Seismic response of reduced-scale modular block and rigid faced reinforced walls through shaking table tests

Seismic response of reduced-scale modular block and rigid faced reinforced walls through shaking table tests

Geotextiles and Geomembranes 43 (2015) 307e316 Contents lists available at ScienceDirect Geotextiles and Geomembranes journal homepage: www.elsevier...

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Geotextiles and Geomembranes 43 (2015) 307e316

Contents lists available at ScienceDirect

Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem

Seismic response of reduced-scale modular block and rigid faced reinforced walls through shaking table tests G. Madhavi Latha a, *, P. Santhanakumar b a b

Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India Indian Institute of Science, Bangalore, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 January 2015 Received in revised form 21 April 2015 Accepted 22 April 2015 Available online 8 May 2015

This paper focuses on understanding the seismic response of geosynthetic reinforced retaining walls through shaking table tests on models of modular block and rigid faced reinforced retaining walls. Reduced-scale models of retaining walls reinforced with geogrid layers were constructed in a laminar box mounted on a uniaxial shaking table and subjected to various levels of sinusoidal base shaking. Models were instrumented with ultrasonic displacement sensors, earth pressure sensors and accelerometers. Effects of backfill density, number of reinforcement layers and reinforcement type on the performance of rigid faced and modular block walls were studied through different series of model tests. Performances of the walls were assessed in terms of face deformations, crest settlement and acceleration amplification at different elevations and compared. Modular block walls performed better than the rigid faced walls for the same level of base shaking because of the additional support derived by stacking the blocks with an offset. Type and quantity of reinforcement has significant effect on the seismic performance of both the types of walls. Displacements are more sensitive to relative density of the backfill and decrease with increasing relative density, the effect being more pronounced in case of unreinforced walls compared to the reinforced ones. Acceleration amplifications are not affected by the wall facing and inclusion of reinforcement. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Geosynthetics Shaking table tests Seismic response Modular block walls Geogrid Retaining walls

1. Introduction Retaining walls reinforced with geosynthetics performed satisfactorily during strong earthquakes as observed by several researchers (Juran and Christopher, 1989; Kutter et al., 1990; Collin et al., 1992; Bathurst et al., 1993; Sandri, 1997; Tatsuoka et al., 1997; Ling et al., 2001). Collin et al. (1992) reported that Geosynthetic Reinforced Soil (GRS) walls survived the Loma Prieta earthquake of 1989 with estimated ground accelerations ranging from 0.3 to 0.7 g. White and Holtz (1997) conducted a survey of three geosynthetic reinforced walls and four geosynthetic reinforced slopes after Northridge earthquake of 1994 to show that these walls and slopes were not subjected to any visual distress after the earthquake. However, there are also many case studies of failures of geosynthetic reinforced retaining walls, a database of 171 of them documented by Koerner and Koerner (2013).

* Corresponding author. Tel.: þ91 80 2293 3123; fax: þ91 80 2360 0404. E-mail addresses: [email protected] (G.M. Latha), santhy.iisc@gmail. com (P. Santhanakumar). http://dx.doi.org/10.1016/j.geotexmem.2015.04.008 0266-1144/© 2015 Elsevier Ltd. All rights reserved.

The use of Segmental or modular block Retaining Walls (SRW) that include dry-stacked concrete block units as the facia system together with extensible sheets of polymeric materials (geosynthetics) that internally reinforce the retained soils and anchor the facia has gained wide popularity in recent times. Studies on SRW in North America were reported by Bathurst and Simac (1994). Several other researchers (Cazzuffi and Rimoldi, 1994; Gourc et al., 1990; Knutson, 1990; Won, 1994) reported the use of these structures in Europe, Scandinavia and Australia. Use of modular block walls has tremendously increased all over the world during recent years. The distinguishing feature of these structures is the facing column that is constructed using mortarless modular concrete block units that are stacked to form a wall batter into the retained soils (typically 3e15 from vertical). Modular blocks of different shapes and sizes are available in market and are well explained by several researchers (Bathurst and Simac, 1994; Ehrlich and Mirmoradi, 2013). Shaking table tests facilitate testing of relatively larger structures and model response can be physically observed in these tests along with measurements of response parameters. Most of the shaking table tests are conducted using reduced scale models in a

