Experimental evaluation of an expanded polystyrene (EPS) block-geogrid system to protect buried pipes

Soil Dynamics and Earthquake Engineering 129 (2020) 105965

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Experimental evaluation of an expanded polystyrene (EPS) block-geogrid system to protect buried pipes M. Azizian, S.N. Moghaddas Tafreshi *, N. Joz Darabi Department of Civil Engineering, K.N. Toosi University of Technology, Valiasr St, Mirdamad Cr, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Repeated loading Buried pipe EPS block Vertical diameter strain (VDS) Pipe’s crown strain

This study presents the results of experimental tests carried on buried uPVC (unplasticized polyvinyl chloride) pipes with an external diameter of 160 mm. The behavior of buried pipes in unreinforced and reinforced trenches by a single layer of HDPE (high-density polyethylene) geogrid and expanded polystyrene (EPS) geofoam block were investigated. To simulate vehicle wheel loadings, 500 cycles of repeated load respectively with an amplitude and frequency of 450 kPa and 0.33 Hz was applied to a loading plate placed over the trench surface. Pipe behavior under cyclic loadings were assessed using vertical diameter strain measurements, circumferential strains and pressures at the crown and springline. Additionally, settlement at the soil surface was measured throughout testing. The testing program is aimed at evaluating the role of different parameters influencing pipe behavior, such as embedded depth of the pipe, implementation of the EPS block and geogrid layer simulta­ neously and separately, and density, width and thickness of the EPS blocks. The results illustrate that the rate of changes in the pipe circumferential strain, vertical diameter strain and soil surface settlement, which increase rapidly in initial loading cycles, decreased as loading progressed. Based on the results, the density, width and thickness of implemented EPS blocks have an impact role in improving the behavior of buried pipes. The use of geogrid reinforcement with an EPS block with density of 30 kg/m3, thickness of 60 mm and width of 1.5 times the pipe diameter showed the most benefit when balancing vertical diameter strain, pipe crown strain, and soil surface settlement.

1. Introduction Buried pipes are one of the most common and critical components of urban infrastructure. The performance of these facilities is heavily dependent on proper design considering a variety of loading conditions, particularly those relating to overburden stresses and vehicular loading. Marston and Anderson [1] pioneered the geotechnical analysis of buried pipes, later facilitating proposed methods for investigating the behavior of buried pipes [2–6]. Conard et al. [7] experimentally investigated buried pipes respond against loads at the center and near their ends for simulating the role of eccentric loads on a shoulder of roads. Their re­ sults show the pipe deflection is slightly higher at the ends of them. Faragher et al. [8] carried out full-scale tests to study the vertical deformation of embedded flexible pipes under repeated loads in field conditions. Their results show that pipe vertical deformation tendency will decrease by progression of cyclic loadings. Moghaddas Tafreshi and Khalaj [9] studied the behavior of buried plastic flexible pipes under repeated loading, finding that a large portion of pipe deformation and

soil surface settlement (SSS) happen in initial cycles of loading, and that soil density and embedment depth of the pipe play a significant role in pipe deformation. Talesnic et al. [10] further demonstrated that with decreased backfill density, the maximum stresses over the buried pipe transferred from the crown to its invert. Chaallal et al. [11] further expanded on the observed behavior of shallow pipes, finding that the upper side of the pipe has a significant role in front of applied loadings with a typical figure of bumping at the crown and flattening in the shoulder. Ameliorating the behavior of buried pipes through use of geo­ synthetics (i.e. EPS blocks, geotextiles, geogrids and geocells) has been the subject of extensive research [12–29]. In all of these researches, implementation of geosynthetics over the crown of buried pipes show meaningful enhancement in buried pipes behavior. EPS geofoam blocks have particularly functional benefits, such as damping of energy and sound, settlement reduction of foundations on soft soil and absorption of stress over buried structures. Meanwhile, the infinitesimal poison’s ratio of EPS blocks reduces the stress over

* Corresponding author. E-mail addresses: [email protected] (M. Azizian), [email protected] (S.N.M. Tafreshi), [email protected] (N.J. Darabi). https://doi.org/10.1016/j.soildyn.2019.105965 Received 7 February 2019; Received in revised form 3 October 2019; Accepted 10 November 2019 Available online 15 November 2019 0267-7261/© 2019 Published by Elsevier Ltd.

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Fig. 1. (a) Grain size distribution for trench soil, (b) A photograph of geogrid used in the tests.

enhancing buried pipe performance. Moghaddas Tafreshi and Khalaj [12] demonstrated that buried flexible plastic pipes subject to repeat surface loading exhibit lower levels of deformation when overlaid by geogrids. Corey et al. [17] conducted four large-scale plate loading tests on shallowly buried steel-reinforced HDPE pipes that included sand and aggregate base sections with or without geogrid reinforcement. Their results demonstrated that the geogrid reduced the longitudinal strains in the plastic shell of the buried pipe subjected to static surface loading. Hedge and Sitharam [19] explored the efficiency of combination geocell and geogrid as a protection of buried pipes through experimental ana­ lyses. In this situation, the results have an indication that the buried pipes experience a substantial reduction in pipe deformation in front of solo reinforcement. Elshesheny et al. [36] experimentally investigated the role of a geogrid layer over the performance of HDPE pipes under the cyclic loading condition. Their results show that the geogrid layer has practical aptitude to reduce the strain at the crown and springline of buried pipes wall and this efficiency could increase by implementing two geogrid layers. Furthermore, the combination of geosynthetics such as geogrid, geocell, geotextile with EPS blocks is a dominant approach in the range of geotechnical applications – e.g. in embankment constructions [31,33, 37–40], over pipes [28] and in culvert [41], retaining walls [37,42,43], and other applications. In these approaches, geosynthetics have wide arrays of benefits such as preparation sufficient beds for EPS blocks, separation layer between EPS blocks and surface pavements, protection of EPS blocks, and as a strength parameter with or without concrete slab loads. Although there has been significant research towards the benefits of using either geogrids, geotextiles, geocells or EPS blocks as a means of enhancing buried pipe performance, an approach has been implemented in the line of amendment and assessment (1) the influence of both sys­ tems (geogrid and EPS block) simultaneously, (2) the effects of these components on pipe deformations, and (3) the effect of EPS blocks on rut surface of trench (soil surface settlement) under cyclic loading, which has not been studied in previous studies. Much of the literature focused on application of EPS blocks over buried pipes has been applied using static loading [e.g. 15,22,28,44], or limited to rubber/geogrid/geocell reinforcement without the presence of an EPS block [e.g. [12,16,17,19,36,45]. Thus, the aim of this study is

