Reduction of Vertical Earth 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 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- 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 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 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 . 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 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 can be reduced over 50 percent when 25cm of TerraFlex®, similar to EPS block, is used with backfill from instrumented 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 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 was 33 degrees. Also, the sand classified as poorly graded sand (SP) according to the Unified 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. In all tests, falling height of sand was 50cm(the relative density 87.5%) and overburden pressure was varied from 5.0 to 15.0tf/m2. Test 3 through 5 was performed for verification of the effectiveness of a compressible material which mobilizes shear stresses. Test 6 through 9 was performed using two layers of EPS blocks and was conducted for verification of the effectiveness of double EPS block layers which can maximize the shearing stresses at both sides of soil prism by widening the vertical shear plane. The performance condition of Test 6 through 9 was that the width of EPS block was fixed at same size of pipe diameter and the spacing between each layer of EPS block varied from same size of pipe diameter to 1.5 times of it. For each test, the earth on buried pipe were continued recording until there was little change over time.

1065 20 20 18 18 16 16 14 14 12 12 10 Test 1 10 Test 1 8 8 Test 2 Test 2 Test 6 6 6

Surcharge(tf/m2) Test 3 Surcharge(tf/m2) Test 7 4 Test 4 4 Test 8 Test 9 2 Test 5 2 Test 3 0 0 0 5 10 15 0 5 10 15 Vertical Earth Pressure(tf/m2) Vertical Earth Pressure(tf/m2)

(a) Test 1 through Test 5 (b) Test 1, Test 2, Test 3 and Test 6 through Test 9 Figure 2. Measured Vertical Earth Pressure in Different Test Conditions

3.4 Instrumented Model Test Results

3.4.1 Stress Distribution of soil bin Figure 2 shows variation of vertical earth pressure observed by pressure transducers. In Test 1 and Test 2, vertical earth pressure with pipe measured up to 8.6 tf/m2 under the 15 tf/m2 of overburden pressure. On the other hand, vertical earth pressure without pipe was measured up to 14.2 tf/m2 under same overburden pressure condition. As these results indicated that an arching area of the top of pipe occurred, the case of Test 2 was smaller magnitude of soil stress than test 1.

3.4.2 Influence of the width of EPS block In the variation of the width of EPS block, Test 3 through Test 5 was performed to investigate the best sizes of EPS blocks in reducing the vertical earth pressure. As can be seen in Figure 2 (a) and Table 2, all the tests using EPS block showed the significant decrease in vertical earth pressure. By evaluating the best width of EPS block, Test 4 width condition, which was 1.5 times longer than pipe diameter, was the best reduction effect of vertical earth pressure. It is because stress transform, occurred on either side of the soil prism over pipe, affects the vertical earth pressure increase of Test 3 in comparison to Test 4.

Table 2. Summary of Laboratory Model Tests Result (surcharge = 15.0 tf/m2) Vertical Earth Pressure Percent Reduction in Thickness and No. of layers (EPS blocks) Vertical Earth Pressure 2 (tf/m ) (%) Test1(without pipe) 14.2 No EPS block Test2(with pipe) 8.6 100 Test 3(1.0D) 3.2 63 EPS block with one Test 4(1.5D) 2.3 73 layer Test 5(2.0D) 2.7 69 Test 6(0.5D) 2.9 66 EPS blocks with two layers Test 7(1.0D) 2.5 71 (The width of EPS Test 8(1.2D) 3.3 62 block was fixed 1.0D) Test 9(1.5D) 4.0 53 * D: Diameter of pipe

1066 Relatively, there was a little increase in vertical earth pressure for Test 5 compared to Test 4. It is note that the use of excessive width of EPS block brings about the increase of vertical earth pressure which reason comes from the larger soil prism condition over the pipe. Although Test 4 shows the most significant vertical earth pressure reduction, the same width EPS block of pipe diameter was selected for tests to evaluate the influence of the spacing of EPS block with considering the cost effectiveness and construction easiness.

