COLLAPSIBLE SOILS

Collapsible soils

These are unsaturated soils that can withstand

relatively large imposed stresses with small

settlement at low in situ moisture content but will

exhibit a decrease in volume and associated

settlement (which could be of large magnitude) with

no increase in applied stress if wetting occurs

Soil grains

Water bridge

Water bridgeWater bridge

Occurrence in the world

Extensive deposits occur world wide

e.g. sensitive clays of Scandinavia and eastern Canada

loess formations of China, Russia and eastern

Berea Red Sands of the southern African east coast.

Residual soils such as the Highveld granites of South Africa

Kalahari sands

The black cotton soils of Northeastern Nigeria, Cameroon

and Chad

etc

Origin

Granite Residual granite

quartz, mica flakes, kaolinite

It is characterized by quartz grains embedded in some silt materials together with fine sand and colloidal matter

Rainfall will leach out the soluble colloidal material leaving a honey comb structure

In dry environment coupled with the little salts in place the joints where the quartz grains meet some moisture will be trapped so the honey comb structure can withstand some considerable force when dry but when saturated collapses and creates a host of collapsible soil problems

Weathered

Characteristics

Have an open structure

Have a high void ratio

Have a low dry density

Have a high porosity

Are geologically young or recently altered deposit

Have a high sensitivity

Have a low inter particle bond strength

The behavior of collapse is illustrated below

Time

settlement

Normal settlement with soil partially saturated

Additional settlement with no change in applied pressure but increase in moisture content

According to Dudley (1970), and Harden et al., (1973), four factors are needed to produce collapse in a soil structure:

1. An open, partially unstable, unsaturated fabric

2. A high enough net total stress that will cause the structure to be metastable

3. A bonding or cementing agent that stabilizes the soil in the unsaturated condition

4. The addition of water to the soil which causes the bonding or cementing agent to be

reduced, and the inter-aggregate or inter-granular contacts to fail in shear, resulting in

a reduction in total volume of the soil mass.

Collapsible behavior of compacted and cohesive soils depends on the percentage of fines, the initial water content, the initial dry density and the energy and the process used in compaction.

Why do we have problems with collapsible soils?

Either one or all of the following problems may allow collapse to be evident in construction

1. Construction was carried out before collapse phenomenon was identified

2. No geotechnical assessment was carried out

3. In case the geotechnical assessment was done, it did not evaluate correctly or idenity

potential collapsible soils within the profile

4. Recommendations given by the Geotechnical engineer was ignored by the parties invoved

in the design

Evaluation and prediction

Field Identification

Observational method

Look for cracking and building distortion

Soil profiling

Recognize a loose or open fabric

Use a hand lens to look for colloidal coatings and clay bridges

Sausage test- Carve out two cylindrical sample of undisturbed material to nearly as

possible to same diameter and height. Wet and knead one sample and remould it to the

same dimensions you had. A decrease in height when compared with the undisturbed

material is indicative of collapsible material

Laboratory testing

Particle size distributionAtterberg limitsDry density

Consolidometer tests

1. Double oedometer Tests

i. Plot the e-log p graphs for both specimens.

ii. Evaluate the in situ effective pressure, Po. Draw a vertical line corresponding to

the pressure Po.

iii. From the e-log p curve of the soaked specimen, determine the pre consolidation

pressure, Pc.

iv. Determine eo, corresponding to Po from the e-log p curve of the soaked specimen.

v. Through point (Po, eo) draw a curve that is comparable to the e-log p curve

obtained from the specimen tested at natural moisture content.

vi. Determine the incremental pressure, ∆p, on the soil caused by the construction of

the foundation. Draw a vertical line corresponding to the pressure of Po + ∆p in

the e-log p curve.

vii. Now, determine es and ec.

viii. The settlement of soil without change in the natural moisture content is

S1 = ∆es/ (1 + eo) x H

Also, the settlement caused by the collapse of the soil structure isS1 = ∆ ec/ (1 + eo) x H

where H = the thickness of soil vulnerable to collapse

The single consolidometer test.

This is a simpler test to perform since

the interpretations of double oedometer test is cumbersome

Only one undisturbed sample is tested

The sample in the consolidometer is loaded to the expected stress from the structure and

then soaked

Consolidation from natural moisture content and the additional obtained from soaking is

calculated

This method is advantageous in that you can monitor the loading and moisture content paths

to which the soil will be subjected in the field.

Disadvantage this method over predicts settlement

The Collapse Potential Test

The collapse potential test is a special case of the single consolidometer test

Sample is saturated at a load of 200 kPa (Schwartz, 1985).

According to Jennings and Knight (1975) the Collapse Potential is not a design parameter,

but is an index figure providing the engineer with a guide to the collapse situation and

whether there is good reason for further investigation.

The table below gives guidance as to the severity of collapse

Triaxial testings

Stress path testing can be done which will be carried out only by training institutions and

not commercial labs

Sampling Procedures

Use representative undisturbed samples for testings

Use block samples cut by hand from a test pit or trial trench

Take samples in the field directly into consolidometer rings.

Insitu Tests

Any in situ test must be designed to compare the stress deformation curves of the soil at its natural moisture content with that of the saturated condition.

Plate loading tests have been used in many instances

Engineering Solutions

1. Precluding the triggering mechanism

Ensure that the water does not reach the collapsing soil horizons.

2. Chemical stabilization

Stabilizing agent may increase the strength of colloidal bridges. Research on this area is limited. Use of sodium silicate and injection of carbon dioxide have been suggested(Semkin et al., 1986).

3. Piled and pier foundationsStructural loads may be transferred through the collapsible soils by means of piled orpier foundations. This method is suitable for soils whose origin is transported. Then in that case the transported soil which is collapsible is shallow and underlain by stable soils or rock.

4. Design for the collapse as quantifiedincreasing structural flexibility by the provision of joints or reducing the bearing pressures to restrict collapse settlement. Raft foundations are suitable for thisMake sure there is no increase in moisture in the underlying soil with time.

5. Densification

For footings of foundations densification should be limited to 1.5 times the minimum plan dimension of footings and the soil should be compacted to sufficient density such that the CP < 1% down to the accepted depth of influence

For road works compact to 90% Mod ASSHTO for 0-0.5 m and 85% ASSHTO for 0.5-1m

This could be combined with removal and compaction

Vibroflotation

Dynamic compaction

In situ densification by surface rolling- Use impact and vibratory rollers

ReferencesExpansive soils : problem soils in South Africa - state of the art by K Schwartz

The occurrence and extent of collapse settlement in residual granite in the Stellenbosch area by NANINE GILDENHUYS

Geotechnical engineering - Principles and Practices of Soils Mechanics and Foundation Engineering