Cyclic and Post-Cycling Anchor Response in Geocell-Reinforced

: 34 Plate anchors are commonly used to resist static, cyclic and monotonic after cyclic-loading uplift loads. Under 35 cyclic loading, progressive sudden failure may occur, characterized by accumulated displacement – even under loads 36 significantly less than the static capacity. Despite extensive usage of geocell for increasing the cyclic resilience, the 37 influence of geocell reinforcements on cyclic uplift capacity is not well-understood. In this study, a series of near-full 38 scale, experimental tests with and without geocell are presented. Results show that the unreinforced system fails 39 cyclically under a load that is almost 70% of its static capacity (P u ), but use of geocell enables stable cyclic resistance 40 of over 100%P u . For the given soil and configurations, a cyclic displacement rate that reaches less than 0.05 mm/cycle 41 tends to highlight a likely stable response. Evaluation of the soil’s response to cyclic loading demonstrates that, with 42 increasing loading cycles, the loading is increasingly transmitted through the soil close to the anchor in the 43 unreinforced case, but that the reinforced case is less prone to this phenomenon. The monotonic post-cycling capacity 44 of both reinforced and unreinforced anchors decreases after application of cyclic loading; however, the unreinforced 45 scenario demonstrates larger decreases in capacity, particularly in the residual capacity. 46 47

D r a f t

117
Each cell in the geocell used is 110 mm long and wide (d) and 100 mm high (h). Fig. 2 shows an isometric view 118 of the geocell spread over the bottom soil layer and the plate anchor. This geocell was manufactured from a nonwoven 119 polymeric geotextile that was thermo-welded without internal perforations within each pocket. The engineering 120 properties of this geotextile, as listed by the manufacturer, are presented in Table 1. In all tests, the ratio of the geocell 121 pocket size (d) to width of anchor plate (B=300 mm) were kept constant. However, the d/B ratio adopted is not 122 necessarily the optimum value and a change in this ratio might increase or decrease the resistance encountered. Future 123 research could investigate optimal dimensions of geocell anchoring. 124

125
To investigate the uplift capacity and upward displacement of plate anchors supported by geocell layers, large-126 scale testing on a square steel anchor plate with width of 300 mm and 2.54 mm thickness attached to an anchor rod 127 with diameter of 50 mm was conducted in an indoor test pit. The test pit, measuring 2200 mm × 2200 mm in plan and 128 1000 mm in depth, contained the soil, anchor, geocell reinforcement and instrumentation (i.e. load cell, LVDTs and 129 pressure cells). The four sides of the test pit were vertical (Fig. 3). Because the width and depth of the test pit were 130 respectively more than seven and three times bigger than the width of the anchor, the boundary effects during testing 131 were not considered to be significant (Consoli et al., 2012). 132 The loading system (Fig.3) consisted of a loading frame, hydraulic actuator, and a controlling unit. The loading 133 frame is comprised of two heavy steel columns fixed in the ground and a horizontal strong reaction beam spanning 134 the width of the test pit that supports a hydraulic actuator. The hydraulic actuator and control unit may produce 135 monotonic or cyclic loads with the capability of applying a stepwise controlled load with a maximum tensile capacity 136 of 100 kN. The loading frequency of the loading system was in the range of 0.05 Hz to 0.5 Hz, but the best performance 137 was at the 0.

139
A custom data acquisition system was developed to read and record applied uplift loading, displacements and soil D r a f t 141 placed between the loading shaft and a rod attached to the plate anchor (Fig. 3a). To measure the displacement of the 142 plate anchor during the loading, a Linear Variable Differential Transducer (LVDT) with the accuracy of 0.01% of full 143 range (100 mm) was attached to the loading shaft and the supporting beam (as shown in Fig. 3). In some tests, two 144 soil pressure cells ("SPC") monitored the soil pressure at a depth of 100mm above the anchor (the depth of the upper 145 edge of the geocell) (Fig. 3b). The pressure cells had a capacity of 1000 kPa and an accuracy of 0.01% (0.1kPa), small 146 enough to not significantly influence measurements. To prevent stress concentrations from asperities on soil grains 147 located adjacent to the pressure cell, each cell was placed in a small bag filled with clay for consistent transferring of 148 stress to the pressure cells. The suffixes "i" and "o" are used to indicate the inner and outer positions 50 and 150 mm 149 away from the center of anchor, respectively. All output data streams (load cell, LVDT and pressure cells) were 150 recorded continuously using a data acquisition system within internal processor. To ensure an accurate reading, all of 151 the devices were calibrated prior to each test series. Fig. 3a illustrates the test installation prior to loading. A schematic 152 cross-section of the experimental set-up containing the test pit, loading system and data measurement system, geocell 153 layer, and the anchor is shown in Fig. 3b.

