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Collapsible Soils

DefinitionsCollapsible soils are also known as metastable soils. They are unsaturated soils that undergo a large volume change upon saturation.

The sudden and usually large volume change could cause considerable structural damage. The volume change may or may not occur due to an additional load.

Behavior under load:

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• ab: is determined from the consolidation test on a specimen at its in-situ moisture content.

• e1: is the equilibrium voids ratio before saturation.

• e2: is the equilibrium voids ratio after saturation.

• cd: is the curve that ensues from additional loading after saturation.

Types of collapsible soils1) Aeolian soils

They are the most common examples of this type of collapsible soil.

Typical aeolian soils are wind-deposited sands and or silts, such as loess, aeolic beaches, and volcanic dust deposits.

They are characterized by showing in-situ high void ratios and low unit weights.

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They are typically granular or only slightly cohesive.

The Dust Bowl winds of the late 1920’s in the USA stripped large areas of

Oklahoma and Texas of their top soils and carried them to

Missouri, Kentucky and Tennessee.

Other areas around the world that have large aeolian deposits are:

South Africa, Rhodesia, Europe and China.

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2) Residual soilsResidual soils, which are a product of the in-situ

weathering of local parent rocks may also have a potential for collapse.

The weathering process produces soils with a large range of particle size distribution.

The soluble and colloidal materials are leached out by weathering, resulting in large void ratios and thus unstable structures.

Identification of the critical parameters.

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The degree of volume change is expressed as the collapse potential Cp,

where a sample is flooded during a 24-hour period under a pressure pw = 200 kN/m2 (30 psi).

• Values of Cp greater than 5% are indications of trouble for a foundation.

• This test provides void ratios before and after flooding and the collapse potential Cp can be calculated with equation:

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where,eo = the natural (in-situ) voids ratio of the soil, and

∆ε = the vertical strain

Foundation Design

The choice of a foundation system depends on whether the foundation will be exposed to wetting or not.

Soils susceptible to wetting:If the expected depth of wetting is about 150 to 200 cm below

the ground surface, then,

The soil may be moistened and recompacted with heavy rollers, or

Compacted with heavy tamping such as deep dynamic compaction, wherein a heavy weight repeatedly dropped upon the ground.

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Alternatively, if conditions are favorable, the foundation trenches could be flooded with solutions of :

•sodium silicate and

•calcium chloride

to stabilize the soil chemically.

The soil will behave like a soft sandstone and resist collapse upon saturation.

This method is successful only if the solutions can penetrate to the desired depth.

Another method used for dry collapsible soils and for wet collapsible soils that are likely to

compress under the added weight of the structure to be built is to:

1) Inject carbon dioxide into the treated stratum to remove any water present.

2) Inject sodium silicate grout.

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When a soil stratum is susceptible to wetting to a depth of about 10.0 m, there are several techniques that may be used to cause the collapse of the soil before foundations are constructed:

a) Vibroflotation: used successfully in free-draining soils (granular).

b) Ponding, flooding an area by building low dikes around the site.

However, some additional settlement of the soil may occur after the foundations are built.

If precollapsing the soil is not practical, the foundations must be extended below the zone of possible wetting, which may require drilled shafts or other type of deep piles.

However, their design must take into consideration the effect of negative skin friction resulting from the collapse of the soil structure around them, and the associated settlement of the critical soil zone when wetted.

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Sensitive SoilsSensitivity (St)

A measure of the change in strength of clays upon disturbance:

For ordinary clays St = 1 to 4,

sensitive clays 4 to 8,

‘quick’ clays 16 - 100.

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The sensitivity ratio St of most clays range from 1 to 8.

However, highly flocculent marine clay deposits may show sensitivity ratios ranging from about 10 to 80!

There are some clays that turn into viscous fluids uponremolding.

These are found mostly in the previously glaciated areas of North America and Scandinavia.

These clays are referred to a “quick” clays.

“Quick” clays

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Table 8: Undrained strength and sensitivity description for clays.

> 64 even 100Extra quick

32 – 64Very quick

16 –32Medium quick

8 – 16Slightly quick

4 – 8Very sensitive

2 – 4Medium sensitive

1 - 2Slightly sensitive

< 1Non-sensitive

Sensitivity Description

Clay Sensitivity

<20Very soft

20 – 40Soft

40 – 50Soft to firm

50 - 75Firm

75 – 100Firm to stiff

100 – 150Stiff

> 150Very stiff or hard

Undrained strength, cu(kPa)

Consistency

Undrained strength classification for clays BS 8004:1986