Performance of Bituminous Coats for Dragload Reduction in Precast Piles

Transcription

1 Performance of Bituminous Coats for Dragload Reduction in Precast Piles M.G. Khare Research Scholar, Indian Institute of Technology Madras, Chennai, India S.R. Gandhi Professor, Indian Institute of Technology Madras, Chennai, India ABSTRACT: The dragload is the load transferred to the pile due to negative skin friction developed on the pile shaft where surrounding soil settles more compared to the pile. Field measurements in the past have recorded magnitude of dragload ranging from 3 kn to 76 kn. Dragload on precast piles can be reduced by applying a coat of bituminous compound on pile shaft. This paper investigates performance of commercially available bituminous compound Shalikote (T-2) and 3-4 grade bitumen to reduce the skin friction between pile and soil. Direct shear tests were carried out to quantify the reduction in interface friction between soil and pile material. Poorly graded sand was sheared against coated and uncoated mild steel surface in direct shear test apparatus. Steel surface was coated with Shalikote (T-2) and 3-4 grade bitumen of 2, 3 and mm thicknesses. Test results indicate that Shalikote (T-2) compound reduced the interface friction between sand and mild steel up to %. Bitumen coat achieved maximum reduction in shear stress. The reduction in shear stress ranged from 8% to 97% depending on normal stress and coat thickness. For all practical purposes bitumen coat of 3mm thickness appears to be sufficient to reduce dragload. Coating pile with bitumen coat is the most practical and economical method reducing dragload on precast piles. 1 INTRODUCTION The dragload is the load transferred to the pile due to negative skin friction developed on the pile shaft where surrounding soil settles more compared to the pile. Pile foundations may fail because of dragload. Literature shows failure of pile foundations when dragload is not accounted in pile design. Several field measurements have recorded the enormous magnitude of dragload experienced by piles ranging from 3 KN (Fellenius, 1972) to as high as 76 KN (Bozozuk and Labreque, 1969). These measurements have shown that dragload on piles can exceed the design loads and may lead to structural failure of piles and/or bearing capacity failure in the bearing soil stratum. Dragload could have adverse effect on the economy of the project and reduction of dragload may become necessary. 2 METHODS OF DRAGLOAD REDUCTION Field experience have shown that dragload on short piles (of length 8m or less) is small and can be neglected in design. Dragload on piles of intermediate length can be resisted by increasing pile capacity, by providing additional piles, or by reducing pile spacing. However, when large dragloads are anticipated dragload mitigation by one of the following methods is necessary: 2.1 Preloading Preloading the site with fill before pile installation reduces relative settlement of soil to pile and the dragload. Preloading technique can be employed in projects with long term planning to accomplish the preconsolidation of soft soil. The time required for preconsolidation can be reduced considerably with sand drains, band drains or granular columns. 2.2 Electro Osmosis The technique of electro osmosis is commonly employed to temporarily reduce the adhesion between clay and steel piles to be pulled out of ground. Bjerrum et al. (1969) demonstrated the technique of electro osmosis to reduce the dragload wherein piles act as cathodes. During the field tests when the direct current passing through the cathode pile was increased from 4 amperes to 8 amperes dragload reduced to negligible values. The treatment of electro osmosis must be continued until the settlement of surrounding soil is completed. Electro osmosis is expensive compared to other methods and therefore rarely used.

