Design of Roadside Barrier Systems Placed on MSE Retaining Walls (2010)

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38 A series of pullout tests were performed to evaluate the influence of rate effect on the pullout capacity of the reinforce- ment. Ten tests were conducted: seven on steel reinforcement strips and three on steel bar mats. The group that ran the pull- out tests was GeoTesting Express in Atlanta. 4.1 Rate of Loading Three testing times to failure were used: 0.05 sec, 5 sec, and 3,600 sec. This covered five log cycles of time to failure. 4.2 Saturation The question arose whether a fully saturated condition should be considered. The argument is that, according to the- ory, a saturated condition will lead to a decrease in resistance when the soil behavior goes from drained behavior to undrained behavior as the rate of loading is increased. The reason is that the pore pressures are higher in undrained con- ditions compared to drained conditions and, as a result, the effective stress is lower. Because the effective stress controls the strength, then undrained behavior leads to lower resis- tances than drained behavior. This effect was verified on pull- out tests performed by Antonio Bobet (22) where the capacity dropped by about 30%. His study was requested by the Indi- ana DOT because some overpasses are in flood areas and the bottom of the wall may be submerged at times. Discussions with Antonio Bobet at Purdue, Pete Anderson at RECO, and Mark McClelland at TxDOT led to the conclusions that (1) the saturated condition is rare and (2) when it happens it usually affects only the bottom of the wall, not the top where the strips loaded by the impact on the barrier are located. Nevertheless, it is prudent to test that condition to cover all cases. Therefore, the soil was tested under two moisture con- ditions: (1) at its optimum moisture content after proper compaction and (2) saturated after it was tested in the unsat- urated condition. 4.3 Fines Highway jobs have strict tolerances on the gradation of the soil used as backfill. Commercial jobs are much less stringent and allow for a much higher percent fines. The tests were lim- ited to a soil that satisfies the DOTs’ common guidelines. There are typically two types of soils used behind MSE walls: well- graded sand or crushed rock (No. 57 stones). The soil tested had the following characteristics: • Well-graded sand • Less than 15% passing sieve No. 200 • Less than 60% passing sieve No. 40 • Largest particle smaller than 76.2 mm (3 in.) The grain size distribution of the sand used is given in Fig- ure 4.1. A compaction test was also performed and is shown in Figure 4.2. 4.4 Reinforcement Geosynthetics represent a minor component of highway wall reinforcement according to the survey. Therefore, the tests focused on inextensible reinforcement. Two types of reinforcing materials were used in the tests. One type was reinforcing strips provided by RECO. The other type was bar mats provided by Foster Geotechnical, now merged with RECO. The dimensions of the reinforcing strips and bar mats are included in Table 4.1. Concrete sand was used in the tests as the backfill material. The concrete sand was purchased from a retail store and meets ASTM C33 requirements. Compaction and gradation tests were per- formed on the concrete sand. The gradation test results indi- cate the concrete sand meets the usual gradation requirements (less than 15% passing sieve No. 200, less than 60% passing sieve No. 40, and the largest particle smaller than 3 in.). C H A P T E R 4 Reinforcement Pullout Tests

