Collapsible soil is defined as soil that is susceptible to a large and
sudden reduction in volume upon wetting (Day, 2001). Collapsible soil
deposits share two main features:
||They are loose, cemented deposits
||They are naturally quite dry
According to Day (2001) collapse behavior could also happen in fill material
as a result of decrease in negative pore water pressure (capillary tension),
when the fill become wet. The collapse potential increases as the dry
density decrease, the moisture content decrease and the vertical pressure
Collapsible soil can withstand a large applied vertical pressure with
small compression, but then show much larger settlement upon wetting,
with no increase in vertical stress.
This behavior can yield disastrous consequences for structures unwittingly
built on such deposits. The process of their collapsing is often called
any of hydroconsolidation, hydrocompression, or hydrocollapse.
Another type of collapsing could occur in saturated soil bearing soluble
mineral upon subjected to continuous leaching, resulted from infiltration
of rainfall or flocculation of water table, for example the Dead Sea Lisan
Marl deposits in Jordan. This mechanism could take relatively long time,
but by the end it will have a negative impact on the stability and integrity
of the existing structures. The geotechnical term that indicate the effects
of leaching soluble salts from soil include chemical piping, or leaching
and collapse (Karakouzian et al., 1996). The failure mechanism
is not the same in the two types of collapse. In hydrocollapse the particles
are arranged in honeycombed structure, held together by small amount of
cementing agent like clay or CaCO3; introducing the water leads
to dissolve or soften the bonds between particles and hence undergoes
denser packing under loading. In leaching and collapse failure; the leaching
of soluble salts from the soil matrix, increased the void ratio and decrease
the strength, which in turn causing the collapse of matrix under the compression
strength (Karakouzian et al., 1996).
Unsaturated soils or soils with negative pore-water pressures can occur
in essentially any geological deposit, such as residual soil, a lacustrine
deposit and soils in arid and semi arid areas with deep ground water table.
The most common types of collapsible soil:
||Alluvial (water deposited) and colluvial (gravity deposited)
||Wind deposited (aeolian) soils are fine sands, volcanic ash tuffs
||Residual soils formed by extensive weathering of parent materials.
For example, weathering of granite can yield loose collapsible soil
TYPICAL SOILS SUSCEPTIBLE TO COLLAPSE
Marl behavior changed under dry and wet conditions; the strength is dropped
by about 85% upon saturation and exhibited collapsible and swelling potential
when exposed to water (Quhda and Yong, 2003).
Lamas et al. (2002) conducted a study on marly soil classified
as CL (USC) or A-6 (AASHTO) used for construction of impermeable core
of earth dam in Spain. The permeability found to be increased with the
increasing of carbonate content; this explained by the leaching or dissolution
of carbonate by the distilled water used in the triaxial cell.
A permeability tests on Sabkha soil form Kuwait classified as ML, having
high percent of sulphate, carbonate and silica; showed that the permeability
coefficient has been doubled after leaching the samples by distilled water
(from 1.75x10-5 m sec-1 to 3.5 x10-5
m sec-1); chemical analysis of the leachate showed that the
chloride is completely dissolved, partial dissolving of sulphate and no
dissolving of calcium carbonate (Ismael, 1993). A decrease is reported
on the bulk density, specific gravity, atterberg limits, unconfined compressive
strength and slight decrease in clay fraction content, while a significant
increase of fine fraction (from 60.4-97.4%) occurred after leaching. The
increase in fine fraction to the breaking of sand-size particles containing
large amount of gypsum (low hardness mineral) into silt size. Nevertheless,
the soil still classified as ML. This result demonstrated also by Al-Amoudi
et al. (1992); where the wet sieving of sabkha soil classified
as SW-SP in Saudi Arabia using distilled water and sabkha brine water
resulted in 32 and 13% fine fraction, respectively; but the soil classification
in this case has been changed to be SW upon using of distilled water.
