Michael W. O'Neill

RESISTANCE FACTORS FOR SINGLE DRIVEN PILES FROM EXPERIMENTS

Present-generation resistance factors for foundations in load and resistance factor design (LRFD) do not necessarily reflect directly the variance in site-specific resistance and bias in making resistance estimates. The purpose of this paper is to describe a process whereby resistance factors can be determined from experimental data at a site and to demonstrate that process for driven piles at a specific site in overconsolidated clay. Eleven pipe piles were tested to failure in compression, and 28 cone penetrometer tests (CPTs) were performed near the piles. The CPT results were characterized through both a geostatistical method and a random sampling technique to estimate resistance factors for ultimate pile resistances using three CPT methods and two α-methods. A first-order, second-moment reliability method was applied using the interpreted bias characteristics of the design methods and the dispersion characteristics of the CPT and pile tests to relate resistance factors to selected reliability indexes. The resistance factors obtained for this site by this process were 0.50 to 0.62 for the three CPT methods and 0.30 to 0.55 for the ॅ-methods for a target reliability index of 3.5, depending on the value selected for the live load factor.

IMPROVEMENT OF THE GEOTECHNICAL AXIAL DESIGN METHODOLOGY FOR COLORADO'S DRILLED SHAFTS SOCKETED IN WEAK ROCKS

Drilled shaft foundations embedded in weak rock formations support a large percentage of bridges in Colorado. Since the 1960s, empirical methods that entirely deviate from the AASHTO design methods have been used for the axial geotechnical design of these shafts. The margin of safety and expected shaft settlement are unknown in these empirical methods. Load tests on drilled shafts provide the most accurate design and research data for improvement of the design methods. Four Osterberg axial load tests were performed in Denver on drilled shafts embedded in soil-like claystone, very hard sandy claystone, and extremely hard clayey sandstone. An extensive program of simple geotechnical tests was performed at the load test sites, including standard penetration tests (SPT), unconfined compressive strength tests (UCT), and pressuremeter tests (PMT). Information on the construction and materials of the test shafts was documented, followed by thorough analysis of all test results. Conservative equations were sugges...

Axial Performance of ACIP Piles in Texas Coastal Soils

Methods were evaluated for assessing axial compressive capacities of augered, cast-inplace (ACIP) piles in the Pleistocene terrace deposits of the Texas Gulf Coast and Recent to Modern alluvial soils. The study involved a combination of data base analyses and the performance of new loading tests on instrumented piles. The results in sand deposits mirrored earlier studies that indicated that common methods used for designing drilled shafts are also appropriate for ACIP piles, although the patterns of load transfer in the tested piles suggested zones of both lower load transfer (running sands below the water table) and higher load transfer (compacted surface soils) than would be expected in drilled shafts. The action of drilling and grouting resulted in net increases in lateral effective stresses in the soil around the tested piles, near the ground surface, where such measurements were made. In overconsolidated clays, slightly higher unit side resistances were inferred for ACIP piles than are predicted by common drilled shaft design methods. However, common drilled shaft design methods, notably the FHWA method, produced generally accurate capacities for mixed soil profiles.

EXPANSIVE CEMENT CONCRETE FOR DRILLED SHAFTS

This research is aimed at developing techniques for the use of highly expansive cement concrete in drilled shafts to produce a stronger bond between the shaft concrete and the surrounding soil, thus strengthening the system to carry a higher load. An expansive cement containing high-alumina cement (HAC) as the Al-bearing material was tested for expansion, strength, and setting characteristics. Although other properties were excellent, the cement showed unacceptably fast-setting behavior. To overcome the rapid slump loss of concrete using this HAC-type expansive cement, a two-stage mixing process with various admixtures is suggested. Although applicable in certain situations, this technique may not be suitable for general field application where quality control is lacking or where a delay in the expansion phase is required. An innovative solution for the problem is suggested in which the HAC is replaced with hydrated HAC (H-HAC) in the preparation of expansive cements. Concrete made with H-HAC expansive cement displayed the required properties before and after setting. This paper reports the properties of a select group of cement pastes and concretes made from HAC-type and H-HAC-type expansive cements that include slump loss, compressive strength, free and two-dimensionally restrained expansion, expansion pressure, and friction stress obtained from especially designed test methods. Some of the expansive concretes tested during this study had compressive strength in the range of 70 MPa (10,150 psi) and developed a self-stress in excess of 8 MPa (1,160 psi).

Phenomenological Study of Model Piles in Sand

A device is described whereby sand can be confined in a depthwise variable manner to permit simulation of lateral earth pressures to depths simulating driving depths of piles. Instrumented model piles were driven into a quartz sand placed at two different densities in the device, and load transfer patterns were measured for varying depths of penetration for compressive and uplift loading. In most respects the load transfer patterns were similar to those measured in full-scale tests in sand.

