Reduction of Earth Pressure on Buried Pipes by EPS Geofoam Inclusions

Transcription

1 Geotechnical Testing Journal, Vol. 33, No. 4 Paper ID GTJ Available online at: H. Kim, 1 B. Choi, 2 and J. Kim 3 Reduction of Earth Pressure on Buried Pipes by EPS Geofoam Inclusions ABSTRACT: This paper presents the experimental study that investigated the applicability of compressible inclusion, expanded polystyrene (EPS) Geofoam panels, placed over a buried pipe. A series of model tests was conducted to identify the optimal EPS geometry. The tests employed multiple sections of EPS for imperfect ditch condition. Full-scale tests were also performed to calibrate the reduction in earth pressure due to the placement of double layers of EPS. In the model tests, one layer of EPS Geofoam reduced the vertical earth pressure acting on the pipe up to 73 % depending on the width of EPS. Double layers of EPS could induce the reduction of vertical earth pressure on the pipe as much as 71 % and horizontal earth pressure on the pipe about 60 % depending on the spacing between Geofoam inclusions. The full-scale test results indicated that the magnitude of the vertical pressure decreased by about 31 % 36 %. In case of horizontal pressure, 37 % reduction was observed for double layers of EPS while only 5 % reduction was found for a single layer of EPS. This study demonstrates that multiple layers of EPS provide better solutions in reducing the earth pressure on a pipe. KEYWORDS: compressible inclusion, EPS (expanded polystyrene), Geofoam, buried pipes, imperfect ditch condition, earth pressure, model test, full-scale test Introduction Buried pipes have contributed extensively to public service in forms of water mains, sewers, spillways, underpasses, etc. A buried pipe is generally designed such that the pipe cross section can resist earth pressure imposed on it. When the earth pressures both at the crown (vertical direction) and at the springline (horizontal direction) are computed, the pressure distribution along the pipe should also be estimated for designing the pipe. Accordingly, both vertical and horizontal earth pressure serve as crucial elements in order to obtain significant savings in the structural pipe design. Earth pressure on a pipe is determined by the two following conditions: The rigidity of pipes and pipe installation methods. For purposes of considering these conditions, design manuals for pipes employ arching factors to be applied to loads in the vertical and horizontal directions (hereinafter VAF and HAF, respectively). The factor is multiplied by the self-weight of soil above the pipe so the stresses acting on the pipe can be computed using Eqs 1 and 2 (ACPA 2007a). V = VAF W s (1) H = HAF W s (2) where: W s =the self-weight of soil per unit area above the pipe, V=the maximum vertical earth pressure (V F for flexible condition, V R for rigid condition), and Manuscript received December 29, 2008; accepted for publication March 26, 2010; published online April Former Researcher, Korea Institute of Construction Technology, Gyeonggi-do , Korea, hobicom@hotmail.com 2 Researcher, Korea Institute of Construction Technology, Gyeonggi-do , Korea, bhchoi@kict.re.kr 3 Research Fellow, Korea Institute of Construction Technology, Gyeonggi-do , Korea, jmkim@kict.re.kr H=the maximum horizontal earth pressure (H F for flexible condition, H R for rigid condition). Figure 1 shows the earth pressure distribution on both flexible and rigid buried pipes under embankment condition. After the researches of Marston (1930) and Spangler (1941) on the earth pressures acting on pipes for the embankment installation condition, AISI (1999) suggested a VAF of 1.0 and a HAF of 1.4 for flexible pipes. For rigid pipes, ACPA (2007b) proposed ranges of for VAF and for HAF depending upon the type of fill soil. Thus, vertical earth pressure on a flexible pipe is much less than that on a rigid pipe, whereas horizontal earth pressure on a flexible pipe is larger than that on a rigid pipe. In both cases, when the fill height above the pipe is sufficiently large, the plane of equal settlement is developed within backfill zone as indicated in Fig. 1. Above the plane of equal settlement, no relative settlement exists between the soil prism and the soils adjacent to the soil prism. Background When high fill is to be built on a buried pipe, the imperfect ditch construction method can be employed to reduce the earth load produced by high fill. The concept of imperfect ditch method is that compressible inclusion induces arching, which partially supports the vertical earth load due to the weight of the soil column over a buried pipe. Thus a