adjacent structures [30]. Kim et al. [15] reported that the width of EPS blocks implemented over the buried pipes have impact role on their performance. Their results show the optimum width of 1.5 times the pipe diameter is the optimum width in absorbing the transferred pres­ sure over the crown of buried pipes. Anil et al. [18] experimentally examined the aptitude of energy absorption in EPS blocks over buried pipes in front of dropping weight over the trench surface. Their results show the impact role of EPS blocks in reducing the deleterious effect of impact loads on the performance of buried pipes. Bartlett et al. [20] conducted experiments on the function of the EPS block which had been implemented over buried pipelines, finding that the EPS system reduced the peak uplift force by a factor of four. Meguid et al. [23] investigated the reduction in contact pressure on the walls of rigid box buried in granular backfill with an overlying EPS geofoam block, resulting in significant pressure reductions at the side and lower walls of the buried structure. Beju and Mandal [28] used both geofoam blocks and jute geotextiles as a means of improving buried pipe performance. They found that the optimal clearance between the EPS block and pipe crown is approximately 20% of the pipe diameter and that use of a geotextile further improved buried pipe performance. In many field projects, implementing a concrete slab over EPS blocks and beneath the surface of pavements is a desideratum point [20, 31–35]. The use of reinforced concrete Load Distribution Slab is well known to be required in EPS sections to (1) protect EPS from permanent deformations and (2) reduce stresses in the depth of backfills (e.g. over the buried utilities). In addition, implementing a concrete slab has own advantages such as a barrier for petroleum spills that can decompose EPS blocks, anchorage for various highway hardware, and for mini­ mizing pavement thickness. Meanwhile the disadvantages of concrete slabs encompass the cost of the concrete slab (20–30% percent of a project), the potential for sliding of the slab during an earthquake, cracks that could be a way of petroleum spills, and the weight of con­ crete slab (dead load) over the EPS [32]. To that end, alternative sepa­ ration layers (geocell, geogrid, geotextile) might be implemented instead of a concrete slab [32] whether they will be practical in spreading the stress to a desirable limit, or the thickness of concrete slabs can be replaced by considering granular material with a ratio of 1–3 for granular material [33]. The application of geogrids has demonstrated benefit towards 2

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characterize the benefit of combined EPS block-geogrid reinforced sys­ tems towards pipe performance under repeated loading through well-instrumented, large-scale testing regime.

Table 1 Characteristics of implemented geogrid.

2. Overview of testing program Owing to the benefit of both geogrids and EPS block to enhance the performance of buried pipes, a testing series was performed on a single uPVC pipe that is 160 mm diameter, buried in unreinforced and rein­ forced trenches with and without an EPS block/geogrid, and subject to repeat surface loading. The testing program was developed to evaluate the role of different factors on pipe performance, such as embedment depth, geogrid placement, and EPS block density, width and thickness. One of the eminent characteristics of EPS block is its compressible and low density material properties. When a layer of EPS block is located above a buried pipe, it can decrease the pressures acting on the pipe walls through arching [15,20,44]. Thus, its size and location may significantly influence underlying pressures. In contrast, the aforemen­ tioned compressibility of the EPS system may result in increased soil surface settlement, but could possibly be mitigated through the use of an overlying geogrid reinforcement. Thus, consideration of the surface settlement of the loading surface and pipe deformation is measured in this experimental setup. In each test, the vertical diameter strain (VDS), found by measuring change in the vertical diameter of the pipe divided by the initial external diameter, the settlement of the soil surface (SSS), circumferential strain of pipe crown and the vertical pressure acting on crown and springline of the pipe are used to interpret the results. It should be noted that 5% vertical diameter change is presented as threshold criteria for pipe deformation in order to avoid buckling [46, 47] and a 2% vertical diameter change is a threshold for structural distress [48], such as cracking.

Description

value

Material Aperture Shape Aperture Size (mm) Thickness (mm) Mass Per Unit Area (kg/m2) Ultimate Tensile Strength (kN/m)

HDPE Hexagonal 27 � 27 5.2 0.695 5.8

Fig. 2. Stress-strain behavior of EPS blocks with nominal density of 10, 20 and 30 kg/m.3.

3. Material properties

3.2. Pipe properties

3.1. Soil properties

The Experiments were carried on uPVC pipes having a ratio of thickness to pipe diameter being in line with the BSI [52] regulation for sewer and drainage system. The pipe has a 160 mm external diameter, a wall thickness of 3.2 mm and length of 980 mm. To efface the effect of friction between the two ends of buried pipe and the trench walls on behavior of buried pipes, the length of the pipes considered as a way providing 10 mm distance between the buried pipes and trench walls. The modulus of elasticity and the Poisson’s ratio of the pipe were 265 MPa and 0.44, respectively. At the onset of every test, a new, un­ disturbed pipe was used. After testing concluded, each pipe was exhumed and inspected for damage.

