COLLAPSIBLE SOILS
PREPARED BY
NAME - RAVINDRA KUMAR BURNWAL
ROLL NO - P19SM013
o It’s basically a silt & sand/ silt or sand with a metastable condition.
o Also called as-
LOESS (WATER CONTENT >LL)
HYDRO-COMPACTIVE SOILS
HYDRO-CONSOLIDATION
HYDRO-COLLAPSE SOIL
SOIL
o Natural collapsible soils are : Mainly Wind deposited sand & silt / silt , eolic
beaches, volcanic dust deposits, debris flows.(1)
o Mineralogical structure : Mainly flocculated (held together by clay bridges or
calcium carbonate.(2)
Fig: Particle size distribution of Algerian yellow and grey loess
Need for attention
o Found in 10% world’s Landmass(mostly in Europe, united states, India
,china ) mainly in aired to semi aired environment.
o challenges (Geotechnical engineering prospective)(3):
1. Identification and characterisation of collapsible soil site.
2. Estimation of the extent and degree of wetting.
3. Estimation of collapsible strain and collapsible settlement.
4. Selection of design and mitigation alternative,(Bearing capacity
decreases to about 50% on wetting(4) ).
(Pavement engineering prospective ) (5)
1. formation of depressions & settlement.
2. Higher collapsible potential of collapsible soil owing to the high salt
content.
3. variability in strength and compressibility leading to differential
settlement.
4. sand movement causing abrasion to existing structures and blockage
of some streets and highways.
Main forms of collapsible soil structure
Factors affecting collapsible potential of soil
o Initial Moisture Content
o Initial Dry Density
o Initial Soil Composition
o Confining Pressure
o Hydraulic & Mechanical History of soil
o Soil fabric
o Permeability
o Collapsible Layer Thickness
o OCR (Over consolidated ratio)
o Age of deposit
Typical Range of engineering properties of collapsible soil (Egyptian, Elgerian
collapsible soil)
Properties Range
Maximum dry density (Mod. proc. Test ) 17- 19.5 KN/m3
Optimum moisture content (Mod. Proc. test) 12% - 16%
Liquid Limit Nearly (20% - 36%)
Plastic Limit 11% - 19%
Plasticity Index 10% - 17%
Natural water content 2% - 15%
Activity 0.45 – 1.71
Coefficient of uniformity (Cu) 15 – 85.7
Coefficient of compression (Cc) 0.27 – 4.9
Specific Gravity 2.6 – 2.75
Note : These geotechnical engineering properties of collapsible soil is very much
site specific so for practical purposes field investigation is essential.
Behaviour of collapsible soil with moisture content
Methods to determine the collapsible potential of soil
Empirical method (6)
Undisturbed soil sample (Cp) = 0.177 – 0.59*Wc
95% compacted sample (Cp) = 0.033-0.11*Wc
Transportation engineering prospective
Foundation engineering problems
Cp (%) Severity of problem
0-1 No problem
1-5 Moderate trouble
5-10 Trouble
10-20 Severe trouble
20 Very severe trouble
Ref : Clemence , s.p. ,and finbarr, A.O.(1981) (ASCE)
collapsible settlement criteria based on dry density
Collapsible potential from void ratio(e) V/S log(P) graph
Cp= e/(1+e0)
Collapsible potential by dry unit weight V/S LL
Holtz and Hilf (1961)
Collapsible potential from Dry unit weight V/S %passing(sieve no-200)
Moghadam et. al.(2006)
Laboratory method to determine the collapsible settlement
Jennings & knight method (1975)
Step 1 : obtain two undisturbed soil specimen for tests in a standard consolidation
test apparatus (oedometer).
Step 2 : place the two specimen under 1 KN/m2 pressure for 24 hours.
Step 3 : After 24 hours , saturate one specimen by flooding and retain the other
specimen at it’s natural moisture content.
Step 4 : after 24hours of flooding ,resume the consolidation test on both specimen
by doubling the load to the desired pressure level.
Step 5 : plot e V/S log(’) graphs for both specimen.
Step 6 : calculate the in situ effective pressure (0‘). Draw a vertical line corresponding
to the pressure (0‘).
Step 7 : from the graph (soaked specimen) determine the pre-consolidation
pressure(c‘).
if , (c‘/0‘) = 0.8 -1.5 ; NCC soil.
(c‘/0‘) > 1.5 ; soil is pre-consolidated.
Step 8 : Determine e0’ , corresponding to 0’ for soaked specimen as shown
in the figure below.
Fig a: Normally consolidated soil Fig b: Over-consolidated soil
Step 9 : Through point (0’ , e0’) draw a curve similar to the e-log(0’) curve
Obtained at natural state.
Step 10: Determine the incremental pressure (’) , on the soil caused by
construction of the foundation. Draw the vertical line corresponding
to the pressure of (0’+’) in the e-log(0’) curve.
Step 11: Now , determine e1 & e2 . The settlement of soil without the
change in the natural moisture content is-
Sc(1) = (e1 /(1+e0’))*(H)
where, H : thickness of soil susceptible to collapse.
Also , the settlement collapse in the soil structure is
Sc(2)= (e2 /(1+e0’))*(H)
Other reported criteria for identification of collapsing soil
Investigator Year Criteria
Denisov 1951 Coefficient of subsidence :
K=(void ratio at liquid limit)/(natural void ratio)
K = 0.5-0.75 ; highly collapsible.
K = 1.0 ; non collapsible loam
K = 1.5-2.0 ; non-collapsible soil.
Clevenger 1958 If ,dry unit weight < 12.6 KN/m3 ; settlement will be
larger.
If ,dry unit weight > 14NK/m3 :settlement will be small.
priklonski 1952 KD =(Wn – PL)/PI ;
KD <0 ; highly collapsible soil
KD >0.5 ; non-collapsible soil
KD >1.0 ; swelling soil
Gibbs 1961 Collapsible ratio, R=(Ws /LL), and this was put
into graph
Soviet 1962 L =(e0 - eL)/(1+e0) ;for natural degree of saturation
building <60% & if L > -0.1 , the soil is a collapsing soil.
code
Feda 1964 KL =(W0 /Sr) – (PL/PI)
For Sr < 100% ,if KL > 0.85, the soil is subsident
soil.
Benites 1968 A dispersion test in which 2g of soil are dropped
into 12ml of distilled water & specimen is timed
until dispersed; the dispersion time of 20 to 30 Sec
Were obtained for collapsing Arizona soil.
Handy 1973 Iowa loess with clay (<0.002mm) contents:
<16 % ; high probability of collapse
16-24 % ; probability of collapse
24-32% ; <50% collapse probability.
>32% ; usually safe from collapse.
Suitable foundations on collapsible soil
o Continuous strip footing
o Spread footing
o Deep foundation etc.
Combating collapsible Potential of a soil
Bowles (1988)
(a) compaction (excavation and replacement) of the soil to
Dry unit wt. > about 15.5 KN/m3.
(b) use of different types of admixture such as Portland
cement during compaction.
(c) use of piles through the collapsible soils to a more
competent underlying stratum.
Other methods to reduce collapsible potential
o Chemical stabilization (using sodium silicate or calcium chloride).
o Deep dynamic compaction
o Flooding or pre-wetting
o Soil cement cushion
o Stone column (vibro-flotation & vibro=replacement)
o Pre-loading
THANK YOU!