155
In order to compact the unreinforced layers and geocell-reinforced layer in the test pit (Fig. 3), a handheld 156 vibrating plate compactor was used. In all the tests, depending on the embedment depth of anchor the unreinforced 157 soil layers were prepared and compacted at thicknesses of either 50 or 100 mm with respectively one or three passes 158 of the compactor to achieve the required density (i.e. dry density of ≈18.78 kN/m 3 in Table 2). As the same the soil 159 filled the pockets of geocell layer was compacted with four passes of compactor to achieve the required density of soil 160 layer (shown in Table 2). This amount of compactive effort was maintained throughout the testing series. The density 161 of the both unreinforced and reinforced layers were checked for compaction specifications through sand cone testing 162 (ASTM D1556-07), performed at least three times per lift. A maximum difference of approximately 1-2% was 163 observed between the measured and desired density of compacted layer. The materials used were compacted at an 164 optimum moisture content of 5%, but the average measured (recovered) moisture content of the layers was between 165 4.8% and 5.2%. The exposed backfill material was covered with a waterproof paper to limit possible moisture loss.

166
To prepare the backfill in the test pit, a 100 mm thick unreinforced soil layer was compacted first. design is approximately three. In all the tests, the initial sustained load (P s ) was applied with a rate of 1.5 201 kPa per second to the both unreinforced and reinforced system (Fig. 4). After reaching the predefined P s , 202 the load is kept constant for approximately 120 seconds as to stabilize anchor movement before applying 203 cyclic loading. To control the rate of 1.5 kPa per second (i.e. the rate of 0.135 kN per second) during 204 monotonic loading, the predefined P s was applied at a fixed duration which was operated by an automated 205 load control system.

ii)
Cyclic loading: After the sustained load is reached, 250 sinusoidal loading cycles of amplitude P c and 10 207 sec. period (0.1 Hz frequency) are applied to the anchor (Fig. 4) D r a f t 247

248
Both the unreinforced and reinforced installations were tested with a fixed static load ratio (SLR=30%) and varying 249 cyclic load ratios (CLR) as shown in Table 3. Two general types of load-displacement behavior were observed under 250 the application of loading cycles, characterized as behavior (1)  , whereas stress states that are less than that required for progressive 260 failure result in long-term, steady-state response where no collapse is observed. Fig. 6b shows the load hysteresis 261 derived from the same test. In most of the tests, a large proportion of total anchor uplift displacement (between 15% 262 to 55% of total displacement) occurs in the first cycle, reaching an eventual stable state under applied cyclic loading.

263
With increased load cycles, the hysteresis loops become more symmetric and loading and unloading paths become 264 closer, implying that the load-displacement response is acting under increasingly elastic conditions.

265
Unstable behavior was observed for unreinforced conditions when CLR was greater than 40% and for geocell 266 reinforced conditions when CLR=70% at embedment depth of D/B=1.5 as shown typically in Fig. 6c

