NON-DESTRUCTIVE SHAKING: INITIAL ANALYSIS AND PROPOSED ISSUES FOR DISCUSSION & DECISION

1. NON-DESTRUCTIVE SHAKING:INITIAL ANALYSIS AND PROPOSED ISSUES FOR DISCUSSION & DECISION Ricardo Dobry (with help from C. Medina and M. Gonzalez) NEESR-SG piles project, 7/26/06 VTC

2. Background • Issue discussed in March VTC • Objective: small-amplitude input shaking not inducing pore pressure buildup nor permanent lateral displ., aimed at measuring dynamic response of sand model (Vs, damping, resonant frequency) • Fundamental frequency in 5-7 Hz range? • Small amplitude random (white) noise banded between 3 and 15 Hz to be considered • Responsibility: M. Pitman, M. Zeghal and A. Elgamal

3. How do we define non-destructive shaking (NDS)? • NDS should not generate any pore water pressure (pwp). For inclined tests, no pwp means no lateral spreading. • No pwp means that cyclic shear strains in soil are equal or smaller than threshold strain = 0.01% • Therefore, one way to define NDS is to look at soil shear strains ≤ 0.01% • Another way is to look at field evidence and calculations relating soil accelerations and liquefaction / pore pressure buildup

4. Evidence relating acceleration to pwp buildup / liquefaction • Evidence below relates pwp to peak horizontal acceleration at soil ground surface, ap • Owi Island field measurements in an earthquake showed that pwp started when ap reached about 0.06g (Ishihara, Finn et al.) • Centrifuge tests at UC Davis showed that pwp started when ap reached about 0.05g (Arulanandan et al.) • Calculations based on threshold strain suggest that for depths shallower than 20 ft, very loose sand and water level at ground surface, pwp start when ap reaches 0.02-0.07g (Dobry et al.) • In summary: ap to start pwp is in range 0.02-0.07g • Table of case histories of liquefaction (Andrus et al.) indicates that lowest value of ap in the field causing liquefaction has been 0.12 g

5. Recent developments in our project(M. Pitman) • Minimum displacement amplitude of input ground motion is about ± 0.01” • Random motion for 3-15 Hz and ± 0.1” already generated by actuators (see next slide) • For pure sinusoidal motion: • ± 0.01” at 3 Hz corresponds to ap = 0.01g • ± 0.01” at 15 Hz corresponds to ap = 0.23g • These of course are input shaking parameters at base of model, not soil shaking parameters

6. ±0.1”, 3-15 Hz random motion generated by actuators

7. Recent developments in our project(M. Gonzalez) • Marcelo Gonzalez ran exactly the same Cyclic 1D model of a 6m inclined test reported in June VTC, changing only input shaking to approximate NDS. • Four runs done, with 10 sec of sinusoidal motion and displ. amplitude of ± 0.01”: • f = 3 Hz (acc. = 0.1g) • f = 5 Hz (acc. = 0.026g) • f = 7 Hz (acc. = 0.05g) • f = 15 Hz (acc. = 0.23g) • Results in next slide

8. M. Gonzalez Cyclic 1D results

9. Some thoughts for discussion • Marcelo’s runs: • confirm close correlation between soil surface acceleration and soil shear strain • suggest that relation between input acc. & soil acc./soil strain is rather complicated • we can get large soil strains and pwp very easily unless we are very careful with NDS • We should continue studying the problem very carefully, including extracting critical information from preliminary tests. Theva will install accelerometers at base and top of soil in next 3m tall preliminary test, in addition to ring instrumentation • Options being discussed: random shaking with disp. amplitude decreasing with f; use of background noise (no actuator motion) as NDS • We need proposal from Pitman-Zeghal-Elgamal group (deadline?) • What sequence of destructive and NDS do we apply in preliminary, 3m tall test?