481
Technical Committee 102 /
Comité technique 102
dry unit weight with increasing gypsum content. Further
more,(Al Heeti 1990) showed an increase in dry unit weight with
increasing gypsum content. This descrepency may be due to the
type of gypsum ( hydriate or anhydriate), type of soil and range
of gypsum content considered in the investigation.
3.1.3.
Soil constiuents and texture
The samples of gypseous soils collected from different parts of
Iraq showed that gypsum exists primearly in Sandy soil and silty
sand and less in silty clay or clayey silt. The presence of appre-
ciable amount of gypsum creats problems in determining the
constituents of the soil. During sedementation test disolution of
gypsum will occur causing the flucculation of silt and clay parti-
cles.Pretreatment with water was suggested by (Al-Khashab
1981 and Mohammed 1993). EDTA was suggested by ( AL-
Khuzaie 1985 and others). Most of the treated agents used cause
distruction of bonds and most likely revealed an unreliable
amount of constituents.
3.2.
Chemical properties
3.2.1
Chemical compsition of gypsum
Pure chemical proportions of gypsum as reported by (Nashat
1990) are
20.9 % combined with water
46.6 % sulphur trioxide SO
3
32.5 % calcium oxid CaO
(Majeed 2000) observed that the alkalinity increases with in-
creasing gypsum content. On the other hand the electrical con-
ductivity, cation exchange capacity and exchangeble sodium
percentage decrease with increasing gypsum content.
3.2.2.
Solubility of gypsum
The most effective parameter in the general behaviour of gyp-
sous soils is the solubility. Gypsum is classified as a moderate
soluable salt. The solubility of the hydrated type in pure water is
2g/l ( Hesse 1971). Some higher values, 2.41g/l and 2.6g/l , were
reported for Iraqi gypseous soils ( Sirwan etal 1989, Seleam etal
1988)
3.2.3
Rate of dissolution of gypsum
The rate of dissolution of gypsum is responsible for the devel-
opment of cavities and sinkholes. It is very complex to be evalu-
ated as it is affected by many environmental conditions such as
temperature,source of water,time, concentration of sodium chlo-
rid and calicum sulphate etc.
3.3.
Geotechnical properties
The geotechnical prperties of gypseous soils cover, compressi-
bility, collapsibility, permeability and shear strength parameters
( c and Ø)
3.3.1.
Compressibility
More than ten researchers have investigated the influence of
gypsum on the copressibility characterstics.(Al-Khashab 1981
and many others) reported a decrease in the copression index
with increasing gypsum content. It is hard to judje about the con-
tradicting results as many parameters such as the placement con-
ditions, degree of disturbance, and testing methodology. Similar
contradicting results were reported for the recompression index.
Most of the researchers demonstrated an increase in the secon-
dary compression index with increasing gypsum content. This
phenomenon is attributed to the contieous dissoltion process of
gypsum with timeas reprted by (Saleam 1988 and Nashat 1990).
The same researchers and many others showed that the coeffi-
cient of consolidation remains unchanged with increasing gyp-
sum content.
3.3.2.
Collapsibility
Gypseous soils are distinguished by their collapsible behaviour
upon wetting. The term collapse potential is used to classify the
hazarduse state of collapsibility. ( Jennings and Knight 1957)
proposed a double oedometer collapse test to predict the col-
lapsibility of the foundation soil. Two identical samples are
tested, one at natural water content and the other after submerge-
ing in water for one day.The collapse potential C.P. is defined as
C.P. = ∆e / 1+ e
o
(1)
Where
∆e is the difference in void raio of the two samples at a specific
stress
e
o
is the natural void ratio
The severity according to the collapse potential is shown in table
1.
Table 1. Collapse identification ( Jennings and Knight 1975)
Severity No prob-
lem
Moderate
Trouble
Severe
Very
Severe
C.P. %
0-1
1-5
5-10
10-20
> 20
(Saleem 1988, Nashat 1990 and many others), found that the
collapse potential under a constant stress of 200kN/m
2
increases
with increasing gypsum content. The gypsum content of the
tested samples ranged between 20 - 80% revealed a moderate
type of 4 % maximum collapse potential.
3.3.3.
Moduluse of deformation
Al Khafaji etal 2009 investigated the deformability of gypseous
soils through plate load tests performed on natural and soaked
soils. The tests were performed on two sites GP-GM soil and
SM soil and socking period extended to 7 -11 days under 300
mm head of water. All types of stiffness moduli were calculated,
the initial tangent moduluse, the permissible secant modulus at
half the yeild, the yeild secant modulus at the yeild and the yeild
tangent modulus after the yeild. The outcomes revealed that
soaking decresed the stiffness moduli in the range of 2 to 5 folds
for GP-GM soil and from 2 to 3.5 for SM soil. The field tests
highlights on the hazardius degree of constructing structures on
gypsious soils without awarness of the expected generated set-
tlements that may result from the contamination of water.
3.3.4.
Hydraulic conductivity
Hydraulic conductivity or coefficient of permeability of gyp-
seous soils is very hard to predict. Standard constant head test on
sandy gypseous soils does not reveal reliable results as the gyp-
sum disolves during flow creating more free space for the soil
particles to reorient themselves to a closer state of packing, caus-
ing a suddent fluctuation of rate of flow during test This phe-
nomenon is very difficult to evaluate as the dissolution process
is influenced by many factores like type and amout of gypsum,
hydralic gradient, initial placement of soil sample. Attempts
were made to perform leaching permeability tests under different
stress levels using Rowe cell (Al-Kaisi 1997 and many others).
(Al- Qaissi 2001 and many others) using triaxial permeability
leaching apparatus cited that the variation in hydraulic gradient
combined with diffusion of gypson encountered serious difficul-
ties in predicted reliable values of the coefficient of permeabil-
ity.
