Soils age. Their engineering properties often improve significantly during aging times of practical interest to engineers. This paper provides examples from research and practice. It includes a review of the triaxial IDS test, followed by examples of its use to investigate the question of whether the aging improvements result from frictional or cohesive effects. Contrary to most current thinking, the soil stiffening and strengthening appears entirely frictional in effect. The aging effects described appear mechanical, resulting from dispersive particle movements and internal stress arching under drained conditions. The paper concludes with suggestions for using these mechanical aging effects in practice.

Three different types of block sizes, namely, bars, plates, and prisms of various dimensions, are related to the classical rock quality designation (RQD) index by means of the volumetric joint count definition. Statistical models are derived analytically by assuming that three sets of fractures at different directions occur according to a negative exponential distribution function of the intact lengths. This distribution function has one model parameter, which is the average number of discontinuities related to the block volume and the volumetric joint count. Necessary charts are presented, which help to identify the representative block size within a rock mass provided that the volumetric joint count and average number of discontinuities are estimated from the field surveys. Furthermore, the global volumetric joint count is defined and related analytically to the nonhomogeneity coefficients in different directions. Finally, relevant analytical expressions are derived and presented in the form of charts for the relationships between RQD, block volume, and volumetric joint count. Accordingly, excellent, good, fair, poor, and very poor rock quality classifications are delineated on these graphs. Finally, the application of the methodology developed herein is presented for a set of discontinuity survey on igneous rocks in the Kingdom of Saudi Arabia.

The influence of saturation and back pressures on the liquefaction resistance of a sand is investigated by conducting cyclic triaxial tests on four sets of sand samples with different degrees of saturation and on five sets of samples with different back pressures. The tested sand is taken from the site of Tong Jaizhi Water Power Station in China. Test results show that the conspicuous effect of saturation on the liquefaction resistance of the tested sand cannot be improved by keeping back pressures on samples during the test. On the other hand, the application of back pressures otherwise results in higher liquefaction resistance. The effect of back pressures is further explained by analyzing the internal interaction of soil particles. It is concluded that the back‐pressure method should not be used in cyclic liquefaction tests for improving the degrees of saturation of sand, and Terzaghi's effective stress principle is not valid unless additional conditions are included for the actions of interparticle forces in soils.

A complete set of algebraic formulas and dimensionless charts is presented for readily computing the dynamic stiffnesses (K) and damping coefficients (C) of foundations harmonically oscillating on/in a homogeneous half‐space. All possible modes of vibration, a realistic range of Poisson's ratios, and a practically sufficient range of oscillation frequencies are considered. The foundations have a rigid basemat of any realistic solid geometric shape. The embedded foundations are prismatic, having a sidewall‐soil contact surface of height d, which may be only a fraction of the embedment depth D. Two numerical examples illustrate the use of the formulas and charts and elucidate the role of foundation shape and degree of embedment on radiation damping for various modes of vibration. A companion paper (Gazetas and Stokoe 1991) presents supporting experimental evidence from model tests. The two papers aim at encouraging the practicing engineer to make use of results obtained with state‐of‐the‐art formulations, when studying the dynamic response of foundations.

Comprehensive experimental results of 54 free‐vibration tests on model footings embedded at various depths in sand and having circular, square, and rectangular base shapes were reported by Erden and Stokoe in 1974 and 1985.The results for vertical and coupled swaying‐rocking oscillations are used herein to verify the homogeneous‐halfspace solutions of the companion paper by Gazetas (1991) in the form of algebraic formulas and graphs for dynamic stiffnesses and damping factors. Particular attention is accorded to selecting appropriate values of S‐wave velocity, using either Erden's original recommendations of 1974 or the measured response of the corresponding surface footings. Close agreement is found between theory and experiment for vertical oscillations (in both frequency and damping values) and for swaying‐rocking (only in frequency values). Measured swaying‐rocking damping averages only half the theoretical estimates. It is demonstrated that: (1) The lower value of modulus around the sides and slippage at the footing side‐soil interface may have contributed to not more than 25% of the shortfall in the experimental values; and (2) boundary wave reflections, at the interfaces between sand and high‐damping soft boundary layer are responsible for at least 75% of the observed discrepancies, having essentially eliminated the damping contribution of the swaying component of motion.

A series of dynamic shear tests was performed on large hollow cylindrical reinforced and unreinforced sand samples. Steel rods were used to reinforce the specimens uniaxially and biaxially. A newly developed test apparatus was used to perform cyclic torsional shear strain control tests with strain amplitudes ranging from $1\times {10}^{-2}\%$ to 7% and frequencies varying between 0.5 Hz and 10 Hz at different levels of confining pressure. Test results indicate that confining pressure and strain level affect the dynamic shear modulus and damping of unreinforced and reinforced sands. Dynamic shear modulus and internal damping were independent of frequency (up to 10 Hz) for both reinforced and unreinforced samples. Reinforcements have no effect on the complex shear modulus of the material up to a strain amplitude of 5%. At large strains (>7%) uniaxial reinforcements inhibited the formation of helical shear bands, which had been observed on the unreinforced specimen. However, vertical shear bands parallel to and equidistant from the reinforcements were observed. Biaxial reinforcements prevented the formation of all rupture surfaces, thus greatly increasing the ductility of the material. The results indicate that the effect of the reinforcements on the dynamic properties of the reinforced soil composite can be ignored when analyzing the dynamic response at small strains, but should be considered when evaluating the dynamic response at larger strains.

There is a lack of strength data on weak rocks where cores for conventional testing can only be obtained with difficulty. In recent investigations, strengths are obtained over a 300-ft (91-m) vertical section for calcareous fanglomerates, typical of the Southwestern United States, by plate testing and correlation with seismic velocity measurements. A 240-ton (213-metric ton) ore truck is used to provide the large reaction load necessary to induce failure. Values for the compressive strength are computed from Boussinesq stresses, Mohr circle analysis, and by correlation with P-wave velocity, and range from 451 psi (3.1 MPa) at a depth of 100 ft (61 m) to 863 psi (5.6 MPa) at a depth of 300 ft (122 m). Interpretation of the results of the plate tests provides an interesting example of the different engineering approaches sometimes needed in rock and soil mechanics. Both the plate tests and seismic correlations give similar results and, when used with care, represent simple and reliable methods for strength estimation.