Development of high precision direct shear apparatus for liquefaction testing

Transcription

1 Japanese Geotechnical Society Special Publication The 5th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering Development of high precision direct shear apparatus for liquefaction testing Sokkheang Sreng i), Hiroki Ishikawa ii),takuya Kusaka iii), Takashi Okui iv) and Akitoshi Mochizuki v) i) Senior Researcher, R&D Center, Nippon Koei Co. Ltd., 234, Inarihara, Tsukuba-shi, Ibaraki 3-259, Japan. ii) Consultant Engineer, Nippon Koei Co. Ltd., 2, Kojimachi 4-chome, Chioda-ku, Tokyo, 2-83, Japan. iii) Researcher, R&D Center, Nippon Koei Co. Ltd., 234, Inarihara, Tsukuba-shi, Ibaraki 3-259, Japan. iv) Professor of Emeritus, The University of Tokushima, 2-, Minamijosanjima-cho, Tokushima , Japan. v) Engineer, Penta-Osean Construction Co.,Ltd., 2-2-8, kouraku, bunkyou-ku, Tokyo, , Japan. ABSTRACT A newly developed direct shear apparatus for cyclic tests is, first, introduced. It is shown that liquefaction strength on Toyoura sand obtained using a newly developed direct shear apparatus and test procedure coincides well with that obtained from cyclic triaxial tests. Then, two series of cyclic direct shear tests were carried out on samples of sandy silt, which was excavated by a grab bucket at a construction site in Naruto City, Tokushima Prefecture. Therefore, considerable disturbance was expected. Results were compared with those from a series of cyclic triaxial tests on undisturbed samples by a triple-tube technique at the site. It was found that liquefaction strength from the direct shear tests coincided well with that obtained from cyclic triaxial tests. It is deduced that disturbance of a sample was well cured in the consolidation process, which is one of the advantages of the newly developed apparatus. Keywords: liquefaction, cyclic loading, direct shear test, triaxial compression test INTRODUCTION The Niigata-earthquake in 963 caused serious damage to infrastructure in that area due to liquefaction of sandy ground. In 966 a cyclic loading test for liquefaction using a traditional triaxial cell was proposed by Seed and Lee (966). Thereafter, a cyclic simple shear test, a cyclic torsion test, a cyclic ring shear test etc., have been developed. However, none of these test methods are free from the problem of sample disturbance, which causes serious underestimation of liquefaction strength, during sampling, shaping of a specimen and even testing as sensitive sand or sandy silt layers are generally a target of liquefaction tests. To obtain an intact borehole sample of over 6 cm-long also makes the testing difficult. It is obvious that the accuracy of testing depends significantly on the degree of sample disturbance. This continues to be an issue. In 967, just after the Seed's proposal, a series of cyclic loading tests on a sand sample under constant volume condition (CU-condition), was presented by Mikasa and Mochizuki (967), in which Mikasa's direct shear apparatus was used (Mikasa, 963). In 998, Oshima et al. succeeded in measuring accurate vertical stress working on a sample avoiding side friction working on a side surface of shear box. Ishikawa et al. (29) developed an apparatus originally in order to observe vertical stresses working on a sample accurately even under low stress conditions. It is remarkable that a small amount of sample is enough to carry out testing, and that a sample of sand or sandy silt, commonly sensitive, has a merit of a kind of curing during a consolidation process in a shear box, as was found in this study. However, the testing method is not generalized even among researchers, and a small number of papers on the subject using a direct shear apparatus has been available up to the present. In this paper, static direct shear tests and cyclic direct shear tests on Toyoura sand were conducted, firstly, in order to verify accuracy of the apparatus by comparing results with those obtained using a triaxial cell. Failure criterion of liquefaction for the developed method was also discussed. Then, two series of cyclic loading tests on a sandy silt sample, which was excavated at a site using a construction bucket, were carried out and compared with results of cyclic triaxial tests on an undisturbed sample using triple tube samplers. It is shown that results obtained using the new apparatus coincide well with those obtained from cyclic triaxial tests. 2 OUTLINE OF THE NEWLY DEVELOPED DIRECT SHEAR APPARATUS AND TEST PROCEDURE Fig. is a schematic diagram of newly developed 268