portion of the soil s self-weight is transferred to the adjacent side soils; as a result, the vertical earth load on a pipe is less than the self-weight of the overlying soil prism (Kim et al. 2002; Kim et al. 2004). Marston (1930) found this imperfect ditch method and presented the mathematical formula for the design based on the principles of mechanics. Engineers have developed the imperfect ditch method by means of adapting several types of compressible material over a buried pipe. Throughout much of the 20th century, leaves (Spangler 1958), baled straw (Larson 1962), sawdust, or woodchips (McAffee and Valsangkar 2004; McAffee and Valsangkar 2005) have been selected as compressible material. It is difficult, however, Copyright 2010 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA

2 2 GEOTECHNICAL TESTING JOURNAL FIG. 1 Earth pressure distribution of buried pipes under embankment condition (modified after Spangler 1941). (a) flexible pipe and (b) rigid pipe. to control the stress-strain behaviors of these materials. In addition, an explosion hazard may result from the methane gas generation that accompanies anaerobic decomposition of organic material in a confined space (Vaslestad et al. 1994a; Horvath 1996). Expanded polystyrene (EPS) Geofoam has gained popularity since its early use as a pavement thermal insulation in the 1960s (Horvath 2003). After the Norwegian Road Research Laboratory (NRRL) introduced EPS as lightweight fill material in the early 1970s, the global use of EPS increased drastically (Horvath 1996; Frydenlund and Aabøe 1996; Kim et al. 2003; Hazarika 2006). EPS as lightweight fill has been shown to be especially beneficial when earth load reduction is necessary due to weak subsoil (Bartlett et al. 2000). The overall growth of EPS use in the geotechnical field has led to research on EPS as a geoengineering material. From the late 1980s, another function of EPS, that of the compressible inclusion, has emerged and become widespread in practice (Partos and Kazaniwsky 1987; Horvath 1996). Vaslestad et al. (1994a) suggested placing EPS as compressible inclusion over a buried pipe to reduce the vertical earth load on the pipe. Since the reliable performance of EPS as a fill material has been documented for more than 40 years, EPS can be a practical alternative to organic materials as a compressible inclusion over pipes. In addition, the mechanical characteristics such as the strength of EPS can be easily regulated in a factory by varying its density (Kim 2002). Although EPS has been suggested as a compressible inclusion material, there are few recommendations in the literature regarding the optimal geometry of compressible inclusion for the imperfect ditch method, and even these few recommendations are not quantitatively consistent. For example, based on three field case studies, Vaslestad et al. (1994b) proposed using a compressible inclusion with a minimum thickness of 50 cm and minimum width larger than 1.5 times the pipe diameter. However, Kim and Yoo (2005) performed an analytical study, showing that no significant load reduction was achieved if the width of the compressible inclusion was greater than 1.5 times of the pipe diameter. Consequently, the optimal geometry of compressible inclusion should be refined with further studies. In addition, the multiple layers of compressible inclusion could be considered to reduce the earth pressure acting on buried pipes. When the buried pipe is installed under high embankment or when the pipe is installed in the imperfect trench condition, the plane of equal settlement is developed within the embankment. Spangler (1950) showed that, in the embankment condition, the plane of equal settlement developed at a location approximately twice the rigid pipe diameter of 1.12 m above the top of the pipe. The result implies that placing an additional compressible inclusion above the plane of equal settlement can induce the relative settlement between the soil prism and the soils adjacent to the soil prism and therefore, mobilize the shear strength between the soil prism and the adjacent side soils. This mobilized shear strength can reduce the overburden pressure further if the embankment is high enough to accommodate a second plane of equal settlement. This concept, in fact, is not new. Spangler (1958) made use of several layers of leaves and loose soil for constructing induced trench condition. However, to date, no research has addressed double layers of EPS compressible inclusions above a buried pipe. Thus, parametric study of EPS compressible inclusions used in the double layer configuration is essential to improve the applicability