Implementation of Bender Elements to Measure In-situ Stiffness of Soft Clays

Transcription

1 4th International Conference on Earthquake Engineering Taipei, Taiwan October 1-13, 006 Paper No. 33 Implementation of Bender Elements to Measure In-situ tiffness of oft Clays Young-Jin Mok 1, Jae-Woo Jung, Man-Jin Han, and Chul-oo Park ABTACT Bender elements, composed of thin piezo-ceramics and elastic shims, have been used to measure shear wave velocities of specimens in laboratories. As a preliminary stage of their field applications, an in-house research of optimizing suitable bender elements and their geometrical arrangement has been carried out in a barrel of kaolinite slurry. Two types of measurement configuration, similar to cross-hole and in-hole seismic testing, have been implemented. Prototype instrumented rods were penetrated into a soft clay layer in the west coast of Korea and excellent shear waves were recorded. Development of penetration device (mandrel) and associated instrumented rods are in progress for deep investigation. Keywords: oft clays, Bender elements, hear wave velocity, Cross-hole, In-hole INTODUCTION Ground improvement for soft marine clay and reclaimed soils is being monitored by piezometers and assessed in terms of dissipation of excessive pore water pressure. The instrumentation is the only usable method to monitor the improvement continuously. However, the method has a critical shortcoming, which is the fact that it measures pore water pressure only, rather than mechanical properties. If a new technique for continuous monitoring of mechanical properties, rather than pore pressure, is developed, it will be very beneficial to engineering profession. A new instrumentation for monitoring the properties is sought by embedding piezo-electric sensors. A three-year research project was launched to develop a field technique to measure stiffness of soft clays and the first year research results are presented here in. The embedded arrangements of bender elements, composed of piezo-materials and metal shims, were developed to measure in-situ stiffness of soft clay. Bender elements have favorable features suitable to field instrumentation such as simple principle of energy conversion, excellent control capability and small physical size. As a preliminary stage of their field applications, an in-house research of optimizing bender elements for the best performance and their geometrical arrangement has been carried out in a barrel of kaolinite slurry (Mok et al., 005; Jung, J.W., 005; Jung et al., 005). Two types of measurement configuration, similar to conventional cross-hole and in-hole seismic techniques, have been implemented. To confirm the performance in rugged field environment, prototype instrumented rods were pushed two meters deep into the coastal mud near PyungTaek-city with bare hands in the west coast of Korea. 1 Professor, Civil Eng. Dpt., Kyung-Hee University, Korea (yjmok@khu.ac.kr) esearch Assistant, ditto

2 Crosshole and in-hole seismic testing were conducted and excellent shear wave signals were recorded. A pushing scheme using a mandrel was developed for protection of bender elements and deeper penetration. DECIPTION OF BENDE ELEMENT A bender element consists of two piezoceramic sheets sandwiching a central metal shim as it is shown in Fig. 1 (antamarina, 001). Bender elements can be made into various shape, size, and arrangement of piezoceramic sheets and metal shims. They can be optimized with proper shape, thickness and material stiffness depending upon usage purposes. Fig. shows the process of manufacturing a bender element by cutting a piezoceramic sheet into proper size, bonding components, connecting electrodes and coating. Metal him Piezoceramic Piezoceramic Electrode Figure 1. tructure of a bender element The bender element can bend as piezoceramic sheet on one side expands while the other side contracts with an applied voltage. Expanding and contracting of the sheets are reversed as the direction of the voltage is changed. Thus, application of an AC voltage enables the element vibrate and be used as an actuator. On the other hand, if the element is deformed by outside force, it generates voltage and can be used as a receiver. There are two types of bender elements: series-connected and parallel-connected. With the same voltage applied, the parallel-connected bender element generates twice the tip displacement as the series-connected bender element. On the other hand, series-connected bender elements can generate twice the voltage as parallel bender elements under the same deformation. Thus, parallel-connected bender elements are used as actuators(source) while series-connected bender elements are better suited as receivers (Jung, M.J., 005). Polyurethane coating Piezo heet him After gluing Co-axial cable Ground Figure. Manufacturing process of a bender element COHOLE In the cross-hole configuration shown in Fig. 3, one rod was mounted with source benders and the other rod with receiver benders, respectively. Each pair of source and receiver benders was used to generate one cycle of harmonic perturbation and to monitor the shear wave motion at each location. As a preliminary stage of their field applications, crosshole arrangement has been carried out in a barrel of kaolinite slurry and excellent shear wave signals were obtained (Mok et al., 006; Jung et al., 005).

