Liquefaction Strength compared with Cone Resistance in Triaxial Specimen considering Aging Affect of Sand containing Non-Plastic Fines

Miniature cone penetration tests (CPT) and subsequent cyclic loading tests are carried out in a triaxial test specimen. An innovative simple mechanism is introduced in a normal cyclic triaxial apparatus enabling a miniature cone to penetrate into the specimen with a constant speed. The two test results are compared to yield direct correlations between penetration resistance (qt) and liquefaction strength (RL) for sands of different relative density containing various amount of fines and cement. A unique curve relating qt and RL has been found irrespective of relative density and fines content Fc. In order to simulate aging effect on the relationship between penetration resistance and liquefaction strength, sand samples mixed with cement are consolidated for a day to make an accelerated test. The test results revealed that the cementation effect tends to increase RL more than qt and higher Fc results in higher RL under the same penetration resistance qt. INTRODUCTION Recently liquefaction strength is evaluated using not only standard penetration tests but also by cone penetration tests (CPT) in engineering practice. If sand contains a measurable amount of fines, liquefaction strength is normally raised in accordance to fines content in most of liquefaction potential evaluation methods. As one of its experimental basis, Suzuki et al. (1995) carried out in situ penetration tests and soil sampling by in situ freezing technique from the same soil deposit and compared the tip resistance qt-value and liquefaction strength RL of the undisturbed samples. The result showed that the higher fines content tends to increase the liquefaction strength for the same penetration resistance. In contrast to their finding, however, quite a few laboratory tests show that liquefaction strength clearly decreases with increasing content of low plasticity fines having the same relative density (e.g. Sato et al. 1997; Kokusho and Komiyama 2001, Hara et al. 2005, Kokusho et al. 2007). ___________________________________________ 1 Professor, Faculty of Science & Engineering, Chuo University, Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551, JAPAN, kokusho@civil.chuo-u.ac. jp 2,3 Graduate student, Faculty of Science & Engineering, Chuo University, Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551, JAPAN Thus, a wide gap seems to remain between the current practice for liquefaction potential evaluation and the laboratory experiment on the modification of the liquefaction strength in sands containing fines despite its importance in engineering design. In order to establish direct correlations between cone resistance and liquefaction strength considering the effect of fines, a systematic experimental study was undertaken, in which cone penetration test and subsequent cyclic loading test were carried out on the same triaxial test specimen with parametrically changing relative density and fines content (Kokusho et al. 2005). An innovative simple mechanism originally introduced by Kokusho et al. (2003) was used enabling a miniature cone to penetrate the sand specimen. In this paper, in addition to the previous tests (Kokusho et al. 2005), a new series of test are undertaken using the same triaxial apparatus for sand specimens containing various amount of fines mixed with cement. In the accelerated tests where the aging effect of natural sand deposits in a long geological period is simulated in a short time, penetration resistance and liquefaction strength are measured in series the same specimen and compared to develop direct correlations between them. TEST APPARUTUS AND TEST PROCEDURES In a triaxial apparatus used in this research, the specimen size is 100 mm in diameter and 200 mm high. In liquefaction tests, the soil specimen can be loaded cyclically by a pneumatic actuator as a stress-controlled test. In order to carry out a cone penetration test in the same specimen prior to undrained cyclic loading, a metal pedestal below the soil specimen was modified as shown in Fig.1, so that a miniature cone can penetrate into the specimen from below. For that goal, the pedestal consists of two parts, a fixed circular base to which the cone rod is fixed and a movable metal cap, through the center of which the cone rod penetrates in the upward direction into the overlying specimen. The annulus between the Water release Water supply Pedestal Cone