of EPS as compressible inclusions over a buried pipe. Figure 2 illustrates the concept of vertical earth load reduction due to one layer as well as two layers of compressible inclusion. As shown in Fig. 2(b), when an additional compressible inclusion is installed above the plane of equal settlement, additional shear forces are also generated, which induces the reduction of the vertical load over the pipe (Kim et al. 2002). The objective of this research was to investigate the optimal geometry of EPS compressible inclusions for the imperfect ditch condition. This was accomplished by performing a series of model tests. The factors considered in the tests included the width of EPS and the spacing between EPS Geofoam inclusions. In the tests, the effect of double layers of EPS Geofoam inclusions was also investigated. Full-scale testing was performed to quantify the reduction in earth load by the EPS Geofoam inclusions over the pipe. Instrumented Model Tests The purpose of the test was to observe the variation in the earth pressure imposed on the pipe with changing the width of the EPS

3 KIM ET AL. ON REDUCING EARTH PRESSURE BY GEOSYNTHETIC INCLUSIONS 3 FIG. 3 Unconfined compression stress-strain curves of EPS used for the model test FIG. 2 Earth load distribution of buried pipes with one layer and two layers of compressible inclusion. (a) One layer of compressible inclusion. (b) Two layers of compressible inclusion. and the spacing of double layers of EPS Geofoam inclusions. Nine different sections were selected for the model tests. Test Pipe Steel pipe mill coated with zinc with a diameter of 10 cm was selected for the model test as corrugated steel pipe was going to be used for full-scale test. In order to find a pipe with stiffness equivalent to that of the 1-m-diameter corrugated steel pipe used for fullscale test, a series of parallel plate tests was conducted. The parallel plate tests characterized the bending stiffness of the pipe. The tests measured the load per unit length when the pipe reached 5%deflection of pipe diameter with 50 mm/min strain rate. From two sets of parallel plate test, a nominal 100 mm (4 in.) outside diameter and a 5 mm thickness were chosen for the model test. The bending stiffness of the pipe determined from the two sets of tests was 433 and 506 kn/m/m. According to Webb et al. (1996), these values are within the typical range for 1 m corrugated steel pipe commonly used in practice. Fill Material Characteristics Soil The model test utilized Jumunzin silica sand obtained at Jumunzin-Eup, Korea. The Jumunzin silica sand was classified SP according to the Unified Soil Classification System (USCS, ASTM D2487) and A-1-b according to the AASHTO classification system (ASTM 3282). Most of the particles passed through No. 40 sieve while only 0.5 % of the test sand retained on No. 100 sieve. The specific gravity was 2.63, the maximum dry density was kn/m 3, and the minimum dry density was kn/m 3.In all tests, the Jumunzin silica sand was pluviated from a 50 cm drop height, resulting in a dry density of 14.8 kn/m 3. The peak internal friction angle as determined from direct shear testing was The water content of the sand maintained about 2 % during the test. EPS Geofoam An EPS Geofoam with 15 kg/m 3 density was chosen for both the model and the full-scale tests. Horvath (1996) indicated that the lower density of EPS (e.g., 12 kg/cm 3 ) was desirable for compressible inclusion as Young s modulus decreases with decreasing density. However, the higher EPS density 15 kg/m 3 was selected out of concerns related to the quality and durability of the material. For all model tests the thickness of EPS was 50 mm. Fig. 3 shows unconfined compression test result of EPS used for the model test. The tests confirmed that EPS satisfied the standard quality designated by ASTM D Based on this stress-strain response, the choice of EPS thickness (i.e., 5 cm) was determined to be large enough to allow the amount of deformation in the overlying soil to mobilize positive arching action over the pipe. Test Facility and Instrumentation The instrumented facility consisted of a steel soil bin with a clear acrylic front wall. The EPS-pipe-soil cross-section was observed through the acrylic wall to check the geometry of the pipe and EPS as placed. The size of the soil bin was 140 cm in length, 100 cm in width, and 90 cm in height. The fill height of the sand above the pipe was 80 cm, which is 8 times the pipe diameter, more than enough to generate one plane of equal settlement with 5 cm EPS thickness. Also, the front wall length of 140 cm provided the pipe with a complete projection condition. After