COLLAPSIBLE SOILS
Collapsible soils
These are unsaturated unsaturated soils that can withstand relatively relatively large imposed stresses with small settlement settlement at low in situ moisture content content but will exhibit a decrease in volume and associated settlement settlement (which could be of large magnitude) with wit h no increase in applied ap plied 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 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 granites of South Africa
Kalahari sands The black cotton soils of
and Chad etc
Northeastern Nigeria, Cameroon
Origin Granite
Weathered Residual granite
quartz, mica flakes, kaolinite
It is characterized 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 moisture will be trapped trapped so the honey comb structure structure can withstand some considerable considerable force force when dry but when saturated collapses collapses and creates a host of collapsible soil problems 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 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 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 b e 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, 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. Constru Construction ction was was carried carried out befo before re collap collapse se phenomen phenomenon on was identi identified fied 2. No geot geotech echnic nical al assessm assessmen entt was was carr carried ied out out 3. In case the the geotechn geotechnical ical assessmen assessmentt was done, done, it did not evalu evaluate ate corr correctl ectly y or idenity potential collapsible soils within the profile 4. Recommendati Recommendations ons given by the the Geotechnical Geotechnical engineer was ignored ignored by the the parties parties invoved invoved in the design
Evaluation and prediction Field Identification Identification Observational Observational method Look
for cracking and building distortion
Soil profiling Recognize Use
a loose or open fabric
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 Tests i.
Plot Plot the the e-lo e-log g p grap graphs hs for both both spec specim imen ens. s.
ii.
Evaluat Evaluate e the in situ eff effecti ective ve pressur pressure, e, Po. Po. Draw Draw a vertic vertical al line line corres correspond ponding ing to the pressure Po.
iii.
From From the e-log e-log p curve curve of the the soaked soaked specime specimen, n, determi determine ne the pre pre consol consolidat idation ion pressure, Pc.
iv.
Determine eo, corresponding to Po from the e-log p curve of the soaked specimen.
v.
Through po point (P (Po, eo) draw a curve that is comparable to the e-log p curve obtained from the specimen tested at natural moisture content.
vi.
Dete Determ rmine ine the the incr increm ement ental al pre pressu ssure re,, ∆p, on the soil caused by the construction of
the foundation. Draw a vertical line corresponding to the pressure of Po +
∆p
the e-log p curve. vii. vii. Now Now, dete determ rmin ine e es and ec. viii. The settlement settlement of soil without without change in the natural natural moisture moisture content content is S1 = ∆es/ (1 + e o) x H Also, the settlement caused by the collapse of the soil structure is S1 = ∆ ec/ (1 + e o) x H where H = the thickness of soil vulnerable to collapse
in
The single consolidometer test.
This the
is a simpler test to perform since
interpretations of double oedometer test is cumbersome
Only The
one undisturbed sample is tested
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 below
gives guidance as to the severity severity of collapse 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. Pr Precl ecludi uding ng the trigg triggeri ering ng mech mechani anism sm Ensure
that the water does not reach the collapsing soil horizons.
2. Chemical stabilization 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 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 joints 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 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 In
compaction
situ densification by surface rolling- Use impact and vibratory rollers
References Expansive soils soils : problem soils in South Africa 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 Geotechnical engineering - Principles and Practices of Soils Mechanics and Foundation Engineering
