FIELD STUDY ON COLLAPSIBLE SOIL BORG EL ARAB REGION-EGYPT Hisham H. Abdelmohsen, Professor, Department of Civil Engineering, Alexandria University, Egypt Naema.A. Ali, Lecturer Higher Institute for Engineering & Technology, Behera, Egypt ABSTRACT: Collapsible soils are generally characterized by their loose structure of bulky shaped grains, often in silt to fine sand size with a small amount of clay. Salts, dried clay or silts and other chemical precipitats often form a significant component of its cementation. Collapsible soils have a loose open, metastructure binded by cementing agents, that which upon wetting become weak and may dissolve causing collapse. The existence of collapsible soils has been reported in the world continents. In Egypt, Collapsible soils zones are located in the northern portion of the western desert including Borg El-Arab region and around Cairo city. The article presents laboratory and field measurements on collapsible soils improved by pre-wetting and compaction prior to construction of foundation system. Results proved that improvement of collapsible soils is possible to prevent / mitigate their risk potentials against sudden settlement when exposed to a source of water. Field investigation was performed in the form of plate loading tests conducted on collapsible soil improved by pre wetting followed by mechanical compaction. INTRODUCTION Almost all naturally occurring collapsible soil deposits are either debris flows deposits or wind deposits (loess). Debris flows are at low density, but are relatively stiff and strong in their natural dry state. Loessial deposits compries a relatively narrow size range of particles usually silt to fine sand, with coatings of small amounts of clay being common 1. Loess exhibits low density, moderately high shear strength and stiffness when dry and is subject to densificantion and collapse upon wetting under load. Collapsible soil exhibits considerable strength when it is dry, when it is inundated, it loses its strength and experiences significant volume reduction. Cementations consist of dried clay and other chemical precipitats that may have been added after deposition. Wetting under load weakens the cementation, reduces the soil suction and causes densification or collapse. In practice of collapsible soils engineering, geotechnical engineers are faced with identification and characterizeation of collapsible soil sites, estimation of collapsible strains and collapsible settlements and selection of design / mitigation alternatives. Site estimation of the extent and degree of wetting can be infield for natural arid region conditions in undeveloped areas; rainfall either runs off suface soils or infiltrates a relatively shallow depth into the soil and then evaporates to the surface. These sources of added soil water include landscape irrigation, broken water or sewer lines, roof run off, poor surface drainage and local ground water reacharge. In collapsible soil areas, numerous cases are reported in which failure or severe damages of public facilities such as schools, highway maintenance buildings, jail facilities, water tanks, roads, 1
and other infrastructures 2, 3. The greatest problems with collapsible soils arise when the existence and extent of the collapsible potential are not recognized prior to construction. Usually, the soil collapses caused by minor change in their water content, weakening of soil cementation in the form of large settlement and losing of bearing capacity. The magnitude of soil collapsibility usually depends on initial porosity. The basic characteristics of collapsible soils are categoried as, high porosity, low saturation, high silt content and rapid softening in the water. Many researchers have been evaluating soil collapsibility by defining various criteria, and also many documanations studied the effeciveness of treatment methods for collapsible soils 9. There are many documentations 4, 5 concerning soil collapsiblity phenomenons. They reported that the most important factor, arguable, is that of inter-particle bonding either cementation, chemical or physical attraction or negative pore pressures (matric suction) 4, 5. The forms of bonding are numerous and include such natural formations of clay bridges and salts and capillary tension (suction). Salts and dried clay and silts often form a significant component of cementation for collapsible soils. Dependening on salt composition, these cementation bonds may be unstable or stable in water 6. In Egypt, collapsible soils were observed to exist in northern portion of the western desert, such as north coast, Borng EL-Arab region