the bin was filled with sand, the steel frame with screw jack and steel loading plate was placed above the top of the fill surface. This was used for adding surcharge on the surface of the sand. A sand pluviator was used for filling the Jumunzin silica sand. The size of sand pluviator was 140 cm in length, 100 cm in width, and 50 cm in height. Since the width and length of the sand pluviator were identical to those of soil bin, the sand pluviator could be operated without traveling along the surface. The size was also beneficial to control the height of the pluviator. Two soil pressure transducers were used to measure the vertical and horizontal earth pressures on the pipe. The transducers were positioned the on pipe crown and on the springline, respectively. The soil pressure transducers had a diameter of five centimeters and a maximum capacity of 200 kpa. Test Procedure Prior to the test, the two soil pressure transducers were mounted on the pipe. Then the pipe was placed on the centerline of the bottom plate. The Jumunzin silica sand was pluviated gently into the soil bin using the sand pluviator with crane until the level of the sand arrived at the level of pipe crown. Next, EPS was placed above the top of the pipe prior to placing the sand above the level of pipe crown. After soil bin was filled with sand, the surcharge was applied to the surface of the sand in three stages: 49, 98, and 147 kpa. The

4 4 GEOTECHNICAL TESTING JOURNAL TABLE 1 Instrumented model test variables (D denotes Diameter of pipe, B E denotes the width of EPS, and S E denotes spacing between EPS inclusions) FIG. 4 The model test procedure. (a) Overview of soil bin facility. (b) Air pluviation of the silica sand. earth pressures acting on the pipe were monitored at each loading step and were recorded until the earth pressure showed little change over time. Fig. 4 shows the photographs of the model test. Table 1 summarizes the instrumented model test variables. Tests 1 and 2 were performed to determine the earth pressure without and with the pipe, respectively. Tests 3 5 were performed to find out the optimal width of a single layer of EPS installed immediately over the pipe. An additional layer of EPS was installed in tests 6 9. These tests were conducted to examine the effect of the placement of double EPS layers. For tests 6 9, a fixed EPS width equal to the pipe diameter was used, while the spacing between EPS Geofoam inclusions was varied from 0.5 to 1.5 times the pipe diameter. For consistency with tests 3 5, the bottom layer of EPS was also placed immediately over the pipe. Model Test Results and Discussion Earth Pressure on the Pipe Figure 5(a) shows the variation of vertical earth pressure for tests 1 5. The maximum vertical pressure (including self-weight of the sand above the pipe) computed for an applied surcharge of kpa was kpa. In test 1, vertical earth pressure at the elevation corresponding to the pipe crown in the other tests was kpa under kpa applied surcharge. It was partly because the pressure acting on the surface is not transmitted to the bottom with the same amount. Also, frictional effects at the boundaries reduced the vertical earth pressure. In test 2, vertical earth pressure on the pipe was only 84.3 kpa at the same surcharge level. This result indicates that deflection of the pipe by itself induced some arching action and reduced the vertical earth pressure at the level of the pipe crown by approximately 39 %. These effects influenced the measured earth pressures in all the The Presence of EPS Geofoam Inclusion Test Factor Variables Test No. No EPS Stress Distribution Sand deposit without pipe Test 1 of fill soil Sand deposit with pipe Test 2 Single layer of EPS Double layers of EPS The width of EPS (Single layer of EPS Geofoam inclusion) Spacing between EPS Geofoam inclusions B E =1.0D Test 3 B E =1.5D Test 4 B E =2.1D Test 5 S E =0.5D Test 6 S E =1.0D Test 7 S E =1.2D Test 8 S E =1.5D Test 9 tests, so the relative magnitudes of earth pressure are comparable. Figure 5(b) shows the variation of horizontal earth pressure for tests 1 5. As observed for vertical earth pressure, the horizontal earth pressure acting at the location of the pipe springline was reduced by 15 % 45 % due to the presence of the pipe. Optimal Width for a Single Layer of EPS Tests 3 5 were performed to investigate the optimal geometry of EPS in reducing earth pressure. All test cases that an EPS Geofoam inclusion was used achieved the significant amount of reduction of vertical earth pressure. The horizontal earth pressures in test 3 at low confinement, however, were larger than those of test 2. This might be due to