Collapsible Soils 2gt51

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

Water bridge Water bridge

Soil grains

Water 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

Weathered 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 ts 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

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 s e t t l e m e n t

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 s 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 distribution Atterberg limits Dry 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 is S1 = ∆ 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 foundations Structural loads may be transferred through the collapsible soils by means of piled or pier 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 quantified increasing structural flexibility by the provision of ts or reducing the bearing pressures to restrict collapse settlement. Raft foundations are suitable for this Make 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 < 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 References Expansive 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