CHAPTER 5: VACUUM TEST WITH VERTICAL DRAINS

Transcription

1 CHAPTER 5: VACUUM TEST WITH VERTICAL DRAINS 5.1 Introduction Using surcharging as the sole soil consolidation mean can take a long time to reach the desired soil settlement. Soil consolidation using prefabricated vertical wick drains can rapidly increase the rate of soil settlement and hence cut the project duration drastically. The prefabricated vertical wick drain core is made of high quality flexible polypropylene which allows a large water flow capacity in the longitudinal direction of the core via perforated grooves or water channels on both sides of the core. The wick drain core is tightly wrapped in a geotextile filter jacket of spun-bonded polypropylene having a very high coefficient of permeability while retaining the fine soil particles. Both the core and geotextile filter jacket have high strength, a high degree of durability in most environments, and high resistance to chemicals. In practice, most soil improvement works are carried out using vertical drains. In this Chapter, vertical drains combined with surcharge or vacuum are investigated to provide a better understanding on vacuum consolidation with drains. Three additional tests (Tests 5, 6 and 7) were performed, at 1g as given in Table 3.3. The test results will be presented and discussed in detail in this chapter. 5.2 Surcharge with Vertical Drains (Test 5) In most soil improvement works, vertical drains are used in conjunction with surcharge to accelerate soil consolidation. Test 5 was conducted to study the consolidation

2 behavior of kaolin clay under surcharge and vertical drains. This test would serve as the bench mark to evaluate the efficiency of vacuum consolidation against that of conventional surcharge method. The schematic diagram of Test 5 is shown in Fig.5.1. In this test, prefabricated vertical drains (PVDs) of 6 mm wide and 3 mm thick in prototype scale (6 mm wide and 3 mm thick in model scale) were installed in a 6m square grid. In this test, eighteen model drains were installed to 17 m of the clay depth. The drain spacing ratio n was 11.9 and the equivalent diameter of PVD of 57 mm (5.7 mm in model scale) was calculated as recommended by Hansbo (1979). The diameter of equivalent soil cylinder was 67.8 mm (Rixner et al.1986). As described in Chapter 3, drain installation was done at 1g. The water table was kept on top of the clay surface throughout the centrifuge test Ground Surface Settlement and Degree of Consolidation The variation of ground settlement with time is shown in Fig.5.2. As expected, the settlement rate is much faster due to the presence of vertical drains. The degree of consolidation was estimated using the hyperbolic method, as shown in Figure 5.3. The gradient of the final straight line in the plot is used to estimate the average degree of consolidation and the ultimate settlement. The variation of degree of consolidation with time is shown in Fig.5.4. At the end of the test, over 9% of consolidation has been achieved and the settlement has almost reached the ultimate stage. The 8% degree of consolidation is achieved in 1.16 years of prototype time. 12

3 LVDTs Sand (8kPa) 33 Clay Vertical Drain PPTs 6m 6m (a) Side View Drain LVDTs 6 2 (b) Plan View Fig.5.1 Schematic diagram of Test 5 (all dimensions in cm) 13

4 Time (Years) Settlement (m) 1 2 3m Left Side of Centre 3m Right Side of Centre 3 4 Fig.5.2 Variation of Settlement with Time S 1, S 2 (.2946).8 t/δ (year/m) Time (Years) Fig.5.3 Hyperbolic Plot 14

5 1 8 Degree of Consolidation (%) 6 4 3m Left Side of Centre 3m Right Side of Centre Time (Years) Fig.5.4 Degree of Consolidation with Time Pore Water Pressures Pore pressure transducers were installed at depths of.5m, 3.5m, 6m, 9m, 15m, and 18m in the clay. All these pore pressure transducers were installed in the middle of selected square grid of the drains. Owing to the presence of vertical drains, the rate of pore pressure dissipation is much faster compared with Test 1. The development of excess pore pressures with time is shown in Figure 5.5. The results show that the dissipation of excess pore pressure is rapid initially and the PPT close to the top surface reaches hydrostatic steady state quickly. 15