314
As observed for unreinforced conditions, a threshold cyclic load ratio may demonstrate a transition from a stable to 315 an unstable condition -use of soil reinforcement may mitigate this phenomenon. Fig. 9 compares the behavior for 316 unreinforced and reinforced conditions for CLR=40%. Unlike the unreinforced case, the reinforced case shows a 317 stable response for CLR=40%. As seen in Fig. 9a, the cumulative displacements for the reinforced installation is 318 well below the corresponding value in unreinforced condition at same cyclic load ratio.  . 11 shows the surface heave at the end of cyclic loading with CLR=40% for unreinforced and reinforced 332 beds. As seen in Fig. 11a, with application of cyclic load to the unreinforced bed, the soil located above the anchor 333 locally displaced upward and cracks propagated though the soil, leading to a reduction of soil resistance and finally 334 to failure of soil-anchor system. On the other hand as seen in Fig. 11b   376 Therefore, the load distribution area increases and upward displacement diminishes, which helps the overall stability 377 of composite layer against static and cyclic loads.
378 Fig. 16 shows the displacement accumulation rate for different cyclic loads and embedment depth ratios. As 379 discussed before, the displacement accumulation rate in the initial cycles (especially in first 10 cycles) is an important 380 surrogate for describing the long-term stability of anchor cyclic behavior. As seen in Fig. 16  (after a small increase in the first few cycles of loading of the reinforced soil). Soil pressure in all reinforced cases is 414 less than in the unreinforced cases regardless of the fact that the cyclic load ratio in all cases is higher for the reinforced 415 system. Evidently, the reinforced system can distribute load over a larger area and this helps to generate a more even 416 and consistent distribution of uplift stress in the overlying soil. As the cyclic load ratio increases, the soil pressure 417 measured by the outer soil pressure cell (SPCo) decreases more rapidly with number of cycles. Another observation 418 is that, as CLR increases, the stress distributed outwards from the anchor centerline remains high when the installation 419 is reinforced. Thus, reinforcement benefit is increased at high load ratios and at more cycles -eventually the 420 installation is adjusting to the loading with more stress being transferred to the geocell layer.

437
After stable cyclic loading, monotonic loading was applied to the anchor until failure occurred, highlighting the 438 influence of cyclic loading on the degradation or increase of the ultimate capacity of anchor systems.  451  19) is same as for the purely static loading (c.f. Fig. 5), but there are some key differences. A distinct peak uplift load 452 was observed for unreinforced conditions whereas no distinct peak was observed for reinforced conditions as shown 453 in Fig. 19. This is also evident for the monotonic-only results (Fig. 5). The geocell-reinforced systems exhibited a 454 stiffer response than the unreinforced system (Rahimi et al., 2018b). Post-cycling monotonic loading, even at small 455 CLR, show a non-negligible reduction in both unreinforced peak and residual loads with the largest cyclic loads 456 resulting in the greatest reduction in subsequent monotonic load capacity.
D r a f t 457 Table 5 shows a detailed summary of post-cycling monotonic loading at different embedment depths and cyclic load 458 levels. Less than a 5% reduction is observed in the uplift capacity of the reinforced bed at the failure load level of the 459 unreinforced case (CLR=40%), i.e. hardly any damage has been caused to the reinforced system under cyclic loading.

460
This advantage is more significant in comparison to the equivalent reduction for the unreinforced installation, which 461 is about 8% but at a much lower cyclic load level. At higher cyclic load levels there is a 15% reduction in both the 462 peak and residual loads for the reinforced bed with CLR=40-70% whereas a 20% reduction occurs for peak and a 20-463 30% reduction for residual loads in the unreinforced bed at CLR=20 and 30%, respectively. This reduction in strength   D r a f t 511  The post-cycling anchor load capacity of both the reinforced and unreinforced systems was less than their 512 respective original static load capacities. The greatest reduction from initial to final monotonic load capacities 513 was found in those installation that had received the largest magnitude of cyclic load amplitude. At the same 514 cyclic stress level, more damage was observed (by means of the reduction from initial to final monotonic -515 peak and residual -load capacities) in the unreinforced than in the reinforced installations.

516
The experimental results were obtained for only one type of soil, one type of geocell characteristics and one    Tables   Table 1 Engineering properties of the geotextile used in the tests Table 2 Densities of soil for unreinforced and geocell-reinforced layers after compaction Table 3 Scheme of the uplift tests on anchor in unreinforced and geocell-reinforced backfills (h=100 mm, b/B=3, SLR=30%) Table 4 Comparison of measured soil pressure in unreinforced and geocell-reinforced systems corresponding to peak of first (1 st ) and last (250 th ) lading cycles List of Figures   Fig. 1 Grain size distribution curves for backfill soil Fig. 2 A view of geocell spread over the anchor plate in the test pit  D r a f t          D r a f t