qatif clay for characterization. Mixtures of sand and Al-Qatif clay were prepared with sand being the base material and with different clay contents (0%, 5%, 10% and 15%). Test results indicate that the shape of SWCC curves changes from the unimodel to the bimodal form of soil water characteristic curve with the increase in clay content indicative of changes in sand/clay mixture micro-and macro-pores.

Design Guide for Rigid Foundation Systems on Expansive Soils

AbstractExpansive soils pose a major nightmare for practicing geotechnical engineers in the semiarid and arid zones of the world. The introduction of rigid substructure is a costly approach but can be made practical and financially tolerable for soils where expansion is characterized within medium to low problematic classes. This paper outlines justifications and design concepts for a rigid substructure foundation of a two-story concrete frame structure. The project was constructed in a district in Saudi Arabia where many buildings experienced serious damage because of expansive soil problems. Upheaval forces likely to act against the proposed structure were determined and used in a finite-element analysis to design an appropriate rigid substructure. Plots of moments and shear forces were determined, and critical sections were pointed out. The main reason for choosing the rigid alternative is to reduce the differential settlement, which is likely to be exaggerated in flexible designs. The settlement measu...

Use of Rigid Foundation System on Expansive Soils

Expansive soils pose a major maintenance nightmare for geotechnical practicing engineers in the semi-arid and arid zones of the world. The introduction of rigid substructure is a costly approach but can be made practical and financially tolerable for soils where expansion is characterized within medium to low problematic classes. This paper outlines justifications and design concepts for a rigid substructure foundation of a two story concrete frame structure. The project was constructed in a district in Saudi Arabia where many buildings experienced serious damage due to expansive soil problems. Upheaval forces likely to act against the proposed structure were determined and used in the finite element analysis and design of an appropriate rigid substructure design. Plots of moments and forces were determined and critical sections were pointed out. Methods and stages of construction were monitored and a baseline vertical movement was established for future verifications. The advantages and limitations of this system are also discussed.

Effects of confining pressure and effective stress on hydraulic conductivity of sand-clay mixtures

The hydraulic conductivity is highly influenced by the state of confinement and effective pressure applied. Hydraulic conductivity evaluated using indirect (i.e., oedometer tests) or direct techniques such as constant or falling head methods induces lateral confinement. The most reliable direct measurement tests are those considering triaxial or flexible wall setup such that field conditions are simulated. The ASTM D5084 and BS1337-6: 1990 are appropriate approaches to give reliable hydraulic conductivity values. Both tests require considerable time due to various stages of saturation, consolidation, and permeation. This study provides a comparison between indirect hydraulic conductivity tests and standard flexible wall tests in order to cut down the time needed to perform the test for different sand-Alqatif clay mixtures. Sand-clay mixtures used as liners for waste containment or water barriers are very sensitive to the hydraulic conductivity and need to be designed for different values of effective pressures. This research presents prediction equations derived from a series of direct and indirect tests carried out for sand-Alqatif clay mixtures. The results indicated that oedometer tests underestimated the hydraulic conductivity by 2 to 3 orders of magnitude at different effective pressures. The hydraulic conductivity was found to vary with effective pressures indicating that near surface material yields higher hydraulic conductivity than deeply seated similar soils.

Swelling Characteristics of Saudi Tayma Shale and Consequential Impact on Light Structures

Expansive soils in Saudi Arabia have received wide attention in recent decades, following the rapid urbanization of rural and agricultural parts of the country. Tayma expansive shale inflicted serious damages to light structures, roads and boundary walls. This research is aimed at studying the engineering characteristics of the shale at this area including the basic geotechnical parameters as well as swelling behavior under oedometric loading conditions. The engineering properties were determined and compared to the local and international characterization charts. Mineralogy study using x-ray diffraction was conducted to investigate clay minerals present. The swell percent and the swelling pressure of the shale were investigated for samples prepared at different initial moisture contents and various dry densities. A close-up view of damages caused by the expansive shale in the town is highlighted and measures to reduce the risk of potential swelling distress for future constructions are presented.

The Influence of Placement Conditions on the Swelling of Variable Clays

The swelling of clay when it is subjected to moisture increase is a complicated process found to be influenced by several factors. The clay’s level of response is highly dependent on its mineralogical composition and structure. Practicing geotechnical engineers use the placement state and general index properties to forecast the swelling behavior of the soils. The purpose of this study was to investigate the influence of placement conditions on various clays and to demonstrate that the swelling of a particular clay type should not be predicted using information and trends obtained for other clays with different origins. Clay from Saudi Arabia was used to investigate the influence of initial moisture content and initial dry density on swelling. The prediction models created by other researchers were compared to the measured values in this study. The swelling behavior of both initially dry and wet prepared bentonite was examined, and the rate at which swelling developed in the bentonite clay was studied. The role of clay content in the volume change of sand–bentonite mixtures was also investigated. It was concluded that the prediction of clay-swelling parameters should not be based on the properties of other clays with different origins and mineralogical compositions. The trends published in the literature should be taken as a general guide only, and the influence of moisture content and dry density on swelling should be verified for individual sites. Because clay content significantly influences the overall volume change, it should be carefully assessed in each case.