3.4.3 Influence of the spacing of EPS block As described section 2, double layers of compressible materials theoretically maximize shear stresses which partially support interior soil body at the vertical shearing interface between interior and exterior prism. In this regard, we conducted Test 6 through 9 for researching the effectiveness of double layers of EPS blocks. Figure 2 (b) indicates the vertical earth pressure readings which were applied to double layers of EPS blocks. Although the value of vertical earth pressure recorded such a relatively small amount that it was hard to evaluate the results, all tests data using two layers of EPS blocks except Test 9 showed a tendency to reduce the vertical earth pressure in comparison to Test 3 which of EPS block size and property was the same as Test 6 through 9. Therefore, it experimentally proved that double layers of EPS blocks could maximize the friction forces, which act upward in direction, compared to the friction forces of one layer of EPS blocks. In case of Test 6 and Test 7, as the spacing of EPS blocks increase, there was a decrease in vertical earth pressure, which came from the widening the vertical shear plane. Test 7 through 9, however, indicated that the value of vertical earth pressure was proportional to the spacing between two layers of EPS block. It showed that although larger spacing between two layers of EPS blocks seemed to be more effective to widening the vertical shear plane, it was no use of reducing vertical earth pressure because the length, spacing between them plus the thickness of EPS block, exceeded the height of the horizontal plane which is the settlements of the interior and exterior prisms of soil are equal(Plane of equal settlement). So, the application of double layers of EPS blocks, which spacing was more than 1.5 times of pipe diameter, was not beneficial to reduction of the vertical earth pressure compared to one layer of EPS blocks.

4 FIELD TEST OF BURIED PIPES

4.1 Instrumentation of field test

A field test was planned to assess the effectiveness of a compressible material in reduction of vertical earth pressure on the site. As shown in Figure 3, cross sections of field test were divided into three classes. CASE A represented the section which was not applied to a compressible material, CASE B represented the section which was applied to one layer of EPS block, and CASE C represented the section which was applied to double layers of EPS blocks as compressible materials.

Figure 3. Cross Section of field tests with Location of Measuring Instruments

1067 A trial construction site was situated in the factory area which produced segmented facing concrete blocks for geosynthetic-reinforced retaining walls. Indeed, we could give a high fill condition to buried pipe with putting concrete blocks on the ground surface. The weight of one concrete block was 40 kilograms and three layers of concrete blocks could be static load which was 5.4tf/m3. Positions of the measuring instruments are shown In Figure 3. A summary of monitoring items is provided in Table 3.

Table 3. Summary of monitoring items for Field Test Measuring Value and Information Device Position(for each section) Settlement of fill soil in construction Settlement Plate 3 points over pipe stage Vertical and horizontal Strain Characteristics of Pipe Strain Rod direction Vertical direction : 1 cell Vertical and horizontal earth pressure Earth Pressure Cell Horizontal direction : 3 cells Strain Characteristics of EPS block Steel Plate Above and below EPS block

As can be seen from Table 3, three earth pressure cells were installed to evaluate the earth pressure for each section. The exposed pressure sensing face of each cell was about 100mm in diameter and the maximum measurement capacity of it was 20tf/m2.

4.2 Properties of Test Materials

The fill material used for the field tests was weathered soil easily obtained from the site. From laboratory tests, it was found that the specific gravity was 2.63, the maximum dry density was 1.94 tons per cubic meters, the was 14.5%, the optimum water content was 12.1%, and the angle of internal friction was 34.7 degrees. Also, the sand classifies as Well graded sand (SW) according to the Unified Soil Classification System and relative compaction in the field was 93%. A compressible inclusion used for field test was the same material proprieties used for instrumented model tests. The density of EPS block is 15kg/m3 and the geometry of EPS block was 0.1×1.0×1.8m. The pipe used for instrumented model test was made of corrugated steel. Also, it was mill coated with zinc(KS D 3503). its diameter and thickness were determined to 1.0m and 2mm geometry of which was widely used and played an important role in the form of drainage system.

4.3 Summary of Field test results

Figure 4 shows the measurements of EPS block deformation. In Figure 4, CASE C-L indicates the EPS block which level was EL -2.5m and C-U indicates the EPS block which level was EL -1.5m.

30

25 End of Construction )

20

15

10 CASE B

Displacement(mm CASE C-L 5 CASE C-U 0 050100150 Time(day)

Figure 4. Measured Deformation of EPS block

1068 20 14 18 End of Construction End of Construction 16 12 14 10 12 10 8 8 6 CASE A CASE A 6 4 Deformation(mm) 4 CASE B Deformation(mm) CASE B 2 2 CASE C CASE C 0 0 050100150 Time(Day) 050100150Time(Day)

(a) Vertical Direction (b) Horizontal Direction Figure 5. Measured Deformation of pipe

As shown in Figure 4, EPS block deformations had similar values which were 19.8mm in CASE B, 24.3mm in CASE C-L and 21.3mm in CASE C-U. Moreover, before the construction was completed, most of EPS block deformations occurred. Percent values deformations between data with end of construction and final measuring data were 64% for CASE B, 61% for CASE C-L, and 74% CASE C-U. Measured Deformation in each direction versus time for pipe is presented in Figure 5. The values indicated that about 70% of total pipe deformation, which rate was similar to EPS block deformation, was occurred during construction. Also, a trend of pipe deformation had a convergence after the end of construction. The vertical maximum amount of pipe deformation was 12.4mm for CEAS A, 13.5mm for CASE B, and 14.3mm for CASE C ranging from 1.2% to 1.4% of the pipe diameter. Since allowable flexible pipe deformation was 5% for pipe diameter in Road Design Guidebook published by Korea Ministry Construction and Transportation, pipe deformation for the field test was stable in connection with allowable flexible pipe deformation. These horizontal deformations exhibited very similar variations with time as did the vertical deformations as shown in Figure 5.

