Horizontally Mounted Bender Elements for Measuring Shear Modulus in Soaked Sand Specimen

Transcription

1 Advanced Materials Research Vols (2014) pp (2014) Trans Tech Publications, Switzerland doi: / Horizontally Mounted Bender Elements for Measuring Shear Modulus in Soaked Sand Specimen Keeratikan Piriyakul 1,a* and Janjit Iamchaturapatr 1,b 1 Center of Excellence in Structural Dynamics and Urban Management, Department of Civil and Environmental Engineering Technology, College of Industrial Technology, King Mongkut s University of Technology North Bangkok, Bangkok, Thailand. a keeratikanp@kmutnb.ac.th, b janjiti@kmutnb.ac.th Keywords: Bender element, Shear modulus, Sand Abstract. New horizontally mounted bender element devices capable of high-quality transmission and reception of horizontally propagated shear waves polarized in orthogonal planes across the midheight of a sand specimen are described. Mounting of these bender elements is on the membrane, attaching on the side wall of the reactor container. This technique is suitable for use on samples down to 80 mm length. The effective fabrication procedures that have been developed are described. The instrumentation systems used to drive and receive signals are outlined, and estimates of the magnitude of the shear strains developed by the bender elements and the accuracy with which shear wave velocities can be determined are discussed. The sand specimen is treated by the solution then its strength is developed. These new bender elements enable shear modulus to be measured before, during and after the treatment. Introduction This research applied a bender element test to measure the shear modulus of sand specimen. The measurement uses the principles of wave propagation showing a direct correlation between the shear wave velocity, V s, and the initial shear modulus, G 0. The G 0 is widely considered to be a fundamental soil stiffness property in earthquake engineering. The reliable determination of G 0 and inferring complete stress-strain curves especially in the small and intermediate strains, offers the possibility of deducing the functional relationship between shear modulus and strain as shown in Fig. 1. At the very small strain domain (the shear strain values below the linear elastic threshold strain of about %), the G 0 does not change in the very small strain range. G 0 Shear modulus, G Very small strains Small strains Larger strains 10-3 % Shear strain, γ 1 % Fig.1 Typical variation of shear modulus with strain for soils. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /05/14,04:42:49)

2 Advanced Materials Research Vols Material and Test Method Sand Reactor. The sand reactor were made of plastic containers with dimensions of 80mm x 80mm x 80mm (width x length x height). The reactors were free drop placed with sieved sand (passed no. 100 and retained no.200) with an approximate height of 40 mm (Fig. 1b) and the density of 1470 kg/m 3 and then filled with 300 ml of nutrient solution contained 250mM of urea, 250mM of calcium ion (by CaCl 2 ), and glucose of 1.5mM. Source of water used for preparing the solution was collected from Northern part of Chaophraya River. Control reactor was made of sand sample mixed with only water. The experiment was performed in ambient condition with average temperature of 25 ± 2 o C. The water level of each reactor was re-marked. An addition of deionized water to each sand reactor was sometimes needed to maintain the constant level of water table and prevent the level falling due to water loss by evaporation. Bender Element Test. Fig. 2a showed the schematic diagram of bender element test. A function generator was used to generate a signal. This signal is sent and received by piezoelectric ceramic sensors placed at opposite ends of the soil sample in Fig. 2b. An oscilloscope is used to measure the arrival time between a sending signal and a receiving signal. A voltage pulse is applied to the sending sensor; this causes it to produce a shear wave. When the shear wave reaches the other end of the soil sample, distortion of the receiving sensor produces another voltage pulse. The receiving sensor is directly connected to the oscilloscope to compare the difference in time between the sending and the receiving signals. The shear wave velocity measurements are usually performed with frequencies ranging between 2 to 15 khz, at strains estimated to be less than %. At low frequencies, signals can be influenced by a near-field effect. At high frequencies, the receiving signal is very weak and difficult to interpret. In most cases, signals are averaged 32 times in order to get a clear signal. The shear wave velocity is calculated from the tip to tip distance between the two sensors and the time required by the shear wave to cover this distance and time as explained in details [1, 2, 3] and shown in Eqs. 1 and 2. After determining the propagation of shear wave velocities, it is possible to calculate the initial horizontal shear modulus, G hh, using the relationship of elastic continuum mechanics in Eq. 3. Vs = L/t req (1) t req = t t - t c (2) G hh = ρ.v s ² (3) where V s is the shear wave velocity in m/s, L is the tip to tip distance between two sensors in mm. t req is the required time to cover this distance in µs, t t is the total travel time in µs and t c is the offset time in µs. G hh is the initial horizontal shear modulus in MPa measured from horizontally propagated, horizontally polarized shear waves and ρ is the soil density in kg/m 3. (a) Fig.2 Schematic diagram of bender element test and experimental set-up: (a) Installation of bender element and oscilloscope for monitoring the strength development and (b) Setup of sand reactor.