Two types of soil, namely "natural soil" and "backfill soil" are used in the testing program. The natural soil at the bedding and two sides of the backfill soil was simulated by a soil which is classified as well-graded sand with clay (SW-SC, ASTM D2487-11 [49]). This soil has a specific gravity of 2.66 (Gs ¼ 2.66), a maximum grain size of 6.35 mm, 9.5% passing the number 200 sieve (0.075 mm), a liquid limit (LL) of 24 and a plastic limit (PI) of 10 as shown in Fig. 1a. Maximum dry density and optimum moisture content of soil, derived from the modified standard proctor compaction test (ASTM D1557-12 [50]) were obtained as 19.05 kN/m3 and 7.7%, respectively. The angle of internal friction (ϕ) and cohesion (c) of the soil, obtained from consolidated undrained triaxial compression testing for a moist density of 18.43 kN/m3 (corre­ sponding to 90% of maximum dry density with moisture content of 7.5%) were respectively ~35� and 11 kPa. The soil used as backfill - around the pipe, above the pipe and above the EPS block – may be classified as a well-graded sand (SW, ASTM D2487-11 [49]), which complies with grain size limits for pipe backfill materials according to ASTM D2321-08 [51]. A grain size distribution as shown in Fig. 1a. This soil has a maximum grain size and mean grain size of 10 mm and 2.22 mm, respectively and a specific gravity of 2.65 (Gs ¼ 2.65). The maximum dry density and optimum moisture content of the soil (ASTM D1557-12 [50]) were approximately 19.52 kN/m3 and 6.7%, respectively. The angle of internal friction (φ) of the soil, obtained from consolidated drained triaxial compression tests was 37.5� for a moist density of 19.16 kN/m3 (corresponding to relative compaction of 92%, similar to the compacted density of the backfill soil layers shown later). The ASTM D2321-08 [51] stipulated the minimum relative compaction of SW soil to 85% in the trench of buried pipes. Owing to this fact, the density of the soil layers in the trench considered 92% of relative compaction.

3.3. Geogrid properties One layer of HDPE (High-Density Polyethylene) geogrid (CE 131, manufactured by MeshIran) was used to reinforce the soil mass over the pipe. Fig. 1b and Table 1 show a photograph and the characteristics of the used geogrid in the tests, respectively. 3.4. Geofoam properties EPS blocks with a rectangular cross-section and nominal densities of 10 kg/m3, 20 kg/m3 and 30 kg/m3 were utilized over the buried pipes. These implementations were utilized to investigate the advantage of using EPS blocks characteristics such as low unit weight and stress dissipating capacity. Fig. 2 represents the results of strain controlled uniaxial tests on cubic EPS blocks with dimensions of 50 mm. As seen in this figure, hardening of strain against of increasing in stress level is obvious [53]. ASTM C578-95 [54] considers 10% strain as a criterion for compressive strength of EPS blocks. Owing to this fact, Fig. 2 shows the nonlinear trend between the EPS blocks densities and their compressive 3

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loading system complex which supports the reaction from the hydraulic cylinder. The hydraulic cylinder could produce monotonic or cyclic loads based on the requisite frequency and magnitude level. Cyclic vertical loads with amplitudes up to 10 kN, and frequencies up to 0.5 Hz, are controlled by the data acquisition system.

Table 2 Physical and mechanical properties of EPS geofoam with different densities. Parameters

Values 3

Nominal Density (kg/m ) Actual density (kg/m3) Modulus of elasticity at 1% strain (MPa) Compressive strength at 1% strain (kPa) Compressive strength at 10% strain (kPa)

10 9–9.8 1.15 11.5 33.9

20 17–19 2.7 27 76

30 28–29.5 5.8 58 134

4.2. Data acquisition system This complex system controls, monitors and records all continues torrents of data from versatile sensors (e.g. Load cell, strain gauge, pressure cell and LVDTs). The precision of the vertical load applied by the hydraulic cylinder to the loading plate on the surface of the soil was measured by a load cell (sensor with a capacity of 10 kN and accuracy of 0.1% of its maximum capacity) that is directly mounted between the loading plate and the cylinder axis. The vertical diameter strain of the pipe and soil surface settlement were recorded using a linear variable displacement transducer (LVDT) with a capacity of 50 mm and accuracy of 0.01% of its maximum capacity. The LVDT that was installed in the pipe was fixed firmly to the surrounding pipe with screws so that it remained vertical during testing. For determination of circumferential strain in the crown of the pipe wall (exactly under the center of surface loading in Fig. 3), a strain gauge (KFG-5-120-C1-16) was affixed to the crown of the pipe. Chaallal et al. [55] addressed the behavior of buried flexible pipes under the live load of trucks, indicating that the circum­ ferential strain of the pipe below the loading center is greater than the longitudinal strains along the length of the pipe. Their results also indicate a decrease in the circumferential strain of the pipe with

strengths. This situation is in agreement with the ASTM C578-95 [54]. Table 2 presents the summary of the EPS properties with nominal den­ sities of 10, 20 and 30 kg/m3. 4. Testing apparatus The testing apparatus consists of a testing tank, loading system, and a data acquisition system to measure the response from various, embedded instruments. The testing tank accommodates model pipe, geogrid layer and EPS block inside a backfill soil with a predetermined density - a schematic of the test and instrument layout is shown in Fig. 3. A brief description of each of three primary components is briefly dis­ cussed herein. 4.1. Loading system A Loading frame, hydraulic actuator and directing unit comprise the

Fig. 3. The schematic representation of the testing apparatus and geometry of model (Not scaled-units in mm). 4

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increasing distance from the loading center. Accordingly, in the exper­ iments, a single strain gauge is affixed over the pipe crown and under­ neath the loading surface that is considered as the critical location for studying pipe behavior. In order to measure (1) the vertical stress transferred to the crown and springline of the pipe and (2) the effect of the geogrid layer and the EPS block on vertical stress reduction, two pressure cells with a diameter of 50 mm were installed on the crown of the pipe and the beside the spring line of the pipe in some selected tests. The pressure cells have a capacity of 1 MPa and a measurement accuracy up to 0.1 kPa. Fig. 3 shows the location of load cell, LVDTs, pressure cells and strain gauges.