6 Excess Pore Water Pressure (kpa) Z =.5 Z = 3.5 Z = 6 Z = 9 Z = 15 Z = 18 Depth Z in meters Time (years) Fig.5.5 Variation of Excess Pore Water Pressure with Time The excess pore water pressure isochrones are shown in Fig.5.6. The maximum excess pore water pressures were observed in the furthest distance from the drainage boundary. It can be seen that the vertical drains combined with 8 kpa of surcharge load had reduced the consolidation time significantly. The variation of undrained shear strength with depth is shown in Fig.5.7. The initial shear strength is about 3kPa (at 1g) and almost uniform through out the clay depth due to the relatively low 1kPa preconsolidation pressure. The effective stress, at 1g, was estimated by adding the effective stress due to the surcharge (q) and self weight (ϒ'z) of the clay. It is assumed that the unit weight of the clay at 1g would be approximately equal to the average unit weight of the clay at 1g, which was measured immediately after spinning down. 16

7 The shear strength of the clay would be approximately.254 times of its effective stress. The shear strength at 1g was then estimated from the final effective stress of the clay. Upon spun down of the centrifuge, considerable swelling has taken place in the clay. Owing to this, noticeable reduction could be observed between the estimated shear strengths at 1g and the measured final shear strengths at 1g though the trend of the two strength profiles are similar. Excess Pore Pressures (kpa) Depth (m) Time t in years t = 2.5 t = 2 t = 1.5 t = 1 t =.5 t = + 2 Fig.5.6 Excess Pore Pressure Isochrones 17

8 Undrained Shear Strength (kpa) Estimated Shear Strength (1g) Depth (m) Initial Strength (1g) Final Strength (1g) 18 2 Fig.5.7 Variation of Shear Strength with Depth 5.3 Vertical Drains Combined with Vacuum In practice, vacuum preloading is commonly used in conjunction with vertical drains in soil improvement works to accelerate soil consolidation. In the present study, Tests 6 and 7 were performed using vacuum with drains of 2 different spacings. Impermeable rubber membrane was used to seal the clay around all edges to prevent vacuum pressure leakage. Light weight geotextiles were used to facilitate the application of vacuum on top of the clay. To enhance the effectiveness of the applied vacuum pressure, the model prefabricated vertical drains were installed prior to the placement of the air-tight membrane. 18

9 5.3.1 Vacuum combined with Drains (Test 6) In Test 6, 18 model PVDs having the same dimensions of those of Test 5 were installed in 6m square grids. The vacuum pressure was applied at 1g in centrifuge. The drains were in contact with the geotextile and the vacuum transferring tube was kept below the geotextile so that vacuum could easily be applied. The soil preparation and instrumentation has been described in Chapter 3. The schematic diagram of Test 6 is shown in Fig Ground Settlement and Degree of Consolidation The variation of ground settlement with time is shown in Fig.5.9. In this test, the settlement rate was noted to be faster than that of Test 5. The hyperbolic settlement plot is shown in Fig.5.1, which was used to evaluate the degree of consolidation. The relationship between degree of consolidation and time is shown in Fig At the end of the test, about 9% of soil consolidation has been achieved. 19

10 LVDTs Surcharge (7.5kPa) +Vacuum (Avg.75kPa) (a) Side View 33 Clay Vertical Drain PPTs Drain LVDTs 6 2 (b) Plan View Fig.5.8 Schematic Diagram of Test 6 (all dimensions in cm) 11

11 Time (years) Settlement (m) 1 2 3m Left Side of Centre 3m Right Side of Centre 3 4 Fig.5.9 Variation of Settlement with Time.5.4 S 1 (.2996) S 2 (.2992) t/δ (year/m) Time (years) Fig.5.1 Hyperbolic Plot 111

12 1 8 Degree of Consolidation (%) 6 4 3m Left Side of Centre 3m Right Side of Centre Time (years) Fig.5.11 Degree of Consolidation with Time Vacuum Pressure and Pore Water Pressure Pore pressure transducers were installed at depths of 5m, 8m, 12m, 15m, 17.5m and 19.5m in the soil. All PPTs were installed in the middle of selected square grids of the drains. The variations of vacuum pressure and pore water pressures at various depths with time are shown in Fig The PPT, which was used to measure suction, functioned properly and the effective vacuum pressure at top was 75 kpa. During this test, the vacuum pressure was practically constant near the top surface. It is noted that all PPTs respond immediately when the vacuum pressure changes. This clearly indicates that the vacuum could reach to the base of soil and all PPTs had sufficient sensitivity to monitor the variation of suction pressure. Similar observation was noted 112

