Anne Lemnitzer

Nonlinear Efficiency of Bored Pile Group under Lateral Loading

A 3×3 bored pile group consisting of nine cast-in-drilled-hole reinforced concrete shafts and a comparable single-shaft were subjected to reversed cyclic, lateral head loading to investigate group interaction effects across a wide range of lateral displacements. The piles had the same diameter of d=0.61 m and similar soil conditions; however, various equipment constraints led to two differences: (1) a fixed head (zero rotation) boundary condition for the single pile versus minor pile cap rotation in the vertical plane for the group and (2) shaft longitudinal reinforcement ratios of 1.8% for the single pile and 1% for the group piles. To enable comparisons between the test results, a calibrated model of the single pile (1.8% reinforcement) was developed and used to simulate the response of a single shaft with 1% reinforcement. Additional simulations of the pile group were performed to evaluate the effects of cap rotation on group response. By comparing the simulated responses for common conditions, i.e., 1...

Nonlinear Load-Deflection Behavior of Reinforced Concrete Drilled Piles in Stiff Clay

AbstractThe results of three full-scale lateral load tests of cast-in-drilled-hole RC piles are evaluated to provide insights into the soil-pile interaction behavior as represented by p-y curves (p = soil reaction/length; y = lateral pile deflection) spanning from elastic conditions to mobilization of the soil capacity. The test sequence enables evaluation of the diameter effects on the p-y behavior as a result of testing two otherwise similar specimens, but with different diameters (0.6 and 1.8 m), and evaluation of the head-fixity effects as a result of testing similar specimens (with a 0.6 m diameter) under flagpole and fixed-head conditions. A constrained exhaustive search procedure that facilitates data interpretation even in the presence of severe shaft nonlinearity is introduced, which was needed because the testing was performed to full structural failure conditions. The p-y curves obtained with the present test data differ from the predictions of a standard (American Petroleum Institute) model, i...

Settlement Estimations of Peat during Centrifuge Experiments

The Sacramento-San Joaquin Delta hosts approximately 1800 km of man-made levees most of which rest atop peaty soils. The cyclic behavior of the levees and the settlement potential of the soft underlying peat are studied via large scale 9m radius centrifuge testing at the NEES@UCDavis equipment site. Given the high compressibility of peat, it is crucial to properly estimate the expected settlements during model construction and spin up due to the increased effective stresses. The consolidation behavior is nonlinear at low effective stress, and traditional calculations assuming a linear relation between void ratio and logarithm of effective stress do not provide reasonable predictions. Preliminary small scale 1m radius centrifuge experiments helped to evaluate the variation of the consolidation index Cc during testing and to iterate a suitable pre-consolidation procedure. Shear strength for normally consolidated peat was also identified as critical parameter, and a simple procedure to calculate a factor of safety against bearing capacity failure during spin- up is proposed. Settlement predictions using the 1-D theory along with models from literature and analysis results from SETTLE3D were plotted against the measured settlements observed during the 9m radius centrifuge study. SETTLE3D analysis, calibrated with Cc parameters measured during the various g-levels from the small scale centrifuge tests, provided the most reliable results and matched the test data very well. Models found in literature under predicted the settlements at low confining pressures but showed similar consolidation indices at larger stress levels.

Centrifuge Testing of Model Levees atop Peat: Experimental Data

Four large-scale centrifuge tests were performed at the NEES@UCDavis equipment site to study the cyclic behavior of levee structures resting atop soft organic peat. The model configurations using a non-liquefiable levee focused on the seismic deformation potential of peat during primary consolidation and secondary compression. The tests performed with a sandy levee studied the liquefaction potential of saturated loose sand fill overlying soft peat as well as the levee-peat-interaction under different loading conditions. The models were subjected to scaled ground motions representative of the Sacramento/San Joaquin Delta. System instrumentation consisted of linear potentiometers, pore pressure sensors and accelerometers. Slow data recorded at 1 Hz document the settlements during spin up, application of ground motions, and spin down. Fast data sampled at 4,167 Hz measured the dynamic response of the system, the excess pore pressure increase and immediate settlements. The project is archived at the NEES data repository under nees.org/warehouse/project/1161.