39 Figure 4.1. Grain size distribution of the sand used in the pullout experiments.

40 Figure 4.2. Compaction curve for the sand tested.

4.6 Procedure (Soil Installation, Rate of Loading, Testing) A pullout test box was used in the tests. The box has approx- imate dimensions of 701 mm × 381 mm × 1.31 m (2.3 ft × 1.25 ft × 4.3 ft). Photos and sketches of the box are shown in Figure 4.3. Sand was first compacted near optimum moisture content to approximately 95% of the maximum dry density to a height 165 mm (6.5 in.) from the bottom of the box, and then the reinforcing strip or bar mat was placed. An additional 216 mm (8.5 in.) of sand was then placed and compacted to the top of the box. A steel plate was placed on the top of the sand. Dead weights were then placed on top of the steel plate to sim- ulate 3 ft of soil overburden. A hydraulic loading system was attached to the front of the strip or bar mat to provide loading for slow- and medium-speed tests. A pneumatic loading sys- tem was used for high-speed tests. Figures 4.3 and 4.4 show the box setup and the loading systems for the strips and bar mat, respectively. Two LVDTs were mounted to the box to monitor the deflection during the tests. A ruler was also attached to the piston to measure the deflection after the LVDTs reached their limit of 38.1 mm (1.5 in.). A load cell was attached to the pis- ton to measure the load during the tests. The LVDTs and load cell were connected to a computer data acquisition system to acquire the data during the tests. 4.7 Results and Conclusion The soil was compacted in layers up to the location of the reinforcement. A standard size steel strip and a standard size bar mat were installed and the compaction process was com- pleted. A surcharge was placed on top of the sand to simulate a total of 0.91 m (3 ft) of soil cover on the reinforcement. Then the reinforcement was pulled to failure. A load displacement curve was obtained for each test. The results are shown in Figures 4.5 to 4.7. Table 4.3 shows a sum- mary of the test results at failure. The rate effect is shown for all tests on Figure 4.8. The data indicate that there is no particular trend in the effect of the rate of loading. Indeed the pullout resistance at the fastest rate is often larger or equal to the resis- tance at slower rates. Therefore, these tests indicate that there is no reason to take into account any rate effect on the pull- out capacity of the reinforcement during barrier impact. The back-calculated F* values for the steel strips ranged from 1.7 to 3.87 and averaged 3.0. This average is well within the range of values obtained in the literature (Figure 4.9) (23). The present AASHTO recommendations for calculating the resistance of MSE wall reinforcement to static loading lead to a predicted reinforcement resistance smaller or equal to the actual reinforcement resistance under impact loading (safe condition). On the basis of these few tests, it is suggested that the current AASHTO recommendations be used as-is to calcu- late the resistance of the reinforcement to impact loads. 41 Reinforcing Material Length (in.) Width (in.) Thickness (in.) Strip 60.0 2.0 0.2 Bar Mat 57.5 24.5 0.4 Table 4.1. Dimension of the reinforcing strip and bar mat. Test No. Reinforcement Target Time to Failure (s) Soil Condition 1 Single Strip 0.05 95% MDD @ OM 2 Single Strip 5.0 95% MDD @ OM 3 Single Strip 3,600.0 95% MDD @ OM 4 Single Strip 0.05 95% MDD Saturated 5 Single Strip 5.0 95% MDD Saturated 6 Single Strip 5.0 95% MDD Saturated 7 Single Strip 3,600.0 95% MDD Saturated 8 Bar Mat 0.05 95% MDD @ OM 9 Bar Mat 5.0 95% MDD @ OM 10 Bar Mat 3,600.0 95% MDD @ OM MDD = Maximum Dry Density; OM = Optimum Moisture Table 4.2. Pullout test matrix. 4.5 Number of Tests A total of 10 pullout tests were performed as outlined below. • Tests: two sets of three tests on the unsaturated backfill and one set of four tests on the saturated backfill • Time to failure: 0.05 sec, 5 sec, and 3,600 sec for each of the two sets on the unsaturated backfill (3 × 2 = 6 tests) and 0.05 sec, two for 5 sec, and 3,600 sec for the set on the satu- rated backfill (4 × 1 = 4 tests) • Soil: well-graded sand as described above; same soil for all 10 tests • Reinforcement: seven tests on the steel reinforcement strips, three tests on the steel bar mats, length of reinforcement = 1.13 m (3.7 ft) • Saturation: optimum water content and maximum dry density for six tests (three on strips and three on bar mats); saturated condition for four tests (on strips). • Box: 701 mm × 381 mm × 1.31 m (2.3 ft × 1.25 ft × 4.3 ft). Table 4.2 summarizes the conditions for each test.

42 (a) Pullout box setup (b) Surcharge to simulate depth (c) Box dimensions and location of strip (d) Box dimensions and dead weight (e) Placing the strip on the sand (f) Strip coming out of the front of the box Figure 4.3. Test setup with steel strip.

43 (a) Grabbing the bar mat (b) Placing the bar mat on the sand (c) Bar mat and box dimensions (d) Dead weight on bar mat (e) Grabbing and loading the strip Figure 4.4. Test setup with bar mat.

44 Tie Back Strip, Unsaturated, 0.00097 in/s estimated displacement from ruler 0 200 400 600 800 1000 1200 0 1 2 3 4 5 6 Displacement (in) Pu llo ut F or ce (lb ) Speed increased to 0.144 in/s Tie Back Strip, Unsaturated, 0.12 in/s estimated displacement from ruler 0 100 200 300 400 500 600 700 800 900 1000 0 0.5 1 1.5 2 2.5 3 3.5 Displacement (in) Pu llo ut F or ce (lb ) Tie Back Strip, Unsaturated, 3.84 in/s 0 200 400 600 800 1000 1200 1400 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Displacement (in) Pu llo ut F or ce (lb ) Figure 4.5. Load displacement curve obtained (tie back strip, unsaturated).

45 Tie Back Strip, Saturated, 0.0016 in/s estimated displacement from ruler 0 100 200 300 400 500 600 700 800 0 1 2 3 4 5 6 Displacement (in) Pu llo ut F or ce (lb ) Speed increased to 0.144 in/s Tie Back Strip, Saturated, 0.156 in/s estimated displacement from ruler 0 200 400 600 800 1000 1200 1400 0 1 2 3 4 5 6 Displacement (in) Pu llo ut F or ce (lb ) Figure 4.6. Load displacement curve obtained (tie back strip, saturated). (continued on next page)

46 Tie Back Strip, Saturated, 3.84 in/s 0 200 400 600 800 1000 1200 1400 1600 0 0.2 0.4 0.6 0.8 1 1. .4 Displacement (in) Pu llo ut F or ce (lb ) Tie Back Strip, Saturated, 0.168 in/s Displacement (in) estimated displacement from ruler 0 200 400 600 800 1000 1200 1400 1600 0 1 2 3 4 5 6 Pu llo ut F or ce (lb ) Figure 4.6. (Continued).

