Peter Lai

Influence of Spatially Variable Side Friction on Single Drilled Shaft Resistance and LRFD Resistance Factors

Load and resistance factor design (LRFD) is a method that aims at meeting specified target reliabilities (probabilities of failure) of engineered systems. The present work focuses on ultimate side friction resistance for axial loads on single cylindrical drilled shaft foundations in the presence of spatially variable rock/soil strength. Core sample data are assumed to provide reliable information about local strength in terms of mean, coefficient of variation and spatial correlation structure (variogram) at a site. The geostatistical principle of support up-scaling is applied to quantify the reduction in variability between local strength and the average ultimate shaft side friction resistance without having to recur to lengthy stochastic finite difference/element simulations. Site and shaft specific LRFD resistance factors (Φ values) are given based on the assumption of lognormal load and resistance distributions and existing formulas recommended by the Federal Highway Administration. Results are efficiently represented in dimensionless charts for a wide range of target reliabilities, shaft dimensions, and geostatistical parameters including nested variograms of different types with geometric and/or zonal anisotropies. Field data of local rock strength is used to demonstrate the method and to evaluate the sensitivity of obtained resistance factors to potentially uncertain variogram parameters.

Measured and Predicted Response of a New Jetted and Grouted Precast Pile with Membranes in Cohesionless Soils

AbstractWith increased urbanization, deep foundation (bridges, signage, walls, etc.) selection is moving toward the minimization of disturbance and installation time, as well as addressing quality control and assurance issues. Unfortunately, many types of deep foundations involve noise and vibration during installation (e.g., driven piles) or integrity and reduced resistance issues (e.g., drilled shafts, both conventional and post grouted tip, continuous flight auger piles). This paper presents a new foundation type, a jetted and grouted precast pile, which uses the advantages of several proven deep foundation installation techniques. The installation of the new pile is comprised of three distinct phases: (1) pressurized water-jetting of a precast pile into the ground; (2) side grouting of the pile; and (3) tip grouting. The pile has two separate side grouting zones, each with its own grout delivery system. Each grout zone is covered with a semirigid membrane, which results in radial expansion of the soil...

Driven Concrete Pile Foundation Monitoring with Embedded Data Collector System

Florida Department of Transportation (FDOT) sponsored research performed at the University of Florida resulted in developing a wireless monitoring and real time static capacity estimate technology for driven piles. This new technology, Embedded Data Collector (EDC), uses two levels of instrumentation, embedded in the body of precast prestressed concrete piles near the head and tip. Strain and acceleration measurements obtained at these instrumentation levels during driving are sent wirelessly to a receiver in the field, and analyzed in real time to provide the operator with estimates of static capacity, stresses in the pile, transfer energy, damping factor, stroke height, and other relevant parameters used to evaluate the pile driving process and the driving system. The EDC system is currently undergoing phase one of a two phase field evaluation program planned by the FDOT to determine the level of reliability that can be anticipated by its use. The first phase consists of monitoring piles with EDC instrumentation and concurrently monitoring them with the Pile Driving Analyzer (PDA), given that there is ample data supporting the reliability of PDA. Measurements of strain and particle acceleration converted to force and velocity traces can then be compared between the two systems, along with the corresponding calculated magnitude of downward and upward traveling stress waves as they move along the pile at any point in time. Selected hammer blows recorded by PDA equipment are analyzed by means of signal matching software (CAPWAP) and the estimated static skin, end bearing, and total resistance obtained are compared against EDC static resistance predictions. The second phase of the evaluation will compare EDC estimates of static capacity against instrumented static load test results. The purpose of this paper, is only to present a summary of results obtained thus far in phase one, and compares EDC estimates of static capacity results with those from PDA and CAPWAP.

Experimental and Numerical Study of Laterally Loaded Pile Groups with Pile Caps at Variable Elevations

A series of lateral load tests were performed on 3×3 and 4×4 pile groups in loose and medium-dense sands in the centrifuge with their caps located at variable heights to the ground surface. Four cases were considered: Case 1, pile caps located above the ground surface; Case 2, bottom of pile cap in contact with the ground surface; Case 3, top of pile cap at the ground surface elevation; and Case 4, top of pile cap buried one cap thickness below ground surface. All tests with the exception of Case 1 of the 4×4 group had their pile tips located at the same elevation. A special device, which was capable of both driving the piles and raining sand on the group in flight, had to be constructed to perform the tests without stopping the centrifuge (spinning at 45 g). The tests revealed that lowering the pile cap elevation increased the lateral resistance of the pile group anywhere from 50 to 250 percent. The experimental results were subsequently modeled with the bridge foundation-superstructure finite element program FLPIER, which did a good job of predicting all the cases for different load levels without the need for soil–pile cap interaction springs (i.e., p-y springs attached to the cap). The analyses suggest that the increase in lateral resistance with lower cap elevations may be due to the lower center of rotation of the pile group. However, it should be noted that this study was for pile caps embedded in loose sand and not dense sands or at significant depths. The experiments also revealed a slight effect for the case of the pile cap embedded in sand with a footprint wider than the pile row. In that case the size of the passive soil wedge in front of the pile group, and consequently the group’s lateral resistance, increased.