2 2.3 Separation of Soil and Pile Shaft Negative skin friction can be eliminated by installing pile inside a predriven casing. Use of casing is not advisable when the piles are required to provide lateral support. Separation of soil and pile can also be achieved with tapered piles. Model tests conducted by Sawaguchi (1982) showed 9% reduction in dragload for tapered piles compared to straight piles. 2.4 Protection Piles around Pile Group The technique consists of a system of closely spaced inner toe bearing driven piles surrounded by protection piles to carry dragload. Okabe (1977) reported successful application of protection pile technique in construction of a marshalling yard in Japan. The protection piles installed at the perimeter of pile group were vertically separated from the footing. Dragload of 3 KN was recorded on outer protection piles while inner piles supporting the foundation were virtually free from dragload. 2. Slip Layer Technique to Reduce Soil-Pile Friction using Bentonite Slurry and Bitumen Coat Test results reported by Brons et al. (1969) and by Bjerrum et al. (1969) indicate that bentonite slurry around the piles can reduce the negative skin friction. Edwards and Visser (1969) have reported a case where the negative skin friction was reduced with bentonite. The investigated pile was protected by a 3 mm to 4 mm thick bentonite layer. The dragload on protected pile was 12 KN compared to 7 to 8 KN for unprotected piles. Coating the pile with bitumen is the most economical method for reducing negative skin friction (Baligh et al., 1978). Measurements by Brons et al. (1969) and by Bjerrum et al. (1969) indicate that a thin layer of bitumen coating is sufficient to reduce the negative skin friction. Test results reported by Bjerrum et al. (1969) show that 1 mm thick bitumen coating reduced the dragload to a magnitude less than 1% of that of uncoated piles. Field tests indicate that bitumen behaves as a nonlinear viscous fluid and viscosity of bitumen depends on the ground temperature. The thickness of the bitumen coating is generally 1 mm to mm. Bitumen coating is susceptible to deformation during storage in hot weather. Cold water sprays or storage of piles under water may become necessary to prevent the melting of the bitumen and effects of temperature variations. It is important to protect the bitumen coating from being scraped off during pile driving in rock or gravel fills. The thickness of the bitumen layer may be increased as a precaution in cases where there is potential of scrapping of bitumen during driving. 3 LABORATORY STUDY TO INVESTIGATE EFFECTIVENESS OF BITUMINOUS COATINGS IN REDUCING DRAGLOAD The effectiveness of coating in reducing dragload depends on characteristics of the pile, the soil and the coating material itself. In case of fine grained soils, the shearing behavior of the coating depends on the average rate of settlement of soil. In case of coarse grained soils, soil particles slowly penetrate into the coat causing significant increase in the negative skin friction. Test results have shown that the negative skin friction for bitumen coated piles in coarse grained soils reaches a maximum value in less than a month and that bitumen at this stage behaves as viscofrictional material with complex properties (Baligh et al. 1981). Coating material should have low viscosity to permit the slippage of soil surrounding pile shaft and at the same time it should adhere to pile shaft during storage and pile driving. A soft and thicker coating results in small dragloads. In India there are no guidelines available to select the coat type and thickness for dragload reduction. The purpose of this laboratory investigation was to find suitable type and thickness of coat for dragload reduction. 3.1 Methodology The maximum unit negative skin friction develops on pile shaft where pile is passing through granular soil. The particle penetration of granular soil in to coating material during pile driving may result in scrapping off the coat. Therefore it is necessary to study the efficiency of coating material in reducing the interface friction between granular soil and pile shaft. The methodology involved modeling of interface friction between granular soil and pile shaft using direct shear apparatus. Pile shaft was represented by a solid mild steel box with size of 8. by 8. by 2.8 cm. The conventional direct shear apparatus was modified to conduct interface friction tests as shown in Figure 1. The properties of granular soil used in study are listed in Table 1. D Table1. Properties of Granular Soil D 1 (mm) (mm) C u C c G s γ max (KN/m 3 ) γ min (KN/m 3 ) The granular soil was classified as poorly graded sand (SP) as per IS and hereafter referred as sand. Two types of coating materials were used in this study namely Shalikote (T-2) and bitumen.