Vacuum Pluviation Device for Achieving Saturated Sand

Saturation of sand specimens during experimental investigations is important to correctly reproduce undrained shearing behavior, including liquefaction. Sand below the water table is often well saturated in situ because any gases trapped during deposition or compaction have had adequate time to dissolve or migrate through the sand. Reproducing this condition on a short time scale in the laboratory often requires use of backpressure or vacuum saturation. However, backpressure and vacuum saturation sometimes cannot be utilized, for example, in centrifuge models containing soils sensitive to the effects of vacuum. This paper focuses on development and validation of a water pluviation device to construct saturated sand levees during a centrifuge testing program for which backpressure and vacuum methods could not be utilized. P-wave velocity, Vp, measurements using an ultrasound system verified the degree of saturation achieved in the fill. Correlations between Vp and B values are discussed. The vacuum saturation system is shown to provide a high degree of saturation (Vp > 1500 m/s), whereas more traditional water pluviation techniques are shown to produce unsaturated fill.

The Influence of RC Nonlinearity on p-y Curves for CIDH Bridge Piers

Abstract The p-y method is one of the most popular methods in pile design and has been calibrated for various boundary conditions using numerical and experimental studies during recent years. Most studies on reinforced concrete (RC) piles have included the impact of flexural nonlinearity, (e.g. nonlinear moment–curvature relations) but not considered associated pile shear deformations when deriving p-y curves from field data. Common p-y curves may be better applicable for piles with flexure dominated failures (e.g. piles with free- head boundary conditions). For piles with fixed head boundaries (i.e. rotation restrained piles) shear deformations could be of significant influence. To study this problem, a coupled shear flexure interaction model for axial-bending-shear behavior coded in OpenSees was applied to a 0.61 m (2 ft) diameter flagpole and a 0.61m (2 ft) diameter fixed head pile specimen to investigate the possible influence of shear deformations to the overall pile responses. The surrounding soil was represented by p-y curves derived from prior large scale testing on piles with similar boundary conditions. Analysis results show that for flagpole piles, shear forces and shear deformations were insignificant. Considerable contributions of pile shear displacements and forces were observed for the fixed head pile, with shear displacements contributing up to 40% of the total pile displacement. Results suggest that nonlinear shear deformations for reinforced concrete piles should be considered for fixedhead or similar conditions, and that currently used p-y curves may underestimate the actual lateral pile displacement and possibly lead to unsafe design for the particular boundary condition.

Large Diameter Soil Pressure Sensors Employed in Dynamic Shallow Foundation Testing

Measurements of distribution and magnitude of static and dynamic earth pressures resulting from self-weight and applied loading is essential to the design, behavior, and performance of many civil engineering-type structures involving soil-structure interaction. Advancements in testing and instrumentation have allowed researchers to improve upon the original, now classic analytical models to predict earth pressures acting on retaining walls, shallow foundations, and various deep foundation elements. Ideally, soil pressure instrumentation should be tailored towards the respective engineering application and comply with the stiffness of the structural system. In addition, it is desirable to provide sufficient sensing surface area to avoid pressure localization, and install the pressure surface flush with its surroundings to minimize arching effects. A new and simple load cell based pressure sensor with a diameter of 10 cm and a capacity of 144 kPa was developed, built, and deployed in a large-scale experimental investigation. Upon describing details pertaining to the design, fabrication, and calibration of the new sensor, results from the large-scale experimental investigation on dynamic shallow foundation performance is presented.

