DESICCATED SOILS(ADVANCED SOIL MECHANICS)

CONTENTS

• What does desiccated soil mean?• Properties of desiccated soils• Desiccation theory for soft cohesive soils• Desiccation Characteristics of soft cohesive soils• Desiccation cracks• Swelling of desiccated clays• Identification of desiccated clays• Case history

Desiccated soils

• Desiccation means To make or become free of moisture,

dry (out), dehydrate.

• So desiccated soil means the soil that is free from all moisture

DIFFERENT PROPERTIES

• Swelling Behavior

• Volume Change Behavior

• Shear Strength

• Cohesion Values

1. SWELLING BEHAVIOUR• Experimental results showed that the volumetric

swell potentials of the desiccated soil specimens were independent of their compaction water contents, but were strongly influenced by their initial void ratios.

• The vertical swell potentials of the desiccated specimens were independent of their initial water contents and initial void ratios.

• Specimens in the shrunken state have a history of four cycles of wetting and drying are referred to as desiccated soil specimens.

CONT……..

• The results of this study indicated the existence of an optimum void ratio for the examined compacted specimens. Specimens compacted to this void ratio exhibit similar volumetric swell potentials in the as-compacted state and after cycles of wetting and drying.

2. VOLUME CHANGE BEHAVIOUR

• Experimental work indicated that repeated desiccation results in interparticle bonding which is also chemical in nature.

• Such bonding results in increased resistance to compression and greater permeability.

• Depending on the degree of bonding, an expansive soil may display either reduced expansiveness or even behave as a non-expansive soil in the undisturbed condition.

3. SHEAR STRENGTH

• The behavior of both saturated and unsaturated soils is affected by the pore-water pressures in the soils.

• Unsaturated soils have the negative pore-water pressures (i.e., metric suctions). Suctions have a major effect on shear strength and volume change for unsaturated soils.

• The very high suction is produced by evaporation near ground surface.

4. COHESION VALUES

• It is now common knowledge that if soil becomes desiccated it demonstrates a significant change in cohesion.

• Preliminary test data from the soil tests undertaken on many projects indicate that the cohesive strengths of a normal saturated clay can increase by as much as 100kN/m² as the soil becomes desiccated.

FIGURES SHOWING DIFFERENT DESICCATED SOILS

• a. Desiccated plants macro remains

b. Nut shells

c. Impressions of cave floor

d. Charred

e. Possible whole nut (top) & details of its X-sec (bottom)

DESICCATION THEORY FOR SOFT COHESIVE SOILS

• The theory provides, for a soft layer undergoing consolidation and desiccation, information about the amount and time rate of settlement, the distribution of pore water pressure and void ratio at various times, and depth and volume of cracks with time.

• One of the main features of the theory is its ability to predict the thickness and strength (from the void ratio) of the crust that forms on the soil surface as a function of the groundwater level and the evaporation rate. The slower the evaporation rate is the deeper the desiccation effect is felt.

CONT……………..

• The theory has four main components: 1.Consolidation under one-dimensional

compression. 2.Desiccation under one-dimensional shrinkage. 3.Propagation of vertical cracks. 4.Desiccation under three-dimensional shrinkage.

Usman

DESICCATION CHARACTRERISTICS OF SOFT COHESIVE SOILS

• The homogeneous states of effective stress across the specimen, obtained when the applied flow rate is low, are used to determine the desiccation compressibility relations for one- and three-dimensional shrinkage

• The heterogeneous states of effective stress across the specimen, obtained when the applied flow rate is high, are accounted for in the analysis of the test data and used to evaluate the desiccation permeability relation

DESICCATION CRACKS

• Desiccation causes cracks in the soil structure

• Some of those are shown NEXT

CRACKS IN BOULDERS

• Splendid desiccation cracks produced by drying out of a layer of mud. The cracks were later filled

DESICCATION CRACKS IN SEDIMENT DEPOSITE

• These cracks are the result of drying of fine grained sediment due to sub aerial exposure

• Finding mud cracks in a bedrock outcrop tells us that the sediments forming the rocks were very close to sea level after their deposition.

SWELLING OF DESICCATED CLAYS

• Desiccated clays are common in the areas having near surface clay deposits and periods of droughts.

• Structures constructed on the top of desiccated soils can be severely damaged by the expansive soil heave.

CONT…………..

• Chen (1988) states “very dry clays with moisture content less than 15% usually indicate danger. Such clays can absorb moisture as high as 35% that results in expansion and ultimately damages to the structure”

• In the figures shown next cracks in the desiccated clays are shown

• CRACKS IN DESICCATED SOILS

IDENTIFICATION OF DESICCATED CLAYS

• Can be identified by the presence of excess of cracks

• These have distinct water content profile where the water content increases with the depth.

• For example in the next figure water content profile w.r.t depth for two different soils is shown.

Failure envelope of a desiccated, unsaturated

silty soil• Direct shear and unconfined compression

test is performed.• Modified direct shear apparatus is used• Air pressure chamber is applied to

completely enclose the shear box to evaluate the ambient air pressure.

TESTS FOR SOIL WATER CHARACTERISTIC CURVES

• Three tests are performed to plot soil characteristic curve

1. A compacted, unsaturated soil was desiccated using a relative humidity equilibrium technique until its water content was below residual water content.

2. Glass desiccator containing saturated salt solutions (i.e., vapor equilibrium technique)

3. Relative humidity technique in relative humidity control chamber

RESULTS

• An unsaturated silty soil in the residual stage of unsaturation has very little water between soil particles.

• The shear strength of a desiccated, unsaturated silty soil shows an essentially horizontal envelope at high soil suctions.

• The failure envelope is composed of two segments, a curvilinear segment followed by a linear segment.

• The failure envelope has a slope of 43 degrees (i.e., equal to effective friction angle in saturation, ')

• The slope of the failure envelope appears to decrease when soil start to desiccate.

CASE HISTORY

• CASE HISTORY IN USA• Desiccation of the clay layer in composite lining systems

was first observed by Corser et al. (1991). Basnett and Bruner (1992) later reported this phenomenon seen during expansion of a landfill where the existing liner, installed 3 years earlier, comprised a 300 mm clay layer, installed to CQA procedures, overlain by an HDPE geomembrane. The bottom of the cell had been covered with a protection layer but the side slopes were left uncovered, due to their gradient (1 in 3), and consequently were exposed to direct sunlight.

• When the HDPE liner was pulled back, to allow joining of the new extension, the clay on the side slopes was found to be highly desiccated, with cracks averaging 12–25 mm wide extending the full depth of the liner. This desiccation was found at least 30–50 m back from the edge of the liner and appeared to be more severe further in to the cell. The condition of the clay in the base of the cell was good.

REFERENCES• herve.peron@epfl.ch• http://lms.epfl.ch/• desiccation of soil. Third Asian Conference

on Unsaturated Soils. 21st-23rd April 2007 • liangbo.hu@duke.edu,• hueckel@duke.edu, • Research Report 66: U.S. Army Snow

Ice and• Permafrost Research Establishment,

Wilmette,