13 in the Yaoqiang Airport runway project, as reported by Tang and Shang (2). In the field project, pore water transducers were only installed down to 1 m of depth. However in the present study, vacuum is observed to reach to 2 m depth. Using the same procedure presented in Chapter 4, the pore pressure drawdowns were estimated. The variation of the final and initial pore water pressure with depth is shown in Fig It can be seen that the final pore water pressure line is very close to the theoretical line. The estimated degree of consolidation in top 1m clay depth is 96% while below 1 m, it is 86%. It can hence be deduced that the suction pressure at the lower part of the clay was also high. Fig.5.13 shows that the excess pore pressure close to the surface has completely dissipated and the pore pressure at the rest of clay depths have also dissipated substantially. Pore Water Pressure (kpa) Depth Z in meters Vacuum Pressure Z = 5 Z = 8 Z = 12 Z = 15 Z = 17.5 Z = Time (years) Fig.5.12 Variation of Vacuum Pressure and Pore Water Pressure with Time 113

14 Pore Water Pressure (kpa) Initial Pore Pressure Hydro Static Pressure Final Pore Pressure Theoritical Suction Pressure Depth (m) Fig.5.13 Pore Pressure Drawdown with Depth The suction variation could be estimated form the pore pressure drawdown using the same approach described in Chapter 4. Fig.5.14 shows the measured and theoretical vacuum variation with depth, the effective stress profile due to 7.5 kpa surcharge plus self weight and the final effective stress profile at 1g. It could be observed that the measured suction pressure was almost constant along the clay depth. A small difference could only be observed between the theoretical and the measured suction pressure profile. In this test, the suction could reach down to 2 m depth and the effectiveness of applied pressure is over 85% at the base of the clay. The final effective stress was estimated by adding effective stress due to surcharge plus self weight with vacuum pressure along the clay depth. Theoretically, the amount of pore water 114

15 pressure reduction at the end of the test by suction would be converted as the effective stress, as discussed in Section 2.3. The initial and the final shear strength variations with depth are shown in Fig The estimated shear strength at 1g is also plotted. The final shear strength increases with depth because the effective stress of the clay increases with depth. It is noted that there is a considerable difference between the estimated and the measured shear strengths because considerable swelling has occurred in the clay after the spinning down the centrifuge. Thus the application of the vacuum pressure together with drains could increase the shear strength significantly. Effective Stress (kpa) Suction Pressure Final Effective Stress 6 Depth (m) Effective Stress due to Surcharge plus Selfweight Fig.5.14 Effective Stress with Clay Depth 115

16 Undrained Shear Strength (kpa) Estimated Strength (1g) 6 Depth (m) Initial Strength (1g) Final Strength (1g) 2 Fig.5.15 Variation of Shear Strength with Depth Vacuum Combined with Drains (Test 7) Test 7 was performed with a drain spacing of 5m square grid. 9 kpa of vacuum pressure was applied and 3 model drains were installed as compared to 18 drains in Test 6. The model set up is shown in Fig Settlement and Degree of Consolidation The variation of settlement with time is shown in Fig.5.17 and the associated hyperbolic plot is shown in Fig The degree of consolidation and ultimate settlement were estimated from the gradient of the hyperbolic plot. The relationship between the degree of consolidation and time is shown Fig

17 LVDTs Surcharge (7.5kPa) +Vacuum (Avg.9kPa) 33 Clay Vertical Drain PPTs (a) End View Drains LVDTs 6 2 (b) Plan View Fig.5.16 Schematic Diagram of Test 7 (all dimensions in cm) 117

18 Time (year) Settlement (m) 1 2 3m Left Side of centre 3m Right Side of centre 3 4 Fig.5.17 Settlement Variation with Time.6.5 S 1, S 2 (.2941).4 t/δ (year/m) Time (year) Fig.5.18 Hyperbolic Plot 118