Table 4. Earth pressure for each section Section CASE A CASE B CASE C Earth Pressure Design value 7.32 7.32 7.32 Vertical Earth Pressure(tf/m2) Measurement 6.67 4.30 3.90

Lateral Earth Pressure(tf/m2) 6.25 6.20 4.70

10 10 9 9 End of Construction End of Construction 8 8 7 7 6 6 5 5 4 ` 4 3 CASE A 3 CASE A 2 CASE B 2 CASE B 1 CASE C 1 CASE C Vertical Earth Pressure(t/m2) Earth Vertical 0 Pressure(t/m2) Earth Lateral 0 050100150050100150 Time(Day) Time(Day)

(a) Vertical Earth Pressure (b) Horizontal Earth Pressure Figure 6. Measured earth pressure on pipe

1069 Figure 6 shows the earth pressure acting on the pipe. As shown in vertical earth pressure from Figure 6 (a), the measurements indicated that both CASE B and CASE C which was applied to EPS block had a significant decrease as compared with CASE A which was not applied to EPS block. Also, although there was small difference in vertical earth pressure between CASE B and CASE C, CASE C which was applied to double layers of EPS blocks had more effectiveness to reduce the vertical earth pressure. Especially, a significant decrease of lateral earth pressure was monitored in CASE C in comparison to CASE B. The CASE C condition, which was lesser mobilized shear stress over pipe, brought about the decrease of lateral earth pressure. Otherwise, all surface settlement measurements from three sections had not distinguished value as compared with each other.

5 CONCLUSION

The objective of the research reported in this paper was to obtain measurements of vertical earth pressure reduction which was applied to EPS blocks as compressible materials and to verify the effectiveness of double layers of EPS blocks. In the instrumented model test, all the tests using EPS blocks showed the significant decrease which came from an arching area of the top of pipe which was applied to EPS blocks. However, the use of excessive width of EPS block brings about the increase of vertical earth pressure which reason resulted from the larger soil prism condition over the pipe. In addition, the same length of EPS block width was recommended as the spacing between EPS blocks which was applied to double layers of EPS blocks. From the field tests with three sections, good agreements between instrumented model tests and field measurements could be seen from measurement trends of vertical earth pressure For further research,

REFERENCES

Filz, G. M. and Reeves, J. N., “Earth force reduction by a synthetic compression inclusion”, A report of research, Geotechnical Engineering Division, Dept. of , Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 57p(2000). Frydenlund, T. E. and Aabøe, R., “Expanded Polystyrene-The Light solution”, Proceeding of the International Symposium on EPS Construction Method (EPS Tokyo '96), Tokyo, Japan, pp.31∼46(1996). Handy, R. L., “The arch in soil arching”, Journal of Geotechnical Engineering Division, ASCE, Vol. 111, No. 3, pp. 302∼318(1985). Horvath J. S., “The Compressible-Inclusion Function of EPS Geofoam : An Overview”, Proceeding of the International Symposium on EPS Construction Method (EPS Tokyo '96), Tokyo, Japan, pp. 71∼81(1996). Horvath J. S., “The Compressible-Inclusion Function of EPS Geofoam”, and , Vol. 15, Nos. 1-3, pp. 77∼120(1997). Kim et al. “Evaluation of EPS block construction method”, Research Report, Korea Institute of Construction Technology, 135p(1998). Marston, A., "The Theory of External Loads on Closed Conduits," Bulletin No. 96, Iowa Engineering Experiment Station, Iowa State College, Ames, Iowa, pp. 5∼8(1930). Schlick, W. J., “Loads on Pipe in Wide Ditches”, Bulletin No. 108, Iowa Engineering Experiment Station, Iowa State College, Ames, Iowa, 48p(1932). Spangler, M. G., “The Theory of Loads on Negative Projecting Conduits”, Proc. Highway Research Board 37(1950). Spangler, M. G. and Handy, R. L, Soil Engineering, 4th edition. Harper & Row, New York(1982). Taylor R. K., “Induced Trench Method of Culvert Installation” , Discussion by M. G. Spangler, Highway Research Board Rec.(1973). Yuichi et al., “The earth Pressure Reduction for Culverts Using EPS , Proceeding of the International Symposium on EPS Construction Method (EPS Tokyo '96), Tokyo, Japan, pp.214∼221(1996).

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