the soil prism which had the same width with the pipe diameter. In test 3, shear stresses were mobilized along the boundaries between the soil prism and the adjacent soils, which made the vertical earth pressure at the springline enlarged and therefore, resulted in the increase of the horizontal earth pressure. However, as the confinement increased, the elongation of the pipe in test 2 took place due to its high vertical stress acting on the pipe and as a result, the horizontal earth pressure in test 2 became larger than that in test 3. With respect to the width of an EPS, test 5 (EPS width of 2.1 times the pipe diameter) showed little additional reduction of earth pressure in comparison with test 4. The small change in earth pressure when the EPS width increased to 2.1 times the pipe diameter suggests that an optimal width of an EPS is approximately 1.5 times the pipe diameter among the three cases with a single layer of EPS. Also, increasing the width of the EPS beyond the optimal FIG. 5 Measured earth pressures in different test conditions (tests 1 5). (a) Vertical earth pressure. (b) Horizontal earth pressure.

5 KIM ET AL. ON REDUCING EARTH PRESSURE BY GEOSYNTHETIC INCLUSIONS 5 FIG. 6 Measured earth pressures in different test conditions (tests 6 9). (a) Vertical earth pressure. (b) Horizontal earth pressure. width not only provides no further benefit but it also negate a portion of the earth pressure reduction because the use of excessive EPS width widens the soil prism over the pipe, thereby diminishing the arching effect. Certainly, it is economical to minimize the use of the EPS, which is more expensive than the typical fill soil. Optimal Spacing for Double Layers of EPS Tests 6 9 investigated the effectiveness of using double layers of EPS (see Table 1). Figure 6 indicates the measured earth pressures in tests 6 9. The earth pressures measured in test 3 (one layer of EPS with width equal to pipe diameter) is also plotted in the figure for the reference. The horizontal pressure in test 6 was not recorded due to a problem with the data acquisition system. Figure 6 showed that all test cases using double layers of EPS except test 9 showed reduced earth pressures below those recorded for test 3. This demonstrates that using double layers of EPS allows the fill soil above the pipe mobilize more shear stresses along the soil prism than using a single layer of EPS does. In tests 6 and 7, the earth pressures decreased as the spacing between EPS Geofoam inclusions increased, suggesting that the additional layer of EPS expanded the mobilized shear zone vertically along the soil prism. Also, the reduced vertical earth pressure resulted in reduced horizontal earth pressure. While in all double layer cases the second EPS Geofoam inclusion reduced the earth pressures below those measured for the single layer cases, the earth pressures in test 8 were slightly larger than the earth pressures in test 7. In test 9, the double layers of EPS did not contribute any additional reduction of earth pressure over the single layer cases. This result suggests that increasing spacing between EPS Geofoam inclusions beyond an optimal spacing approximately equal to the pipe diameter diminishes the effect of the second layer of EPS. Note that these results are limited to cases with the EPS width equal to the pipe diameter. The results may be different depending on the width of EPS. Optimal Geometry of EPS Geofoam Inclusions The model tests found that the width of EPS and the spacing between EPS controlled the reduction amounts of earth pressures. Figure 7 presents the percent reduction in earth pressures for all tests compared to test 2. In other words, the figure demonstrates the change in earth pressures normalized by the earth pressures of test 2 in which no EPS on the pipe was installed. Figure 7 indicated that both tests 4 and 7 condition could minimize the earth pressures within all test cases. Note that in test 4 condition, the width of the EPS Geofoam inclusion was 1.5 times the pipe diameter and that in test 7 condition, the widths of EPS Geofoam inclusions were equal to the pipe diameter. Full-Scale Test The objective of the full-scale test was to validate the effectiveness of double layers of EPS compressible inclusions above a buried pipe, especially compared to a single layer of an EPS Geofoam inclusion. In case of double layers of EPS since the model tests were conducted with the EPS width equal to the pipe diameter, the 1 m width of EPS was used for the full-scale test. FIG. 7 Percent reduction in earth pressures. (a) Normalized vertical earth pressure. (b) Normalized horizontal earth pressure.