Table 3 Scheme of tests for buried pipes.

4.3. Testing tank A trench with a width of 600 mm settled in agreement with the recommendation of AASHTO [47] suggesting the minimum width of the trench should be greater than the maximum values of 1.5 times the outside pipe diameter plus 305 mm or pipe outside diameter plus 406 mm. Meanwhile, the proposed trench width is in line with ASTM D2321-08 [51] and BSI [56] which suggest the minimum value of the trench width should not be less than 1 and 1.25 times the external pipe diameter plus 300 mm, respectively. The trench contains the model pipe, EPS block, unreinforced and reinforced soil and had a width of 600 mm, length of 1000 mm (longitudinal pipe axis) and various embedment depth of pipe in a line of multipurpose tests (See Fig. 3). The model trench was constructed in a rigid test box with dimensions of 1000 � 1000 � 1000 mm (Fig. 3). The model/trench dimensions (including backfill thickness and natural soil thickness) fulfilled the recommended values by Thakur et al. [57] and Moghaddas Tafreshi et al. [58] who indicated that a horizontal and a vertical dimension of about 7 and 2–2.5 times the loading plate, respectively (which in this study are 6.7 and 3.7–4.5) would be sufficient to prevent possible stress redistribution induced from two sides and bottom of the test box. Also, DeMerchant et al. [59] used the text box with width and depth of 7.2 and 2.8 times the loading plate width, respectively for studying geogrid-reinforced foundation. They reported no influence on the results by the sides and bottom boundaries of text box. Along with the above suggestion, Hegde and Sitharam [60] reported the depth of 1.6B (B is the width of loading surface) for pressure dispersion depth that the pressure is less than 10% of the applied surface pressure. Thus the dimensions of the test box employed here are more than sufficient on the basis of previous researchers’ results and do not interfere in the results.

Test Condition

z

wg (mm)

hg (mm)

EPS Density (kg/m3)

No. of Tests

Purpose

Unreinforced

1.2D, 1.5D, 2D

–

–

–

3þ 3*

Geogrid reinforced

1.2D, 1.5D

–

–

–

2þ 2*

EPS block in addition to geogrid layer

1.5D

2D

100

10, 20

2þ 2*

1.5D

D, 1.5D, 2D

30, 60, 100

30

9þ 6*

To evaluate the effect embedment depth of pipe To evaluate the effect of geogrid layer To evaluate the effect of EPS block density in geogridreinforced backfill To evaluate the effect of width and thickness of EPS block in geogridreinforced backfill

**The tests which were performed two or three times to verify the repeatability of the test data. For example, in unreinforced tests, a total of 6 tests were per­ formed, including 3 independent tests plus 3 replicates.

placed in the positions as show in Fig. 3. In the tests with an EPS block, a rectangular EPS block with length equal to the pipe were also installed over a 20 mm soil layer (at a distance of 20 mm from the top of the pipe). In reinforced tests, a geogrid layer with a size of 10 mm less than trench dimensions on all sides, was laid at depth of 0.33B (B ¼ diameter of loading plate) beneath the loading surface [12]. By reaching the pro­ posed surface level of trench, a rigid loading plate with a diameter of 150 mm and thickness of 25 mm, representing the vehicle wheel contact with the road surface, was placed over the center of the trench and embedded pipe and the related instruments (LVDT and load cell) were adjusted. Repeated loading was applied through the hydraulic actuator to the loading plate and recorded data were measured through the data acquisition system. The amplitude of the maximum applied repeated surface load was selected based on the AASHTO [61] to represent the twin tires of a typical H 15–44 truck. The recommended axle load truck was about 10886 kg, over two pairs of twin wheels, which create a stress of approximately 5 kg/cm2 (about 500 kN/m2) base on wheel inflation pressure. Overlaying pavement as a robust layer that could dissipate the stress was not available in the test series. Therefore, using KENPAVE software (Huang [62]), and assuming a 50 mm asphalt layer with a Young’s Modulus of 2.5 GPa, a pressure of 500 kPa can be reduced to 450 kPa, representative of the pressure transferred to the trench surface. Although the frequency of repeated loading might have a direct effect on the response of the system, a wide range of frequencies (e.g. 0.01–10 Hz) have been used by previous researchers [57,63–65]. Gonzalez-Torre et al. [65] reported that high frequency loading has no significant ef­ fect on pavement behavior and the lower the frequency, the higher impact will the loading have. Also, Powell et al. [66] reported that the use of 500 axle passages is a reasonable approximation for the number of vehicle passes. Thus, to simulate the aforementioned truck vehicle loading, all tests were carried out under 500 load cycles with an amplitude of 450 kPa and frequency of 0.33 Hz which is a reasonable choice, within the limits of the above studies. Thus, the simulated con­ dition could be considered representative of overlying pavement under an applied pressure of 500 kPa.