2 direct shear apparatus. Base concept of the apparatus follows Mikasa's original one with a loading system where vertical stress is loaded from the bottom of a sample so as to avoid friction force working between an upper and a lower shear boxes (Mikasa, 963). A symbol in the figure shows a shear box, which is composed of an upper and a lower shear box. A loading cell to measure vertical stress, σ N, is fixed at the upper shear box using linear-slide bearings. Reaction force from the bottom of a sample is supported by an upper reaction plate. The system can avoid not only friction working at the upper and lower shear box surfaces, but also side friction working between a specimen and the shear boxes. It is also sure not to be influenced by weight of the upper shear box in order to measuring accurate vertical stress. A check load cell, 3, is also attached at the end of the rod. A torque motor, 4, controls vertical force constantly for a CD-test or vertical displacement to be zero in order to keep a specimen s volume constant in a CU-test. For a cyclic loading test (CU-condition), constant height of specimen volume is controlled by fixing a vertical rod at a plate of symbol 7.This is because the torque motor system under very low stress condition cannot control it with sufficient precision. Cyclic shear stress is loaded using a torque motor, 2, and shear stress is measured using a load cell, 6, causing loading triangulate force-waves which simulate that of an actual earthquake. of the newly developed direct shear apparatus. Then, a series of cyclic direct shear tests on Toyoura sand, No.5, was performed, and compared with that of test No.4. An undisturbed sample for No.6 were obtained from a depth of GL-7.5~8.5m (σ'=78.6 kn/m 2 ) using a triple tube sampler at the site, and results of cyclic triaxial tests, following JGS regulation, JSFT54, were used as an index strength of liquefaction of the sample. Samples for No.7 and 8 (σ N =5, kn/m 2, respectively) were obtained using a construction bucket (Fig.2()) and then, thin-wall tubes measuring φ 8.9 L2cm were driven into the block of sandy silt by hand (Fig.2(2)), so, considerable disturbance to samples was expected. A method of a cyclic direct shear test, No.7 and 8, is as follows; After τ d was found from a static direct shear test under CU-condition, test conditions, such as σ, τ d /σ N, shear speed (.2mm/min) and double amplitude of shear displacement, Dδ, were set. Here, shear ratio of cyclic load (τ d /σ N ) is the same parameter as σ d /2σ in a triaxial cyclic test, and "Dδ" is defined in section 5.. A specimen was pushed into the shear box from a cutter ring, and consolidated until a time defined by "4t (see Soil test method of JGS 29)". Cyclic force controlled by a servomotor was loaded until horizontal displacement reached over Dδ. As accuracy and validity of the apparatus was already verified from comparison of tests No.4 and 5, then, results of No.7 and 8 were compared with those of No.6 in order to observe influence of disturbance on samples レールレール 5 Table. Main properties of samples and test condition. Test Drain w Sample Test name σ (kn/m2) Fc(%) ρs e No. condil. (%) Sr(%) CD 5,, normal TA 2 Toyoura CD 5,, developed DS CU 3, 6, Cyclic-TA 32 Toyoura.85 5 Cyclic-DS CU 6 Naruto Cyclic-TA Naruto2 Cyclic-DS CU *: Naruto=sampled by a triple sand sampler, Naruto2=sampled by a construction bucket *2: TA=triaxial compression test, DS=direct shear test 3 4 Fig.. A newly developed direct shear apparatus. 3 SAMPLES AND TEST CONDITION Three series of static tests No. to 3, and cyclic tests No.4 and 5 on Toyoura sand, and three series of cyclic loading tests No.6 to 8 on sandy silt samples from a construction site in Naruto, Tokuhsima were performed. The main properties of samples and test conditions are shown in Table. Static tests No. and 2 (triaxial, CD) and No.3 (direct shear, CU) were carried out following JGS regulations in order to verify accuracy and validity Fig.2. Sampling of the Naruto-2 samples. 269

