A NEW SPECIMEN PREPARATION METHOD ON CALIBRATION CHAMBER IN SILTY SANDS

Transcription

1 A NEW SPECIMEN PREPARATION METHOD ON CALIBRATION CHAMBER IN SILTY SANDS Huai-Houh Hsu 1, An-Bin Huang 2, Yu-Jie Huang 2 and Chung-Ci Chen 3 1 Department of Civil Engineering 3 Institute of Mechatronoptic Systems Chienkuo Technology University Chang-Hua, TAIWAN 2 Department of Civil Engineering National Chiao Tung University Hsin-Chu, TAIWAN ABSTRACT The sand on the West Coast of Taiwan contains significant amount of silt. Due to lack of cohesion, it is difficult to obtain undisturbed samples for sand. In situ tests are often used to determine the engineering properties of sand. The cone penetration test (CPT) is a widely used in situ test. The interpretation rules of CPT data are mostly developed based on chamber calibration tests, and limited theoretical considerations. Most of the available empirical rules came from Europe or Northern America, based on tests in uniformly graded, clean sand. The difference between uniformly graded clean sands and silty sands can be significant. It is not desirable to directly adopt the empirical CPT interpretation rules developed in the West to the silty sand on Taiwan. The preparation of a uniform chamber specimen is important. When the sand contains fines, the particle segregation during deposition process should be minimized. A new deposition method has been developed to prepare a uniform chamber specimen of silty sands. This paper introduces this method and presents test results. INTRODUCTION The silty sand on the west coast of Taiwan comes from soft rocks of shale and mudstone. Many coast structures are constructed on this kind of sand layers. When the fines content (FC, particles which pass through sieve No. 2 or smaller than.75 mm) increases, the engineering properties will be changed. Many researches show that the fine particles in sand will affect its dilatancy, shear strength, liquefaction potential, and explanation of in situ test results. The calibration tests in sand usually use clean, uniform, silica or quartz sand, which is refered to as academic sand. Lots of 178

2 empirical rules and analytical models are established which are based on the academic sand test results. The in situ sand layers are not academic sand, their engineering properties will be affected by the fines mineral contents, particle shapes, and the types of soil gradation (Ishihara, 1994). Accordingly, it is not appropriate to apply the empirical equations derived from academic sands to explain the behavior of silty sands. The calibration chamber test data of cone penetration test (CPT) in silty sand are still limited at present. Because the volume and weight of sand specimen have significant difference between triaxial test and calibration chamber test, it can not use the same way to reconstitute both of them especially for silty sands. The triaxial test results indicate that while coarse sands are surrounded by fines, the compressibility and strength are strongly correlated with fines characteristics (Thevanayagam et al., 22). Been et al. (1988) used Kogyuk sand to conduct triaxial tests. Under the same relative density and confining pressure, the increase of FC could make Kogyuk sand s dilatant behavior change into contraction. Regarding the relationship between FC and soil liquefaction potential, there presents obviously different consequences by researchers. Some scholars (Vaid, 1994; Koester, 1994; Zlatovic and Ishihara, 1997; Lade and Yamamuro, 1997) show that the resistance liquefaction strength will be decreased, while FC increasing. On the contrary, Some scholars (Chang et al., 1982; Dezfulian, 1982; Amini and Qi, 2) show that the resistance liquefaction strength will increase, while FC increasing. But another test results indicate a concave curve with the increase of FC (Law and Ling, 1992; Thevanayagam, 1998; Polito and Martin II, 21). Huang and Ma (1994) used distinct element method (DEM) coupled with boundary element method (BEM) to simulate CPT in a granular material with infinite boundary. Results demonstrated by Huang and Ma (1994) have indicated the small particles have higher stress than the large particles during cone penetrating. The cone tip resistance (qc) correlated closely with dilatancy of sand. The failure mode around cone tip is affected by dilatancy, stress level, and boundary conditions of calibration chamber. Stark and Olson (1995) suggested that when using qc to evaluate liquefaction potential, it should be modified according the percentage of fines content. The friction ratio (Rf, ratio of sleeve friction resistance and qc) increases with FC and soil plasticity (Suzuki et al., 1995). Robertson and Wride (1998) adopted modified qc and Rf to define a soil behaviour type index (Ic), and propose the relationship between FC and Ic. The above studies on silty sands indicate the engineering characteristics are affected by geological history, mineral contents, soil sampling process, and reconstitute specimen method. It is important to establish an explanation way for silty sand in Taiwan. 179

