LATERAL SPREADING OF SLOPES Data report on tests SKH-5-12 S.K. Haigh 1 CUED/D-SOILS/TR317 (2002)

Transcription

1 LATERAL SPREADING OF SLOPES Data report on tests SKH-5-12 S.K. Haih 1 CUED/D-SOILS/TR317 (22) 1 Research Associate, Cambride University Enineerin Department

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3 Table of Contents Table of Contents i List of Fiures ii List of Tables ii 1. Introduction 1 2. Experimental Procedure Model Preparation Test Procedure 3 3. Instrumentation 5 4. Results Test SKH Earthquake Earthquake Earthquake Test SKH Earthquake Earthquake Test SKH Test SKH Test SKH Test SKH Conclusions Acknowledements References 39 -i-

4 List of Fiures Fiure 1: Particle size distributions for fraction E and fraction B silica sands. 3 Fiure 2: Instrument layout for test SKH-5 5 Fiure 3: Instrument layout for test SKH-8 6 Fiure 4: Instrument layout for test SKH-9 6 Fiure 5: Instrument layout for test SKH-1 7 Fiure 6: Instrument layout for test SKH-11 8 Fiure 7: Instrument layout for test SKH-12 9 Fiure 8: Accelerations measured in test SKH-5 earthquake 1 13 Fiure 9: Pore pressures measured in test SKH-5 earthquake 1 14 Fiure 1: Accelerations measured in test SKH-5 earthquake 2 15 Fiure 11: Pore pressures measured in test SKH-5 earthquake 2 16 Fiure 12: Accelerations measured in test SKH-5 earthquake 3 17 Fiure 13: Pore pressures measured in test SKH-5 earthquake 3 18 Fiure 14: Accelerations measured in test SKH-8 earthquake 1 19 Fiure 15: Pore pressures measured in test SKH-8 earthquake 1 21 Fiure 16: Accelerations measured in test SKH-8 earthquake 2 22 Fiure 17: Pore pressures measured in test SKH-8 earthquake 2 24 Fiure 18: Accelerations measured in test SKH-9 25 Fiure 19: Pore pressures measured in test SKH-9 27 Fiure 2: Accelerations measured in test SKH-1 29 Fiure 21: Pore pressures measured in test SKH-1 31 Fiure 22: Accelerations measured in test SKH Fiure 23: Pore pressures measured in test SKH Fiure 24: Accelerations measured in test SKH List of Tables Table 1: Properties of fraction E silica sand. (after Tan, 199) 2 Table 2: Properties of fraction B silica sand. (after Lee, 199) 2 Table 3: Instrumentation coordinates for test SKH-5 5 Table 4: Instrumentation coordinates for test SKH-8 6 Table 5: Instrumentation coordinates for test SKH-9 6 Table 6: Instrumentation coordinates for test SKH-1 7 Table 7: Instrumentation coordinates for test SKH-11 8 Table 8: Instrumentation coordinates for test SKH ii-

5 1. Introduction The many lare earthquakes of recent years, includin those at Northride (1994), Kobe (1995), Turkey (1999), Taiwan (1999) and Gujarat (21) have shown the capacity of these lare-manitude events to cause massive damae to infrastructure and enormous loss of life. The concentrations of population in seismically active reions of the world, includin the Pacific Rim, California and northern India has put many millions of people at risk when these lare earthquakes strike. Whilst nothin can be done to prevent these earthquakes from occurrin, efforts must be made to find ways to minimise the impact of these earthquakes on the population and infrastructure. In many of these major earthquakes, lare-scale damae due to liquefaction and especially due to the lateral spreadin of liquefiable slopes has been observed. In the Niiata Earthquake of 1964, lateral spreadin of up to 5 m was observed alon the Shinano River (Hamada 1992). These lare manitude deformations have been shown to have devastatin effects on structures founded on these slopes and lifelines passin over or throuh them. This research was hence undertaken to investiate the lateral spreadin of liquefiable slopes and to develop experimental techniques to be used in order to study the interaction of these spreadin slopes with pile foundations. The research consisted of a series of eiht beam centrifue tests (SKH-5 to 12) two of which (SKH-6 & 7) were unsuccessful due to failure to achieve saturation and will hence not be discussed further. Full details of the analysis of these test results is iven by Haih (22). -1-