3 τd/σn τd/σn τd/σn σ'/σn Friction angle, ( ) δ(mm) L=3cm H' 2=.32L 5 45 CD (Mikasa's DS) CD (developed normal DS) CU (Mikasa's DS) CU(developed low stress DS) CD (Triaxial CD) 2L 4 H' =.25L δ=.3l δ=.6l Fig.3. Fig.3 Development of shear of band. shear band Void ratio, e ()τ d /σ N ~N N( 回数 ).4 (4)τ d /σ N ~σ /σ N Fig.4. Comparison ofφ' orφ CD (3)δ~N -2 ~N N( 回数 ).5. (2) σ N /σ N ~N ~N (5)τ d /σ N ~δ φ= σ'/σn δ(mm) Fig.5. Results of a cyclic DS test:e =.85,τd/σN=.2. 4 RESULTS OF STATIC TESTS N O.,N O. 2, ON TOYOURA SAND Fig.4 shows a comparison of φ cd or φ' obtained from static triaxial compression tests and static direct shear tests. For the latter, data from two types of apparatus were plotted; one was obtained using Mikasa's apparatus and the other was the authors' newly developed apparatus. It was found that strength, φ', using the newly developed apparatus on e =.85 samples of Toyoura sand coincides well with those obtained from triaxial compression tests and also φ' from CU-test using Mikasa's apparatus. It was clarified that strength, φ', obtained from direct shear CU-tests coincides with that of φ CD (Mochizuki, 992, Ishikawa, 29). It was found that newly tested data show less friction angles measuring -.3~-.2 than those of previous data. This result proved the viability of the newly developed direct shear apparatus. 5 RESULTS OF CYCLIC LOADING TESTS 5. Criterion of double amplitude displacement, Dδ It is necessary to define "double amplitude of shear displacement, Dδ", in a cyclic direct. This is the same as the criterion, "double amplitude (DA=5%)", in a cyclic triaxial test. If we can know the developed height of shear band in a direct shear test, "double amplitude of shear displacement, Dδ " can be defined taking into consideration that an axial strain of 5% (=DA) corresponds to a shear strain, γ xy of 7.5% (or γ xy =±3.75%). Fig.3 shows development of shear band in a two-dimensional shear box test which was performed by our research group (Ofuji et al.,999), and two assumptions regarding "height of shear band (H')" were proposed. 27

4 τd/σn σd/2σ τd/σn σd/2σ τ/σn σd/2σ () Assumption : H' =.25L for Dδ In Fig.3, vertical height of the shear band was observed as.25l. Here, L is referred to as length of shear box. Then, height of shear band in a case of 6mm diameter (=L) specimen is evaluated as Eq.(). H' = 6mm.25, and ± d /H' 2 =±.375. Then, Dδ =.25mm () (2) Definition 2: H' 2 =.32L for Dδ 2 In Fig.3, taking into consideration that a slope of.2 angles against the horizontal of shear band and that height of the shear band, H' 2, measured as.32l, then, Dδ 2, is estimated as Eq. (2); H' 2 = 6.32, and ±[ d 2 cos.2 ]/H' 2 =±.375. Then, Dδ 2 =.5mm (2) Both criteria of Dδ are adopted at time, and their validity will be discussed in the following sections. 5.2 Test results on Toyoura sand Fig.5 shows an example of test results obtained from test No.5. It should be noted that symmetric displacement under loading of symmetry force, as was expected, is clearly observed. Fig.6 and Table 2 show a comparison of liquefaction strengths of test No.4 and 5. Here, liquefaction strength, σ d /2σ or τ d /σ N, were obtained using a regression technique of curve fitting. It was found that liquefaction strengths based on the two different criteria coincide well with each, which shows that the difference of criteria does not have significant influence on liquefaction strengths. Liquefaction strengths of direct shear tests coincide well, or show a small gap of -.3%, with those obtained from the cyclic triaxial tests. The gap can be acceptable in practice use. A liquefaction strength calculated from Ishihara's equation (σ d /2σ =.42D r (%), here, D r =28.9% on Toyoura sand, is calculated as.2, which also coincides acceptably well with that of the test data (Ishihara et al., 997). These results on Toyoura sand proofs accuracy and applicability of the newly developed direct shear apparatus. 5.3 Test results on disturbed samples Very sensitive soils are often a target of liquefaction tests. The strength of which is commonly underestimated due to disturbance of a specimen in every process of sampling, shaping of a specimen or testing. However, it was expected that deterioration of a sample would have less influence when using a cyclic direct shear test than when using a cyclic triaxial test due to the difference of methods in the process. Thus, two series of cyclic direct shear tests under σ N =5 and kn/m 2 were performed on the samples obtained by a bucket at the Naruto site Fig.6. Comparison of liquefaction strength. 図 -6 一面 三軸液状化試験 : 豊浦砂 ( 気乾 e=.85).5 DS Dδ () N =5N/m 2 Toyoura sand, e =.85 ()σ N =5N/m 2 (2) N =N/m 2 図 -7(2) 繰返し一面 (σn=kpa) Fig.7. Comparison 繰返し三軸の比較 of liquefaction strength ( 撫養 -2 on 試料 Naruto ) samples. Table 2. Results of cyclic loading tests. DS Dδ 図 -7() 繰返し一面 (σn=5kpa) 繰返し三軸の比較 DS ( 撫養 Dδ - 試料 ) (2)σ N =N/m 2 Test σ DA or Lique. cyclic DS Sample No. (kn/m2) Dδ strength /cyclic TA 4 5%.8 - Toyoura.25mm mm Naruto 5% mm mm Naruto2.25mm mm *: cyclic TA=cyclic triaxial test, cyclic DS=cyclic direct shear test 27