Editorial Note – Issue 2-3 (2017) – Award Issue

We are pleased to publish this special award issue featuring a set of outstanding manuscripts submitted to the DFI 2017 Young Professor and Student Paper competitions. Since 1985, DFI and the DFI Educational Trust have held an annual Young Professor Paper Competition as a means to help bridge the gap between practice and study. This year the range of submission covered a broad technical spectrum, ranging from numerical modeling to large scale experimental testing of axially and laterally loaded pile foundations, statistical analyses and uncertainty predictions using big data analysis methodologies, state-of-the-art evaluation of regional, national and international design recommendations, laboratory and model scale studies on soil behavior, ground improvement and installation effects during foundation construction, as well as QA/QC analysis pertaining to drilled shaft installation and long term shaft integrity. The winning papers as well as a set of manuscripts that received a special recognition are published in this issue. The editors would like to thank the competition reviewers for their constructive feedback to all manuscripts and the substantial amount of time spent on more than 15 paper submissions. We are so grateful for the successful competition and sincerely appreciate your service to DFI and your encouragement of our rising and promising young professionals. Mobley and Costello, a graduate student co-author team from the University of South Florida won the 2017 student paper competition for their research work on “The Effect of Slurry Type on Drilled Shaft Cover Quality”. Sarah Mobley and Kelly Costello, both PhD candidates in the Civil and Environmental Engineering Department at USF, study and research under the supervision of Prof. Gray Mullins. The research team examined 24 tremie-placed laboratory drilled shaft specimens, constructed using bentonite, polymer or natural slurry to identify correlations between slurry type and laitance channel formation. The authors received their award during DFI’s annual conference in NewOrleans and delivered a stellar presentation during the conference’s technical sessions. Mobley and Costello’s work is a critical contribution to increasing the long term resilience of drilled shaft foundations by identifying and quantifying the effects of shaft surface degradation triggered by the use of different slurry types. Closely followed, with extremely high review scores are two runner-up researchers in the student paper category: Van Wijngaarden received a runner-up award for his work on the “Modelling of Pore Pressure Developments below Cyclically Loaded Offshore Gravity Foundations”, a study that investigates the effects pore pressure increase and dissipation on the stability of the foundation system. Martijn van Wijngaarden recently graduated from the Delft University of Technology in the Netherlands and currently works at Volker Staal en Funderingen in Rotterdam. In his paper, he outlines an approach to assess the time dependent pore pressure development below a marine foundation by establishing load spectra due to wind, waves and turbine operations as well as the dynamic amplification of the loading. Van Wijngaarden concluded that a reliable prediction of excess pore pressures can only be accomplished by utilizing a large set of generated time series, as the irregular nature of cyclic loading results in significant spread in maximum pressures below a foundation. Ostrowsky won a runner-up paper award for her manuscript presenting “A New Approach for Evaluating the Ductility, Volumetric Stiffness and Permeability of Cutoff Wall Backfill Materials”. Jennifer Ostrowsky is a PhD student at Utah State University working with her graduate advisor Prof. John Rice. The research team developed a laboratory testing procedure to quantify the ductility of soilcement and plastic concrete relative to changes in permeability (hydraulic conductivity) with strain. The group performed extensive testing on soil-cement specimens with various cement and bentonite contents. The test results showed that the proposed method is effective in illustrating and quantifying the differences in 2017 DFI Board of Trustees

Centrifuge Testing of Levees: Saturation Techniques during Model Construction

Levees in the Sacramento-San Joaquin Delta, located in Northern California, were constructed by placing soils dredged from adjacent channels in an unengineered manner atop highly compressible organic soils (i.e. peat). Two 9 m radius centrifuge experiments were performed at the NEES@UCDavis facility with the overall objective to evaluate (1) the seismic deformation potential of peat, (2) levee-peat interaction (analogous to soil-structure interaction), and (3) the liquefaction potential and seismic deformations of saturated loose sand fill overlying soft peat. This paper focuses on development and validation of a water pluviation device constructed to form saturated levee fill. P-wave velocity measurements using an ultrasound system were utilized to verify the degree of saturation achieved in the levee material by various construction methods. Correlations between P wave velocities and B-values are discussed. The vacuum saturation system is shown to provide a high degree of saturation, whereas more traditional water pluviation techniques are shown to produce unsaturated fill.

Experimental Assessment of the Passive Resistance of a Bridge Abutment System with various Backfill Heights

Two full scale lateral load tests were performed on abutment backwall systems 4.57 m wide, 2.8m tall, and 0.91m thick using silty sand backfill with 1.68m and 2.4m heights. The backfill material type and compaction (median of 96% modified Proctor relative compaction) satisfied California bridge design standards. The loading system operates under displacement control and provides horizontal wall displacement with minimal vertical displacement. Wingwalls are furnished with PVC sheets to minimize side friction, providing for 2D plane strain conditions during the experiments. Testing was performed under quasi-static lateral loading up to displacements equal to 10% of the wall height, with unloading and reloading at several displacement levels. In the 1.68 m and 2.4 m tests, peak resistances occurred at horizontal displacements of 0.03H and 0.06H and produced peak passive earth pressure coefficients of 16 and 24, respectively. Following testing, trenching of the backfill material revealed log-spiral shaped principal failure surfaces in both tests along with relatively minor shear surfaces at shallower depths. Results from various analytical models are compared with the experimental results.