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1 g field (Bathurst et al., 2001; Koseki et al., 2003; Panah et al., 2015) that are possibly subjective to scale effects due to the influence of stress levels and the lack of reasonable scaling techniques. Most of the model studies on seismic behavior of GRS walls have been performed on very small-scale models where scale effects are expected to have a major influence on measured response. Some examples include: Wang et al. (2015), H (model wall height) ¼ 0.7 m; Lo Grasso et al. (2005), H ¼ 0.35 m; Watanabe et al. (2003); Kato et al. (2002) and Koseki et al. (1998), H ¼ 0.5 m; Latha and Krishna (2008), H ¼ 0.6 m. There are also some seismic tests on larger models: El Emam and Bathurst (2007), Matsuo et al. (1998) H ¼ 1 m; Sakaguchi (1996), H ¼ 1.5 m and Ling et al. (2005), H ¼ 2.8 m. In the present study, height of the model walls is 0.6 m. Though scale effects prevail in these tests, relative performance of rigid faced and modular block walls at varying earthquake shaking conditions can be derived from the observations, providing insights to the effect of various parameters on the seismic performance of these walls. Several studies on segmental retaining walls are available in literature. Yoo and Kim (2008) investigated the effect of surcharge loads on segmental retaining walls by carrying out a full-scale load test and a 3D finite element analysis on a two-tier, 5 m high, geosynthetic reinforced segmental retaining wall. Bathurst et al. (1997) presented full scale tests on geosynthetic reinforced retaining walls constructed with a column of dry-stacked modular concrete units and wrapped face. It was concluded that hard facing column is a structural element that acts to reduce the magnitude of strains that would otherwise develop in a wall with a flexible facing. Ramakrishnan et al. (1998) presented shaking table test results of geotextile wrap faced and geotextile-reinforced segmental model retaining walls. Segmental retaining wall was found to sustain approximately twice the critical acceleration of the wrap-faced wall. Huang et al. (2003) used multi-wedge method based on Newmark's sliding block theory to analyze four geosynthetic reinforced modular block walls in the 1999 chiechi earthquake. Ling et al. (2005) presented shaking table tests on three large scale 2.8 m high modular-block geosynthetic-reinforced soil walls subjected to significant shaking using the Kobe earthquake motions. The reinforcements used were polymeric geogrids, which were frictionally connected to the facing blocks having a front lip. It was observed that the wall performance under earthquake shaking could be improved by increasing the length of the top reinforcement layer, reducing vertical reinforcement spacing, and grouting the top blocks to ensure firm connection to the reinforcement. Koerner and Soong (2001) carried out extensive survey of existing geosynthetic reinforced segmental walls and reported major reasons for excessive deformations and collapse of some of these walls. Yoo and Jung (2006) investigated the case history of a failed geosynthetic reinforced segmental retaining wall in Korea. Finite element analysis of the wall and laboratory tests carried out on backfill and reinforcement revealed that the main reasons for failure were inappropriate design and low quality backfill, apart from the rainfall infiltration. Liu (2012) carried out extensive finite Table 1 Properties of backfill sand. D10 D30 D60 Coefficient of uniformity Cu Coefficient of curvature Cc Specific gravity G Maximum void ratio emax Minimum void ratio emin Maximum unit weight gdmax Minimum unit weight gdmin