19 1 8 Degree of Consolidation (%) 6 4 3m Left Side of centre 3m Right Side of centre Time (year) Fig.5.19 Degree of Consolidation with Time Vacuum Pressure and Pore Water Pressures In this test, pore water pressure transducers were installed at depths of 3m, 6m, 9m, 12m, 15m, and 18m.The final position of the PPTs were measured and they were used to calculate the final hydrostatic pressures. The variations of vacuum pressure and pore water pressures with time are shown in Fig.5.2. The initial and final pore water pressure readings and the theoretical suction variation with depth are shown in Fig

20 25 2 Depth Z in meters Pore Water Pressure (kpa) Z = 18 Z = 15 Z = 12 Z = 9 Z = 6 Z = 3 Vacuum Pressure Time (year) Fig.5.2 Variation of Vacuum Pressure and Pore Water Pressure with Time Pore Water Pressure (kpa) Initial Pore Pressure Hydro Static Pressure Final Pore Pressure Theoretical Suction Pressure Depth (m) Fig.5.21 Pore Pressure Drawdown with Depth 12

21 Discussion As expected, owing to the larger preloading pressure and the smaller drain spacing, the rate of settlement is faster than that of Test 5 and Test 6. In Test 7, 8% degree of soil consolidation was achieved in.785 years of prototype time. In this test, about 9 kpa of vacuum pressure was applied and maintained through out the experiment. The PPTs at depths 3m and 6m showed negative readings because the pore water pressures at these depths were less than the vacuum pressure. It was intended that the PPTs would be installed at the midpoint of the drains square grid at specific depths. However, it was difficult to install the PPT precisely at a specific location. Hence certain PPTs might be installed fairly close to the PVDs. This could be the reason for the fluctuations in pore water pressure readings, which could be seen in Fig.5.2. Fig.5.21 shows that the excess pore water pressures have almost dissipated. The estimated degree of consolidation in the first 1m of the clay depth is about 97% while below 1m, it is 88%. From the pore pressure drawdown, the suction variation along the clay depth was estimated. This was then used to estimate the final effective stress. The estimated suction variation with depth and the final effective stress at the end of test are shown in Fig Fig.5.22 shows that the suction effect was fairly consistent throughout the depth and about 89% of the applied pressure was measured at the base of clay sample. This suction variation is similar to that of Test 6. The undrained shear strength variation along the clay is shown in Fig The vane shear tests were done in the mid point of the drains square grid. Fig.5.23 shows that the final shear strength increases with depth. Owing to the high applied suction pressure, a considerable improvement in shear strength was observed in the clay. 121

22 Effective Stress (kpa) Suction Pressure Final Effective Stress Depth (m) Effective Stress due to Surcharge plus Selfweight Fig.5.22 Effective Stress with Clay Depth Undrained Shear Strength (kpa) Estimated Strength (1g) 6 Depth (m) Initial Strength (1g) Final Strength (1g) 2 Fig.5.23 Variation of Shear Strength with Depth 122

23 5.4 Comparison of Test Results (a) Summary Tables Most soil improvement works, especially for consolidation of soft soil, are done by using vertical drains combined with surcharge. However, in many cases, the sand surcharge method may not be applicable due to some practical limitations. In some projects, large amount of sand surcharge is required to improve very soft soil and the height of the surcharge required would be very large. Also, in some cases, the surcharge would be more expensive as compared to other methods. In such cases, vacuum consolidation may be considered as an alternative method. However, it is necessary to identify the effectiveness of this method over the conventional surcharge method. In the present study, three tests were done with vertical drains to compare the surcharge versus vacuum consolidation of soft clay. The time taken to reach 8% degree of consolidation was adopted to evaluate the effectiveness of the 2 methods. A summary of the 3 test results is given in Table 5.1. From the experimental data, the time taken to achieve 9% of consolidation and settlement at that time were also estimated. The ultimate settlement in each test was also calculated. These results are tabulated in Table 5.2. (b) Degree of Consolidation Table 5.1 reveals that the time taken to achieve 8% degree of soil consolidation by vacuum combined with drains are faster than that by surcharge with drains. For the vacuum preloading with drains, any soil element in the ground to be improved reduces its volume not only in the vertical direction but also in the lateral direction, because 123

