Michael McVay

LOAD AND RESISTANCE FACTOR DESIGN (LRFD) FOR DRIVEN PILES USING DYNAMIC METHODS-A FLORIDA PERSPECTIVE

The parameters for load and resistance factor design (LRFD) of driven piles using dynamic methods are presented based on a database of 218 pile cases in Florida. Eight dynamic methods were studied: ENR, modified ENR, FDOT, and Gates driving formulas, Case Analysis with Wave Analysis Program (CAPWAP), Case Method for Pile Driving Analyzer (PDA), Paikowskys energy method, and Sakais energy method. It was demonstrated that the modern methods based on wave mechanics, such as CAPWAP, PDA, and Paikowskys energy methods, are roughly twice as cost effective to reach the target reliability indices of 2.0 to 2.5 (failure probability = 0.62 to 2.5%) as the ENR and modified ENR driving formulas. The Gates formula, when used separately on piles with Davisson capacities smaller or larger than 1779 kN, has an accuracy comparable to the modern methods. The utilizable measured Davisson capacity, defined as φ/λ (ratio of resistance/mean capacity) obtained from testing at beginning of redrive (BOR), is only slightly larger than the end of drive (EOD) values. Furthermore, past practice with driving formulas reveals the existence of a large redundancy in pile groups against failure. The latter suggests the use of a lower relatively reliability target index, βT = 2.0 (pf = 2.5%) for single pile design. Also, the utilizable measured Davisson capacity, φ/λ, for all the dynamic methods studied, is quite similar to published values (Lai et al. 1995; Sidi 1985) for static estimates from in situ tests.

MEASUREMENT AND PREDICTION OF PORE PRESSURES IN SATURATED CEMENT MORTAR SUBJECTED TO RADIANT HEATING

Evaluation of airfield pavement degradation and fire safety evaluation of concrete structures are examples of situations that involve moist porous media (concrete) subjected to severe thermal loadings. When a saturated (or partially saturated) porous medium is subjected to a high temperature heating source, pore pressures large enough to initiate explosive spalling can be developed within the pore spaces of the material. The level to which these pore pressures ultimately rise depends on the saturation and permeability of the medium as well as the rate at which heat flows into the material. In this paper, experimental and numerical studies involving the measurement and prediction of pore pressures in porous media are presented. Pore pressure data are presented for experimental tests in which saturated cement mortar specimens were subjected to high temperature radiant heating conditions. A numerical modeling technique is then presented and is used to numerically simulate the experimental work. Close agreement is shown between the pore pressures and temperatures recorded experimentally and those predicted through simulation.

Centrifuge Testing of Fixed-Head Laterally Loaded Battered and Plumb Pile Groups in Sand

Centrifuge tests were conducted on driven in-flight fixed-head plumb and battered 3 × 3 pile groups at three-diameter (3D) and five-diameter (5D) spacings. The piles simulated 430-mm-diameter by 13-m-long pipe piles founded in medium loose (Dr = 33%) and medium dense (Dr = 55%) sands. The battered groups were in an A frame arrangement, i.e., each pile in a given row battered the same. Results of the tests showed that fixed-head plumb groups have a 30 to 55% higher lateral resistance than free-headed piles depending on soil density and pile spacing. For the battered 3D-spaced group, the lateral resistance was greater by 20 to 50% than the fixed-head plumb response in the medium dense sand; however, in the medium loose sand, the battered and plumb fixed-head group response was very similar. The latter is attributed to the limited axial tension capacity of the piles in the medium loose sand (Dr = 33%). Increasing the dead load on a battered group to 45% of its axial capacity resulted in a 30% increase in the groups lateral resistance. The lateral resistance of the 5D-spaced battered groups were in all cases greater than their 3D counterparts.

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.

Centrifuge Modeling of Laterally Loaded Pile Groups in Sands

Presented is a novel apparatus to drive and laterally load groups of six to nine piles in flight. The main component of the device is a flat plate with multiple apertures which are opened or closed through pneumatic solenoids while being raised or lowered with stepping motors via a PC. Presently, both lateral load versus deformation and load distribution within the rows can be measured. Testing on both loose and medium dense sands at three and five-pile-diameter spacings have been completed for free-headed groups of nine piles. It was found that as the soils density increased, so did the groups lateral resistance as well as the variation from row to row. The five-pile-diameter groups carried larger lateral loads and resulted in a more even load distribution from row to row than its three-diameter counterpart. Work is underway to test larger groups as well as inclined piles.