0.215 mm 0.37 mm 0.71 mm 3.30 0.896 2.65 0.828 0.5022 17.22 kN/m3 14.21 kN/m3

element analysis of geosynthetic reinforced segmental retaining walls and concluded that the deformation of reinforced soil zone was largely governed by reinforcement spacing and reinforcement stiffness, whereas the lateral displacement at the back of reinforced soil zone was governed by the reinforcement length. To understand the performance of geosynthetic reinforced soil (GRS) walls during strong shaking, a series of shaking table tests on reinforced soil model walls with dry sand backfill are performed in the present study. This research effort had the goals of providing insight into the seismic response of geosynthetic reinforced soil walls under controlled dynamic base shaking, with the variation of parameters like type of facing, backfill relative density, reinforcement layers, and frequency of base motion. 2. Equipment and materials used in the experiments This study presents the performance of rigid faced and modular block walls at varying earthquake shaking conditions, providing insights to the effect of various parameters on the seismic performance of these walls. To understand the performance of geosynthetic reinforced soil (GRS) walls during strong shaking, a series of shaking table tests on reinforced soil model walls with dry sand backfill are performed in the present study. This research effort had the goals of providing insight into the seismic response of geosynthetic reinforced soil walls under controlled dynamic base shaking, with the variation of parameters like type of facing, backfill relative density, reinforcement layers, and frequency of base motion. 2.1. Shaking table A computer controlled servo hydraulic single axis shaking table with payload capacity of 1000 kg and foot print of up to 1000 mm  1000 mm was used in this study. To minimize the boundary effects on model structures, a laminar box was designed and built for the shaking table facility. Laminar box is a large sized shear box consisting of several horizontal layers, built such that the friction between the layers is minimized. The layers move relative to one another in accordance with the deformation of the soil inside. The laminar box used in this study is rectangular in cross section with inside dimensions of 500 mm  1000 mm and 800 mm deep made up of fifteen rectangular hollow layers machined from solid aluminum compose. The gap between the successive layers is 2 mm and the bottommost layer is rigidly connected to the solid aluminum base of dimensions 800 mm  1200 mm and 15 mm thickness. The layers were separated by linear roller bearings arranged to permit relative movement between the layers with minimum friction. Accelerometers, soil pressure sensors and Ultrasonic Displacement Sensors (USDT) were used for instrumenting the model retaining walls. 2.2. Back fill material Backfill material used for the model construction is locally available dry sand. The sand is classified as poorly graded (SP) according to the Unified Soil Classification System. Physical properties of the sand are reported in Table 1. 2.3. Reinforcement Backfill sand is reinforced with two different types of geogrids, stronger biaxial geogrid (SG) and weaker biaxial geogrid (WG). These geogrids are made up of polypropylene, biaxially oriented integrally extruded geogrids with rigid junctions and stiff ribs. Properties of both the geogrids are presented in Table 2.

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2.4. Modular blocks Concrete blocks used for model facing were 125 mm wide  100 mm long  150 mm deep with a positive mechanical interlock in the form of concrete lip of 125 mm width  25 mm length  50 mm thickness located at the back bottom of each block. Each modular block has a 75 mm wide  50 mm long  150 mm deep rectangular hollow section created by 25 mm thick reinforced concrete. The dimension details of the block are shown in Fig. 1. Modular concrete blocks were made according to the specifications of National Concrete Masonry Association (NCMA), ASTM codes which are presented in the Table 3. The blocks were casted with high quality ordinary Portland cement of grade 53 for achieving minimum of 28 days characteristic compressive strength of 30 MPa. The maximum nominal aggregate size used is 6 mm. The maximum water content used is 40% with a workability slump of 50e75 mm under severe exposure condition. The mix design ratio by weight is 1:1.2:2.36. The blocks were casted by moulds made up of wood coated with red oxide paint. These blocks were reinforced with galvanized iron wires of 4 mm diameter. Properties of the modular block are presented in Table 4.

Fig. 1. Dimensions of modular block.

Table 3 Specifications of modular blocks. Minimum compressive strength Water adsorption Dimension tolerance Maximum horizontal gap between erected units

30 MPa 8% 3 mm 3.25 mm

2.5. Model construction Model retaining walls were constructed in the laminar box to a size of 700 mm  500 mm in plan and 600 mm height. All model walls are backfilled with sand, constructed in lifts of equal heights. Wall facing was either rigid or modular block facing. Rigid faced wall models were constructed using 12 hollow rectangular steel box sections of 50 mm height and 25 mm width each bolted together using two vertical steel rods at two ends, both bolted to the bottom plywood base to form a 600 mm high rigid panel of 25 mm thickness with a fixed bottom condition. The reinforcement materials were made to run through the rods firmly fixed between two rectangular box sections. Modular block walls were made up of concrete blocks of size 125 mm  100 mm  150 mm with a positive mechanical interlock in the form of concrete lip of 125 mm  25 mm  50 mm located at the back bottom of each block. The model wall forms an inward batter of 7.2 . The bottommost layer of the wall is fixed to the wooden frame which in turn is firmly attached to the base. A typical rigid facing wall and a typical modular faced model wall after construction are shown in Fig. 2. Backfill sand was placed in the laminar box using dry pluviation technique to achieve the uniform density. In case of reinforced wall models, geosynthetic layers were placed at the specific depth while filling the backfill sand. Minimum reinforcement length (Lrein) of 0.7H (420 mm) corresponding to minimum required for reinforced earth structures (FHWA, 2001) is maintained from the wall facing in all the tests. The schematics of typical reinforced rigid faced wall and modular block wall with instrumentation are shown in Fig. 3. The retaining wall models were subjected to specific sinusoidal motion of 20 cycles. Dynamic response of wall models in terms of accelerations, facing displacements, vertical displacements, horizontal soil pressure were measured using accelerometers, displacement sensors, soil pressure sensors respectively and the