24 vacuum consolidates the soil isotropically whereas conventional surcharge method preloads the soil in vertical direction only. Table 5.1 Summary of Test Results Type Degree of Consol idation (%) Settlement at U = 8% (m) Time Taken for U=8% (year) Change of Undrained Shear Strength (at mid depth) (kn/m 2 ) Change of Water content (at mid depth) (%) Change of unit weight (kn/m 3 ) 1.Surcharge (8kPa) + Vertical Drains (6m spacing) From 3.15 to 26.7 From 89 to 66 From to Vacuum(75kPa) + Surcharge(7.5kPa) + Vertical Drains (6m spacing) From 3.5 to 27.4 From 84 to 63 From to Vacuum(9kPa) + Surcharge(7.5kPa) + Vertical Drains (5m spacing) From 2.8 to 3.9 From 87 to 62 From to15.9 Table 5.2 Summary of Estimated Results Type Ultimate Settlement (m) Settlement at U = 9% (m) Time Taken for U = 9% (Yr) 1.Surcharge (8kPa) + Vertical Drains (6m spacing) 2.Vacuum(75kPa) + Surcharge(7.5kPa) + Vertical Drains (6m spacing) 3.Vacuum(9kPa) + Surcharge(7.5kPa) + Vertical Drains (5m spacing)

25 Owing to this isotropic effect, the rate of consolidation is faster in the vacuum tests. A similar observation was reported by Tang et al. (2) for the Yaoqiang Airport runway project in which vacuum preloading took shorter time to reach the maximum settlement than that of surcharge preloading. Matsumoto et al. (2) and Yee et al. (21) also noted that the isotropic consolidation in vacuum preloading would increase the rate of consolidation considerably. The comparison of rate of consolidation between each test is shown in Fig Degree of Consolidation (%) Test 5 Test 6 Test Time (year) Fig.5.24 Comparison of Degree of Consolidation 125

26 As expected, due to the high applied suction and smaller drain spacing, the rate of consolidation in Test 7 is higher than that of Test 6. In practice, much more time would be needed to consolidate the soft clay using the conventional surcharge method because the surcharge should be applied in stages to avoid bearing capacity failure of the subsoil while vacuum could be applied almost at once. Thus the use of vacuum pressure with drains could reduce the project time considerably. (c) Settlement The average surface settlement in the vacuum combined drain test (Test 6), when 8% and 9% of consolidation was achieved, is slightly lower than that of surcharge combined drain test (Test 5). The maximum estimated ultimate settlement is also slightly lower than that of the surcharge test. A major distinction between the preloading and vacuum consolidation methods is the direction of lateral displacement. In the surcharge case, the lateral movement will be outward whereas in the vacuum case it would be inward. This effect was observed in many field projects such as Yaoqiang Airport runway by Tang et al. (2), Oil storage station in China by Chu et al. (2) and others. The maximum settlement observed under vacuum preloading in the Yaoqiang Airport runway project was smaller than that of equivalent surcharge preloading. A similar effect was also observed in the oil storage project in China. This appears to support the evidence of the ultimate settlement in Test 6 is smaller than that of Test 5. The ultimate settlement in Test 6 is slightly lower than that of Test 7 because the applied suction pressure in Test 7 is higher than that of Test 6. The settlement comparison among the 3 tests is shown in Fig.5.25, which reveals that the settlement rate for the vacuum tests is faster than that of the surcharge test. 126

27 Time (Year) Settlement (m) 1 2 Test 5 Test 6 Test Fig.5.25 Comparison of Settlements (d) Pore Pressure Drawdown The suction effect along the clay depth, at the end of the experiment, was estimated form the pore pressure drawdown. A comparison of final pore water pressure drawdown among all 3 tests is shown in It shows that the measured final pore water pressures in Test 5 were closer to the hydrostatic pressures. However, a small difference could be observed especially in the deeper layer due to the stagnation of excess pore water pressures. The final pore pressures in Tests 6 and 7 are less than its hydrostatic pressures because of suction. It can be seen that the final pore pressure drawdown in Test 7 is higher than that of Test 6. As the applied vacuum pressure is high in Test 7, the reduction of pore water pressures in Test 7 is high. 127