5. Preparation of model test and loading pattern To simulate the natural soil at the bedding and the two sides of the backfill soil (Fig. 3), the soil layers with thickness of 20 mm was com­ pacted, each uniformly by through mechanical compaction using a weight of 5.5 pounds from height of 304.8 mm (according to the stan­ dard proctor compaction test, ASTM D1557-12 [50], over a steel plate with dimensions of 300 � 100 � 20 mm. The density of the soil layers was controlled by weighing and rulers mounted on the trench walls. To provide the soil masses either side of the trench with an approximate vertical side (Fig. 3), a vertical stiff wooden shutter was employed. The soil was compacted so as to ensure that a moist density of 18.43 kN/m3 was achieved. In the case of disturbance of the natural soil by the loading of previous test or during removing the backfill soil, the natural soil was reconstructed prior to the next test. Before backfill preparation, LVDT was installed inside the pipe (Fig. 3) and the pipe placed on the surface of the simulated natural soil (Fig. 3). The backfill trench around and over the pipe was prepared at moist density of 19.16 kN/m3 (corresponding to relative compaction of 92%) with a moisture content of 6.7%. Soil was placed in multiple lifts, each approximately 20 mm in thickness and was compacted uniformly by through mechanical compaction as described. When the soil level reached to the springline and crown of the pipe, the pressure cells were 5

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Fig. 4. Example trends of VDS, Crown Strain Gauge, Crown Pressure and SSS during repeated loading.

6. Testing program

1.5D in unreinforced backfill. This figure shows that the rate of change of peak VDS, SSS, pipe crown strain and pressures reduced as the number of loading cycles progressed. These trends are parallel with the results of previous researchers who investigated the behavior of buried pipes under live loads [8,9,19,46,67]. These figures shows that the variations of VDS, pipe crown strain and exerted pressures stabilized earlier than the SSS variation. This suggests that the displacement and densification of soils surrounding the pipe occurs quickly with repeated loading, reaching a semi-elastic state. However, the limited confinement at the surface results in continued settlement under repeated loading.

In this testing program, 16 independent tests under different con­ figurations were carried out, summarized in Table 3. The effect of the embedment depth of the pipe (z), single layer of geogrid reinforcement, width, thickness (wg and hg, respectively) and density of EPS block were investigated. Out of these 16 tests, 13 tests were repeated carefully to ensure the performance of the apparatus and the accuracy of the mea­ surements. The results of two repeated tests throughout 500 cycles of loading shows a close match, with a maximum difference of 4–6%, considered consistent.

7.1. The effect of pipe embedment depth

7. Results and discussions

The variation of the VDS, SSS and pipe crown strain under repeated loading is shown for three pipe burial depths, 1.2D, 1.5D and 2D, shown in Fig. 5. For these tests, the pipes were placed under unreinforced condition. As shown in the figure, the magnitude of VDS decreases significantly with increased depth. For example, after 500 cycles of loading, VDS was approximately 6% at a depth of 1.2D and 2.2% at a

The results of the laboratory model tests are expounded in this part. The distinctive trends of vertical diameter strain of pipe (VDS), pipe crown strain, soil surface settlement (SSS) and exerted pressures to wall pipe under repeated loading (loading, unloading and reloading) are shown in Fig. 4. For the presented test, the pipe was placed at a depth of 6

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Fig. 5. Variations of the maximum (a) VDS, (b) pipe crown strain and (c) SSS for unreinforced system at three burial depths of 1.2D, 1.5D and 2D during repeated loads.

depth of 2D - a reduction of 63.3%. Furthermore, the rate of VDS accumulation decreased with deeper pipe burial. This owes to enhanced stressed dissipation from the surface loading with depth. By increasing the depth of the buried pipes from 1.2D to 2D, the crown strain of the pipe was decreased by about 78.2% at the conclusion of loading. In addition, with increasing burial depth, the amplitude of cyclic circum­ ferential strain variation range decreased owing again to decreased stresses imparted on the pipe crown with deeper burial depth. Lastly, increasing the depth of pipe burial resulted in fewer cycles required to achieve steady pipe deformation behavior. In contrast with VDS amplitudes, increasing the burial depth of the pipes resulted in increased soil surface settlement (SSS). For example, at the end of the loading, SSS was 25.37 mm for a pipe burial depth of 1.5D as compared to 32.35 mm for a pipe burial depth of 2D. That is, increasing the soil cover over the buried pipe resulted in a larger zone of soil prone to deformation and lessened influence of the stiffer pipe element. However, despite the lessened vertical deformation under­ neath the loading plate with decreased burial depth, considerable

deformation in vertical pipe diameter when the pipe was located closer to the surface, contributing to some of the settlement at the surface. Thus, the net compression of the soil while including pipe vertical deformation was 28.57 mm for 1.2D burial depth, approximately 3.2 mm larger than the SSS at buried depth of 1.5D. These results corroborate findings from Chaallal et al. [11] and Hegde and sitharam [19]. 7.2. The effect of geogrid reinforcement without an EPS inclusion A layer of geogrid was implemented beneath the loading surface with a space of u/B ¼ 0.33 (recommended by Ref. [12]) for amending the soil surface settlement and pipe behavior. The results in Fig. 6 compare maximum values of VDS, pipe crown strain and SSS with loading cycles for pipe embedment depths of 1.2D and 1.5D with and without geogrid reinforcement. It is obvious that the rate of deformation in VDS, SSS and pipe crown strain decreases with the accumulation of the load cycles. For the reinforced tests, the maximum VDS at the end of the loading 7

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Fig. 6. Variations of the maximum (a) VDS, (b) pipe crown strain and (c) SSS for burial depths of 1.2D and 1.5D in unreinforced and reinforced conditions during repeated loading.

(Fig. 6a) was reduced by 19% and 15.5% compared to unreinforced conditions for embedment depths of 1.5D and 1.2D, respectively. A reduction in VDS owes to a reduction in the transmitted horizontal stresses (shear stresses) from the overlying soil/fill to the top of the buried pipe stemming from the presence of a deforming geogrid layer [68] and distribution of the applied surface load due to geosynthetic deformation [69]. As seen in Fig. 6b, the crown strain of the pipes demonstrates similar behavior with and without the presence of a geo­ grid, however, the reinforcement demonstrates an average reduction in strain of approximately 21%. Soil surface settlement (SSS) is reduced from geogrid reinforcement (Fig. 6c) by approximately 30% and 12% compared to the unreinforced condition for embedment depths of 1.5D and 1.2D, respectively. Membrane effect of the overlying geogrid, providing a vertical reaction the surface, has impact role in this reduc­ tion [41].