Table 2 Properties of geogrids. Property

WG

SG

Ultimate tensile strength (kN/m) Yield point strain Aperture size Mass per unit area g/m2

20 16.27% 35  35 220

40 9.2% 30  30 230

Table 4 Properties of modular block units. Compressive strength Water adsorption Overall dimensions Hollow section Weight of each unit Maximum aggregate size Sand Cement grade Reinforcement

35 MPa 5.5% 125  100  150 mm/64.5  100  150 mm 75  50  150 mm/14.5  50  150 mm 3.55 kg/1.75 kg 6 mm Zone II OPC 53 Galvanized iron wire of 4 mm diameter

same were recorded using data acquisition system of the shaking table. Out of the four accelerometers, one accelerometer A0 was fixed to the base of the shaking table. Remaining three accelerometers A1, A2, A3 are embedded in backfill material at elevations 150, 300, 600 mm respectively from the base at a constant distance from 100 mm. Three soil pressure transducers P1, P2, P3 are placed inside the wall in contact with facing at elevations 175, 325, 475 mm respectively from the base. To measure the horizontal displacement of wall facing, three ultrasonic displacement transducers (USDT) D1, D2 and D3 were placed at elevations 200, 350, 500 mm respectively from the base. To measure the vertical displacement of the backfill sand, one USDT D4 is placed at a distance 200 mm wall facing. Vertical displacement at other locations along the length of the backfill was measured using dial gauges. 3. Similitude laws Shaking table tests in this study are 1 g model studies carried out on reduced scale models. The stresses and deformations measured in the experiments do not truly represent the stresses and deformations in field because of low confining pressures and boundary effects in model studies. Hence it is essential to apply proper similitude rules for the experiments in order to apply the results to actual field conditions. Iai (1989) presented similitude laws for the 1 g model tests from basic definitions of effective stress, strain and constitutive law, overall equilibrium and mass balance. A geometric scale factor, l, was defined as the proportionality constant between the model and prototype geometry. Similar

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Fig. 2. Retaining wall facings used in the study a) Rigid facing b) Modular block facing.

Fig. 3. Schematics of test set-up and instrumentation a) Rigid faced reinforced wall b) Modular block reinforced wall.

proportionality equations were assumed for other parameters such as stress-strain and pore water pressure. For the present study, the geometric scale factor, l, is taken as 8. Accordingly, the height of the model wall is kept as 0.6 m, corresponding to 4.8 m in field. The

scaling factors computed for relating various physical quantities in models to those in prototype are given in Table 5. Dimensions of the modular blocks in the model tests are 0.125 m  0.1 m  0.15 m (L  B  H), corresponding to prototype

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Table 5 Similitude laws for shaking table model tests (Iai, 1989). Parameter

Model Parameter

Equation for scaling (Prototype/Model)

Scaling factor

Prototype Parameter

Acceleration (g) Unit weight of sand (kN/m3)

0.2 15.48 (47% RD) 16.02 (65% RD) 0.5  0.6

1 1

1 1

l

8

0.2 15.48 (47% RD) 16.02 (65% RD) 4  4.8

0.125  0.1  0.15

l

8

1  0.8  1.2

2 tm

1/l3/4

0.21 4.75 8

0.42 4.75  tm 8  sm

Dimensions of the wall B  H (m) Dimensions of the modular blocks L  B  H (m) Frequency (Hz) Time Stress

l3/4 l

sm

blocks of dimensions 1 m  0.8 m  1.2 m. Bathurst et al. (1996) specified that the maximum dimensions of proprietary modular blocks are 1.8 m  0.8 m  0.6 m (L  B  H). Height of the prototype blocks corresponding to the present study is higher than these specifications, though the length and width fall in the

specified range. Several other researchers have used modular blocks of similar height in the model tests (Ehrlich and Mirmoradi, 2013; Ling et al., 2005). Scaling of reinforcement tensile strength is not attempted in this study. Hence the geogrids used in the study simulate very strong

