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Dr. Asmaa Moddather – Soil Improvement – Spring 2016

Dr. Asmaa Moddather

Soil Mechanics and Foundations

Faculty of Engineering – Cairo University

Spring 2016

Dr. Asmaa Moddather – Soil Improvement – Spring 2016Dr. Asmaa Moddather – Problematic Soil – Spring 2016

Introduction

� Many collapsible soils are mudflows or windblown

deposits often found in arid or semiarid climates such as

deserts.

� A collapsible soil at natural water content may support a

given foundation load with negligible settlement, but when

water is added to this soil the volume can decrease

significantly and cause substantial settlement of the

foundation, even at relatively low applied stress or at the

overburden pressure.

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Introduction

� The amount of settlement depends on the initial void ratio,

stress history of the soil, thickness of the collapsible soil

layer, and magnitude of the applied foundation pressure.

� Collapsible soils exposed to perimeter watering of

vegetation around structures or leaking utility lines are

most likely to settle. Collapse may be initiated beneath the

ground surface and propagate toward the surface leading to

sudden and nonuniform settlement of overlying facilities.


Introduction� Soil collapse forms a major hazard in many parts of the

world.

� Human activities continue to increase in regions underlainby collapsible soils, so that the hazards posed, and theeconomic impacts are increasing.

� In Egypt, recent extensions of urban communities towardsthe desert have exposed the Egyptian engineer to relativelynew geotechnical challenges, among which is thecollapsible soils.

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Types and Formations� Collapsible soils: are metastable material, traditionally

defined as an unsaturated soil that experiences a radicalrearrangement of particles and significant reduction ofvolume upon wetting with or without additional loading.

� A wide range of soils fall into this category of materialincluding:

� Windblown deposits

� Water-laid deposits

� Residual soils

� Highly Saline Soils (Sabkha)

� Man-placed fills made of sands.


Types and Formations1. Windblown deposits

� Consist of materials transported by wind which form dunesand loess.

� The sweep of wind across large sand covered areas, whetheroutwash plains, beaches, flood plains of broad rivers, or evendesert plains, moves the sand and silt sized particles butleaves the gravel behind.

� The sand grains are rolled over each other or bounced shortdistances into the air and piled up to form dunes, whereas thesilt-sized grains are blown away.

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Types and Formations1. Windblown deposits

� The process of selection by wind sorts the sand intoassemblages of very uniform grain size.

� Sand becomes finer with increasing distance from the source.

� High porosity is encountered 50% – 60%.

� Low unit weight.


Types and Formations2. Water-laid deposits

� Alluvial depositions produce high void ratio and low densitydeposits, which are relatively strong in their natural state.

� Cementation consists of dried clay binding the coarserparticles together and chemical precipitates, which may havebeen added during deposition.

� These deposits consist primarily of loose water depositedsediments which form braided streams, alluvial fans, andflood plain deposits.

� The particle size of these deposits depends on the velocityand rate of flow, and the distance from the source.

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Types and Formations3. Residual Soils

� Residual soils are the product of weathering, i.e., thedisintegration and mechanical alteration of the componentsof parent rocks.

� The particles of residual materials may vary in size from largefragments to gravel, sand, silt, colloids, and in some cases,organic matter.

� The collapsible grain structure has developed as a result ofleaching out of soluble and colloidal material. This leachingout of the soluble and fine material results in a high void ratioand unstable structure.


Types and Formations4. Saline Soils (Sabkha)

� Found in salt encrusted flat areas that are the result ofevaporation and sedimentary environment that dominatedthe Arabian Peninsula and parts of the Middle East.

� They are highly cemented by excessive salts present in boththe sediments and their shallow groundwater. Sabkhasediments are common in the coastal and inland areas.

� In general terms, Sabkha is loose to moderately densesilt/sand material of varying size, composition, texture, andorigin.

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Types and Formations4. Saline Soils (Sabkha)

� Mud and clays are often interbedded with the sands and silts,as seams or pockets, or may be found down below towards thebottom.

� At most times, and in open terrain, the hard surface ofSabkha flats is sufficiently strong to take heavy traffic loads.

� However, if the surface becomes wet due to occasionalrainfall, flash floods, or storm tides, the soluble salts whichprovide the cementation in the crust, dissolve.


Types and Formations5. Man-Placed Fill of Sand

� Most of the literature on collapsing soils is concerned withnaturally occurring collapsing soils that are essentially foundin arid or semi-arid climates, and cover a significant area ofthe earth’s surface.

� However, there is a wide range of artificially placed soils, suchas compacted soils that may also exhibit collapse behaviorupon wetting.

� Evidences exist on cases where sand layers compacted dry ofoptimum moisture content experienced collapse uponwetting.

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Collapsible Soils in Egypt

� Geological references on Egypt do not seem to point out

clearly to the presence of classical collapsible deposits such

as loess.

� However, there are geological information on some types of

deposits which, under certain situations, exhibit collapse

behavior. Among these are Aeolian deposits such as sand

dunes, fluvial deposits such as sand sheets, and high

salinity deposits such as sabkha.



1. Sand Dunes

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Sand Dunes



Sand Dunes

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Qena and Aswan Asyout and Suhag

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Giza Menya and Beni-Suef



Delta

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� Backfilled Quarries

Deep pits resulting from previous mining activities are some

times backfilled with random construction debris.

Buildings constructed in these areas can suffer from severe

structural damages when foundation material is wetted by

waters from irrigation systems and leakages from water

supply and wastewater networks.

� Al-Darrassah, and Nasr City.

� The thickness of fill layers in these areas exceeds 20 m.



� Compacted earth fills

Earth fills that are formed by mere dumping of random or

selected material can experience collapse under load when

wetted. Backfilling between cast footings is sometimes not

given adequate attention in compaction.