Using of distilled water as testing media (soaking) in the oedometer
test for sabkha soil did not affect the compressibility (Al-Amoudi et
al., 1992), while by allowing the distilled water to percolate through
the sample; the compressibility significantly increased (Cc and Cs increased
by 50%) and the void ratio increase from 1.14 to 1.23 as reported by Ismael
(1993). Slight decrease in preconsolidation pressure and effective friction
angle as a result of dissolving of salt cementing was recorded also. Depending
on the above it is concluded that the collapse potential should be evaluated
for these types of soil rather than the compressibility in any geotechnical
Karakouzian et al. (1996) stated that an insufficient amount of
fresh water is used to evaluate the effect of soluble minerals on the
engineering behavior of soil; the present salts may not dissolved (named
salt saturation condition), which could lead to underestimation of the
long term impacts of water in contact with the soil. This could explain
the findings of Al-Amoudi et al. (1992) and Al-Amoudi and Abduljauwad
Al-Nouri and Saleam (1994) studied the compressibility characteristics
and the collapse potential of Gypseous silty sand soil in Iraq. The gypsum
is hydrated calcium sulphate (CaSO4.2H2O) with intermediate
solubility in water (0.2%) but the amount of dissolved is much greater
if the water contain salt. The collapse settlement of gypseous sandy soil
upon wetting in a plate load test could amount to 50% of the total measured
settlement, while if water circulation allowed it could amount to 76-90%
of the total settlement. Even the capillary water may cause the collapse
of the soil structure in gypseous soils. The study by Al-Nouri and Saleam
(1994) conduced on soil samples with gypsum content of 26, 60 and 80%,
the gypsum found as cementing material while gypsum lumps (1-15 mm diameter)
found also in the samples that have high gypsum content. In the collapse
test, a sudden compression observed upon submersion which indicates a
collapsible soil, on the volumetric strain-log P graph, this compression
appears as vertical line, followed by steep curve resulting from further
dissolution of gypsum. It is noticed also that the collapse potential
increased progressively with the increase in pressure, which imply that
the gypsum dissolution increases in rate as the stress level increases.
Effectiveness of leaching by changing the grain size distribution, density
and hydraulic gradient has been evaluated by Al-Sanad (1990), on a non
plastic calcareous rounded medium to fine sand soil classified as SP in
Kuwait. The cementing materials in that soil are the chloride, sulphate
and carbonate (the carbonate content equal to 8.3%). The leaching process
increased by increasing the relative density and the fine fraction; this
behavior was explained to the increase in the contact area between soil
Changing the hydraulic gradient from 0.77 to 10 insignificantly affect
the leaching process as it was clear from the leachate analysis, where
the electrical conductivity values ranged between 3.03 to 3.63 m mho cm-1
through the test for all hydraulic gradient values. Brackish water posses
electrical conductivity of 4.69 m mho cm-1; has slightly less
capability of dissolving salt than distilled water. Leaching was more
effective - in relatively short time-for soil rich with NaCl or Na2SO4
rather than CaCO3 or MgCO3 which show low solubility.
James and Lupton (1978) found that the rate of solution of both anhydrite
and gypsum is increased by NaCl and by Carbonate and CO2, so
it is important to know the chemical composition of ground water in regions
of calcium sulphate minerals.
Soluble salts or minerals that are commonly found in soils can be classified
based on the degree of solubility in water as readily soluble, moderately
soluble and weakly soluble. Figure 1 shows the solubility
for the commonly occurring soluble soil minerals (Lide, 1994). In general,
the graph shows that minerals with the highest solubility are Chlorides.
||Solubility of common soil minerals (re-produced after
LABORATORY DETERMINATION OF COLLAPSE POTENTIAL
The quantification of volume change occurs when soil undergoes collapse
is obtained from oedometer test. Once the geotechnical engineer recognizes
the possibility of collapsible soils is present, this mainly done depending
on the density and consistency limits measurements as shown in (Fig.
2); one or more of the following oedometer tests shall be conducted
on undisturbed sample.
Single oedometer collapse test: The undisturbed soil specimen
at natural moisture content loaded in the conventional oedometer to a
stress level ranging between 200 and 400 kPa and then inundation by distilled
water is applied to induce collapse. Abelev (1948) used stress level of
300 kPa and defined the collapse potential (Ie) as:
||Change in void ratio resulting from saturation
||Void ratio just before saturation
while, Jennings and Knight (1975), recommended the using of stress level
of 200 kPa and calculate the collapse potential according to the following
||Change in void ratio resulting from saturation
||Natural void ratio
The stress level of 200 kPa was adopted by (ASTM D 5333-96, 2000) to classify
the severity of the collapse problem (Day, 2001).
Since the idea behind this test is to predict the amount of deformation
that a foundation may experienced upon subsurface wetting; a loading to
the anticipated field loading conditions is recommended.
||Commonly used criterion for determining collapsibility
(Lutenegger and Saber, 1988)
||Typical result from single oedometer test
A typical result
obtained from this test is shown in Fig. 3.
Double oedometer collapse test: Two identical samples are placed
in oedometers; one tested at in-situ natural moisture content and
the other is fully saturated before the test begins and then subjected
to identical loading. Two stress versus strain curves are generated. The
difference between the compression curves is the amount of deformation
that would occur at any stress level at which the soil get saturated.
Results from double oedometer test are shown in Fig. 4.
The collapse potential can be determined at any required stress level.