Table 6 Parameters varied and the corresponding test code. Test code

Facing type

Relative density of backfill (%)

Type of reinforcement

Number of reinforcing layers

UT1 UT2 MUT1 MUT2 RT1 RT2 RT3 RT4 RT5 RT6 RT7 RT8 MRT1 MRT2 MRT3 MRT4

Rigid Rigid Modular Modular Rigid Rigid Rigid Rigid Rigid Rigid Rigid Rigid Modular Modular Modular Modular

47 65 65 65 47 47 65 65 47 47 65 65 47 47 65 65

e e e e WG* WG WG WG SG** SG SG SG WG WG WG WG

e e

2 3 2 3 2 3 2 3 2 3 2 3

WG*: Weaker biaxial geogrid; SG**: Stronger biaxial geogrid.

Fig. 4. Variation of horizontal displacement of wall with change in relative density of backfill a) Rigid faced wall b) Modular block wall.

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Fig. 5. Horizontal displacement vs. normalized height of the wall with change in relative density of backfill for rigid faced and modular block walls.

prototype geogrids. However, comparing their relative tensile strength, these geogrids are referred to as weak and strong geogrids in this paper. 4. Results and discussion Rigid faced unreinforced and reinforced retaining walls and unreinforced and reinforced modular block walls were tested under acceleration of 0.3 g and frequency of 2 Hz for two different relative densities 47% and 65%. Parameters varied in the tests and the corresponding test codes are shown in Table 6. 4.1. Unreinforced walls Effect of backfill density on wall performance of unreinforced and reinforced retaining wall models was studied by conducting tests at two different relative densities, 47% and 65%. Fig. 4 shows

Fig. 7. Variation of vertical displacements of wall with change in relative density of backfill a) Rigid faced wall b) Modular block wall.

the comparison of wall displacements with relative density for unreinforced rigid faced walls and unreinforced modular block walls. It is observed that the displacement of the wall increased with the elevation of the wall. Increase in the backfill density reduced the deformations significantly for both rigid faced and modular block walls. At any specific relative density, modular block walls showed lower deformations compared to rigid faced walls as observed in Fig. 4. This is because of the dissimilarities between the base widths of the rigid faced and modular block walls. The base width of modular block walls was 125 mm, five times the base width of the rigid faced walls (25 mm). To eliminate the effect of base width, horizontal displacements for the rigid faced and modular block walls are plotted with respect to the normalized height (H/Base width) of the wall in Fig. 5. From this figure, it is very clear that at any specific normalized height of the wall, modular block walls deformed more compared to the rigid faced walls. This is because the facing is much rigid in case of rigid faced wall

Fig. 6. Variation of RMSA amplification factors of wall with change in relative density of backfill a) Rigid faced wall b) Modular block wall.

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Fig. 8. Variation of horizontal displacement of wall with type and quantity of reinforcement a) Rigid facing with 47% relative density b) Rigid facing with 65% relative density c) Modular facing with 47% relative density d) Modular facing with 65% relative density.

compared to the modular block walls. The facing consists of a stack of rigid steel panels set in position by running them through two rigid steel rods at both ends in case of rigid faced walls, whereas the facing is made of a stack of interlocking hollow concrete blocks, allowing it to deform more.

Fig. 9. Horizontal displacement vs. normalized height of the wall for unreinforced and reinforced rigid faced and modular block walls.

