Predicting Future Performance of Improved Soils from Today’s Test Data

1. Pre-Blast Range (7 tests) 0 2 4 6 8 10 12 14 1.5 One Week Range (6 tests) One Week Range (6 tests) 2.5 Month Range (3 tests) 2 2.5 3 3.5 Depth, z (m) 4 4.5 5 Sand Aging Overview Sand aging describes the time dependent strength gain following disturbance of clean, loose, saturated sand layers. An example below shows changes in cone penetration test (CPT) tip resistance following explosive compaction at the site of Jebba Dam on the Niger River near Jebba, Nigeria. Effect of time on the cone penetration tip resistance of sand following explosive compaction at the Jebba Dam site (Mitchell and Solymar, 1984). Project Overview While most previous aging research has focused on determining the phenomenon’s underlying mechanisms, there has been less research on how to account for sand aging in design. Existing aging metrics do not easily account for many factors known to influence sand aging, such as soil type and disturbance method. This project will use one soil/site for several testing methods in order to isolate some of the factors known to be important in the aging process (disturbance method and aeration of the pore fluid, for example). A sand and gravel quarry in Griffin, IN was chosen for field testing and samples from that site were used in a comprehensive laboratory study. Prior to disturbing the soil, extensive soil characterization was performed using the CPT and Marchetti dilatometer (DMT). Shear wave velocity (Vs) was also determined using the seismic CPT (SCPT), down-hole testing, cross-hole testing, and spectral analysis of surface waves (SASW). By comparing results of these tests before disturbance and at various times after disturbance, the effects of sand aging could be quantified. University of Michigan Cone Penetration Testing Rig Field Testing Two methods of disturbing the soil were employed: explosive compaction and vibroseis shaking. Explosive compaction releases more energy than vibroseis shaking and aerates the pore fluid, both important differences with regard to aging. Explosive compaction at the Griffin, IN quarry. Network for Earthquake Engineering Simulation (NEES) vibroseis from the University of Texas. CPT tip resistance, DMT horizontal stress index (KD), and Vs all decreased immediately after the blast. This behavior is typical following explosive compaction. Following vibroseis shaking, there was no discernable decrease in these readings. Comparison of CPT tip resistance before and immediately following explosive compaction. CPT tip resistance increased with time following the blast, especially in the weakest layer between 1.5 and 5 meters in depth. DMT KD and Vs both show greater time-dependent increases following the blast. Because previous research has shown that higher energy disturbances cause greater aging changes, we predicted less time-dependent change following vibroseis shaking. As expected, there were slight increases in DMT KD and Vs following vibroseis shaking, but little noticeable change to CPT tip resistance. Time-dependent increase in CPT tip resistance following explosive compaction. Time-dependent increase in Vs following explosive compaction, as measured by down-hole testing. Laboratory Testing Previous research has shown that soil mineralogy, angularity, and grain size distribution, among other factors, influences sand aging. Therefore, a laboratory study characterized soil samples taken from the field testing site. Additionally, cyclic triaxial testing on fresh and aged samples is on going. Initial results show that liquefaction resistance increases with time after sample preparation. CKC cyclic triaxial device at the University of Michigan’s F.E. Richart Soil Dynamics Laboratory. Summary Sand aging is more than simply an academic curiosity because construction projects can be delayed if in-situ test results do not meet quality assurance metrics. A more complete understanding of sand aging will allow engineers to account for aging effects. This project provides an excellent opportunity to improve on current aging metrics. Tip Resistance, qc (Mpa) Depth, z (m) Predicting Future Performance of Improved Soils from Today’s Test Data Shear Wave Velocity, Vs (m/s) Depth, z (m) David A Saftner – PhD Candidate, Department of Civil & Environmental Engineering, University of Michigan Russell A Green –Associate Professor, Department of Civil & Environmental Engineering, Virginia Tech Roman D Hryciw –Professor, Department of Civil & Environmental Engineering, University of Michigan