Critical stress (σcr) represents the stress level at which
the dry sample loose structure breaks down and beyond it the two curves
converge. This behavior could be explained also by that at high stress
level, the limiting void ratio for the saturated sample is approached
for particles packing (Lutenegger and Saber, 1988). It is common for natural
soil that the initial void ratio of the two samples are not initiating
from the same point; in this case adjustment of the two curves according
to the procedure proposed by Jennings and Knight (1975) is adopted for
the normally and overconsolidated clays.
The above mentioned procedure is applicable for the soils that do not
include high percentage of soluble minerals in its matrix.
||Typical results from double oedometer test
||Collapse potential of clay soil- Dead Sea-Jordan
For soils containing
high concentration of soluble salts or minerals (Fig. 1),
the conventional inundation of the soil specimen in the oedometer could
lead to under estimation of the collapse potential since the amount of
water might be not enough to dissolve all the present salts and the water
get salt saturated. In this case leaching out of these salts shall be
carried out prior to testing. Figure 5 shows the results
of double oedometer test conducted by the authors on clayey soil collected
from the Dead Sea area. These deposits contain high content of soluble
minerals such as Halite and Gypsum as well as high ions concentration
in pore water, noting that the salinity of the Dead Sea brine is about
10 times the salinity of the normal seas. It is clear that soaking of
the sample failed to induce collapse. While the collapse potential reached
around 10% upon allowing water circulation.
Rowe cell: Leaching process could be performed in Rowe Cell (Fig.
6). The load in this cell is applied hydraulically and it is used
to carry out consolidation and permeability tests.
||Schematic diagram of Rowe cell
Since this is the case,
it considered suitable to conduct the leaching on the samples to be evaluated
for the collapse potential.
The leaching process could be done according to the following steps:
||Setup the sample under a seating pressure
||Apply a pressure head using an overhead de-aired water reservoir
to saturate the sample
||Connect the overhead reservoir to the back pressure line of the
||Allow the water to percolate through the sample from the bottom
and drain from the top
||Collect the leachate at regular intervals for measuring the electrical
||Terminate the leaching process when the EC measurements show no
Modified oedometer: Al-Amoudi and Abduljauwad (1995) modified
the oedometer to allow for water percolation by making two holes beneath
the specimen; an inlet for the distilled water supply and an outlet for
the overflow to maintain a constant fluid head (Fig. 7).
The leachate is collected at regular intervals for measuring the electrical
conductivity and the water flow (leaching) is terminated when the EC measurements
show no further decrease. In both method of leaching, the leaching could
be started at the overburden pressure and continue the consolidation test
according to the related standard and then compared with the stress versus
strain curve resulted from other identical sample tested without leaching,
or under any desirable pressure (200 kPa for example).
||Modified oedometer setup (after Al-Amoudi and Abduljauwad
||Compacted soil permeability apparatus
Compacted soil permeability apparatus: This device used successfully
by the author (Fig. 8). The sample sandwiched between
two porous disks and the water allowed to percolate from the top and issue
from the bottom. When the leaching completed the sample carefully transferred
to the oedometer for testing. The disadvantage of this device is that
the sample can`t be leached under applied pressure and consequently care
shall be taken if the sample could exhibit swelling behavior.
||Proposed flow chart for collapse potential evaluation
Procedure for collapse potential evaluation: The flow chart shown
in Fig. 9 could be used as a guideline regarding the
steps and the tests to evaluate the collapse potential of the concerned
In summery and based on this research and literature study, the following
conclusions can be drawn:
||The collapse evaluation test shall be conducted on undisturbed samples
to account for the effect of soil fabric
||Inundation in oedometer is satisfactory for the measuring of the
collapse potential when small amount of CaCO3 and/or Clay
is acting as the cementing agent
||Inundation of soil specimen in the consolidation test for the purpose
of measuring the soil collapsibility could under estimate the collapse
potential; if the soil bears high concentration/percentage of soluble
||If the soil contains soluble salts; all the salts shall be leached
out in the testing apparatus, by using the Rowe cell or the Modified
Oedometer; for more accurate evaluation of the collapse potential
||The proposed flowchart (Fig. 8), could provide
a simple guide line for better evaluation of soil collapse potential.
The flowchart differentiates between the two types of collapse mentioned
in the introduction. However, the procedure or the route to be followed
in case of the presence of soluble minerals suit also the soil possess
collapse potential but don`t include soluble salts
||In case of cohesive soil single oedometer is not recommended
because of the decreasing of permeability during the loading (200
kPa) which will increase the time required for full leaching
||Single oedometer test is recommended if the soil is not homogeneous
The authors would like to acknowledge Arab Potash Company, Jordan, for
the permission to access the site and for the logistic and financial support
during the site visit. The contribution of Mr. Talal Abu Baker, the Partner
and Manager of Triple Corporation, Jordan and Sinohydro company, China
(the contractor of dike 18 remedial measures project), in site sampling
is highly appreciated.