Improving the relative density of the backfill resulted in reduction in wall deformations for both rigid faced and modular block walls, the benefit being more pronounced in case of modular block walls. In case of backfill density of 47%, maximum wall face deformation is reduced from 22.33 mm to 18.22 mm with increase in relative density of the backfill from 47% to 65% for the rigid faced walls g as shown in Fig. 5(a). In case of modular block walls, horizontal deformations of the wall reduced from 14.65 mm to 6.14 mm with increase in relative density of the backfill from 47% to 65%. Maximum reduction in displacement with the increase in relative density was 18.4% for rigid faced walls and 58% for modular block walls. When the backfill is loose, it tries to deform more under cyclic loading, thus exerting higher pressure on the wall. When the density of the soil increases, the friction angle increases, leading to a reduction in the active earth pressure coefficient and in turn the pressure exerted on the wall. To simplify the presentation of acceleration response at different elevations of the slope, Root mean square acceleration amplification factor (RMSA) is used. RMSA amplification factor is the ratio of response acceleration value in the soil to that of corresponding value of the base motion (Kramer, 1996). Accelerations are amplified more at the top of the wall. RMSA factors are slightly higher for walls with denser backfill as observed in Fig. 6. However, facing type has no significant influence on the RMSA amplification factors. Incremental residual pressures observed at the end of dynamic excitation along the height of the wall in different rigid faced unreinforced model walls did not show any consistent trend. Not

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Fig. 10. Variation of RMSA amplification factors with type and quantity of reinforcement a) Rigid facing with 47% relative density b) Rigid facing with 65% relative density c) Modular facing with 47% relative density d) Modular facing with 65% relative density.

much variation in the pressures with change in facing type was observed. Fig. 7 shows the variation of vertical displacement along the backfill surface measured at three different locations for tests with different relative densities. Settlement of the backfill increased with the increase in the distance from the facing, which means that the backfill sand settled less near the facing. Vertical displacement of the backfill decreased by about 30% on increase in relative density of the backfill from 47% to 65% for both rigid faced and modular block walls. 4.2. Reinforced walls Two type of geogrids, namely weaker geogrid (WG) and stronger geogrid (SG) were used in the model studies. Rigid faced walls were reinforced with both these types of geogrids in different model tests (Table 5). However, modular block faced walls were reinforced only with weaker geogrids because the deformations were negligible even when the walls were reinforced with weaker geogrid. Length of reinforcement was 420 mm (0.7 H) from the wall facing in all the models. Relative density of backfill was varied as 47% and 65% in reinforced model tests also. The model walls were subjected to 20 cycles of sinusoidal shaking motion of 0.3 g acceleration and 2 Hz frequency. Model tests RT1 e RT8 were intended to study the effect of type and quantity of reinforcement on the performance of rigid faced soil wall models, whereas tests MRT1 e MRT4 were used to study the performance of reinforced modular block walls.

Fig. 8(a) and (b) show the horizontal displacements for unreinforced and reinforced rigid faced walls at relative densities of 47% and 65% respectively. These plots present the comparison of wall deformations without reinforcement and with two and three layers of weak and strong geogrid reinforcement. Fig. 8(c) and (d) show similar comparisons for modular block walls. Compared to unreinforced walls, wall deformations reduced significantly on inclusion of reinforcement for both the types of walls. Maximum displacement of 22.33 mm observed in case of unreinforced rigid faced wall with backfill density of 47% (UT1) was reduced to 3.71 mm upon inclusion of 2 layers of stronger geogrid and it further reduced to 1.68 mm upon inclusion of 3 layers of stronger geogrids. On inclusion of 2 layers of weaker geogrid, horizontal displacements in modular blocks were restricted to 5.95 mm and the deformations further reduced to 1.34 mm on the inclusion of 3 layers of weaker geogrid. Similar decrements in deformation were observed with the inclusion of reinforcement at 65% backfill relative density also. Horizontal deformation of the modular block wall with three layers of weaker geogrid reinforcement at 6% backfill relative density is less than 1 mm. Since the deformations in modular block walls were very low even with weaker reinforcement, further tests with modular block walls reinforced with stronger geogrids were not planned. A comparative plot of the deformations of rigid faced and modular block walls with and without reinforcement at 47% relative density of the backfill is presented in the form of horizontal displacement of the wall against normalized height of the wall in Fig. 9. This figure indicates that though the unreinforced modular block wall deformed more compared to the unreinforced rigid