6 6 GEOTECHNICAL TESTING JOURNAL FIG. 8 Cross-section details of the full-scale test with the location of measuring devices: (a) Case A; (b) Case B; (c) Case C. Material Characteristics Corrugated steel pipe mill coated with zinc was used for the fullscale test. The pipe had a diameter of 1 m and a thickness of 2 mm. A coupling band and pad were used to connect pipe segments of 6 m length. The fill material used for the full-scale test was residual soil located at the site. The fill soil was classified as well-graded sand (SW) according to the USCS and contained 3.7 % particles passing No. 200 sieve. The specific gravity of the solids was An angle of internal friction of 34.7 and a cohesion intercept of 45 kpa were measured from a direct shear test. Based on the standard compaction test (ASTM D698), a maximum dry density was 19.0 kn/m 3 and an optimum water content was 12.1 %. The natural water content at the site was 14.5 %, and the compacted in place dry unit weight of the soil was 19.9 kn/m 3 in average, resulting in a relative compaction of approximately % in the field. The EPS used for the full-scale test had the same material proprieties used for the model test. The thickness of EPS Geofoam inclusions used for the full-scale test was 10 cm. The width selected for the EPS was equal to the pipe diameter. Thus, the geometry of EPS was 0.1 m thick 1.0 m wide 1.8 m length. Test Procedure Figure 8 shows the three different configurations investigated on the full-scale tests. Figure 8(a) shows the buried pipe without EPS (case A), Fig. 8(b) illustrates a single layer of EPS above the pipe (case B), and Fig. 8(c) represents the buried pipe with double layers of EPS (case C). Figure 8 also indicates the types and positions of instrumentation used in the test. Table 2 provides the summary of instrumentation for the fullscale test. In order to investigate the vertical and horizontal pressures imposed on the pipe, we used nine earth pressure cells, of which the pressure sensing face was about 100 mm in diameter, and the maximum capacity was 200 kpa. For each case, four deformation rods were attached to face of the pipe inside to measure the deflection of the pipe. Three settlement plates were installed to measure the settlement of the fill soil above the pipe. Additionally, as shown in Fig. 9(a), two settlement plates were positioned 3.0 m away from the pipe crown to investigate the settlement ratio. The plates were square with side length of 30 cm. Steel plates of these dimensions were also placed on both faces of all EPS Geofoam inclusions to measure their vertical deformations. Figure 9 shows the section overview of the full-scale test. Although the foundation area where the pipe was placed was excavated, the width of ditch was 5.5 m B d /B c =5.5. Since the width sufficiently exceeded the transition width that was slightly less than 2.0 for any settlement ratio, the pipe had the condition of projecting-type conduit. In addition, in order to reproduce the condition of the pipe installed under high fills, three layers of concrete blocks were stacked on the top of the fill surface [see Fig. 9(b)]. One layer of concrete blocks weighed 17.7 kpa. Totally, the surcharge of 53.0 kpa was applied on the fill surface. Two different types of compactor were used for filling the soil. A 4.5 ton single drum vibratory roller was used for most of the compaction, while the fill soil adjacent to the pipe was compacted with a 650 kg tandem roller (small double drum compactor). A nuclear density meter was used for the quality control of the compaction in the test. Test Results and Discussion Earth pressures and pipe deflections Figure 10 plots the measured earth pressures acting on the pipe. The construction Instrumentation TABLE 2 Summary of instrumentation for the full-scale test. Earth pressure cell Settlement plate Steel plate Deformation rod Measurement Vertical and horizontal earth stresses Settlement of the fill soil Deformation of EPS Geofoam inclusion Vertical and horizontal deformation of the pipe Position (For Each Section) Vertical direction: 1 ea/case Horizontal direction: 2 ea/case 3 locations/case 2 locations/case Above and below the EPS Vertical and horizontal direction of the pipe inside

7 KIM ET AL. ON REDUCING EARTH PRESSURE BY GEOSYNTHETIC INCLUSIONS 7 FIG. 9 Section overview of full-scale test. (a) Plan view and (b) side view. was completed at 15 days. For all cases, the vertical earth pressures were less than the self-weight of the fill soil plus the concrete blocks because the pipe used for the study was the flexible pipe, which reduces the vertical earth pressure acting on it by its deformation. After the end of construction, the average vertical earth pressure of Case B was 49.0 kpa and the average vertical earth pressure of case C was 40.5 kpa. Without EPS (case A), the average vertical earth pressure measured 66.1 kpa. These results indicate that the application of EPS in both cases B and C reduced vertical earth pressures compared to the vertical earth pressure in case A. On the other hand, the horizontal pressure cell in case B did not evidently show reduction in earth pressure compared to the horizontal earth pressure of case A. The result showed good agreement with the research by McAffee and Valsangkar (2005), who mea- FIG. 10 Measured earth pressures on the pipe. (a) Vertical earth pressure. (b) Horizontal earth pressure.