6 2. Experimental Procedure The test series consisted of seven oil-saturated tests (SKH-5 to 11) and one dry test (SKH-12) carried out as a control. The models were all prepared and tested followin the procedure outlined in the next section, with the exception of the dry model, in which the procedures for model saturation were obviously not required. 2.1 Model Preparation All tests from SKH-5 throuh to SKH-12 comprised of slopes of loose liquefiable fraction E silica sand with properties as shown in Table 1 and particle size distribution as shown in Fiure 1. The models were prepared to a nominal void ratio of.8 or an R D (relative density) of 5% by air pluviation from an overhead hopper. Density was controlled by adjustin both the heiht of the hopper and the rate of pourin. End reservoirs of Fraction B silica sand, with properties as shown in Table 2 and particle size distribution as shown in Fiure 1, were provided at the top and bottom of the slope to ensure a plane strain seepae condition throuh the model. These are separated from the rest of the sand by stainless steel mesh and are filled at the same rate as the rest of the model. Table 1: Properties of fraction E silica sand. (after Tan, 199) Property Value φ crit 32 D 1.95 mm D 5.14 mm D 6.15 mm e min.613 e max 1.14 k with water at e =.72.98E-4 m/s G s 2.65 Table 2: Properties of fraction B silica sand. (after Lee, 199) Property Value φ crit 36 D 1.84 mm D 5.9 mm D mm e min.495 e max.82 G s

7 Fiure 1: Particle size distributions for fraction E and fraction B silica sands. Once sand has been poured to the final level, a lid is bolted to the top of the ESB box, with pump rease bein applied to the joint to achieve a seal. A vacuum of reater than 95 is then applied to the box to remove trapped air from the pore spaces. This ensures even saturation of the model as the silicone oil does not need to displace air from the pores and also increases the speed of seepae due to the enhanced pressure radient between the oil reservoir and the model container. This does however introduce problems of sand boilin if oil is admitted at too reat a speed. Silicone oil is then admitted to the base of the model throuh feed pipes fitted with needle valves to control flow-rate. A flow rate of.5 khr -1 per feed pipe was found to be sufficiently low to prevent pipin of the model whilst still allowin saturation to be achieved in approximately 1 hours. The feed pipes are located in the reservoirs at top and bottom of the slope and consist of 8 mm diameter pipe with holes drilled at intervals to spread the oil across the model. 2.2 Test Procedure After model preparation was complete, the SAM actuator and counterweiht were loaded onto the centrifue arm. The SAM was placed on a wooden block to make it han level. The ESB box and shakin table were then ballasted with weihts to make them han horizontally from the crane and were lowered into place on the SAM. The SAM and ESB box were loaded separately onto the centrifue in order to minimise model disturbance, as the oil-saturated slope is fairly unstable at 1. Once the ESB box was in place on the SAM, wires from the instruments present in the model were connected to the junction boxes on the SAM and cable-tied into place. After pre-fliht checks had taken place, includin such items as checkin that enouh pressure was available in the accumulator to fire earthquakes, the centrifue was started and accelerated to 8 at which point both swinin platforms should have sat back aainst the end of the centrifue arm. This was monitored by pairs of micro-switches at either end of the arm which are closed by the swin sittin aainst the end of the beam. The centrifue speed at which this should occur for each swin -3-

