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Reduction of Vertical Earth Pressure on Buried Pipes by EPS Blocks

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Reduction of Vertical Earth Pressure on Buried Pipes by EPS Blocks Reduction of Vertical Earth Pressure on Buried Pipes by EPS Blocks H. B. Kim Assistant Researcher, Inst. of Construction Technology, Hanjin Heavy Industries and Construction. [email protected] J. M. Kim Senior Researcher, Dept. of Geotechnical Engrg, Korea Institute of Construction Technology. [email protected] S. D. Cho Research Fellow, Dept. of Geotechnical Engrg, Korea Institute of Construction Technology. [email protected] T. S. Joo Senior Researcher, Inst. of Construction Technology, Hanjin Heavy Industries and Construction. [email protected] B. H. Choi Researcher, Dept. of Geotechnical Engrg, Korea Institute of Construction Technology. [email protected] S. Y. Oh Researcher, Dept. of Geotechnical Engrg, Korea Institute of Construction Technology. [email protected] ABSTRACT: This Paper presents experimental data of vertical earth pressure, which is reduced by the compressible inclusion function of EPS blocks placed to the top of a pipe. Previously, Spangler & Handy (1982), Vaslestad(1994), Horvath(1996) and Yuichi. et al.(1996) showed that the insertion of one layer of EPS block was applicable to compressible materials and reduced vertical earth pressure over pipes. In this study, A series of instrumented model indicated that the section which was applied to EPS block was a significant decrease as compared with the section which was not applied to EPS block. Also, a field tests in three conditions concluded that double layers of EPS blocks as well as single layer of EPS block could be effective system for reduction of earth pressure. 1 INTRODUCTION Pipes are generally used in construction projects in the form of sewers, gas lines, water mains, underpasses, conduits, etc. Throughout which installation conditions of pipes become aggravated gradually, the loads which apply to buried pipes tend to increase. The design of buried pipes is accomplished by analyzing equilibrium of forces and moments with considering the load distribution and magnitude acting on pipes after devising construction plan. Since pipes under high earth fills support heavy overburden pressure, the geometry of pipes, such as thickness, require strong section which serves as the key element of a cost-effective design. Important work associated with buried pipes has been theoretically and experimentally studied by many researchers. Marston(1930) published the mathematical derivation of the formula for loads on pipes. Schlick(1932) accomplished experimental studies of loads on rigid pipe in wide ditches. Spangler(1950) conducted field measurements of settlement ratios of various types of pipes. With other research results, the design method of buried pipes has developed and established. In connection with reduction of vertical earth pressure on pipes in high fills, it was eminent that induced- trench procedure, known as the imperfect-trench method, could transmit a large portion of vertical earth pressure, which results from the arching action to the side soil above the top of pipes. Taylor(1973) conducted 1063 the experiments of observing the settlement ratios for the induced-trench method of installation. On the other hand, early in the 20th century, bales of hay or organic material were used to compressible inclusion over pipes, which played an important role in inducing positive arching action. However, their stress-strain behaviors are unpredictable and uncontrollable. Also, it can result in a potential explosion hazard as a result of the methane gas generation that accompanies anaerobic decomposition of organic material in a confined space(Horvath, 1995). Therefore, instead of using the straws, haystacks or organic earth which was a compressible material, a method for paving an EPS Block on the top surface was suggested to reduce the vertical earth pressure applied to buried pipe. EPS blocks first applied as lightweight fill material by the NRRL(Norwegian Road Research Laboratory) on 1972 are widely used on a global scale after having found the advantageous method for that which load reduction are essential in projects. In addition, EPS blocks are the best product for compressible material in comparison with other further-suggested materials in that EPS Blocks have been verified as a fill material for geotechnical engineering. With regard to EPS Blocks which is applied to compressible material, several experimental works have been conducted. Vaslestad(1994) measured the earth pressure on concrete pipes backfilled with well compacted sandy gravel beneath high rock-fills. The tests show that the vertical earth pressure on top of the pipes was reduced to less than 30% of the overburden and the compression of the expanded polystyrene is 26- 27%. Yuichi et al.(1996) and Kim et al.