3 Solid Mild Steel Box/Cement Mortar Block Normal Load 8. cm Top half of shear Apparatus Grid Plate Coat Sand 2.8 cm inside the 6 cm by 6 cm mould placed on top of mild steel box. The Shalikote (T-2) took more than 24 hours to cure. The end of curing was indicated by change in color of coat from brown to black. Sand was then placed directly on top of cured coat at 7 percent relative density by pluvial deposition technique. After placing sand on top of coat the desired normal stress was applied through sand. The top half of direct shear apparatus was then lifted with the help of three lifting screws so that it remains just above the top of coat as shown in Figure 1. The soil was then sheared against the coated mild steel box at.2 mm/min rate of shear. The coating of 2, 3 and mm thickness were used. All tests were conducted at an ambient temperature of 31 C. Figure1. Schematic diagram of direct shear test on coated mild steel box and sand. Shalikote (T-2) is dispersion of selected grades of bitumen in water. It is used as a protective coating over steel to prevent rusting. It has semi solid consistency and can be applied cold on a surface. It can withstand temperature variations and vibrations. The bitumen coat used in present study had a penetration value between 3 and 4 and softening point between C and 6 C. The first set of tests was conducted to measure the residual shear stress at the interface of mild steel box and sand. The top half of direct shear apparatus was placed on solid mild steel box and secured in position with locking pins. Sand was placed in top half of direct shear apparatus at 7 percent relative density by pluvial deposition technique. The apparatus used for pluvial deposition was calibrated to get the required density of sand in direct shear apparatus. The calibration curve of height of fall and relative density is shown in Figure 2. Normal stress was then applied through the soil to the sand and mild steel interface and sample was sheared. All tests were conducted at.2 mm/min rate of shear. The second set of tests was conducted to study the reduction in shear stress by coating the mild steel box with 3-4 grade bitumen and Shalikote (T- 2). Bitumen was heated to 1 C and poured in a 6 cm by 6 cm mould placed on top of mild steel box. The coat was allowed to remain in mould for 24 hours. After a period of 24 hours the mould was removed and the top half of direct shear apparatus was carefully placed on mild steel box so as not disturb the coat. Sand was then placed directly on top of bitumen coated mild steel box at 7 percent relative density. In case of Shalikote (T-2), the semi solid coat was thoroughly mixed and applied at uniform thickness ) Relative Density (% Height Of Fall (cm) Figure2. Pluvial deposition calibration curve 3.2 Results The reduction in shear stress was considered as a measure of effectiveness of coat. Tests with Shalikote (T-2) showed initial increase in interface friction followed by substantial reduction as sample was sheared. In case of bitumen, interface friction increased as particles penetrate in to coat and then remained almost constant. Tests showed that full interface friction is mobilized at a relative movement of few millimeters. However where dragload mitigation is required soil undergo large settlements with respect to pile. Residual shear stress has more significance than the peak shear stress in calculating the dragload on coated piles. Therefore in present study residual shear stresses are considered to compare performance of Shalikote (T-2) and bitumen. The results of Shalikote (T-2) coated steel box and sand are presented in the form residual shear stress versus normal stress plots in Figure 3. Reduction in residual shear stress with respect to thickness for Shalikote (T-2) at different normal stresses is presented in Figure 4.

