The Instantaneous Strain of Frozen Sand

Abstract

ABSTRACT In engineering application, creep strain of frozen soil was modelled by a pseudoinstantaneous strain and a secondary creep law. The pseudo-instantaneous strain was often neglected by researchers because it is usually less than 10% of the total strain in a long term situation. Nevertheless, many cases of frozen soil structural analysis require computation of pseudo-instantaneous strain. There are two ways to determine the pseudo-instantaneous strain: (1) after separating the pseudo-instantaneous strain into an elastic strain and a plastic strain, compute the elastic strain by dividing the stress by an elastic modulus, and compute the plastic strain with a power law; (2) use a simplified approach to obtain the pseudoinstantaneous strain by dividing the stress by a secant modulus. This project investigated the pseudoinstantaneous strain of frozen sand through a serious of uniaxial compressive tests including both constant stress (creep) and constant strain rate tests. These experiments were conducted in two large size freezers. A front-end loading machine with dead weight was used to conduct creep tests. An uniaxial compression machine with constant displacement rate control was used to conduct constant strain rate tests. Two sample sizes, 2.94 in. (74.7 mm) diameter by 7.25 in. (184.2 mm) in length and 1.45 in. (36.8 mm) in diameter by 4.375 in. (111.1 mm) in length were used. Ottawa sand passing #30 sieve and retained on #140 sieve was used. The coefficient of uniformity was approximately 1.5. Distilled water was used to mix the. samples. Sand concentration were 64 and 70 percents by volume. The sand volume fraction was determined by preweighing the amount of sand and water. Samples were frozen to desired temperature before testing. A temperature probe was embedded at the top portion of each specimen to monitor its temperature during testing. A thin membrane was used to protect the specimen from evaporating. Specimen temperature were from −5 to −11 degree C. Temperature variation during each test was within 0.5 degree C. In creep test, specimens were loaded until the straight line portion of creep curves (secondary creep) was reached.; Then, the straight line portion of the curve was projected back to vertical axis at time zero to obtain the pseudoinstantaneous strain. The secant modulus was obtained by dividing the applied stress by the pseudo-instantaneous strain. The relation between the secant 'modulus and temperature is shown in figure 1. The secant moduli were found not sensitive to sand concentration. In constant strain rate tests, a deformation rate 0.113 inches per minute was used. Elastic-modulus was obtained from the initial slope of the stress-strain curve. The elastic modulus was shown in Table 1. The secant modulus was about 1 to 3 percent of the elastic modulus. The plastic strain was set equal to the pseudo-elastic strain since the elastic strain is negletable. Relation between the applied stress, the plastic strain, and temperature is shown in figure 2.