7.3. The effect of EPS block in geogrid-reinforced system Ultra lightweight, compressibility inclusion and energy dissipation of EPS geofoam blocks in comparison to dominant materials are unde­ niably fruitful characteristics that are viable means of reducing pressure transferred to the crown of the buried pipe through active soil arching, and as a result reducing pipe deflection. In this part, the role of the density, thickness and width of EPS blocks placed above buried pipes are examined. 7.3.1. The effect of EPS block density on the geogrid-reinforced system To investigate the effect of EPS block density on the pipe behavior in a geogrid-reinforced system, tests were carried out with EPS blocks with a thickness of 100 mm (hg ¼ 100 mm) and width ratio of 2 (wg/D ¼ 2) at three densities of 10, 20 and 30 kg/m3. In these tests, the pipe was installed at a depth of 1.5D. Fig. 7 shows the maximum VDS, pipe crown strain and SSS with loading cycles for different EPS densities. As shown 8

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Fig. 7. Variation of the maximum (a) VDS, (b) crown strain and (c) soil surface settlement of the buried pipes in unreinforced backfill, geogrid reinforced backfills with and without EPS block with the repeat loading.

in this figure, with increasing EPS density, VDS, pipe crown strain and SSS values decrease significantly. This may be attributed to the increased elastic modulus and energy absorption of EPS with increasing density. For the lowest EPS block density (10 kg/m3), excessive settle­ ment and pipe rupture occurred in the initial cycles of loading (failure at less than about 50 cycles). This deleterious behavior may be attributed to insufficient strength and flexural resistance of low density EPS (10 kg/ m3), where punching occurred and consequently a large stress transfer to the pipe walls ensued. As shown in Fig. 7a, the final reduction in VDS values for the system with EPS blocks and geogrid reinforcement compared with just geogrid reinforcement were approximately 61% and 84% for EPS density of 20 and 30 kg/m3, respectively. The corre­ sponding reduction in pipe crown strain were about 31% and 89% (Fig. 7b). The decrease in VDS, pipe crown strain and SSS with increasing EPS density are attributed to higher energy absorption

capacity and higher flexural rigidity of EPS block. According to Fig. 7c, although the inclusion of EPS blocks result in increased surface settle­ ments compared with unreinforced backfill and geogrid-reinforced backfills, an EPS density of 30 kg/m3 yields a settlement of 32 mm at the conclusion of loading, satisfying a limit of 30–70 mm for unsealed low volume roads recommended by AASHTO [70]. The undesirable ef­ fects of EPS block compressibility on surface settlements can be attrib­ uted to the high compressibility of the EPS material. The variation of VDS and SSS at the conclusion of loading for varying EPS density are shown in Fig. 8. As shown, increasing EPS block density results in a significant decrease in both VDS and SSS. This owes to the increased stiffness of higher EPS density systems, resulting in lowered deformation under the same loading conditions for lower density EPS blocks. Further, the high resistance and lowered deformation of denser EPS blocks causes decreased VDS as mitigates the stress transfer to the 9

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Fig. 8. Variations of maximum VDS and SSS at last cycle of loading with EPS density (a) VDS, (b) SSS.

pipe was reduced to 9.5 and 4.5 kPa that are about 22% and 27% of those obtained for unreinforced and geogrid reinforced trenches, respectively.

Table 4 Maximum vertical pressure for different tests at the end of cyclic loading (z ¼ 1.5D, wg/D ¼ 2, hg ¼ 100 mm). Test Condition Unreinforced Geogrid reinforced (No EPS) Geogrid reinforced (with EPS)

EPS density (kg/m3)

– – 10 20 30

Vertical Pressure (kPa) Crown

Springline

45 35 125 31 9.5

20 17 37 15 4.5

7.3.2. Effect of EPS block width and thickness To investigate the effect of EPS block width and thickness on the performance of buried pipes in geogrid-reinforced soil, the tests were carried out with EPS block with a density of 30 kg/m3, which were used in three widths (wg) of D, 1.5D and 2D and three thicknesses (hg) of 30, 60 and 100 mm. In these tests, the embedment depth of the pipe was 1.5D. Fig. 9 shows the maximum VDS of the pipe with loading for different widths and thicknesses of the EPS block. As seen, there is a significant improvement in the maximum VDS values owing to the presence of the EPS block and geogrid reinforcement in comparison to geogrid-reinforced and unreinforced systems, irrespective of block width and thickness. Fig. 10 shows the maximum pipe crown strain under cyclic loading. These results demonstrate that VDS is significantly reduced from the presence of an EPS block, particularly for thicker blocks and in comparison to the unreinforced system. Fig. 11 shows the variation of SSS with the cyclic loading. In contrast to the beneficial behavior of the EPS block system on reducing VDS and pipe crown strains, an undesirable increase in surface in comparison to geogridreinforced and unreinforced cases is observed. Fig. 11 also shows that EPS inclusions with appropriate width and thickness in a geogrid rein­ forced systems could lessen SSS value as compared with unreinforced system. In order to have a clear and direct understanding of the influence of EPS block on the system behavior, the variation of VDS and SSS with EPS block width (wg/D) for three different block thicknesses at the end of loading are shown in Fig. 12. The variation of the VDS with wg/D ratio in Fig. 12a shows that VDS decreases whenever any EPS block is placed above the pipe, regardless of hg and wg/D ratio. This can be attributed to the compressibility of the EPS block and its associated arching effects. Fig. 12a demonstrates that EPS blocks with larger thickness and width are more effective in reducing VDS, particularly when the width was greater than 1.5 times the pipe diameter (wg/D � 1.5). For example, in the unreinforced and geogrid reinforced installation with no EPS block present, the maximum VDS value at last load cycle are 3.56% and 2.88%, respectively. With an EPS block thickness of 60 mm, the

