Ricardo Dobry

Evaluation of pile foundation response to lateral spreading

The effects of liquefaction on deep foundations are very damaging and costly, and they keep recurring in many earthquakes. The first part of the paper reviews the field experience of deep foundations affected by liquefaction during earthquakes in the last few decades, as well as the main lessons learned. The second part of the paper presents results of physical modeling of deep foundations in the presence of liquefaction conducted by the authors and others at the 100g-ton RPI centrifuge. In the last decade centrifuge modeling has been identified as a key tool to identify and quantify mechanisms, calibrate analyses and evaluate retrofitting strategies for pile foundations. Results are presented of centrifuge models of instrumented pile foundations subjected to lateral spreading, including single pile and pile groups, 2- and 3-layer soil profiles, mass and stiffening elements above ground to incorporate the effect of the superstructure, and evaluation of proposed retrofitting strategies. Interpretations of these centrifuge experiments and their relation to field observations and soil properties.

Mechanics of Lateral Spreading Observed in a Full-Scale Shake Test

This paper examines in detail the mechanics of lateral spreading observed in a full-scale test of a sloping saturated fine sand deposit, representative of liquefiable, young alluvial and hydraulic fill sands in the field. The test was conducted using a 6-m tall inclined laminar box shaken at the base. At the end of shaking, nearly the whole deposit was liquefied, and the ground surface displacement had reached 32 cm. The presented analysis of lateral spreading mechanics utilizes a unique set of lateral displacement results, DH, from three independent techniques. One of these techniques—motion tracking analysis of the experiment video recording—is especially useful as it produced DH time histories for all laminar box rings and a complete picture of the lateral spreading initiation with an unprecedented degree of resolution in time and space. A systematic study of the data identifies the progressive stages of initiation and accumulation of lateral spreading, lateral spread contribution of various depth ranges and sliding zones, their relation to the simultaneous pore pressure buildup, and the soil shear strength response during sliding. DOI: 10.1061/ASCEGT.1943-5606.0000409 CE Database subject headings: Soil liquefaction; Residual strength; Hydraulic fill; Full-scale tests; Lateral displacement. Author keywords: Liquefaction; Residual strength; Hydraulic fill; Full-scale tests; Lateral displacement.

Centrifuge and Large-Scale Modeling of Seismic Pore Pressures in Sands: Cyclic Strain Interpretation

AbstractCentrifuge modeling of pore pressure buildup in a sand deposit as a result of shaking is evaluated by comparison with a large-scale experiment. In large-scale Test SG-1, a 5.6-m-thick, mildly sloping deposit of hydraulic fill clean Ottawa sand of Dr=40%, was subjected to 5 s of low-intensity base shaking (<0.02g) that induced excess pore pressures short of liquefaction. Three centrifuge experiments using various soil deposits and saturation fluids were conducted and compared with the large-scale test. One of these centrifuge simulations used the same Ottawa sand and Dr=40% of the prototype, a viscous pore fluid, and dry pluviation deposition, which created a soil fabric stiffer than the prototype. The other two centrifuge simulations used silty sand saturated with water. The pore pressure buildup in one of the silty sand tests was in good agreement with the prototype, while the other two centrifuge deposits did not develop any excess pore pressure. The various responses in the four tests are expla...

Cyclic Triaxial Strain-Controlled Testing of Liquefiable Sands

Two series of undrained cyclic triaxial strain-controlled tests were performed on two different Imperial Valley, California, silty sands which liquefied during an earthquake in 1981. Both intact and reconstituted specimens were tested, and the testing procedures are described. The experimental data confirm that cyclic shear strain is the fundamental parameter governing pore pressure buildup, because strain-controlled tests essentially eliminate the influence of specimen fabric and sample disturbance. Also, the results indicate that the cyclic triaxial test can be used to model cyclic simple shear (similar to seismic field conditions), if the cyclic simple shear strain, γ c y , is related to the cyclic triaxial axial strain, ∈ c y , by either of two similar analytical expressions: γ c y = 1.5 ∈ c y or γ c y = 3 ∈ c y . Consequently, a unique pore pressure model is developed and recommended to simulate the seismic pore pressure buildup at the site. This model is applicable to reconstituted and intact specimens of the two sands, despite their different void ratios and nonplastic silt contents, and is valid for both cyclic triaxial and cyclic simple shear strain-controlled conditions.

CPT-Based Evaluation of Liquefaction and Lateral Spreading in Centrifuge

Two series of centrifuge model tests were conducted using Nevada sand. Four saturated models placed in a mildly inclined laminar box and simulating a 6-m-thick deposit were shaken inducing liquefaction effects and lateral spreading. The sand was deposited at a relative density, Dr =45 or 75%; two of the 45% models were subjected to overconsolidation or preshaking. The second series involved in-flight measurements of static cone tip penetration resistance, qc , simulating the standard cone penetration test (CPT) 36-mm cone. Values of qc increased with Dr , overconsolidation, and preshaking. A normalized resistance, qc1N , was assigned to each of the four liquefaction/lateral spreading models. Increases in Dr , overconsolidation, and preshaking decreased liquefaction and ground deformation, but relative density alone captured these effects rather poorly. Conversely, qc1N predicted extremely well the liquefaction and lateral spreading response of the four models, confirming Seed’s hypothesis to explain the s...

