ABSTRACT
Large areas of Amman, the capital of Jordan, are covered with expansive clay soils, which derived mainly from marl, limestone and chert. The swelling and shrinkage potential of these soils caused by mineralogical constituents, and the surrounding environment, create serious problems to different types of construction. In order to determine the consolidation coefficients, swelling potential and Young’s modulus of these soils, the Odometer tests were performed. The Odometer tests showed that the swelling of soils ranged between 0.16% and 1.89% at natural water content, and between 0.34% and 0.94% at saturation. The consolidation coefficients ranges between 0.1-0.2 m2/year and 0.4-1.6 m2/year for the dry soil samples, and 0.2-0.4 m2/year for the wet soil samples. As well the soil suction was predicted from consolidation stress versus void ratio curve from three different ways. The first value was 166KPa, the second was 264KPa, and the third value was 87KPa. The average of these three values was taken as 169 KPa.
KEYWORDS: consolidation,swelling, void ratio, soil suction
INTRODUCTION
Many parts of the world suffer from constructing problems that associated with expansive clay soils. These problems include, cracking, break-up of pavements, heaving, and damaging building foundations.
Superficial clay soil with the potential to shrink or swell with different degree of expansion covers large areas of many cities in Jordan such as Amman and Irbid. The thickness of these superficial deposits varies from few centimeters to more than 12 meters in the above mentioned cities. In the study area, this thickness ranges between 2 and 6 meters.
Many buildings in Jordan include homes, schools, and offices which have been damaged by the movements of the superficial expansive clays, because the foundation design has not taken into consideration the expansive properties of the soil. This study is part of an effort to lead to improved geotechnical engineering practices by providing the engineering properties of these soils.
TEST RESULTS
The conventional odometer test was performed to predict the compression of the soil when the loads have been applied on the soil. As well the heave prediction when the soil unloaded. The theoretical concept of the consolidation process for saturated clays under load was stated.
Four undisturbed samples from two boreholes 76-mm in diameter were used. Sample no. 1 from BH-1 at depth 1.25 m, sample no. 2 from BH-1 at depth 1.7 m, sample no. 3 from BH-2 at depth 1.2 m and sample no. 4 at depth 1.65 m. The samples were trimmed to 70 mm in diameter and placed into confining ring. The diameters to height ratio for the specimen were between 3.62 and 3.66.
The loading sequence which was used for samples number 1, 3, 4 was 26, 52, 104, 208, 416KPa, while for sample 2 was 52, 104, 208, 416KPa. The elapsed time sequence was 0, .17, 2.25, 4, 9, 16, 30.25, 64, minutes and so on for at least 24 hours, for each load increment, and load decrement.
The tests were carried out for each sample as the follows.
Sample number 1; loaded at natural water content to final load (416KPa) then unloaded at natural water content to the initial load (26KPa), after that the sample inundation and loaded wet to the final load, then the load decrement to the initial load, finally reloaded wet for two load stages. Sample number 2; was loaded at natural water content to final load then flooding and unloaded wet to (52KPa), after that loaded wet to final load, then unloaded to (52KPa), finally reloaded wet for one load stage. Sample number 3; loaded at natural water content to final load, then unloaded dry to (104KPa), the sample inundation and loaded wet to final load, then unloaded wet to initial load, finally reloaded wet for three load stages. Sample number 4; was loaded at natural water content to final load, then unloaded at natural water content to initial load. After that the sample inundation and loaded wet to final load, then unloaded wet to initial load, then the sample-reloaded wet to final load, after that unloaded wet to initial load. This specimen-reloaded wet for second time finally unloaded wet.
The dial gage readings were taken for each sample at each load increment for each stage at the elapse time.
The void ratio was calculated for each sample as the follows.
|
(1) |
Where H is the thickness of specimen calculated for each load increment by subtracting the reduction or increasing in thickness recorded by the dial gauge from the initial thickness.
: Equivalent
height of the solid particles and calculated as follows:
|
(2) |
Where
is the dry mass of specimen,
is the specific gravity of the soil particles,
is the area of the specimen, and
is the density of water.
The dial gage readings were plotted on a linear scale against
the elapsed time on a logarithmic scale. The dial reading-time were converted to
consolidation-time curve by first determining the zero consolidation point as usual ( described below for the sake of completeness).
Select time t1, and a time t2 that is 4t1 for initial branch of the curve. Measuring the ordinate between t1 and t2 then the distance between t1 and t2 was laid off above t1. The horizontal line was drawn through this point. Intersect this line with deal reading ordinate is D0, and 100% consolidation () by the equation
|
(3) |
The time for 50% primary consolidation () was determined by projecting the value of D50 on the curve and dropping a vertical line to the log axis and reading t50 as shown in Figures 1 and 2. The
was used to calculate the consolidation coefficient
(
for the soils by the following equation:
|
(4) |
Where is the average thickness of the specimen (Bowles, 1992).
Figure 1. Stiffness vs effective consolidation stress
Figure 2. Consolidation Coefficient Histogram
Also, the "elastic modulus" (i.e., the tangent Young’s modulus) of the soil was calculated for each specimen for each load increment and load decrement by the following equation:
|
(5) |
Where is the change in vertical stress, and
is the vertical strain calculated by the following equation.
|
(6) |
where DH is the change in thickness, and H is the specimen thickness. Table 1 shows the values of DH, H, e0, and E0.