Structural Resistance Factors for Drilled Shafts Considering Construction Flaws

The design of any foundation should involve consideration of the structural ultimate limit state. In the case of drilled shafts, this usually involves the use of methods published in the various structural design codes for the design of super-structural beam-columns. However, drilled shafts are cast-in-situ elements that are subject to a much more uncertain construction history than an element in the superstructure. Voids, soil inclusions, offset and corroded reinforcing steel, and weakened concrete can exist in the finished product without the knowledge of the designer. Large voids and soil inclusions, occupying more than 15% of the cross-sectional area of the shaft, can usually be detected with state-of-the-practice nondestructive evaluation methods. In such case the shaft is repaired or replaced if the defect is in the critical location. However, smaller voids, corroded and offset steel, and slightly weak concrete cannot be detected reliability. The premise of this paper is that the effects of such flaws must be included in the design process. A series of experimental and analytical studies were performed to evaluate appropriate structural resistances for drilled shafts that have some probability of containing undetectable construction flaws. The tests included field tests on large-scale drilled shafts constructed with small flaws, structural laboratory tests on 0.305-m-diameter modes and on 0.76-m diameter specimens to evaluate scale effects. The laboratory and field tests were modeled using a calibrated section analysis model in order to extend the experimental results over a larger spectrum of flaws and flaw combinations than were feasible in the experimental study. A simple probability model was then used to evaluate the final structural resistance factors, based on the frequency of flaws observed at selected field sites, reported in the literature, and as represented via expert opinions. This paper summarizes the probabilistic aspects of the research and proposes structural resistance factors for use in evaluating axial and bending of drilled shafts based on simple modifications of existing code equations for structural beam columns. The recommended factors multiply the capacities computed from nominal structural design equations.

Corrosion of Reinforcing Steel in Drilled Shafts with Construction Flaws

The construction process for drilled shafts sometimes introduces structural flaws such as voids or soil inclusions within the shaft. In that case, the reinforcing steel loses concrete cover and comes in contact with the surrounding soil medium, which creates a favorable environment for corrosion. Corrosion of steel inside concrete, as well as in soil media, has been investigated in the literature. However, the specific case of steel passing through different media (concrete and soil) simultaneously has received little attention. The results of an experimental program designed to investigate the effect of two different media of substantially different pH values on the rate of corrosion of steel reinforcement in sand and clay soils and with different anode-to-cathode-area ratios are presented. Preliminary results suggest that galvanic currents driven by dissimilar media could increase the corrosion rate of exposed steel by 3.3 to 5.6 times, particularly in environments with otherwise low corrosion potential.

GEOPHYSICAL SURVEY TECHNIQUES TO DETERMINE LENGTHS OF PILES AND DRILLED SHAFTS

Three different types of seismic techniques were tested to determine the lengths of pile(s) and drilled shafts of bridge bents in overconsolidated clay soils in the Houston, Texas area. These methods were the seismic-wave reflection survey (SWRS), the transient forced vibration survey (TFVS) and the parallel seismic survey (PSS). They were conducted at bridge bends located along I-45, southeast of downtown of Houston, and on existing half-scale foundations at the University of Houston national Geotechnical Experimentation Site (NGES-UH). The purpose of the tests was to ascertain whether any or all of these methods would be reliable in determining the lengths of old or poorly documented foundations whose lengths are not known in overconsolidated clays of the type typically found in Houston, Texas. All three methods measured the depths to the toes of pile groups or individual drilled shafts to an accuracy acceptable for most bridge maintenance purposes, within approximately 0.3 m (1.0 ft) to 1.1 m (3.6 ft) of the actual pile lengths shown on the as-built plans. The most accurate prediction was achieved with the SWRS tests, while the least accurate prediction was achieved with the TFVS test.

SIMBASE: A Decision Support System for Economic Justification of Automated Construction Technology

Methodology for the economic justification of automated technology has already been developed in the manufacturing arena. This paper discusses the modifications required for application of this methodology to the construction industry. Modifications are necessary due to the industrys conservative nature in adapting new technology, the fact that the construction process resembles more a job shop production system rather than a flow shop (more difficult to automate), and due to the fact that the majority of construction contractors, relatively speaking, are small in size and capital poor. An economic justification methodology appropriate for the construction industry is presented with four modules: Cost and Performance Information Module (CPIM), Cost and Performance Evaluation Module (CPEM), Project Bids Estimation Module (PBEM), and Total Expected Profit Module (TEPM).

Residual Load Development in ACIP Piles in a Bridge Foundation

It has traditionally been assumed that when cast-in-situ piles, such as drilled shafts or ACIP piles, are constructed, the residual stresses after curing are zero. When conducting load tests on instrumented cast-in-situ piles, therefore, the readings in the strain gauges at the beginning of the load test are assumed to represent an unstressed condition. The implementation of ACIP piles in a bridge foundation in the Texas DOT system recently has allowed for the measurement of residual loads in several ACIP piles during curing. The authors discuss site conditions, pile installation and monitoring, instrumentation, grout mix design, load tests, grout strength and modulus tests, a procedure for converting strain to load, and residual loads and load transfer analysis. These residual loads were found to be relatively small, but not insignificant, and appear to have been related to site-specific soil profile characteristics. The residual loads were computed by using vibrating wire strain gauges to read strains along the piles at several times, both during and after casting, developing an approximation of time-dependent grout modulus, and computing stress and load increments from the incremental strains.