8 was calculated in the balance calculations and was compared with observed values as a safety check. In eneral (with SAM packaes) the blue-end swin (SAM and ESB) swins up at 3 rpm and the red-end swin (counterweiht) at 4 rpm. Once both swins have swun up, the centrifue was accelerated in staes to the required speed, in the case of the work described here 16 rpm or 5. Pore pressures were monitored durin this acceleration stae in order to check the interity of the packae. When 5 was reached, pumpin commenced, with oil bein admitted to the reservoir at the top of the slope and the sump bein kept empty. Pore pressures within the model were monitored and oil was continuously admitted until a steady-state seepae reime was achieved with the water-table close to the surface of the slope. When this condition was achieved, the model was ready to be subjected to an earthquake. Threephase power was turned on to the SAM actuator and an offset was driven, chanin the lever-arm lenth joinin the packae to the SAM. This was monitored by an LVDT (linearly variable displacement transducer) allowin a predictable earthquake manitude to be achieved. Once this had been carried out, the compressed air supply to the clutch centrin mechanism was turned on, forcin the clutch into a central position. The timer controllin the solid-state relays on the SAM packae was then started, ivin a ten second delay and then firin the earthquake. The timer also starts the data-acquisition system, which collects data durin the earthquake and for a period afterwards. After data-acquisition was complete, the data was uploaded from the CDAQS box and the decision was taken as to whether or not to fire subsequent earthquakes. If no further earthquakes were to be fired, the centrifue was slowed and stopped. The SAM was then blocked up to han level and the model was examined, measured and photoraphed. -4-

9 3. Instrumentation The models tested were instrumented with Birchall A23 accelerometers (ACC s) and Druck PDCR81 pore pressure transducers (PPT s) arraned throuhout the sand of the model. Schematic instrumentation layouts for the centrifue tests toether with the coordinates of the instruments are shown in Fiures 2 to 7 and Tables 3 to 8 respectively. The coordinate system used is x bein the downslope coordinate from an oriin at the end of the box, z bein a vertical coordinate from the base of the ESB box and y the transverse coordinate from the ede of the box. Fiure 2: Instrument layout for test SKH-5 Table 3: Instrumentation coordinates for test SKH-5 Instrument x (mm) y (mm) z (mm) ACC ACC ACC ACC ACC ACC ACC876 ESB box bottom rin PPT PPT PPT PPT PPT

10 Fiure 3: Instrument layout for test SKH-8 Table 4: Instrumentation coordinates for test SKH-8 Instrument x (mm) y (mm) z (mm) ACC3441 ESB box base rin ACC ACC ACC ACC ACC ACC ACC ACC PPT PPT PPT Fiure 4: Instrument layout for test SKH-9 Table 5: Instrumentation coordinates for test SKH-9 Instrument x (mm) y (mm) z (mm) ACC ACC ACC ACC ACC8879 ESB box bottom rin ACC ACC ACC ACC PPT PPT PPT

11 Fiure 5: Instrument layout for test SKH-1 Table 6: Instrumentation coordinates for test SKH-1 Instrument x (mm) y (mm) z (mm) ACC ACC3477 ESB box bottom rin ACC ACC ACC ACC ACC ACC ACC ACC PPT PPT PPT PPT

12 Fiure 6: Instrument layout for test SKH-11 Table 7: Instrumentation coordinates for test SKH-11 Instrument x (mm) y (mm) z (mm) ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC982 ESB bottom rin PPT PPT PPT PPT6672 PPT PPT

13 Fiure 7: Instrument layout for test SKH-12 Table 8: Instrumentation coordinates for test SKH-12 Instrument x (mm) y (mm) z (mm) ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC982 ESB bottom rin -9-

14 4. Results Time-histories of acceleration and pore-pressure for the centrifue tests are shown in Fiures 8 throuh 27. Only a brief discussion of the results will be made here, further discussion can be found in Haih (22). 4.1 Test SKH-5 This model was subjected to two earthquakes of approximately 15% manitude. The final measured round displacement was 2 mm (1 m at prototype scale). From in-fliht video, this was seen to have been accrued almost entirely durin the first earthquake Earthquake 1 The results of test SKH-5 earthquake 1 are plotted in Fiures 8 & 9. The data acquisition system crashed after.5 s, so the data for the end of the earthquake is not available. It can however be seen that the model was subjected to a 15% earthquake of.5 s duration. Pore-pressures consistent with full liquefaction built up over approximately four earthquake cycles Earthquake 2 The results of test SKH-5 earthquake 2 are plotted in Fiures 1 & 11. This was a 2% manitude earthquake of.5 s duration. Pore-pressures consistent with full liquefaction were seen to be achieved after approximately two earthquake cycles Earthquake 3 The results of test SKH-5 earthquake 3 are plotted in Fiures 12 & 13. This was a 17% manitude earthquake of.5 s duration. Pore-pressures consistent with full liquefaction built up over approximately three earthquake cycles. 4.2 Test SKH-8 This model was subjected to two earthquakes of approximately 2% manitude. The final measured round displacement was 12 mm (.6 m at prototype scale) Earthquake 1 The results of test SKH-8 earthquake 1 are plotted in Fiures 14 & 15. This was a 25% manitude earthquake of.5 s duration. Pore-pressures consistent with full liquefaction were seen to be achieved after approximately two earthquake cycles with rapid drainae bein seen both durin the earthquake and afterwards relative to the other tests discussed here. This is due to the relative thinness of the liquefiable layer in this test. Examination of accelerations recorded close to the head of the slope (ACC s 3477 & 8925) shows the hiher harmonics of the earthquake bein severely -1-