buried pipe walls in Fig. 8a. This corroborates findings by Beju and Mandal [28]. where use of higher density EPS blocks demonstrated better pipe performance than lower density EPS. Fig. 8b shows that although the combination of EPS block and geogrid layer have beneficial effect in decreasing the VDS values, the SSS values were higher than those obtained for corresponding unreinforced and geogrid-reinforced systems. The response of the SSS is also a function of thickness and width of EPS block which has been described in the following section. Table 4 demonstrates the maximum vertical pressures transferred to the crown and springline of the pipes at the end of load cycles for the unreinforced, reinforced, and reinforced EPS systems with densities of 10, 20 and 30 kg/m3. In the unreinforced system, the vertical pressure at the crown and the springline of the pipe were attained 45 and 20 kPa, respectively. Greater reductions in stress were measured when a single layer of geogrid was present (with no EPS), where pressures on the crown and the springline of the pipe were reduced to 35 and 17 kPa (about 78% and 85% of the unreinforced condition, respectively). As seen in Table 4, for a combination of geogrid and low density EPS block (10 kg/cm3), the transferred pressures on the crown and the springline of the pipe are much larger than the unreinforced case. This is attributed to insufficient flexural resistance of the EPS block and the ensuing punching failure of the system, corresponding to observations of excessive SSS and VDS in the first cycles of loading mentioned previ­ ously (Fig. 7). It is also seen from Table 4 that with an increase in EPS block density, the pressure on crown and springline of the pipe signifi­ cantly decrease, with an EPS block of 30 kg/cm3 showing the least pressure. In this case, the pressure on the crown and the springline of the 10

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Fig. 9. Variations of maximum VDS with number of load cycles for unreinforced backfill and geogrid reinforced backfills with and without EPS block (a) wg//D ¼ 1, (b) wg//D ¼ 1.5, (c) wg//D ¼ 2.

maximum VDS are about 2.61%, 1.06%, and 0.86% for the same system with EPS width ratios of 1D, 1.5D and 2D. The marginal reduction in VDS when the EPS width changes from 1.5 to 2 times the pipe diameter suggests that an optimal width of an EPS is approximately 1.5 times the pipe diameter (wg/D ¼ 1.5). The reduction in VDS with EPS presence is due to a decrease in pressures transferred to the wall of the pipe, as shown in Table 6. The VDS value of 1.06% belongs to the installation of EPS block with a thickness of 60 mm and width ratio of 1.5D over the pipe in geogrid-reinforced backfill (Fig. 12a), less than a 2% VDS required for avoiding signs of structural distress [48]. Fig. 12b shows the variation of the SSS in the EPS block system with varying wg/D ratios and thicknesses (hg). As seen in this figure, increasing EPS block thickness in the geogrid-reinforced installation worsens SSS behavior. For example, for an EPS block with thickness of 100 mm, irrespective of EPS block width, larger settlements occur, even compared with unreinforced backfill. However, it should be noted that for geogrid-reinforced system containing EPS block with thicknesses of 30 and 60 mm and width ratio of 1.5D and 2D, the SSS value is less than

the unreinforced system. In context of this trend, appropriate use of EPS blocks with a geogrid layer can provide a significant reduction in VDS with acceptable SSS. Fig. 12b shows that for the geogrid-reinforced system and a fixed EPS block thickness, the value of SSS decreases with wg/D. The reason for this phenomenon might be the inability for a narrow block with less width in the all-direction flexibility and as a result, concentration of stress on it make the geofoam block have more strain and finally this parameter effect on the SSS value. At a fixed EPS width, increasing the thickness of EPS block (hg) increases the amount of the SSS. It may be attributed to the increase in thickness of the compressible EPS block and thus more settlement beneath the loading surface will occur. Table 5 shows the maximum strain in EPS-blocks with the width (wg/ D) and the thickness (hg) of EPS block. The maximum strain in EPS blocks, beneath the center of surface loading, were measured after the end of the tests by using a mechanical dial gauge. The results show that there is no visible strain (less than 1%) in blocks having 30 mm thick­ ness, irrespective of width of EPS block. In this situation, the blocks are 11

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Fig. 10. Variations of maximum crown strain of the pipe with number of load cycles for unreinforced backfill and geogrid reinforced backfills with and without EPS block (a) wg//D ¼ 1, (b) wg//D ¼ 1.5, (c) wg//D ¼ 2.

so far from the imposed surface load that they may have insignificant contribution to the surface settlements and remained in the elastic domain. The EPS blocks with thicknesses of 60 mm and 100 mm showed permanent deformations (over 1% as elastic limit) with decrease in the wg/D values. The significant point in this regard is for the EPS blocks having a thickness of 60 mm and wg/D ¼ 1.5 and 2. In these tests, the EPS blocks didn’t show any permanent strains beside credible amend­ ments in the behavior of embedded pipes (Fig. 12a) and soil surface settlements (Fig. 12b). The EPS blocks having 100 mm thickness showed permanent strains (more than 1%) besides the most effectual decreases in the VDS of pipe (Fig. 12a). The reason for this phenomenon is that the blocks experienced more stress because of their distance to the surface load and results in punching of concentrated loads into the EPS geofoam. Table 6 shows the maximum vertical pressures acting on the crown and springline of the pipes at the end of cyclic loading for the unrein­ forced and reinforced systems considering EPS blocks with different

thicknesses and widths. The vertical pressure on the crown and spring­ line of the pipe are significantly reduced with an increase in EPS block thickness and width. For the reinforced system with EPS block 100 mm thick (hg ¼ 100 mm) and wg/D ¼ 1 (width of 160 mm, equal to pipe diameter), the arching phenomenon causes a redistribution in vertical stress within the system, resulting in increased at the side of the pipe in comparison to the unreinforced and geogrid reinforced with no EPS block. As the EPS block width increases, a portion of transferred pressure is redistributed away from the pipe and thus the pressure on crown and springline of the pipe are decreased. The rate of reduction of pressures on the crown and the springline of the pipe were reduced with increasing width and thickness of overlying EPS blocks. From Table 6, as the EPS block thickness increases, leading to an increase in mobilization of shear strength in the infill and diminished the transferred pressure onto the pipe walls [28]. For example, the mini­ mum vertical pressure on the pipe provided by a 320 mm wide EPS block 12