3 498 KKU International Engineering Conference Fig.3 Horizontally mounted bender element. Fig. 3 shows the horizontally mounted bender element test. A bender element device is mounted on the aluminum plate by using the superglue. The aluminum plate is directly glued by using the silicone at the opposite side of the membrane. The membrane is attached to the reactor sidewall by using the silicone. The reactor sidewall need to be drilled a hole of 20mm x 20mm (width x length). This new technique is modified the frictional bender element technique as described by [4], allowing the possibility to perform the bender element test in the soaked condition without water leakage. Results and Discussion (a) (b) (c) (d) (e) Fig. 4 Measurement of shear wave velocity: (a) 0 day, (b) 1 day, (c) 2 day, (d) 3 day and (e) 4 day.

4 Advanced Materials Research Vols Fig. 4a shows the example result of the shear wave velocity measurement in sand specimen at 0 day. Measurement result found that the t t value is 710 µs and t c value is 4 µs. Thus, the required time (t req ) is 706 µs according to Eq. 3. The tip to tip distance (L) between the transducers is 79.5 mm. Therefore, the shear wave velocity (V s ) of m/s is obtained by using the Eq. 1. Then the shear modulus of MPa is calculated with the sand density of 1470 kg/m 3 by using Eq.3. In the similar ways, the shear wave velocity and the shear modulus results from 0 to 4 day are shown in Table 1. Fig. 5 shows that the shear wave velocity results were increased with increasing treatment time. The calculated V s indicated that the shear wave velocity of treated sand was increased about 2.5 folds in comparison with before treatment. In the same ways, Fig. 6 shows that the shear modulus results were also increased with increasing treatment time. The reason for increasing V s and G hh with time may be due to the change in particle contact condition, such as increase in interparticle friction or cohesion Vs[m/s] Time [day] Fig. 5 Shear wave velocity versus time. Fig. 6 Shear modulus versus time.

5 500 KKU International Engineering Conference Table 1. Shear wave velocity and shear modulus results. Time [day] Shear wave velocity, V s [m/s] Shear modulus, G hh [MPa] Summary The new horizontally mounted bender element devices capable of high-quality transmission and reception of horizontally propagated shear waves polarized in orthogonal planes across the midheight of a soaked sand specimen. The new technique is directly mounted bender elements on the membrane allowing the possibility to measure the shear wave velocity in soaked condition without water leakage. This technique is suitable for use on specimens down to 80 mm length. The sand specimen is treated by the solution then its strength is developed. This new horizontally mounted bender element enable shear wave velocity and shear modulus to be measured before, during and after the treatment. Acknowledgement The authors would gratefully like to acknowledge the support research financial support by the College of Industrial Technology, King Mongkut s University of Technology North Bangkok. References [1] R. Dyvik, C. Madshus, A.K.M. Mohsin, Laboratory measurement of G max using bender elements, D.W. Airey, Automating G max measurement in triaxial tests, The ASCE Annual Convention, Detroi, USA, (1985), [2] E.G.M. Brignoli, M. Gotti, K.H. Stokoe, Measurement of shear waves in laboratory specimens by means of piezoelectric transducers, Geotech. Testing J. 19 (1996), [3] K. Piriyakul, A development of a bender element apparatus, Journal of King Mongkut s University of Technology North Bangkok. 20 (2010), (in Thai). [4] V. Fioravante, R. Capoferri, On the using of multi-directional piezoelectric transducers in triaxial testing, Geotech. Testing J. 24 (2001),