3 ASSESSMENT OF RECONSTITUTED SAND SPECIMEN METHODS Due to lack of cohesion, it is difficult to obtain in situ undisturbed samples in sand. The reconstituted specimen is frequently used in the laboratory tests. However, the preparation of a remold specimen should consider its uniformity, repeatability of test result as well as simulate the field state (Kuerbis and Vaid, 1988). Sand Specimen of Triaxial Test Many methods have been proposed to prepare the triaxial sand specimen, these methods include (Ishihara, 1993; Kuerbis and Vaid, 1988; Tatsuoka et al., 1986): 1. dry deposition (DD) 2. air pluviation (AP) 3. moist tamping (MT) 4. moist vibration (MV) 5. water sedimentation (WS) 6. water vibration (WV) 7. slurry deposition (SD) The sand fabric prepared by different method will not totally the same. Tatsuoka et al. (1986) compared the resistance liquefaction strength of different sand specimen reconstituted methods (AP, MT, MV, and WV), the MV method had the highest resistance strength and AP method got the lowest resistance strength. Amini and Qi (2) evaluated the differences between uniform and layered specimen. They found that the difference of resistance liquefaction strength was not obvious. Yamamuro (24) adopted WS and DD methods to prepare the specimen of Nevada Sand to conduct cyclic triaxial tests. Test results show the sand behavior was dilatant and not easy liquefaction while using WS method. On the contrary, the sand behavior was contractive and easy liquefaction while using DD method. Chamber Calibration Specimen The concept of a calibration chamber test is to prepare a large sand specimen in the laboratory, consolidated to a desired stress level, and then perform the experiment under given boundary conditions. Since the entire experiment is conducted in the laboratory, the test quality can be readily controlled. The large sand specimen, with uniform deposition and known engineering properties, provides reference values for the interpretation and thus calibration of the in situ test method. Specimen preparations in a calibration chamber are mostly made by pluvial deposition. Two types of pluviator systems have been reported for the preparation of sand specimens: the stationary and traveling sand pluviators (Salgado, 1993; Fretti et al., 1995; Salgado et al., 1998). 18

4 As shown in Figure 1, the stationary sand pluviator consists of a hopper, a rainer plate, a shutter plate, and diffuser meshes. The test sand, whose total weight must be heavy enough to form the desired specimen, is placed initially in the hopper. The holes in the rainer plate are closed by the shutter plate. The diffuser meshes are placed inside the chamber at an appropriate height from the chamber bottom. After the rainer plate is opened, the sand falls from the hopper through the holes of the rainer plate and down to the diffuser meshes. The diffuser meshes are positioned at a constant distance over the surface of sand deposition by a pulley. When the falling sand jets generated by the rainer holes pass through the diffuser meshes, the cross sieves spread them out to form a uniform deposit. Figure 1. The stationary sand pluviator, from Sweeney and Clough (199) The travelling pluviator is comprised of four components: a solid metal plate with a rectangular aperture, a set of interchangeable openings, a stepping motor, and the sand hopper (Lo Presti et al., 1993). The test sand is loaded in the hopper and drops from the opening that is mounted in the rectangular aperture. During pluviation, the pluviator moves back and forth horizontally above the open chamber and the falling sand is like a wide brush to sweep over the deposition surface. The stationary pluviator is economical and easy to operate. However, when the sand contains fines, segregation can be significant. The sand or gravel sized particles spread over a wider area than the fine grains. Silt columns or silt concentration is observed right beneath the holes or nozzles of the rainer plate (Lo Presti et al., 1992). Although the traveling pluviator can reduce the phenomenon of segregation, both of them are air pluviation methods. 181

5 Rahardjo (1989) used slurry consolidation method to prepare chamber specimen of silty sand. The silty sand and water are mixed together by a concrete mixer, and consolidated to the desire stress level of performing CPT. It takes a long time to form a chamber specimen by the slurry consolidation method. For a FC=4% chamber specimen, 1.5m in diameter and 1.5m high, it will take 18 to 2 days to complete the consolidation process to reach the confining pressure of 7 kpa. THE WET SPRAY METHOD The wet spray (WSp) method adopts the concept of both air pluviation and slurry deposition. During silty sand pluviation, sands pass through a water spray zone to make coarse and fine particles attach to each other, then move dowward through a film of water, and finally deposit inside the chamber. The wet spray pluviation system consist of a sand pluviation device and a water spray equipment. Figure 2 demonstrates the schematic view of this system. Figure 2. Schematic view of the wet spray pluviation system Properties of Mai Liao Sand EVALUATION OF WET SPRAY METHOD A batch of silty sand from Mai Liao, the west coast of Taiwan are used to provide specimens for laboratory experiments. Table 1 shows the physical properties of natural Mai Liao Sand (MLS). 182