47 Bar Mat, Unsaturated, 0.00168 in/s estimated displacement from ruler 0 500 1000 1500 2000 2500 3000 3500 4000 0 0.5 1 1.5 2 2.5 3 3.5 Displacement (in) Pu llo ut F or ce (lb ) Speed increased to 0.144 in/s Bar Mat, Unsaturated, 0.144 in/s estimated displacement from ruler 0 500 1000 1500 2000 2500 3000 3500 0 0.5 1 1.5 2 2.5 3 3.5 Displacement (in) Pu llo ut F or ce (lb ) Bar Mat, Unsaturated, 1.08 in/s 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 0.05 0.1 0.15 0.2 0.25 Displacement (in) Pu llo ut F or ce (lb ) Figure 4.7. Load displacement curve obtained (bar mat).

48 a) Tie Back Stripa, Unsaturated Test Condition Pullout Speed Dry Density of Sand Normal Load Specimen Width Length of Embedment Pullout Force Elapsed Time at Peak Displacement at Peak Pullout Resistanceb (in./s) (pcf) (lbs) (ft) (ft) (lbf) (s) (in.) (lbf/ft) Unsaturated 0.00097 101.5 2946 0.17 3.7 1212 (1163) 1287 (862) 1.24 (0.75) 7269 (6841) Unsaturated 0.12 101.5 2946 0.17 3.7 933 5 0.35 5602 Unsaturated 3.84 101.5 2946 0.17 3.7 1141 0.05 0.12 6848 Notes: a Tie Back Strip Dimension: L = 60 in., W = 2 in., Thickness = 0.2 in., Thickness at rib = 0.3 in., length of embedment = 3.7 ft b Pullout resistance = Pullout Force (lbf) / width of tie back strip (ft) For samples whose peak value occurred after 0.75 in., values in parentheses represent values at 0.75 in. displacement. b) Tie Back Stripa, Saturated Test Condition Pullout Speed Dry Density of Sand Normal Load Specimen Width Length of Embedment Pullout Force Elapsed Time at Peak Displacement at Peak Pullout Resistanceb (in./s) (pcf) (lbs) (ft) (ft) (lbf) (s) (in.) (lbf/ft) Saturated 0.0016 101.5 2946 0.17 3.7 650 66 0.09 3903 Saturated 0.156 101.5 2946 0.17 3.7 1306 (1286) 8 (6) 1.18 (0.75) 7835 (7720) Saturated 0.168 101.5 2946 0.17 3.7 1508 4 0.40 9049 Saturated 3.48 101.5 2946 0.17 3.7 1357 0.06 0.17 8143 Notes: a Tie Back Strip Dimension: L = 60 in., W = 2 in., Thickness = 0.2 in., Thickness at rib = 0.3 in., length of embedment = 3.7 ft b Pullout resistance = Pullout Force (lbf) / width of tie back strip (ft) For samples whose peak value occurred after 0.75 in., values in parentheses represent values at 0.75 in. displacement. c) Bar Mata Test Condition Pullout Speed Dry Density of Sand Normal Load Specimen Width Length of Embedment Pullout Force Elapsed Time at Peak Displacement at Peak Pullout Resistanceb (in./s) (pcf) (lbs) (ft) (ft) (lbf) (s) (in.) (lbf/ft) Unsaturated 0.00168 101.5 2946 2 3 3655 (3129) 1811 (799) >1.5 (0.75) 1837 (1565) Unsaturated 0.144 101.5 2946 2 3 3180 (2996) 12 (6) 1.48 (0.75) 1590 (1499) Unsaturated 1.08 101.5 2946 2 3 3900 0.17 0.09 1950 Notes: a Bar Mat Dimension: L = 57.5 in., W = 24.5 in., Bar Thickness = 0.4 in., Joint Thickness = 0.7 in., length of embedment = 3 ft, 5 bars parallel to direction of force, 4 cross-bars (3 embedded) b Pullout resistance = [Pullout Force (lbf) * number of bars per unit width (2.5 bars/ft)] / number of bars parallel to direction of force (5) For samples whose peak value occurred after 0.75 in., values in parentheses represent values at 0.75 in. displacement. Table 4.3. Pullout test results.

49 100 1000 10000 0.01 0.1 1 10 100 1000 10000 Tie Back Strip, Unsaturated Tie Back Strip, Saturated Bar mat, Unsaturated Time (sec, log) Pu ll- ou t F or ce (lb f, l og ) Figure 4.8. Pullout load at failure versus time to failure for all tests. Source: The Reinforced Earth Company (23) Figure 4.9. Values of apparent coefficient of friction (f*) from pullout tests.