15 attenuated resultin in near sinusoidal accelerations bein measured after.3 s. This is explained in terms of inter-particle slidin by Haih (22) Earthquake 2 The results of test SKH-8 earthquake 2 are plotted in Fiures 16 & 17. This was a 26% manitude earthquake of.9 s duration. Pore-pressures consistent with full liquefaction were seen to be achieved after approximately three earthquake cycles. The accelerations measured at the head of the slope aain show the filterin of hiher harmonics of the motion by slidin and become very asymmetric, peak positive (upslope) accelerations bein three times those measured in the neative direction for ACC Test SKH-9 This model was subjected to one earthquake of approximately 15% manitude. The final measured surface round displacement was found to be approximately 15 mm (.75 m at prototype scale). Marker lines at depth revealed that sub-surface deformations of 25 mm (1.25 m) were occurrin at 2 mm (1 m) depth. This could be a feature of the rotation of surficial soil due to -field curvature or due to toe restraint. This is further discussed by Haih et al. (2). The results of test SKH-9 are plotted in Fiures 18 & 19. The same filterin of hiher harmonics as was seen in test SKH-8 can be seen from ACC Test SKH-1 This model was subjected to one earthquake of approximately 22% manitude. The final measured surface round displacement was found to be approximately 26 mm (1.3 m at prototype scale), thouh subsurface peak displacements were measured as up to 6 mm (3 m). The results of test SKH-1 are plotted in Fiures 2 & 21. Some filterin of hiher harmonics can be seen from ACC7427. Unfortunately the data acquisition system crashed after.5 s, so later acceleration information is not available. 4.5 Test SKH-11 This model was subjected to one earthquake of approximately 2% manitude. The final measured round displacement was found to be approximately 28 mm (1.4 m at prototype scale). The results of test SKH-11 are plotted in Fiures 22 & 23. Lookin at the response of ACC876 at the end of the earthquake reveals different behaviour on odd and even cycles. This is revealed in FFT s as a component at 25 Hz, half of the earthquake frequency. This has been explained as bein a feature of the natural frequency of the liquefied soil column, which was calculated based on shear wave velocities to be approximately 23 Hz by Haih (22). -11-

16 4.6 Test SKH-12 This dry test was carried out as a control in order to hihliht the importance of liquefaction to the displacements and dynamic behaviour of these slopes. This model was subjected to one earthquake of approximately 2% manitude. No sinificant round displacement was measured. The results of test SKH-12 are plotted in Fiure 24. Almost identical accelerations were measured at all instrument locations, hihlihtin that the base accelerations necessary to fail these shallow slopes in the absence of liquefaction are very hih. -12-

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33 2 1 Max= 1.9 ACC Min= Max= 5.75 ACC Min= Max= 5.71 ACC Min= Max= 6.34 ACC982 5 Min= Max= 7.15 ACC Min= TEST SKH 1 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

34 2 Max= 1.9 ACC3477 Min= Max= 8.6 ACC3441 Min= Max= 7.71 ACC8915 Min= Max= 7.6 ACC8932 Min= Max= 5.2 ACC717 Min= Max= 5.49 ACC8925 Min= TEST SKH 1 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