LOAD AND RESISTANCE FACTOR DESIGN, COST, AND RISK: DESIGNING A DRILLED-SHAFT LOAD TEST PROGRAM IN FLORIDA LIMESTONE

Currently there are few if any guidelines on estimating the number of load tests in the design of drilled-shaft foundations in Florida limestone. For instance, for many sites there may be a similar number of field load tests but a significantly different number of design shafts. Moreover, little if any information exists on risk or reliability versus cost of drilled-shaft foundations or on the cost of field load testing. The collection of a large database of drilled-shaft tests (more than 25 with Osterberg and Statnamic devices), in situ laboratory data, drilled-shaft construction costs, and field load testing costs for Florida limestone is reported on. From the field load tests, the average unit skin friction for various sites was found, as well as the predicted values based on the Florida Department of Transportation recommended design approach. Next, using load and resistance factor design (LRFD), the resistance (ϕ) values were found for various reliabilities (risk or probability of failure). Once the factored design loads were known (from plans), drilled-shaft lengths were estimated on the basis of the computed LRFD ϕ-values for different reliabilities (i.e., risk). From the linear length of the designed shaft as well as the expected cost per meter, a plot of total foundation cost versus reliability (risk) was generated for each site. On the basis of the latter plot, acceptable risk, and the cost of field load testing (bid and itemized), the designer can identify the cost savings of load testing and the appropriate number of tests to be performed.

Flow of Nitrogen and Superheated Steam Through Cement Mortar

This work concerns quantifying the error of Darcys expression over Klinkenbergs equation for gas flow in cement under elevated temperatures.

Dynamic Finite Element Analysis of Vessel-Pier-Soil Interaction During Barge Impact Events

Current specifications for highway bridge design provide empirical relationships for computing lateral impact loads generated during barge collisions; however, these relationships are based on limited experimental data. To better understand and characterize such loads, dynamic finite element analysis techniques were employed to simulate vessel impact conditions never before tested experimentally. Descriptions of the methods used to model a hopper barge, bridge piers, and soil–structure interaction are given. Impact simulation results, including time histories of impact loads and barge deformations, are presented and compared with data generated using current bridge design specifications.

The Florida Pier Analysis Program Methods and Models for Pier Analysis and Design

The Florida Pier Analysis Program (FLPIER) was developed by the University of Florida Department of Civil Engineering in conjunction with the Florida Department of Transportation (FDOT) Structures Division. The program was developed in order to give pier designers a comprehensive model development and analysis tool to optimize pier designs. The current version is a nonlinear, static, soil-structure interaction suite of programs that run on a personal computer and include group pile effects, layered soil, pier columns and cap, high mast lighting, sound, and retaining walls. The program was designed to allow input to be specified graphically using “designer variables” such as spacing, offsets, number of columns, and so forth. Its use has reduced the time for model development and analysis from days to under an hour. The numerical modeling techniques used have been compared with experimental data and give highly accurate results leading to an improved overall design and reduced costs.

CALIBRATION OF LOAD AND RESISTANCE FACTOR DESIGN: RESISTANCE FACTORS FOR DRILLED SHAFT DESIGN

The AASHTO load and resistance factor design specification was approved for use in 1994 with phi factors determined from fitting allowable stress design (ASD). Unfortunately, the latter did not provide resistance factors for shafts founded in sand, gravel, and rock or identify the influence of construction (dry, wet, and cased). Using a database of 273 shafts, of which 185 failed (settlement equal to 5% of the diameter) and have sufficient soil information, the resistance factors were determined for a number of different AASHTO design methods based on probability of failure. Effects of skin friction, combined skin and end bearing, and method of construction were investigated for different soils/rock. It was found that all the methods had resistance factors between 0.3 and 0.6, which correspond to a factor of safety of 2.5 to 4.5 in ASD. Current FHWA design methods gave the highest resistance factors in soils with the cased method of construction. In the case of rock, the dry hole method of construction and the intermediate geomaterials design method gave the highest resistance factors.