4 Results for bitumen coated steel box are presented in Figures and 6. be attributed to component of adhesion of Shalikote (T-2). Shear Stress (kpa) Uncoated 1mm shalikote 1.36mm shalikote 2.16mm shalikote Shear Stress (kpa) Uncoated 2mm Bitumen 3mm Bitumen mm Bitumen Normal Stress (kpa) Figure 3. Shear stress versus normal stress plot for sand and Shalikote (T-2) coated mild steel box Normal Stress (kpa) Figure. Shear stress versus normal stress plot for sand and bitumen coated mild steel box. 1 Residual Shear Stress (kpa) 1 Normal Stress=2 kpa Normal Stress=kPa Normal Stress=7 kpa Shalikote Thickness (mm) Figure 4. Variation in shear stress with thickness of Shalikote (T-2) at different normal stresses. Analysis of test results suggests that bitumen coat achieved maximum reduction in residual shear stress for all normal stresses and all thicknesses. At normal stress of 2kPa, residual shear stresses for specimens coated with 1mm and 1.36mm thick Shalikote (T- 2) were marginally higher than those obtained for sand and uncoated mild steel box. This behavior may Residual Shear Stress (kpa) Normal Stress =2kPa Normal Stress=kPa Normal Stress=7 kpa Thickness of Bitumen Coat (mm) Figure 6. Variation in shear stress with thickness of bitumen coat at different normal stresses. The residual stresses for specimens coated with Shalikote (T-2) were 7% and 47% to that of uncoated specimens for normal stresses of kpa and 7kPa respectively. Shalikote (T-2) coating showed substantial reduction in coat thickness due to

5 shrinkage. Initial coating thickness of 2mm, 3mm and mm reduced to 1mm, 1.36mm and 2.16mm after curing. Shrinkage of coat may pose problem of cracks when applied to surface of prototype piles in field. For Shalikote (T-2) percentage reduction in shear stress ranged from 23% to 6%. Bitumen coated specimen showed 8% to 97% reduction in shear stress when compared to uncoated specimen. As the coat thickness increased from 2mm to mm the shear stress decreased substantially. For a coating thickness of 3mm the percentage reduction in shear stress was 9% or more and would be adequate for all practical situations. Therefore considering the economic aspects and design requirements 3mm coat would be efficient in reducing the dragload. 4 CONCLUSIONS In this study, an experimental investigation was carried out to compare performance of Shalikote (T- 2) and 3-4 grade bitumen of 2, 3 and mm thicknesses in reducing the dragload on piles. Direct shear tests were carried out to measure reduction in interface friction of sand and mild steel box when coated with bitumen and Shalikote (T-2). Initial coat thickness of 2mm, 3mm and mm were used. Shalikote (T-2) achieved 3% to % reduction in shear stress. Shalikote (T-2) may develop shrinkage cracks after applying it on pile. The bitumen coat achieved maximum reduction in shear stress. The reduction in shear stress ranged from 8% to 97% depending on normal stress and coat thickness. It was found that as coating thickness increased from 2mm to mm shear stress decreased for a given normal stress. For all practical purposes coat thickness of 3mm appears to be sufficient to reduce dragload. Based on this study coating pile with bitumen coat is the most practical and economical method reducing dragloads on precast piles. Performance of Deep Foundations, ASTM STP No. 444, pp Brons K.F., Amesz A.W., and Rinck J. (1969). The Negative Skin Friction along Shaft of a Foundation Pile. Specialty Session 8, Negative Skin Friction and Settlements of Piled Foundations. 7th Int. Conference on Soil Mechanics and Foundation Engineering, Mexico City, Paper 2. Fellenius, B.H. (1972). Downdrag on Piles in Clay due to Negative Skin Friction. Canadian Geotechnical Journal, vol.9, No.4, pp Okabe, T. (1977). Large Negative Friction and Friction-free pile methods. Proc. 9th Int. Conference on Soil Mechanics and Foundation Engineering, Tokyo, vol.1, pp Sawaguchi, M. (1982). Model tests in relation to a method reduce negative skin friction by tapering pile. Technical Note, Soils and Foundation, vol. 22, no.3, September 1982, pp REFERENCES Baligh, M.M., Vivatrat V., and Figi, H. (1978). Downdrag on Bitumen-Coated Piles. Journal of Geotechnical Engineering, ASCE, vol.14, No.11, pp Bjerrum, L., Johannessen, I.J., and Eide, O. (1969). Reduction of Negative Skin Friction on Steel Piles to Rock. Proc. 7th Int. Conference on Soil Mechanics and Foundation Engineering. Vol.2, pp Bozozuk, M. and Labrecque, A. (1969). Downdrag measurements on 27-ft. Composite Piles.