Tests on Samples 3 and 4 (Table 1 cont'd)
DISCUSSION OF THE RESULTS
The swelling rate range between 0.21% and 0.4% for Sample no. 1 at the natural water content, and 0.51% and 0.65% at saturating. As well the swelling for Sample no. 2 range between 0.54% and 0.62% of wet state. The swell rate for Sample no. 3 at dry state range between 0.16% and 0.54% and between 0.22% to 0.71% at wet state. For Sample no. 4 that value was between 0.94% and 1.89% at dry state and between 0.34% and 0.53% for the wet state.
The elastic modulus (E0) for the dry sample no.1 range between 9 MPa and 77 MPa and range between 4 MPa and 40.7 MPa at wet state.
The values of elastic modulus for Sample no. 2 range between 5.6 MPa and 16 MPa at dry state and range between 1.9 MPa and 60 MPa at wet state. That value range between 7.5 MPa and 130 MPa for Sample no. 3 at dry state, whilt it ranges between 3.7 and 142 MPa at wet state.
The elastic modulus (E0) for Sample no. 4 at the dry state range between 10 and 48 MPa, while its value ranges between 5.8 and 53.6 MPa at wet state.
The values of elastic modulus (E0) were plotted as in Figure 1 to determine the distribution of the elastic modulus (E0).
The elastic modulus (stiffness) "E0 " of the studies soil was taken between 8Mpa and 21MPa for the dry soil, and between 10MPa and 52MPa for the wet soil Figure 1. The above analysis shows that the elastic modulus of the wet soils is higher than that for the dry soils.
The consolidation coefficient (cv) for Sample no. 1 at dry state range between 0.009 and 2.05 m2/y, and it ranges between 0.03 and 0.49 m2/y at wet state. That value for Sample no. 2 at dry state ranges between 0.1 and 6.54 m2/y and at wet state ranges between 0.034 and 0.88 m2/y. While it ranges between 0.047 and 6.5 m2/y at dry state and between 0.016 and 15.7 m2/y at wet state for sample no. 3. Sample no. 4 show that the consolidation coefficient at dry state ranges between 0.18 and 1.6 m2/y and between 0.035 and 1.7 m2/y at wet state. The values of the consolidation coefficient were plotted as histogram (Figure 2). The consolidation coefficient (Cv) values were taken, as 0.1-0.2, 0.4-0.8, and 0.8-1.6m2/year for the dry soil, while for the wet soil is 0.1-0.2, and 0.2-0.4m2/year. The consolidation coefficient for the dry undisturbed samples is much higher than that for the wet one.
The consolidation coefficient of the studies soils was compared with the consolidation coefficients for the soils in other countries in the world (table 2). This table show that the consolidation coefficient (CV) for the expansive soil in Jordan at wet state is almost have the same consolidation for the Swedish medium sensitive clays and the Mexico city clay. While that values for the studies soils are much less than the consolidation coefficient of Boston blue clay and Chicago silty clay.
Table 2. Typical Values of the Coefficient of Consolidation (cv) in the world.
Soil Type |
cv (m2/year) |
|
Boston blue clay (CL) (Load, and Luscher;1965) |
12 ± 6 |
|
Organic silt (OH) (Lowe, Zacheo, and Feldman;1964) |
0.6 - 3 |
|
Glacial lake clays (CL) (Wallace, and Otto, 1964) |
2 - 2.7 |
|
Chicago silty clay (CL) (Terzagi, and Peck; 1967) |
2.7 |
|
Swedish medium sensitive clays (CL-CH) (Holts, and Broms; 1972) |
0.1 - 0.2 |
|
San Francisco Bay Mud (CL) (Leonards, and Girault; 1961) |
0.6 - 1.2 |
|
Mexico City clay (MH) (Leonards, and Girault, 1961) |
0.3 - 0.5 |
|
Jordan (Amman city) clay (CH) (The Authors) |
Wet soil |
0.1 - 0.4 |
Dry soil |
0.1-0.2, 0.4-0.8, 0.8-1.6 |
The void ratios as well were determined for these samples. The void ratio (e0) for Sample no. 1 at dry state ranges between 0.586 and 0.647, while it ranges between 0.585 and 0.622 at wet state. That value ranges between 0.615 and 0.668 for Sample no. 2 at dry state and ranges between 0.608 and 0.639 at wet state. For sample no. 3 the void ratio at dry state ranges between 0.552 and 0.612 and between 0.54 and 0.57 at wet state. This value for sample no. 4 ranges 0.661 and 0.714 at dry state and between 0.682 and 0.709 at wet state.
The values of the void ratios show that increasing with the depth for all samples at the wet and dry state. The void ratios against the effective stress were plotted to show the behavior of the dry and wet studies soils. Figures 3 and 4 show that the behaviors of the studies soil are non-elastic, and non-linear. The slope of the curve for reloading is much smaller than that of loading curve, which indicate that the soil on reloading is less compressible than on the first loading.
Figure 3. Compression curves loading, unloading and reloading
Figure 4. Compression curves loading, unloading and reloading
The soil suction for those soils was determined by three different ways. The first value was determined by drawing straight trend line, between the last two void ratio values of the loading, unloading and reloading curve (Figure 5). The value of the soil suction from this way was taken as 166KPa. The second value was determined by drawing a fit straight line between the last four values of the void ratio (Figure 6). The soil suction from this way was taken as 264KPa. As well the third value was determined as in (Figure 7) by drawing a fit curve between the last four values of the loading, unloading and reloading curve. The soil suction from this way was taken as 87KPa.
Figure 5. Soil suction predict from last two points linear line
Figure 6. Soil suction predict from the last four points linear line
Figure 7. Soil suction predict from the last four points curve line
The soil suction was taken as the average of these three values, which is taken as 169KPa.
REFERENCES
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