8 8 GEOTECHNICAL TESTING JOURNAL FIG. 11 Measured deflections of the pipe. (a) Vertical direction. (b) Horizontal direction. sured higher horizontal earth pressure than the vertical earth pressure imposed on the pipe under the imperfect ditch installation condition. They pointed out that a redistribution of the load from the crown of the pipe to the sides developed shear forces along the soil prism above the pipe and. therefore, the generation of shear stresses along the soil prism caused the increase of horizontal earth pressure on the pipe. While case B showed little or no reduction in horizontal pressure, a significant decrease of horizontal earth pressure compared to case A was measured in case C. This result indicates that the double layers of EPS contributed the reduction in horizontal earth pressure due to relatively less mobilized shear stresses along the boundaries of the larger soil prism. In other words, because the size of the soil prism over the pipe was larger in case C than in case B, the shear forces generated over the pipe in case C were distributed over a larger spreaded border zone, result in lower shear stresses. Thus, the double layer of EPS brought about the decrease of horizontal earth pressure compared to case B. Figure 11 presents the measured pipe deflection in both the vertical and horizontal directions. In the figure, positive deflection indicates the compression of the pipe, while the negative deflection indicates the elongation of the pipe. In all cases, about 70 % of total deflection occurred during the compaction of fill soil. After the end of construction, the pipe deflections converged. Although both the smallest vertical pipe deflection and largest horizontal pipe deflection from three cases were measured in case A, the deflections from three cases exhibited similar variations with time. The maximum amount of vertical pipe deflection was 12.4 mm in case A, 13.5 mm in case B, and 14.3 mm in case C, ranging from 1.2 % to 1.4 % of the pipe diameter. It should be noted that neither pressure nor settlement readings changed significantly over a period of after the completion of construction. Settlement of the Fill Soil and Deformation of the EPS Geofoam Inclusions Figure 12 plots the measured deformation (in the vertical direction only) of the EPS Geofoam inclusions. In the figure, case C-L indicates the EPS deformation installed at 2.5 m elevation, while case C-U expresses the EPS deformation placed at 1.5 m elevation. Total EPS deformations were similar: 19.8 mm was measured in case B, while 24.3 and 21.3 mm were measured in cases C-L and C-U, respectively. Nearly all of the EPS deformation took place during construction in cases B and C-U, while 65 % of EPS deformation occurred during construction in case C-L. Table 3 presents the measured settlement of fill soil in terms of the elevation and the location. In most settlement plates, approximately 70 % of total settlement occurred during construction. The data at elevation 1.0 m demonstrates that the plane of equal settlement exists at about 1 m height since the measurable differential settlement was not found between the test section and the instrumented areas 2.5 m away from the test section. Optimal Configuration of EPS Geofoam Inclusions Compared to the vertical pressure in case A, the vertical pressures of cases B and C were reduced by 31 % and 36 %, respectively. In comparison with the computed vertical pressure by its fill soil and the surcharge, case B showed 45 % of the computed vertical pressure and case C showed 42 % of the computed vertical pressure over the pipe. Based on Eq 1, the VAFs in case A, case B, and case C were 0.65, 0.45, and 0.42, respectively. With respect to the horizontal pressure acting on the pipe, case B displayed only 5 % horizontal earth pressure reduction compared to the horizontal pressure of case A. However, case C showed 37 % reduction of the horizontal earth pressure compared to the horizontal pressure of case A. Based on Eq 1, the HAFs in case A, case B, and case C were 0.62, 0.59, and 0.39, respectively. This suggests that double layers of compressible inclusion can be especially advantageous for reducing horizontal earth pressure. Conceptually, the compressible inclusion can be divided into several layers depending on the fill height and the stress level. Based on the results of this study, it appears that using two (or possibly more) layers of compressible inclusion can be more effective way to reduce the earth pressures acting on the pipe. FIG. 12 Measured deformation of EPS Geofoam inclusions.