28 Pore Water Pressure (kpa) Test 5 Test 6 Test 7 Hydro Static Pressure in Test 5 Depth (m) Hydro Static Pressure in Test 6 and Test 7 Fig.5.26 Comparison of Final Pore Pressure Drawdown The effective depth of vacuum was investigated in both tests. It can be seen that the distribution of pore pressure drawdown is fairly independent of depth, indicating that the PVDs function effectively as the transfer medium for the vacuum pressure. Similar observations are noted by Choa et al. (1989) in the Tianjin Harbor project and by Chu et al. (2) in the Oil storage station project in China. One of the common concerns in the use of vacuum preloading is that vacuum is effective only up to depths of about 1m. The test results reveal that vacuum preloading is still effective even for soils 2 m below the ground surface. There are two process involved in vacuum preloading with vertical drains. The first is pumping water from the vertical drains, and the second is increasing the effective stress to consolidate the soil by reducing the pore water pressure in the soil. In the former, the effect could not exceed 1m depth. However, in 128

29 the later, the limit in the depth will depend on the well resistance. However, this well resistance is normally small and be negligible (Chen, 1987). The reduction in the pore water pressure was almost uniform throughout the whole depth of soil. The estimated average degree of consolidation along the clay depth in vacuum tests is shown in Table 5.3. In both tests, the overall degree of consolidation along the clay depth is over 9%. Also, the estimated average degree of consolidation below 1m is comparably high (more than 85%). This clearly shows that the vacuum pressure is effective until the base of the clay layer. Table 5.3 Average Degree of Consolidation along the Clay Depth Test Test 6 Test 7 Elevation (m) Degree of Consolidation (%) Elevation (m) Degree of Consolidation (%) Elevation (m) Degree of Consolidation (%) Bergado et al. (1998) observed in the Bangkok international Airport project, that the vacuum pressure at depth 15m was around -15kPa and it was only one-fourth of the pressures at the ground surface even though the depth of PVD installation was 15m. Similar effect was observed by Choa (1989) for the Tianjin Harbour Project in China, that the vacuum preloading appears to be less effective at depths greater than 14m. In the project, the vertical drains were installed to about 2 m below the surface of the reclaimed land. In field projects, controlling vacuum leakage was a great problem. 129

30 Owing to these limitations, the effective depth could not be clearly estimated in the field. (e) Undrained Shear Strength The comparison between undrained shear strength in each test is shown in Fig It could be seen from Fig.5.27 that the improvement in undrained shear strength in Test 5 and Test 6 are close to each other because approximately equivalent load was applied in both tests. In Test 7, the applied suction was higher than the other two tests. Therefore, higher shear strength was observed in Test 7. It can also be noted that the trend of undrained shear strength profiles in Test 6 and Test 7 is similar to that of Test 5. This also confirms that the suction pressure was effective until the bottom part of the clay layer. Undrained Shear Strength (kpa) Test 5 Test 6 Test 7 6 Depth (m) Fig.5.27 Shear Strength Comparison 13

31 5.5 Summary Some conclusions can be derived form the results of Tests 5 to 7: It was identified that the effectiveness of the vacuum pressure is almost constant along the clay depth installed with vertical drains. Therefore, vacuum consolidation can be used to consolidate beyond 1 m depth in clay. In the vacuum preloading, the rate of settlement is faster than that of equivalent surcharge preloading. Over 9 % degree of consolidation could be observed in the whole clay layer. The application of vacuum preloading method for the improvement of soil with about 2 m of very soft clay was reported. The vacuum preloading can be used effectively for the improvement of very soft clays, which could be difficult to be treated using fill surcharge. From the results, the initial water content range of 84%-89% is reduced to 62%-66% after improvement. The average soil strength after improvements is about 26-31kPa. These improvements show that the soil has achieved sufficient strength to support higher applied loads. 131

32 In extremely soft soil conditions, the vacuum method enables the preloads to be applied quickly. About.88 years of vacuum preloading with drains would produce approximately equivalent and increase the shear strength as compared with vertical drains with 8 kpa surcharge after 1.16 years of surcharge consolidation. However in field projects, some more time would be taken because surcharge should be applied gradually to prevent bearing capacity failure. Vacuum preloading can be used to eliminate excessive surface settlement in most sites. Owing to the inward lateral soil movement in vacuum preloading, the maximum settlement would be smaller than that of equivalent conventional surcharge preloading. By computing lateral soil movement of the soil, the maximum soil settlement could be estimated. From the results of this study, it can be shown that vacuum preloading consolidation could be used with confidence in soil improvement works for very soft soil. 132