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� A large factory in Sadat City

� The design depth of footings ranged between 4 and 10 m

below finished floor level.

� Although clean sand was used as backfilling material, the

process was not accompanied with nearly any compaction.

Layers 2 to 3 m thick had been placed in one go, hoping

that the movement of trucks on the top surface of dumped

layers was sufficient to compact them.




� The factory was completed and the production process

went on for a few weeks with the normal use of water for

cooling.

� The slab-on grade experienced severe deformations, and

large cavities were observed under the slab at many

locations due to collapse of un-compacted fill under its

own weight.

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Assessment of Collapsible Soils� There are four steps that must be followed prior to arriving

at a final foundation design on a collapsing soil:

1. Identification - determine whether potentially collapsingsoils exist.

2. Classification- if collapsing soils exist, what degree ofattention needs to be paid to them.

3. Quantification - if soils are sufficiently prone to volumechange, a rational assessment of numerical values ofprobable vertical movement should be made.

4. Evaluation of design alternatives.


Why Soil Collapse?� Soils subject to collapse have a honeycombed structure of

bulky shaped particles or grains held in place by a bondingmaterial or force.

� Common bonding agents include soluble compounds suchas calcareous or ferrous cementation that can beweakened or partly dissolved by water.

� Removal of the supporting material or force occurs whenwater is added enabling the soil grains to slide or shear andmove into voids.

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Why Soil Collapse?

Different Inter-Particle Bonds in Collapsible Soils


Why Soil Collapse?

A basic Unit of Loess Structure

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Why Soil Collapse?

A basic Unit of Loess Structure


Why Soil Collapse?� Collapse Potential (CP): the relative magnitude of soil

collapse when water is added at stress level of 200 kN/m2.

� This term is used as a relative indicator of collapsepotential, beneficial for identification but not forestimating potential settlements for specific in situconditions unless the point of interest in the field happensto be stressed at about 200.0 kPa.

CP = ∆ec/(1+eo) (%)

eo = void ratio at natural water content

∆ec = change in void ratio due to wetting

Severity of CollapseCP (%)

NegligibleModerate trouble

TroubleSevere trouble

Very severe trouble

0 – 11 – 5

5 – 1010 – 20

> 20

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Identification� Field Identification.

� Collapsible criteria based on Simple Soil Properties.

� Collapsible criteria based on Laboratory Tests.


Field Identification� Field Identification

� Dry or slightly moist soil

� Loose or open fabric

� Coatings and clay bridges

� Identification of the origin of the soil

� Several simple field tests have been proposed to identifythe collapse phenomenon.

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Field Identification� The sausage test (Jennings and Knight ,1975)

� Carve two cylindrical samples of undisturbed soil as nearlyas possible to the same diameter and length.

� Wet and knead one sample and remold it into a cylinder ofthe original diameter.

� An obvious decrease in length when compared with theundisturbed twin sample will confirm a collapsible grainstructure.

� A similar reduction in volume may be observed by backfillingin a pit or trial hole. If the soil a collapsible grain structure, itwill fail to fill the pit completely.


Field Identification� Dispersion test (Bnites, 1968)

� Few grams of soil is dropped at its natural water content intoa glass of water.

� Time required for the soil to disperse completely isrecorded.

� Collapsible soils typically have a dispersion time of 20 sec to30 sec.

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Collapsible Criteria based on

Simple Soil Properties

� Typical collapsible soils are:� Lightly colored.

� Low in plasticity with liquid limits below 45, plasticity indicesbelow 25.

� Relatively low dry densities between 10.0 and 16.5 kN/m3.

� Porosity of 40% to 60%.

� Most criteria for determining the susceptibility of collapseare based on relationships between the void ratio, watercontent, and dry density.




� Denisov (1953)

� Coefficient of subsidence (K)

� K = eL/eo

eL = void ratio at liquid limit

eo = natural void ratio

K = 0.50 – 0.75 : Highly collapsible soil

K = 0.75 – 1.00 : Collapsible is likely

K > 1.0 : Non-collapsible loam

K = 1.5 – 2.0 : Non-collapsible soil

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Simple Soil Properties� Clevengar (1958):

� γd ≤ 12.6 kN/m3 : soil is likely to be highly collapsible

� 12.6 ≤ γd ≤ 14.0 kN/m3 : soil may be collapsible

� γd ≥ 14.0 kN/m3 : soil is not likely to be collapsible

� Elmamlouk (1985)

� γd < 90% γd max (Standard Proctor Test)

: soil is likely to be highly collapsible




� Gibbs and Bara (1962)

� A soil is susceptible to collapse if:

γd (kN/m3) ≤ 25.5/(1 + 0.026 wL)

� Priklonski (1952)

� KD = (w – PL)/ PI

KD < 0 : Highly collapsible soil

KD > 0.5 : Non-collapsible

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� Feda (1966)

� Proposed a critical void ratio (ec) as follows:

ec = 0.85 eL + 0.15 eP

� A soil is susceptible to collapse if eo > ec



Simple Soil Properties� Handy (1973): for Iowa loess by Clay content

: < 16% : high probability of collapse

: 16 – 24% : probability of collapse

: 24 – 32% : less than 50% probability of collapse

: > 32% : usually safe from collapse

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Simple Soil Properties� Czechslovak Standard

Collapse may occur when:

� Silt > 60%

� Clay < 15%

� S < 60% and LL < 32%

� N > 40%

� W < 13%



Simple Soil Properties� Jenning and Knight (1975):

� Introduced the concept of critical degree of saturation (Sc)above which collapse would not occur, suggested values:

� Fine gravel : 6 – 10%

� Fine silty sand : 50 – 60%

� Clayey silt : 90 – 95%

collapsible soil_part i

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