6 Table 1. Physical properties of natural Mai Liao Sand Mineral content muscovite, chlorite, and quartz FC (%) 15 Specific gravity % (G S ) 2.69 Effective size, D 1 (mm).65 Coefficient of uniformity (C u ) 2.15 Maximum void ratio (e max ) 1.58 Minimum void ratio (e min ).589 Uniformity of the Specimens To verify the uniformity of gradation, a specimen was prepared following the wet spray procedure and then saturated the triaxial cell. The confining stress is then released and the specimen removed from the triaxial cell. The specimen was sliced into five equal layers, each layer was further divided into inner (Zone I) and outer zone (Zone II) (see Fig. 3). Figure 3. Schematic view of specimen sliced layers and zones Table 2 shows the FC measured for each of the five layers. For average FC=14.4% and 17.6%, the difference amongst layers within ±1.1%. Figure 4 demonstrates the vertical distribution of FC in the specimen. Table 3 shows the FC measured at inner and outer zone for each of the five layers. For average FC=14.4% and 17.6%, the difference between inner and outer zone of each layer within ±.5%. Figure 5 presents the horizontal distribution of FC in the specimen. Test results indicate the uniformity of spatial distribution of specimen prepared by WSp method. The particle segregation is not obvious. 183

7 Table 2. FC measured at each layer (vertical direction) Test No. T1 T2 T3 T4 T5 T6 Depth, mm FC, % ~28(Top) ~ ~ ~ ~14(Bottom) Avg., % Bottom<< Layer No. >>Top Test No. T1 T2 T3 T4 T5 T Fines content, % Figure 4. Distribution of FC at each layer (vertical direction) 184

8 Table 3. FC measured at each zone (horizontal direction) Test No. (Zone) T3 I T3 II T3 Avg. T6 I T6 II T6 Avg. Depth, mm FC, % ~28(Top) ~ ~ ~ ~14 (Bottom) Avg., % Bottom<< Layer No. >>Top Zone FC(Avg., %) I 14.4 II 14.5 I 17.3 II Fines content, % Figure 5. Distribution of FC at each zone (horizontal direction) Triaxial test results Repeatability of test result Under the same void ratio and boundary conditions, two cyclic triaxial tests are performed to verify repeatability of test results. Figure 6 demonstrates the measured data during cyclic test process, these two specimens show similar behavior. 185

9 4 Deviator, kpa 2-2 Excess Pore Pressure, kpa Axial Strain, % MLS FC=15%, e c =.89, σ d =3kPa N=12 N= Time, sec Figure 6. Cyclic triaxial test results to verify the repeatability of WSp Soil behavior of different sand specimen reconstituted method A series of isotropically consolidated undrained triaxial (CIU) tests with excess pore pressure measurements were performed on MLS. The triaxial specimens were prepared by WSp method and sheared by axial compression (AC). The test results are compared with MT method (Tsai, 22). Figures 7 and 8 illustrate the CIU test results under two levels of confining pressure (5 and 1 kpa). In the lights of test results, WSp and MT methods present different soil behavior. The specimens indicate strain hardening behavior which prepared by WSp method. For MT method, the strain softening is occurred before reaching 1% axial strain. σ v -σ h, kpa MLS, FC=15%, CIU, σ C '=5kPa e c =.79, WSp e c =.64, MT(Tsai, 22) e c =.71, MT(Tsai, 22) 1 Excess Pore Pressure, kpa Axial Strain, % Figure 7. Comparison of WSp and MT with FC=15% (confining pressure = 5 kpa) 186

10 1 8 MLS, FC=15%, CIU, σ C '=1kPa e c =.68, WSp e c =.66, MT(Tsai, 22) σ v -σ h, kpa Excess Pore Pressure, kpa Axial Strain, % Figure 8. Comparison of WSp and MT with FC=15% (confining pressure = 1 kpa) CONCLUSIONS A wet spray pluviation system has been developed, and a series of tests have been performed to verify its function. The conclusions are as follows: 1. The sliced layers show the variation of FC are within ±1.1% in vertical direction and within ±.5% in horizontal direction among layers and zones. This result indicates WSp method can achieve a high degree of spatial uniformity. 2. Two specimens was performed cyclic triaxial tests under same boundary condition, results illustrate their curves of deviator stress, excess pore pressure, and axial strain are close. The soil tests performed by the specimen of WSp method are repeatable. 3. According to the CIU triaxial test results, WSp method presents a strain hardening behavior and MT method shows a strain softening behavior. The different reconstituted specimen method will affect test results and the explanation of characteristics of silty sands. ACKNOWLEDGEMENT This study was funded by the National Science Council of Republic of China under contract NSC E Its support is greatly appreciated. 187

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