9 KIM ET AL. ON REDUCING EARTH PRESSURE BY GEOSYNTHETIC INCLUSIONS 9 TABLE 3 Measured settlement of fill soil (mm). Test section Adjacent to test section Depth Case A Case B Case C Case A-B a EndofCaseC a 0.5 m m m m a Position of settlement plates are indicated in Fig. 9(a). Summary and Conclusions The optimum configuration of EPS compressible inclusions for the imperfect ditch condition was investigated in a series of both model-scale and full-scale tests. The study investigated the effect of the geometry of an EPS Geofoam inclusion as well as the potential benefits of using double layers of EPS Geofoam inclusions over to using a single layer of an EPS. Multiple sections of EPS Geofoam inclusions were employed in the model test to identify optimal geometry for the imperfect ditch condition by changing the width of a single EPS layer and the spacing of double EPS layers. A full-scale test was also carried out to quantify the reduction in earth load expected due to the placement of double layers of EPS. The model test showed that the width of an EPS and the spacing between EPS Geofoam inclusions affected the reduction amounts of earth pressures. In case of a single layer of EPS Geofoam inclusion, the EPS could induce the reduction of the vertical pressure acting on the pipe by as much as 73 % depending on the width of EPS. The double layers of EPS could induce the reduction of vertical earth pressure on the pipe as much as 71 % and horizontal earth pressure on the pipe about 60 %. If the same width of the EPS Geofoam inclusion was considered, double layers of the EPS with the spacing equal to the pipe diameter were more beneficial than a single layer of the EPS to reduce the earth pressures in the tests. The full-scale test indicated that compared to the pipe without the EPS, the magnitude of the vertical pressure decreased by about 36 % depending on the number of layers of EPS (i.e., whether one or two layers). In the case of double layers of EPS, 37 % horizontal pressure reduction was observed while only 5 % reduction of horizontal pressure was found in the pipe installed under a single layer of the EPS. Based on Eq 1, the VAFs in case A, case B, and case C were 0.65, 0.45, and 0.42 and the HAFs were 0.62, 0.59, and 0.39, respectively. These results show that for pipes installed under high fills, double layers of EPS provide better solutions in reducing the earth load on a pipe The maximum amount of vertical pipe deflection only ranged from 1.2 % to 1.4 % of the pipe diameter and was not found to be significantly affected by the use of EPS Geofoam inclusions. It is important to note that the results in this research are subject to several limitations with respect to the pipe stiffness, the number of compressible layers, the thickness of EPS, and the EPS width with double layers of EPS. Especially, when the full-scale tests were conducted, only the EPS width equal to the pipe diameter was applied to the test because it was the case that the model-scale tests were performed with double layers of EPS Geofoam inclusions. Thus, the results may be different depending on the width of EPS. More refined model study with respect to the changing the rigidity of the pipe, the number of EPS, the thickness of EPS, the width of EPS with double layers cases, the density of EPS, and the density of fill soil is underway. Especially, further research should be carried out to investigate the applicability of double layers of EPS Geofoam inclusions that has 1.5 times of the pipe diameter. 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