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Fig. 11. Variations of maximum SSS with number of load cycles for unreinforced backfill and geogrid reinforced backfills with and without EPS block (a) wg//D ¼ 1, (b) wg//D ¼ 1.5, (c) wg//D ¼ 2.

(wg/D ¼ 2) in a geogrid reinforced trench was reduced by 78% and 73% in comparison with unreinforced and geogrid reinforced trenches, respectively. Similar results in reducing of pressure over the crown of the pipe have been reported in the literature, where an EPS thickness of 1.5 times the pipe diameter was found optimal [15,44]. However these studies did not consider the inclusion of any reinforcing geogrid layer.

conclusions are made: (1) With increasing pipe burial depth, the vertical diametric strain (VDS) and the crown strain of the pipe are significantly reduced. For example, VDS value range from about 6% at a depth of 1.2D to 2.2% at a depth of 2D (63.3% reduction). In contrast, by increasing the burial depth of the pipes, the soil surface settle­ ment (SSS) was increased since the compressible soil layer thickness increased. (2) For the geogrid-reinforced system, the maximum VDS at the end of the loading were reduced by 19% and 15.5% compared to the unreinforced condition for embedment depths of 1.5D and 1.2D, respectively. The reductions in pipe crown strains were similar. The maximum SSS of reinforced system was reduced by about 30% and 12% compared to the unreinforced condition for embedment depths of 1.5D and 1.2D, respectively.

8. Summary and conclusions Expanded Polystyrene (EPS) geofoam blocks have beneficial char­ acteristics being a conducive approach to use as a stress dissipater for buried structures such as buried pipelines. This research describes the results of cyclic load tests that highlight the role of various factors on the buried-pipe performance, such as pipe embedment depth, the influence of EPS blocks of varying thickness, width and density, and the presence of a geogrid reinforcement. Based on the results, the following 13

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Fig. 12. Variations of VDS and SSS at last cycle of loading with width ratio and thickness of EPS block for geogrid-reinforced systems (a) VDS, (b) SSS.

(5) The use of geogrid reinforcement with an EPS block with density of 30 kg/m3, hg ¼ 60 mm and Wg¼1.5D showed the most benefit when balancing VDS, pipe pressures, and SSS. Although these values are not the minimum in among all the tests, the VDS re­ mains lower than the 2% distress criteria discussed in code.

Table 5 Maximum EPS-blocks’ strains for different tests at the end of cyclic loading (z ¼ 1.5D, EPS density ¼ 30 kg/m3). Test Condition

hg

wg/D

Strain

Geogrid reinforced (with EPS)

100

1 1.5 2 1 1.5 2 1 1.5 2

�20% �12% �8% �8.5% <1% (elastic) <1% (elastic) <1% (elastic) <1% (elastic) <1% (elastic)

60 30

This study provides insight into the behavior of the buried pipes subjected to repeated loading (such as traffic loading) protected by geogrid reinforcements and EPS blocks. The tests results are obtained for only one type of pipe (uPVC pipe with 160 mm external diameter), one type of EPS material, one type of geogrid, one type of backfill soil and one type of natural soil. Hence, it should be noted that the test results applied in this paper might be limited to the size and type of the pipe, soil properties, geogrid and EPS material. Hence, additional investiga­ tion to confirm the results of this study are more widely applicable should be considered in the future. Furthermore, the findings herein would be better supplemented by even larger model sizes. Nonetheless, the large-scale testing performed herein does demonstrate the behavior of the system and may be considered a resource for future testing efforts.

Table 6 Maximum vertical pressure for different tests at the end of cyclic loading (z ¼ 1.5D, EPS density ¼ 30 kg/m3). Test Condition Unreinforced Geogrid reinforced (No EPS) Geogrid reinforced (with EPS)

wg/D

– – 1 1.5 2 1.5

hg (mm)

– – 100 30 60 100

Pressure (kPa) Crown

Side

45 35 29.5 14 9.5 31.5 18.5 14

20 17 27.5 6.5 4.5 20.5 11 6.5

Declaration of competing interest There is no conflict of interest in the choice of current reviewers. Nomenclature Specific gravity Gs Liquid Limit LL Plastic Limit PI Cohesion c Soil angle of internal friction φ Pipe diameter D Diameter of loading surface B Embedded depth of geogrid layer below the loading surface u Thickness of EPS block hg Width of EPS block wg Embedment depth of pipe Z Vertical Diametric Strain VDS Soil Surface Settlement SSS

(3) When geogrid reinforcements and EPS blocks are used in parallel, a considerable benefit is presented for reduction of VDS, pipe crown strains and pipe pressures, however it results in increased SSS. (4) With increasing EPS block density, the VDS, pipe crown strain and soil surface settlement values decrease, showing the largest decrease for the densest EPS block (30 kg/cm3). The lowest EPS block density (10 kg/m3) exhibited a punching failure in the initial cycles of loading (less than about 50th cycles), causing both the pipe rupture and excessive settlement. 14

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References

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