(1998) concluded that EPS blocks as a compressible material are applicable to reduce vertical earth pressure through model tests and trial construction. Reeves & Filz (2000) showed that the lateral earth pressure can be reduced over 50 percent when 25cm of TerraFlex®, similar to EPS block, is used with backfill from instrumented retaining wall tests to simulate a basement wall conditions which are stationary and cyclic-moving. The purposes of this paper are; (1) to perform instrumented model tests and field tests for evaluating the effectiveness of EPS blocks as compressible materials over pipes; (2) to investigate the factors, the geometry of EPS blocks, which affect the reduction of earth pressure. 2 PRINCIPLES OF VERTICAL EARTH PRESSURE REDUCTION The induced-trench (imperfect trench) method of pipe installation, called negative projecting pipes, is used to reduce the loads on pipes under a high fill. The basic concept of the induced-trench method is originated with soil arching, which is that a part of the weight of the soil and any surcharge is transferred between the soil “prism” over the pipe and adjacent soil “exterior prism”. For making in effect a negatively projecting pipe, the soil exterior prisms on both sides of the pipe are compacted more than the soil interior prism above the pipe (Spangler & Handy (1982)). This phenomenon can result in loads that are significantly less (positive arching). In these principles, the presence of a compressible material effectively serves to reduce vertical earth pressure. Although the method has been used successfully with pipes under some unusually high fills, the magnitude of the reduction in load achieved by the induced trench has not been clearly established. Also, the application of double compressible materials is not yet introduced and verified. Fig. 1 shows the schematic diagram of buried pipes with and without EPS blocks. W s W s W s F2 F2 EPS F1 F1 F1 F W' W' W' 1 EPS EPS K(W s+F1) K( W s+F1+F2) W' = W s W' = W s - 2F1 W' = Ws - 2(F1+F2) (a) No EPS block (b) One layer of EPS block (c) Double layers of EPS blocks Figure 1. Schematic Diagram of buried pipes with and without EPS blocks 1064 3 INSTRUMENTED MODEL TEST 3.1 Description of Test facility The instrumented soil bin facility consisted of steel and the acrylic wall front which it makes possible to observe inside. The size of soil bin was 1.4m long, 1.0m wide and 0.9m high. The steel frame with screw jack and steel loading plate enabled the surface of fill soil to surcharge the overburden pressure. The overburden pressure plate was applied for overcoming low stress level that was limited by small size of soil bin. The size of sand bucket for filling soil in the bin was 0.5m long, 1.0m wide and 1.3m high. Four soil transducers attached to buried pipe were used to observe the earth pressure. The diameter of soil transducer was 50mm and the maximum measurement capacity of soil transducer was 20tf/m2. 3.2 Fill and Pipe Material Characteristics The fill material used for the instrumented model tests was Jumunzin silica sand obtained from Jumunzin eup, Korea. From laboratory tests, it was found that the specific gravity was 2.63, the maximum density was 1.68 tons per cubic meters, and the angle of internal friction was 33 degrees. Also, the sand classified as poorly graded sand (SP) according to the Unified Soil Classification System. The density of EPS block was 15kg/m3 and the thickness of EPS block was 5cm. Selection of EPS block density and thickness of EPS block were based on the consideration of its engineering property, economical efficiency and previous studies suggested by Vaslestad(1994), Horvath(1996) and Kim et al.(1998). The pipe, the corrugations run helically around, used for instrumented model test was made of steel and mill coated with zinc. Since the pipe dimensions were determined by considering stiffness condition for the field test, its diameter and thickness were determined to 100mm and 0.5mm respectively. 3.3 Instrumented Model Test Procedures First, the pipe which is equipped with four soil transducers was placed on the center line of bottom plate and the fill material was poured gently into the soil bin by means of a sand bucket with crane. EPS block as a compressible material was placed above the top of the pipe prior to placing the fill soil. Two soil transducers were mounted the side of the pipe for measuring lateral earth pressure and others were mounted the top of the pipe for measuring vertical earth pressure. Nine tests were performed for making the mechanism of a compressive material clear. A summary of test conditions is provided in Table 1. Table 1. Instrumented Model Tests Performed The presence of EPS block Test Items Factors No EPS block Stress Distribution of soil bin(Test 1-Test 2) with or without pipe The width of EPS(Test 3-Test 5) 1.0D, 1.5D, 2.0D EPS blocks Two layers of EPS blocks(Test 6-Test 9) 0.5D, 1.0D, 1.2D, 1.5D * D: Diameter of pipe As can be seen from Table 1, Test 1 and Test 2 were performed for finding out the difference of stress distribution between with and without pipe.
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