and close to Cairo city such as Six-of-October area and Tenth of-ramdan district 7. At or near saturation, loess soils in Borg-El-Arab exhibit high susceptibility for collapse, also referred to as hydroconsolidation or hydrocompaction. The loess possesses the three factors needed to produce collapse in soil, loose unsaturated soil structure, a cementing agent (clay in this case, which stabilizes the soil in an unsaturated condition) and a high enough applied or existing stress component to induce collapse. The basic objective of this paper is to investigate the collapsibility characteristics and behaviour of the materials with particular refernce to their response to wetting under different stress levels in the field using plate load test. In this test, the water is introduced to the loaded soil and the resultant displacement due to wetting is recorded. The study also provides solution to improve collopsibility potential risk before construction of foundations. Statement of the problem The arid conditions and deep water table favor the foundation on collapsible problematic soil in Borg EL-Arab (Egypt). The detrimental volume change for collapsible soils is always triggered by increasing of soil water content, inparticular in developed and urban regions. The curent study investigates characteristics and collapsibility potential of natural soil of Borg-El-Arab area, west of Alexandria Egypt. Various ground modification methods can be used to prevent or limit collapse from occurring, or cause the collapse to occur befor construction. Densification of the collapsible soil in place, such as by prewetting followed by dynamic compaction to cause pre-mature settlement before construction are considered in this study. Characteristics of natural soil are experimentally evaluated on undisturbed samples recovered from the site. Soil characteristics and experimental program Experimental investigation program has been carried out to determine the index properties and behaviour of natural deposits at Borg EL-Arab area near Alexandria city, north of Egypt. Undisturbed soil samples were collected for laboratory tests by means of block sampling cut from 2
pit excavated at diffeent sites from ground surface down to 2.0 m depth. Samples were carefully trimmed and waxed. Undisturbed soil samples represented the collapsible soil at four locations at engineering town in Borg EL-Arab-second discrit. A laboratory testing program was designed to determine the geotechnical properties. Soil composition and collapsiability characteristics of the soil samples were tested for water content, unit weight, grain-size / (including hydrometer analysis) and atterberg limits using (ASTM) standard procedures 8, 9. Also, single-oedometer test (ASTM D5333) was conducted through loading the specimens incrementally to a specificed vertical stress and allowing the sample to come to equilibrium under the applied pressure. The sample is then flooded with water and deformation is measured with time. The Collapsibility Potential C p is defined as a ratio between the change in height of the soil sample upon inundation with water at a particular confining stress in consolidation ring, dh, and the initial height of the specimen, H o. Table 1 shows geotechnical properties of the collapsible soils of Borg EL-Arab region based on results of a laboratory testing program on undisturbed soil samples recovered from the test sites. Table 1. Index properties and Collapsibility Potential of Borg EL-Arab undisturbed soil samples Soil properties Sample 1 PL A,B Sample 2 PL C,D Sample 3 PL1,2 Sample 4 PL 3,4,5 Initial Water Content (%) 6.3 8.6 10.2 12.0 Natural Unit Weight ( kn/m 3 ) 14.4 15.5 16.4 17.2 Specific Gravity G s 2.65 2.68 2.72 2.72 Degree of Saturation, S (%) 17 26 33 42 Liquid Limit (%) 32 30 34 35 Plasticity Index (%) 9 7 13 15 Percentage of Sand (%) 22 30 36 30 Percentage of Silt (%) 72 62 54 58 Percentage of Clay (%) 6 8 10 12 Collapsibility Potential C p (%) 12.6 11.2 9.3 8.2 In addition, compaction tests were carried out on soil samples in accordance with modified Proctor procedure, ASTM D1557. Maximum dry unit weights of compacted samples were found to vary from 16.2 to 19.2 kn/m 3 with an average value of 17.7 kn/m 3 at optimal water content of 14.4% to 18.4%. Water was carefully mixed with the soil to reach the desired water content. From the laboratory results, soil collapsibility potential C p of undisturbed test samples varied from 8.2% to 12.6% with an average value of 10.4%. The natural soil classified as problematic / severly problematic 7. As shown in Table 1., the higher is the degree of saturation and clay content, the less is the collapsibility potential C p, where both values have reverse effect on water influence on de-bonding of soil particles. Field tests program 3