Liquefaction-Induced Lateral Load on Pile in a Medium Dr Sand Layer

One-g shake-table experiments are conducted to explore the response of single piles due to liquefaction-induced lateral soil flow. The piles are embedded in saturated Medium Relative Density (Dr) sand strata 1.7–5.0 m in thickness. Peak lateral pile displacements and bending moments are recorded and analyzed by uniform and triangular pressure distributions. On this basis, the observed levels of pile bending moment upon liquefaction suggest a hydrostatic lateral pressure approximately equal to that due to the total overburden stress. Using the experimental data, comparisons with current recommendations are made, and the Showa Bridge case history is briefly assessed.

Laminar Box System for 1-g Physical Modeling of Liquefaction and Lateral Spreading

Details of a large scale modular 1-g laminar box system capable of simulating seismic induced liquefaction and lateral spreading response of level or gently sloping loose deposits of up to 6 m depth are presented. The internal dimensions of the largest module are 5 m in length and 2.75 m in width. The system includes a two dimensional laminar box made of 24 laminates stacked on top of each other supported by ball bearings, a base shaker resting on a strong floor, two computer controlled high speed actuators mounted on a strong wall, a dense array advanced instrumentation, and a novel system for laboratory hydraulic placement of loose sand deposit, which mimics underwater deposition in a narrow density range. The stacks of laminates slide on each other using a low-friction high-load capacity ball bearing system placed between each laminate. It could also be reconfigured into two smaller modules that are 2.5 m wide, 2.75 m long, and up to 3 m high. The maximum shear strain achievable in this system is 15 %. A limited set of instrumentation data is presented to highlight the capabilities of this equipment system. The reliability of the dense array sensor data is illustrated using cross comparison of accelerations and displacements measured by different types of sensors.

Liquefaction Potential of Recent Fills versus Natural Sands Located in High-Seismicity Regions Using Shear-Wave Velocity

AbstractThe liquefaction potential of clean and silty sands is examined on the basis of the field measurement of the shear-wave velocity, Vs. The starting point is the database of 225 case histories supporting the Andrus-Stokoe Vs-based liquefaction chart for sands, silts, and gravels. Only clean and silty sands with nonplastic fines are considered, resulting in a reduced database of 110 case histories, which are plotted separately by type of deposit. A line of constant cyclic shear strain, γcl≈0.03%, is recommended for liquefaction evaluation of recent uncompacted clean and silty sand fills and earthquake magnitude, Mw=7.5. The geologically recent natural silty sand sites in the Imperial Valley of southern California have significantly higher liquefaction resistance as a result of preshaking caused by the high seismic activity in the valley. A line of constant cyclic shear strain, γcl≈0.1–0.2%, is recommended for practical use in the Imperial Valley. Additional research including revisiting available Vs-...

Micromechanical Aspects of Liquefaction-Induced Lateral Spreading

This paper reports the results of model-based simulations of 1-g shake table tests of level and sloping saturated granular soils subject to seismic excitations. The simulations utilize a transient fully coupled continuum-fluid discrete-particle model of water-saturated soils. The fluid (water) phase is idealized at a mesoscale using an averaged form of Navier-Stokes equations. The solid particles are modeled at the microscale as an assemblage of discrete spheres using the discrete element method (DEM). The interphase momentum transfer is accounted for using an established relationship. The employed model reproduced a number of response patterns observed in the 1-g experiments. In addition, the simulation results provided valuable information on the mechanics of liquefaction initiation and subsequent occurrence of lateral spreading in sloping ground. Specifically, the simulations captured sliding block failure instances at different depth locations. The DEM simulation also quantified the impact of void redistribution during shaking on the developed water pressure and lateral spreading. Near the surface, the particles dilated and produced an increase in volume, while the particles at deeper depth locations experienced a decrease in volume during shaking.

Liquefaction Resistance of a Silty Sand Deposit Subjected to Preshaking Followed by Extensive Liquefaction

AbstractThe effect of extensive liquefaction on the liquefaction resistance of heavily preshaken saturated silty sand is studied using a centrifuge experiment. The base of a 6-m homogeneous deposit was subjected to a total of 91 shaking events of different horizontal base accelerations and durations. Three event types were used in alternating patterns: mild preshaking Events A, stronger preshaking Events B, and strong liquefying Events C. The experiment was divided in two stages. In Stage One, reported in a previous publication, 66 preshaking Events A and B, were applied. In Stage Two, which is the focus of this paper, 25 additional shakings were applied which included two Events C in addition to continuing the same pattern of Events A and B. Stage One resulted in a significant increase in liquefaction resistance of the deposit. In Stage Two, extensive liquefaction produced by Events C resulted in a dramatic immediate reduction in liquefaction resistance of the deposit to a level comparable to that before...