5 Fig.7 and Table 2 show test results of the two types, which are compared with those of a cyclic triaxial test. Liquefaction strengths defined by Dδ (=.25mm) appear to be slightly weaker than those by Dδ 2 (=.5mm) in test No.7, as it is expected, though the gap between them is not so much recognized in practical use. In test No.8, the strength by Dδ coincides well with that of Dδ 2. Here, it should be noted that reconsolidation stress of the sample at kn/m 2, is higher than that at a sampling depth (σ'=53 kn/m 2 ). Next, results of test No.8 were compared with those of test No.6, and it was found that slightly low strength measuring gap of -% was obtained compared with that of No.6. This gap is small enough to be acceptable in practical use when samples are considerably disturbed. It is deduced that disturbance of a sample was well cured in the consolidation process, which is one of the advantages of the newly developed apparatus. A sample of the test measuring φ6 H2mm is also advantageous to that of a triaxial test measuring φ 5 L2 mm as the former can avoid unexpected large deformation in the shearing process. 6 CONCLUDING REMARKS Traditional triaxial cells have been used for testing liquefaction strength of silty sand samples up to today. However, there has been concern about unavoidable disturbance of samples with high sensitivity during sampling, forming and even setting a specimen into a triaxial cell. This disturbance causes uneconomical design of structures due to evaluation of less liquefaction strength. In this paper, a newly developed direct shear apparatus was proposed as a test technique that can reduce the influence of sample disturbance in liquefaction tests. In order to verify the issue, three series of cyclic loading tests were performed using the direct shear apparatus and a traditional triaxial cell on two kinds of samples; Toyoura sand and Naruto samples of sandy silt. Cyclic loading tests with the direct shear apparatus used a block sample which was excavated using a grab bucket, so considerable disturbance of a soil block was expected during the sampling. On the other hand, a technique of triple tube sampling was adopted for samples of cyclic triaxial tests. It was found that liquefaction strength from direct shear tests coincide well with that of triaxial tests in each series. This means that a disturbed silt sample was cured during a consolidation process in a shear box of direct shear apparatus, and caused that the direct shear testing for liquefaction strength was equivalent to that of triaxial testing. Taking into consideration that the developed direct shear apparatus has advantages in not only for ease of testing for liquefaction strength, but also its ease of maintenance of the system. It is concluded that the technique of direct shear testing should be adopted more in both testing of liquefaction strength for practical purpose and that for research purpose. REFERENCES ) Ishikawa H., Liu Y., Mochizuki A., Okada S., Sreng S. (29): Development of a new direct shear apparatus and discussion on its effectiveness, Japanese Geotechnical Journal, JGS, Vol.4, No., -9 (in Japanese). 2) Ishihara K. (997): Simple method of analysis for liquefaction of sand deposits during earthquake. Soils and Foundations, Vol. 7, No.3, -7. 3) Japanese Geotechnical Society (29): Method for one-dimensional consolidation properties of soils using incremental loading, , (In Japanese). 4) Mochizuki A. (992): N value of STP and strength parameters, c and φ, (approach and application of them), Section 7 & 8, JGS., 33-6 (in Japanese). 5) Mikasa M., Kishimoto Y., and Mochizuki A., (967): Cyclic shear test on Toyoura sand, Proceedings of the 22nd Japanese National Conference on Geotechnical Engineering, JGS, (in Japanese)). 6) Mikasa M.(963):Newly developed apparatuses for direct shear test and triaxial tests, Proc. of the th Annual Symposium on Geotech. Eng., JSSFE, 7-23(in Japanese). 7) Ofuji H., Ueno K., and Mochizuki A., (999): Deformation of sandy specimen with direct shear, Proceedings of the 54th annual conference of JSCE, Vol.3 (A), (in Japanese). 8) Oshima A., Takada N., Sumi T., and Hongo T. (998): Comparison of cyclic shear tests on sandy by direct box shear tests. Proc. 33rd JNC on Geotech. Eng., JGS, No.358, (in Japanese). 9) Oshima A., Takada N., and Sakamoto Y. (996): Comparison between ordinary and true split box shear tests under constant pressure condition, Proceedings of the 3st Japanese National Conference on Geotechnical Engineering, Yamaguchi, Japan, No.333, (in Japanese). ) Seed, H.B. and Lee, K.L. (966): Liquefaction of saturated sands during cyclic loading, ASCE, Vol.92, No. SM6,