Plate loading tests were performed at depth 2.0 m below the ground surface on circular rigid plates having diameters of 300 and 450 mm to study the field behavior of natural collapsible soils with particular reference to their response to wetting under different stress levels. The bearing plate tests were conducted on natural soil before and after flooding with water and compaction at test level. In these tests, the water is introduced to the loaded soil and the resulting displacement due to wetting is recorded. After the loaded plate is removed, the depth of wetted zone was measured. Assuming that the collapse strain is confined within the wetted zone, the average vertical strain is thus computed as the settlement divided by the vertical depth of wetted zone 10. Nine plate loading tests were carried out in accordance with ASTM D1194 8, 9, to determine pressure settlement response of collapsible soil in-situ. Also, in-situ dry unit weight and water content were mesured versus depth below subgrade surface. Table 2 illustrates the field tests program. Case ID Test A Test B Test C Table 2. Details of testing program on Borg EL-Arab collapsible soil Conditions Natural Unsaturated Collapsible Soil (D plate =450mm.) Flooded Without Compaction (water was allowed to cover the top soil surface by about 50mm thick for a number of hours, loose soil was removed) (D plate =450mm.) Natural Unsaturated Collapsible Soil/ Flooded during test (D plate =450mm.) Test D Prewetting and compacting/ Flooded at stress level of 200kN/m 2 ( soil surface was sprayed with uniform quantity of water ) (D plate =450mm.) Test 1 Test 2 Test 3 Test 4 Test 5 Field test results and discussion Dry Compaction (D plate =300mm.) Prewetting / Compaction duration 8 hr. (D plate =300mm.) Prewetting / Compaction duration 16 hr. (D plate =300mm.) Flooded / Compaction duration 16 hr. (D plate =300mm.) Flooded / Compaction duration 8hr. (D plate =300mm.) Test A was conducted on natural subgrade, whereas Test B was conducted on collapsible soil after flooding the soil. Figure 1. shows that settlement increases upon flooding. Following the common procedure for calculating the bearing capacity as intersection point between initial and final straight lines of the curves, bearing capacity was found to drop to 65% upon wetting. As expected, water flooding causes reduction in bearing capacity of collapsible soil. To mitigate the effect of water on collapsible soils, a new safety factor against bearing capacity failure should be developed to account for water infiltration due any source during structure life time. This means that, the allowable bearing capacity of natural collapsible soils is recommended to use conservatively one and half the factor of safety recommended by various codes for normal soils to account for flooding effect on collapsible soils 6. Figure 2. shows the stress settlement relationship of Test C (natural sample) and Test D (prewetted sample). The loose structural arrangement of the particles for natural collapsible soils is the key element leading to collapse phenomenon 7. The figure shows that the collapsible soil settlement increases after wetting. When a collapsible soil becomes wet, influxes of water breaks down soil arrangement causing loss in strength of binders which causes large and sudden reduction in soil volume and soil to densify. The collapsibility potential 4
decreases to about 0.18 of the original value at dry conditions due to pre-wetting and compaction as indicated in Figure 2. Compaction reduces collapsibility potential of soils, dropping the risk by more than 80%. The water content under the plate in Test C is measured after removing test assembly. In this test, where artificial wetting is applied to cause settlement collapse, the water content before and after wetting are given in Figure 3. The upper compacted subgrade layer has been excluded from the subgrade stratum. This exclusion is deriveds from the fact that the silty layers when compacted lose their entire collapsible characteristic. Moreover, these layers assist in reducing the amount of water penetration into the uncompacted subgrade. Water penetration through soil is directly proportional to permeability. The more is the clay content, the less is the permeability and the peneration depth. For the curent case, water penetration reashed as far as 4.5 plate diameter. Figure 4-a. shows the comparison between the collapsible soil settlements versus