Long -Term Deformation Of Some Gypseous Soils

: Time-dependent deformation and stress relaxation in soils are important in a variety of geotechnical problems where long-term behavior is of concern. Previous studies on soils showed that the magnitude of delayed compression (creep) is controlled by compressibility and soil sensitivity in addition to preconsolidation. In this paper, the time-dependent behavior of gypseous soils is investigated. The soils used in this study were brought from three locations at Al-Tar region west of Al-Najaf city in Iraq. These soils had gypsum content of (66%, 44% and 14.8%). The mineralogical and chemical properties of the soils were determined. Two series of tests were performed. In the first, collapsibility characteristics were investigated for a long period (60 days) by conducting single and double oedometer tests. In the second series, the effect of relative density on collapse with time was investigated. The samples were compacted to 40%, 50% and 60% relative density and then tested. The results of collapse tests showed that the relationship between the strain and logarithm of effective stress has two vertical lines. The first one represents the collapse settlement taking place within 24 hours, while the second one represents the long-term collapse. The collapse potential in both single and double oedometer tests increases when the gypsum content increases from (14.8%) to (66%) and when the initial void ratio increases. The results of double oedometer tests showed that the relationship between the collapse potential and logarithm of time, for samples loaded to 800 kPa for 60 days, consist of three distinct segments. The first segment is represented by a curve concave downward in which the compressibility gradually increases. The second segment is a straight line with a higher increase in the strain. The third segment which refers to creep collapse depends on the gypsum content. Gypseous soil with low gypsum content (14.8%) exhibited significant decrease (5.21% at 24 hours to 7.16% at 60 days) in collapse potential with time .

-Atterberg Limits: The liquid limit test was carried out according to the British Standards (BS 1377: 1975 test No.2, A) using the cone penetrometer method, while the plastic limit test was according to (BS 1377: 1975 test No.3).
-Grain Size Analysis: Grain size analysis was determined by sieve analysis, which was conducted in accordance to the procedure described by Bowles (1978), but Kerosene is used instead of water.
-Minimum and Maximum Density: Minimum density test was performed according to (BS 1377(BS : 1975)), to get a lower density possible to be achieved by slow pouring of the preweighed dry soil in air through a graduated cylinder.Then maximum void ratio can be calculated from which the relative density can be estimated.
Maximum density test was performed according to the procedure described by Bowels (1978).In this test, three trials of maximum density are made by placing the oven-dry soil in the standard mold in five layers, confining the layer with a round steel block at least ( The total soluble salts, SO3% and gypsum content were determined according to British Standards (BS 1377).Table (2) shows the results of some chemical and composition tests conducted on the samples.

X-Ray Diffraction Tests
The results of these tests indicate that gypsum and quartz are the dominant nonclay minerals, while the montmorillonite, palygorskite, illite and chlorite dominant clay mineral components.

Results of Oedometer Creep Tests 1) Collapse Test
The results of collapse tests (CT), shown in Figure (2), are drawn as the vertical strain (ε) versus logarithm of effective stress (log σv).Two vertical lines are noticed in these figures.The first line refers to the settlement that occurs suddenly when water is added to the soil sample under 200 kPa.
The loading period for the first line is 24 hours.The change in strain upon flooding in water points out that the soil is collapsible.The bonds start losing strength with the increase of the water content and at a critical degree of saturation, the soil structure collapses, (Jennings and Knight, 1957 and Barden et al., 1973).A summary of data is given in Table (3).
It can be noticed that the collapse potential, C.P., increases with the increase of gypsum content and initial void ratio.The second line represents the additional settlement caused by creep that occurs under 800 kPa for 60 days.
In Figure (3) the vertical strain is plotted versus time in normal scale for the second line of the three samples.The curve is linear during the entire creep period for sample N3.For samples N1 and N2 a change from the initial linear behavior to another with higher rate of strain is observed and a sudden soil collapse is noticed within the creep stage.This may be explained by high heterogeneity of soil with the existence of gypsum crystals in these samples.
The same results are plotted in Figure (4) in which the time is plotted on a logarithmic scale.No apparent difference is obtained when the results are drawn on a semi-logarithmic scale.
Al-Aithawi (1990) found similar results in his analysis of axial strain versus log time for gypseous soils.A linear behavior was found at (100 and 200) kPa stress intensity.This linearity step was followed by the collapse occurrence.