35 PPT Max= 43.9 Min= 2.32 PPT Max= 1.1 Min= 2.68 PPT Max= 35.6 Min= 2.64 PPT Max= 2.9 Min=.19 PPT Max= 35.5 Min= 3.41 PPT Max= 38.9 Min= 1.59 TEST SKH 1 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

36 PPT Max= 43.9 Min= 3.62 PPT Max= 1.3 Min= 2.68 PPT Max= 35.6 Min= 2.64 PPT Max= 2.9 Min=.19 PPT Max= 35.5 Min= 3.41 PPT Max= 38.9 Min= 1.59 TEST SKH 1 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Lon Term Time Records 1

37 2 1 Max= 8.19 ACC982 1 Min= Max= 5.1 ACC Min= Max= 2.58 ACC8915 Min= Max= 6.29 ACC Min= Max= 1.6 ACC734 1 Min= TEST SKH11 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

38 2 Max= 8.19 ACC982 Min= Max= 8.28 ACC717 Min= Max= 8.12 ACC876 Min= ACC Max= 1.2 Min= Max= 6.25 ACC1572 Min= ACC Max=.78 Min= 1.36 TEST SKH11 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

39 ACC Max= 8.19 Min= 1.1 PPT Max= 35 Min= PPT Max= 55.1 Min=.114 PPT Max= 45 Min=.256 PPT Max= 48.7 Min=.434 PPT Max= 24.9 Min=.273 PPT Max= 45.9 Min=.946 TEST SKH11 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

40 ACC Max= 8.19 Min= 1.1 PPT Max= 35 Min= 8.51 PPT Max= 55.1 Min=.114 PPT Max= 45 Min=.256 PPT Max= 48.7 Min=.434 PPT Max= 24.9 Min=.273 PPT Max= 45.9 Min=.946 TEST SKH11 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Lon Term Time Records 1

41 ACC Max= 9.86 Min= 11.2 ACC Max= 8.55 Min= 9.26 ACC Max= 1.2 Min= 11.4 ACC Max= 8.98 Min= Max= 1.39 ACC7427 Min= ACC Max= 9.59 Min= 11.1 ACC Max= 9.43 Min= 13.8 TEST SKH 12 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

42 2 Max= 9.86 ACC982 Min= ACC3478 Max=.946 Min= Max= 9.5 ACC717 Min= Max= 9.34 ACC8131 Min= Max= 9.38 ACC8925 Min= Max= 9.11 ACC8932 Min= TEST SKH 12 Scales: Model 8th order Butterworth Filter at 5Hz Earthquake Fiure No. FLIGHT 1 Short Term Time Records 1

43 5. Conclusions It has been shown that centrifue modellin can be used to investiate the dynamic behaviour of liquefyin slopes. Dynamic effects such as resonance and filterin of hiher harmonics were observed which are further discussed by Haih (22). It was also observed that peak round surface displacements may not be equal to the reatest displacements occurrin within the soil layer. 6. Acknowledements The author would like to acknowlede the financial contributions of Shimizu Corporation, Japan, and of the Enineerin and Physical Sciences Research Council to this work. The author would also like to acknowlede the contribution of Dr Gopal Madabhushi to this research. 7. References Haih, S.K., Madabhushi, S.P.G., Soa, K., Taji, Y. & Shamoto, Y. (2) Lateral spreadin durin centrifue model earthquakes, Proc. GeoEn2, Melbourne, Australia Haih, S.K. (22), Effects of liquefaction-induced lateral spreadin on pile foundations in slopin round, PhD Thesis, Cambride University, UK Hamada, M. (1992), Lare round deformations and their effects on lifelines: 1964 Niiata earthquake, in Case Studies of Liquefaction and Lifeline Performance Durin Past Earthquakes, Hamada & O Rourke eds., Technical Report NCEER-92-1, NCEER, Buffalo, NY Lee, S.Y. (199), Centrifue modellin of cone penetration testin in cohesionless soils, PhD Thesis, Cambride University, UK Tan, F.S.C. (199), Centrifue and theoretical modellin of conical footins on sand, PhD Thesis, Cambride University, UK -39-