3 Power Amplifier Function Generator Oscilloscope ource eceiver Figure 3. Crosshole configuration Bender elements of 1.5cm long and 1.5cm wide were mounted at every 0.5 meter on source and receiver rods as shown in Fig. 4. A small metal plate( same size as bender elements) was also mounted right below each element to protect the element during penetration. The source and receiver rods were pushed meters deep into coastal mud near PyungTaek-city with bare hands. The distance between source and receiver rods was 0cm. Excellent shear wave signals were recorded at the depth of 0.5, 1.0, 1.5 and.0 meters and travel times were picked as shown in Fig. 4. The arrows in the figure indicate the first arrivals of shear wave energy. hear wave velocities were calculated by dividing travel distance (here, 0 cm) by travel time at each depth and plotted in Fig Vs(m/s) m Travel = 6.19 msec m 1.5m Travel = msec Travel = msec Depth(m) Travel = msec.0m Figure 4. Field crosshole testing.5 In crosshole testing, travel distance should be as accurate as travel time to calculate shear wave velocity at each measurement depth. A pair of extra bender elements was mounted at each measurement depth to measure shear wave along inclined travel path as shown in Fig. 5. Horizontal travel distance, D can be calculated using travel time through horizontal travel path, t₁and travel time along the inclined travel path, t₂. The shear wave velocities(vs) of horizontal and inclined travel paths can be calculated by eqs. 1 and, respectively.

4 D V s = (1) t 1 D H = () t + V s where, H is the vertical offset between source and receiver. Horizontal travel path, D can be obtained by eliminating Vs from eqs. 1 and. On the other hand, the third equation to calculate shear wave velocity can be derived as eq. 3 by eliminating D from the equations. V s = H t t (3) 1 D, t1 Travel - t1 t H Travel - t Figure 5. Geometrical arrangement of sensors for horizontal and inclined wave measurements hear waves of inclined travel paths were measured with varied inclined angle as shown in Fig. 6. At inclined angle of 30, the signal of shear wave was so good that the first arrival time is very clear as indicated by an arrow in the figure. It is difficult to pick the first arrival times from the signals recorded with inclined angle more than 45. Thus the bender elements were arranged with the inclined angle less than 45 for field application. Horizontal 30 Travel = msec Travel = msec Figure 6. hear wave measurements of inclined travel paths

5 As described above, the crosshole configuration requires extra efforts to track down the accurate travel distance. To avoid this difficulty, in-hole arrangement was adopted as below. IN-HOLE In an in-hole configuration, pairs of source and receiver benders were mounted on one rod. It has advantages over crosshole such as the use of one rod only and the fixed distance between source and receiver. However, the noise through the rod is a formidable price paid for using one rod only and should be eliminated. everal installation schemes of bender elements were attempted in the laboratory and the final version was adopted for field testing. The first scheme was the direct mounting of the bender elements on the rod as shown in Fig. 7. The noise, which looked like the same as input, was predominated and any discernible shear wave energy could not be identified. The second scheme was separation of source and receiver using separate two rods. The signal was excellent as expected. However, this scheme is not realistic to be implemented in the field and was discarded. Two arms were attached to the rod and source and receiver benders were mounted at the end of each arm, facing each other. urprisingly the noise through the rod almost disappeared and shear wave energy propagated through soil was dominated and became discernible. But the mounting arms are not amiable in pushing the rod into the ground. The final version is using swing arms, which can be folded during penetration and unfolded for measurements. Fig. 8 shows swing-arm scheme with the arms unfolded and the shear wave signal recorded in the coastal mud. The noise, with high frequency and small amplitude, arrived before the major shear wave energy through the mud. However, the noise does not interfere in picking the first arrival of shear wave. 10cm Figure 7. Direct mounting scheme and measured signal 10cm Travel Figure 8. wing arm scheme and measured signal