2) Double Oedometer Test
Figure (5) shows the variation of strain with logarithm of vertical stress obtained from the double oedometer test (DOT).The compressibility of the soil is low when loaded under unsaturated condition.The loading of the unsaturated soil is the same as the loading of the saturated one from 25 to 800 kPa.From Table (3) it can be seen that the collapse potential for samples N2 and N3 obtained from double oedometer tests are greater than those obtained from collapse test at stress level of 200 kPa, while for sample N1, the collapse potential obtained from DOT was smaller than that obtained from collapse test.
The relationship between the collapse potential and logarithm of time from the DOT at 800 kPa for 60 days are plotted in Figure (6).The percents of collapse potential versus log time curves for the N1 and N2 samples consist of three distinct segments.The first segment is represented by a curve concave downward.In this stage, the compressibility gradually increases.
The second segment is generally a straight line with a higher increase in the strain.In the third segment, the rate of strain increases again to form approximately another straight line opposite to C.P. axis for the sample N1 and parallel to time axis for the sample N2 where the rate of strain remains constant after the day (37) to the end of the test.But the curve of the sample N3 differs from the curves of N1 and N2 samples.The curve consists of two segments, the first is a straight line where the strain increases rapidly and the other is a curve concave upward where, the strain decreases.This expansion may be attributed to the presence of montmorillonite clay mineral, since that gypsum is not expansive.Another source of this expansion can be attributed to rearrangement of sand particles which when loaded to higher stresses show some increase in volume.Dudley (1970) showed that the collapse in fine sand (with 14% montmorillonite) increased with the increase of initial water content until it reached values higher than 10%, the collapse then became less at the same stress level and dry unit weight.Sand deformation response is directly related to the parent minerals, Lambrechts and Leonards (1978).Hossain (2001) found that the swelling pressure decreases with increasing gypsum content for Al-Qatif clay.
The effect of gypsum content on collapse potential for long term under 800 kPa is well illustrated by Figure (7).The relation obtained indicates that creep values increase as the gypsum content increases.It may be ascribed to the continuous dissolution of gypsum.
As shown in Figure ( 8), the collapse potential increases with the increasing stress for the three samples.When the stresses are low (below 100 kPa), this effect is not apparent.
The collapse may be caused by break-down of the interparticle bonds under high loads.It can be seen that for N1 sample where the gypsum content is 66 %, the same rate of increase of C.P., % takes place at all stress levels.On the other hand, for samples N2 and N3 a continuous increase in C.P. rate occurs when the stress level increases.

Effect of Relative Density
Two identical specimens with relative densities 40%, 50% and 60% for the N1 soil are tested independently; one in the natural moisture content, the others are soaked by water from the beginning.The soil specimens were statically compacted in the oedometer ring to a dry unit weight that maintains the desired relative density.The results of double oedometer test are presented as strain versus logarithm of the effective stress as shown in Figure (9).A summary of data is given in Table (4).
It can be observed that the effect of relative density on collapse strain at 200 kPa is not clear.The effect of relative density is evident at 800 kPa.The strain increases with the increase of relative density from 40% to 60%.
Figure (10) shows the collapse potential-logarithm of time relationship for the three relative densities under 800 kPa.The curves indicate that the C.P increases with time for the soil sample compacted to a relative density 60% and the increase is higher than those compacted to 40% and 50%.The curves start with a straight line then a curve concave downward with a high strain is observed.For samples compacted to 40% and 50% relative densities, the curves interrupted by little soil collapses where another curve setup following the collapse is noticed.
Figure (11) shows the variation of collapse strain with time for dry N1 soil at different relative densities.It can be noticed that the strain decreases seriously when the dry soil is compacted at higher relative density.
A decreases of about (43 %) was noticed when the relative density changes from 40% to 60% at long times, while the inverse behavior was noticed for soaked samples as illustrated in Figure (12).The strain increases when the wet soil is compacted at higher relative density.
The relationship between the collapse potential and relative density at 800 kPa for 60 days, shown in Figure (13), indicates an increase in collapse potential with the increase in relative density for soaked samples.

Coclusions
From the results and analysis of the tests presented in this paper on samples of gypseous soil having percentages of gypsum of 66%, 44% and 14.8%, the following conclusions could be drawn: 1.The results of collapse tests showed that the relationship between the vertical strain and logarithm of effective stress has two vertical lines.The first one represents the collapse settlement taking place within 24 hours, while the second one represents the long-term collapse.2. The collapse potential in both single and double oedometer tests increases significantly when the gypsum content increases from (14.8%) to (66%) and when the initial void ratio increases.3. The results of double oedometer tests showed that the relationship between the collapse potential and logarithm of time, for samples loaded to 800 kPa for 60 days, consist of three distinct segments.The first segment is represented by a curve concave downward in which the compressibility gradually increases.
The second segment is a straight line with a higher increase in the strain.The third segment which refers to creep collapse depends on the gypsum content.Gypseous soil with low gypsum content (14.8%) exhibited significant decrease (5.21% at 24 hours to 7.16% at 60 days) in collapse potential with time.4. As the relative density increases from 40% to 60%, the collapse potential increases with time for the same gypsum content at high stress (800 kPa).The strain decreases seriously when the dry soil is compacted at higher relative density (60 %).A decrease of about (43 %) was noticed when the relative density changes from 40% to 60 % at long times.The saturated samples exhibited opposite behavior; the strain increases from 24.3 % to 29.2 % at 60 days when the saturated soil is compacted at higher relative density, (from Dr = 40 % to 60 %).
: Results of double oedometer test on semi-log scale.6) : Variation of collapse potential with time for DOT.