CENTRIFUGE TESTING OF PLUMB AND BATTERED PILE GROUPS IN SAND

Pile groups are generally used under structures subject to heavy axial loads or large lateral forces with or without scour. The focus in this paper is only on pile groups subject to large lateral forces. Currently, little, if any, full-scale lateral load data exist on pile groups that vary pile head fixity or batter. Reported here is the summary of a series of centrifuge tests on free- and fixed-head plumb and battered pile groups. Influence of pile head constraint, pile spacing, soil density, and vertical dead load is reported for groups ranging from 3 × 3 to 3 × 7 in size. Results reveal a significant lateral resistance of fixed- over free-head pile groups; fixed-head piles develop significant axial forces; battered piles without vertical dead loads are generally no better than plumb piles; and in the case of plumb piles, the use of multipliers to represent group interaction is valid.

Experimental Group Behavior of Grouted Deep Foundations

Post-grouting of deep foundations following installation is a proven technique for enhancing axial resistance. The grouting is performed either below the tip of the foundation only (e.g., post-tip-grouted drilled shafts) or at both the side and the tip (e.g., jetted, side, and tip-grouted precast piles). The interaction of such foundations in group placement is currently unknown. This research focused on the group behavior of post-tip-grouted drilled shafts and jetted, side, and tip-grouted piles at a center-to-center spacing of three times the pile/shaft diameter. The study revealed that the post-tip-grouted drilled shafts acted independently within the group (i.e., negligible group interaction), whereas jetted, side, and tip-grouted piles behaved as a block under axial loading. It was determined that side grouting of a foundation prior to tip grouting significantly increases the grout pressure developed during tip grouting and helps in the formation of a tip grout bulb via a spherical cavity expansion process. Thus, the side and tip grouting of adjacent foundations within a group increases the confining stress and relative density of the soil mass within the group, resulting in block behavior under top-down loading. In contrast, tip-only grouted foundations showed little if any increase in radial stress and radial soil displacement, resulting in minimal improvement of the soil stiffness between shafts; as a result there was no block behavior and a negligible group effect at the tip.

An Innovative Prefabricated Pile Installation Method Utilizing Jetting and Pressure Grouting

The majority of Florida Department of Transportation (FDOT) highway structures (e.g., bridges, signage, etc.) are founded on deep foundations, i.e., driven piles and drilled shafts. In the past, the most commonly used foundation type was driven piles. With increased urbanization, the noise and vibrations associated with pile driving has resulted in more drilled shaft usage due to its less intrusive characteristics. Unfortunately, due to horizontal soil stress relief during installation, drilled shafts suffer reduced axial and torsional resistance when compared to similar sized driven piles. Additionally, as-built drilled shaft integrity is generally more questionable than precast piles. To address these concerns, FDOT investigated post grouted drilled shaft tips, which not only increases the end bearing, but also proof tests every shaft. However, the issues of quality control (i.e., the structural integrity of the cast-in-place shaft) and the reduction in side friction due to horizontal stress relaxation still remained. Thus, the FDOT funded a research project to develop an innovative new deep foundation type which ensures both the quality control of a precast member with the lateral stress increase similar to a driven pile but with the minimum intrusion of a drilled shaft. It is referred to as a jet-grouted precast pile or jet-grout pile. The research was conducted in FDOTs new cylindrical 3.66 by 10.67 meter in-ground geotechnical test chamber, which is capable of monitoring soil stresses and deformations. The first trials involved jetting precast concrete piles with multiple sets of embedded grout delivery pipes. The early tests revealed higher horizontal versus vertical distribution of the compaction grout, as well as poor bonding between the grout and pile. Subsequently, the grout delivery system was re-designed; a new colloidal grout mix was developed along with the introduction of grout membranes on the outside of the pile. Small scale testing showed complete envelopment of the pile with excellent grout bond to the pile. Finally, a full scale (0.406m x 0.406m x 6 m) pile was jetted and subsequently grouted along its length in two stages with no tip grouting. The pile was then torque tested to 610 kN-m with 15° of rotation, resulting in a unit torsional side resistance of 77 kPa. Next, the tip of the pile was grouted and a top down compression test yielded 1,335 kN of resistance at 0.25 mm of vertical pile head movement that exceeded the resistance of a traditional similar size driven pile.

Influence of Spatially Variable Side Friction and Collocated Data on Single and Multiple Shaft Resistances

AbstractReliability-based design, such as LRFD, aims at meeting desired probability of failure levels for engineered structures. The present work attempts to contribute to this field by analyzing the influence of spatially variable soil/rock strength on the axial resistance uncertainty of single and multiple shafts in group layouts. This includes spatial variability over the individual shaft surfaces, effects of limited data, random measurement errors, and workmanship. A possible correlation between boring data inside or near the footprint of a foundation and the foundation itself is considered. In a geostatistical approach, spatial averaging (upscaling) and a degenerate case of ordinary kriging are applied to develop variance reduction charts and design equations for a series of foundation group layouts (single, double, triple, and quadruple). For the potential situation of an unknown horizontal correlation range at a site, the worst case scenarios are identified and demonstrated in an example problem. R...

A Practical LRFD Design Method for Deep Foundations Using Side Friction and End Bearing

One of the uncertainty components in Load and Resistance Factor Design (LRFD) stems from spatial variability in design parameters, such as local soil / rock strength or modulus. A solution is presented for uncertainty propagation of effective rock mass modulus through O’Neill’s (non-linear) equations for drilled shaft settlement in intermediate geomaterials. Associated tip resistance is combined with side friction resistance (fully mobilized) of one or more geological layers to form total ultimate shaft resistance with respective uncertainty. Based on this, a graphical iteration chart (existing for side friction in a single layer without end bearing) is generalized to find shaft length as a function of given reliability (probability of failure), factored design load and site conditions. Results are demonstrated by a practical example.