6 PENETATION CHEME The instrumented rod requires additional penetration device to protect the bender elements during deep penetration, where it can encounter shells and gravels scattered in the mud. A mandrel of 38 mm outside diameter can be used to house the instrumented rod during pushing and be pulled out after penetration to the final depth. Fig. 9 shows the schematic diagram of the installation of instrumented rods for crosshole testing. The swing arms were unfolded by pulling the cables connected to them during the withdrawal of mandrel out to the ground surface. Each pair of source and receiver benders was facing each other at each measurement depth. Unfolded ensors were reached further outside of the mandrel and were intimately contacted with surrounding soil. The penetration scheme was tried in the coastal mud and the shear wave signals were recorded at the depth of 0.5 m, 1.0m, 1.5m and.0m as shown in Fig. 10. The signals show the clear-cut first arrival times of the shear wave as indicated by the arrows in the figure. PVC Pipe OD 38mm Mandrel ource Bender eceiver Bender 0cm hoe Figure 9. chematic diagram of installed crosshole configuration Travel = 8.14 msec Vs = 5.7 m/s Depth(m) Travel = 6.73 msec Travel = 7.78 msec Travel = 7.54 msec Vs = 31.3 m/s Vs = 6.8 m/s Vs = 7.7 m/s Figure 10. hear wave signals measured from crosshole testing in coastal mud

7 In in-hole testing, the same penetration scheme as in crosshole was used in the coastal mud as shown in Fig. 11. The swing arms were unfolded after pulling the mandrel out. The source and receiver benders were facing each other with the distance of 10 cm. A typical shear wave signal measured in the mud is shown in Figure 1. The use of swing arm and mandrel enables the sensors be protected during penetration and be contacted intimately with the surrounding soil. Presently, the development of a mechanical device similar to cone penetration test is in progress. OD 38mm Mandrel Unreleased Bender wing Arm ource Bender 10cm eceiver Bender Figure 11. chematic diagram of installed in-hole configuration Vs = 36.5 m/s Travel =.88 msec Figure 1. A typical shear wave signal measured from in-hole testing in coastal mud CONCLUION Bender elements have favorable features suitable to field instrumentation such as simple principle of energy conversion, excellent control capability and small physical size. Crosshole and in-hole measurement configurations, replacing conventional source and receiver with bender elements, have been implemented. The instrumented rods were pushed into coastal mud as deep as meters by bare hands and excellent shear wave signals were recorded. A penetration scheme using a mandrel and a mechanical pushing device is in progress. The present findings of the on-going three year research venture are as below: 1) In crosshole testing, excellent shear wave signals can be measured using bender elements as source and receiver. However the testing arrangement requires extra efforts to track down accurate travel distance between source and receiver.

8 ) In-hole testing arrangement is possible by eliminating the noise through the rod using a swing-arm mounting scheme. It shows promising future for field application because it requires one rod only. 3) A mandrel penetration scheme was useful to protect bender elements during penetration and a mechanical device for the mandrel is in progress. ACKNOWLEDGEMENT The research has been supported by a grant(contract No.; C105A A ) from KICTTEP. The authors also would like to thank Dr. K.H. tokoe, II and Mr. M.J. Jung at the University of Texas at Austin for their help to initiate the research. EFEENCE Mok, Y.J. et al. (005), A Feasibility tudy of eismic Cone in Offshore Environment, ep ort(written in Korean), KODI. Mok, Y.J. et al. (006), Implementation of Piezoelectronics to ustained Evaluation of Mechanical Characteristics of oft Clay, Annual eport(written in Korean), KICTTEP. Jung, J.W. (005), A Pilot tudy of Bender Elements in tiffness Measurements of Civil Engineering Materials, Master Thesis(written in Korean), Kyu ng Hee University, Korea. Jung, J.W., Y.J. Mok, I.. Jang (005), A Pilot tudy of Implementing Bender Elements to In-situ Civil Engineering Measurements, Journal of the Korean Geotechnical ociety(written in Korean), 1(5), Jung, M.J. (005), hear Wave Velocity Measurements of Normally Consolidated Kaolinite Using Bender Elements, Master Thesis, University of Texas at Austin. antamarina, J.C. in collaboration with K.A. Klein and M.A. Fam, (001), oils and waves, Chichester ; New York : J. Wiley on.