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Reinforced retaining walls showed lesser vertical settlements in case of rigid as well as modular block walls at both the relative densities. As observed from Fig. 11a and c, vertical deformations of the rigid faced walls were not affected by the type of reinforcement. However, increasing the quantity of reinforcement resulted in the decrease of settlements for all the model tests. Maximum vertical settlement measured in unreinforced rigid retaining wall with 47% relative density was about 46 mm and it was reduced to about 19 mm by the inclusion of three layers of weaker or stronger geogrid. In case of modular block walls with 47% relative density, maximum vertical deformation of unreinforced wall was about 42 mm and the inclusion of three layers of weaker geogrid brought down the maximum settlement to 15 mm. Similar reduction in settlements was observed also for walls with 65% relative density. Reduction of settlements was about 60% with three layers of geogrid for rigid as well as modular block walls at both the relative densities tested. Frequency of shaking and interface shear between stacked modular blocks are other important parameters that could influence the seismic response of modular block walls, which are not considered in this study. 5. Conclusions

Fig. 11. Variation of vertical displacements with type and quantity of reinforcement a) Rigid facing with 47% relative density b) Rigid facing with 65% relative density c) Modular facing with 47% relative density d) Modular facing with 65% relative density.

block wall, modular block wall reinforced with 3 layers of weaker geogrid deformed less compared to the similarly reinforced rigid faced wall. Reinforcement is more effective in case of modular block walls in reducing the deformations. Comparison of RMSA amplification factors for rigid faced and modular block walls with different quantities of geogrid reinforcement at both the relative densities tested is shown in Fig. 10. Reinforced retaining walls exhibited increased acceleration amplifications and the acceleration amplification is non-uniformly distributed along the height of the wall. Through series of centrifuge tests on geosynthetic reinforced soil structures, Yang et al. (2013) demonstrated that the acceleration amplification factor is larger than 1.0 and non-uniformly distributed with height when the base acceleration is less than 0.4 g. Results from the present study confirm this observation. Accelerations are amplified more at lower levels of the retaining walls by the inclusion of reinforcement and at the top most point of measurement, not much difference in the acceleration amplifications was observed between unreinforced and reinforced retaining walls for at both the backfill densities tested, as seen from Fig. 10. RMSA amplification factors estimated for the reinforced retaining walls in the present study ranged between 1.1 and 1.3, which is same for unreinforced walls. Fig. 11 shows the variation of vertical displacement along the backfill surface measured at three different locations for unreinforced and reinforced model walls with different relative densities.

The following major conclusions are drawn from the results obtained from the 1-g shaking table studies carried out on rigid faced and modular block retaining walls. Displacement of the retaining wall during base shaking increases with the elevation of the wall for both rigid faced and modular block walls. Increase in the backfill density reduced the deformations significantly for both rigid faced and modular block walls, the benefit being more pronounced in case of modular block walls. Maximum reduction in horizontal displacement with the increase in relative density from 47% to 65% was 18.4% for rigid faced walls and 58% for modular block walls. Vertical displacement of the backfill decreased by about 30% on increase in relative density of the backfill from 47% to 65% for both rigid faced and modular block walls. At any specific normalized height of the wall, modular block walls deformed more compared to the rigid faced walls. Reinforcement is more effective in case of modular block walls in reducing the deformations. Reinforced retaining walls exhibited increased acceleration amplifications and the acceleration amplification is non-uniformly distributed along the height of the wall. Vertical deformations of the rigid faced walls were not affected by the type of reinforcement. Increasing the quantity of reinforcement resulted in the decrease of settlements for all the model tests. With the inclusion of 3 layers of geogrid, the vertical deformations were reduced by about 60% in both rigid faced and modular block walls. References Bathurst, R.J., Simac, M.R., Christopher, B.R., Bonczkiewicz, C., 1993. A Database of Results from a Geosynthetic Reinforced Modular Block Soil Retaining Wall, Proceedings of Soil Reinforcement: Full Scale Experiments of the 80's. ISSMFE/ ENPC, Paris, France, pp. 341e365. Bathurst, R.J., Simac, M.R., 1994. Geosynthetic reinforced segmental retaining wall structures in North America. In: Proceedings of the Fifth International Conference on Geotextiles, Geomembranes and Related Products, Singapore, September 1994, pp. 1e24. Bathurst, J., Cai, Z., Pelletier, M., 1996. Seismic design and performance of geosynthetic-reinforced segmental retaining walls. In: Proceedings of the 10th Annual Symposium of the Vancouver Geotechnical Society, Vancouver, BC, Canada, June 1996, pp. 1e26.

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