applied stress for Tests 1 through 4, Table 2. Rollers are used to compact soil surface at foundation depth to densify the collapsible soil with and without wetting. The figure indicates the effect of prewetting, followed by compaction on collapsible soil. The figure depicts that the strength of natual collapsible soil decreases upon prewetting and compacting. In Figure 4-b. load-settlement values have been plotted on a double logarithmic scale, to fall on two approximately straight lines. Ultimate bearing capacity values of the four tests are 210, 170, 140 and 110 kn/m 2. respectively with reduction of 0.80, 0.66 and 0.52 with respect to dry natural collapsible soil (unsaturated soil). The curent engineering practice in treating collapsible soils is pre-wetting followed by dynamic compaction. Although such process can eliminate some of the future settlement upon wetting, it can not fully mitigate future occurance of collapsibile settlement. Prewetting causes soil bonds to collapse (fail) and a new soil pack to form which would be subject to subsequent rearangement and thus movement upon further wetting. The effect of compaction duration is well manifested by Figure 5., It is evident that addatinal 8 houres duration of dynamic compaction improved the soil. Settlement has decreased and ultimate bearing capacity has increased by about 1.25. Figure 5., shows the good news; compaction can eliminate collapse settlement, increases soil unit weight and thus increases the shear strength of the soil. Compaction can reduce collapsibility risk in shallow depth. However, compaction is a relativly shallow treatment. However, less effect is expected on deep collapse of soil. Flooding followed by compaction seems approperiat process to mitigate collapsibility potential risk. D =Plate diameter 5
Figure1. Settlement- stress relationship under loaded plates - Test A & B D =Plate diameter Figure 2. Settlement Stress relationship under loaded plates - Test C & D Figure 3. Water content / Percent of clay content with depth under plate following removable of Plate assembly Test C 6
4.a 4.b Figure 4., Settlement- Stress relationship beneath loaded plates - Test 1 / 4 Figure 5., Settlement- Stress relationship beneath loaded plates - Tests 4 and 5 Curves in Figures 4 and 5 indicate that the effect of pre-wetting and slight compaction is inappreciable. The figures depicted that, there is a significant effect of pre-wetting and complete compaction duration in gaining partially the original dry strength of collapsible soils. The compaction on the wetting side is more effective than that on the dry side at the same stress level. The safe foundation load can be estimated, based on the ultimate bearing capacity at saturated state (significantly less than that at natural unsaturated state). CONCLUSIONS Results of conducted field tests on collapsible soils of Borg Al-Arab area Alexandria, north of Egypt indicated that compaction treatment provides effective means of reducing future settlement associated with increase of water content. Flooding followed by compaction, increases compaction efficiency and uniformity which reduce settlement beneath future foundation system. The total amount of collapsibility potential depends on initial moisture contant, extent of wetting zone beneath loaded plate, duration of wetting and the pattern of water migration. Prewetting with water is easy; however, it is ineffective in reducing collapsibility potential for shallow foundation. Prewetting without preloading via compaction is not sufficient to prevent future foundation settlement. For compacted collapsible soils, the increase in initial water content decreases collapsibility potential risk. It is thus recommended to compact collapsible soils at water content slightly higher than optimal water content requried by laboratory compaction test. The results of field Plate Loading Test indicat that the bearing capacity of collapsible soil decreases in average to about 65% from original value estimated at natual dry condition as a result of pre-wetting and without compaction. Immediate foundation failure may result due to inundation, if inundation pressure exceeds the ultimate soil strength at saturated condtion. In order to obtain a safe design load for a shallow foundation resting on natural unsaturated collapsible soil, evaluation of load-settlement response at fully saturated condition is recommended. One and half the safety factor against bearing capacity failure seems proper to mitigate failure risks associated with future wetting of collapsible soils. 7
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