David Horhota

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.

Influence of Base Clearance on Subgrade Resilient Modulus of Florida Roadway Pavements

The resilient modulus of pavement subgrade materials is an essential parameter for mechanistically based flexible pavement design procedures. “Base clearance” is defined as the clearance between the ground-water level and the pavement base layer within a pavement system. A high pavement moisture content, which is strongly influenced by the base clearance, causes detrimental effects on the resilient modulus of pavement subgrades. The determination of a pavement base clearance is one of the most important steps toward setting up grade lines in a roadway design. This paper presents an experimental study to evaluate the effects of base clearance on the resilient modulus of pavement subgrades. Full-scale dynamic pavement tests were conducted in test pits to simulate vehicle dynamic impact on field pavements. The level of base clearance was adjusted by raising or lowering the water level within the pavement layer in the pit. Ten types of Florida subgrade material were tested at different base clearances for this study. The dynamic plate load test results were compared with the resilient modulus obtained from the laboratory triaxial test by using layer theory. The differences between the resilient modulus from the laboratory test and the plate load test were typically about 20%. Resilient modulus measured from the laboratory triaxial test could be used to predict the resilient deformation of the pavement subgrade layers. The experimental results showed that, at lower base clearances, the high pavement moisture content caused a significant reduction of the resilient modulus of pavement subgrade layers. The resilient modulus of subgrade materials decreases with the decrease of base clearance.

Instrumenting Pencel Pressuremeter Control Unit to Simplify Data Collection, Reduction, and Analysis

The Pencel pressuremeter control unit was instrumented to simplify digital recording, data reduction, and analysis of soil parameters. Digital signals from the instrumentation were collected through a commercially available data acquisition package, known as an automated pressuremeter (APMT), which produced reduced stress-strain data and allowed operators to determine the critical parameters. The instrumented system saved significant time. Data from numerous tests were evaluated for accuracy. Both clays and sands were included in the comparison. A digital pressure transducer was plumbed into the control unit, and a linear potentiometer was connected directly to the piston to produce digital volumes. The digital equipment improved the accuracy of the pressures more than ten-fold and eliminated gear backlash associated with the existing volume counter. The backlash affects the moduli more than the limit pressures, especially those associated with unload-reload loops of stiffer soils. Pressure-versus-time data for each injected volume increment were evaluated to determine when the pressures stabilized. APMT indicated that the pressure stabilized between 10 and 80 s for the sands and clays evaluated. The volume increment stabilization period was defined as the time required for the pressures to stabilize.

Measuring Rock Strength While Drilling Shafts Socketed into Florida Limestone

AbstractThe focus of this research was the real time assessment of rock strength during drilled shaft installations in Florida. Measures of unconfined compressive strength, a function of rock stren...

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...

Shear Wave Velocity Profiles of Roadway Substructures from Multichannel Analysis of Surface Waves and Waveform Tomography

Assessment of roadway subsidence caused by embedded low-velocity anomalies is critical to the health and safety of the traveling public. Surface-based seismic techniques are often used to assess roadways because of data acquisition convenience and large depths of characterization. To mitigate the negative impact of closing a traffic lane under traditional seismic testing, a new test system that uses a land streamer is presented. The main advantages of the system are the elimination of the need to couple the geophones to the roadway, the use of only one source at the end of the geophone array, and the movement of the whole test system along the roadway quickly. For demonstration, experimental data were collected on asphalt pavement overlying a backfilled sinkhole that was experiencing further subsidence. For the study, a 24-channel land streamer and a propelled energy generator to generate seismic energy were used. The test system was pulled by a pickup truck along the roadway and the data were collected with 81 shots at every 3 m for a road segment of 277.5 m, with a total data acquisition time of about 1 h. The measured seismic data set was analyzed by the standard multichannel analysis of surface waves (MASW) and advanced two-dimensional (2-D) waveform tomography methods. Eighty-one one-dimensional shear wave velocity (VS) profiles from the MASW were combined to obtain a single 2-D profile. The waveform tomography method was able to characterize subsurface structures at a high resolution (1.5- × 1.5-m cells) along the test length to a depth of 22.5 m. Very low S-wave velocity was obtained at the repaired sinkhole location. The 2-D VS profiles from the MASW and waveform tomography methods are consistent. Both methods were able to delineate high- and low-velocity soil layers and variable bedrock.

Dual Tip Penetrometer—A New In Situ Approach to Identify Cemented Sand

The Dual Tip Penetrometer (DTP), a novel in-situ test, was developed at the University of Florida (UF) to help identify cemented sands. Pairs of DTP and standard Cone Penetration (CPT) tests were performed at multiple Florida Department of Transportation (FDOT) project sites in Florida. These sites included the West Bay Bridge, and the Port Orange Relief Bridge. Cemented sand layers were present at different depths at these sites. Both DTP and CPT results were analyzed to screen cemented sand identifiers (Tip Resistance qc, Friction Ratio Rf, and the ratio between the second and first resistances T2/T1). The ratio T2/T1 was determined to be best identifier for identifying cemented sand. Based on the above test results and analyses, it appears that the DTP could be an efficient tool to identify cemented sand and better predict pile capacity.

Long-Term Evaluation of Geosynthetic Reinforcement of Flexible Pavements Constructed over Thick Organic Soil Deposits

Many regions throughout Florida have thick deposits of organic soil. Roadways built over these deposits often exhibit differential settlement, significant rut depths, and extensive cracks in a relatively short period of time. The Florida Department of Transportation constructed an experimental project on a realigned portion of State Road 15 on the southeastern shore of Lake Okeechobee in Palm Beach County to evaluate the effect of geosynthetic reinforcement on pavement performance. This roadway traverses farmlands with deep layers of organic material just beneath the surface and has a history of poor performance. The experimental project included four 500-ft sections with combinations of geogrids and geotextiles placed below the base and above the organic material. A fifth section of similar length was constructed with no geosynthetic reinforcement and served as a control. In addition to geosynthetic reinforcement, the alignment was surcharged before construction. The investigation showed that surcharging alone significantly improved the pavement performance compared with historical pavement condition and rehabilitation records. Geosynthetic reinforcement doubled the equivalent single-axle loads allowed on the unreinforced section. A rigid geogrid or woven geotextile placed below the base appeared to provide a slightly stiffer and better-performing pavement than a flexible geogrid. This paper documents the research program and provides details on the improvement resulting from surcharging and geosynthetic reinforcement.

Seismic Waveform Tomography at Test Site with Open Chimneys

An application of two-dimensional time-domain waveform tomography to map the extent of open chimneys at a variable karstic limestone site is presented. The seismic surface wave fields were measured next to three open chimneys and inverted with a full-waveform inversion technique, which was based on a finite-difference solution of two-dimensional elastic wave equations and the Gauss–Newton inversion method. Both the compression wave (P-wave) and shear wave (S-wave) velocities were inverted independently and simultaneously to increase the credibility of the characterized profiles. The waveform analysis successfully profiled embedded low-velocity zones and highly laterally and vertically variable limestone. The inverted results are consistent with the known open chimneys observed from the ground surface. The Poissons ratio determined from the inverted P-wave and S-wave velocities is consistent with soil types: high values of 0.3 to 0.5 for silt and clay and low values of 0.1 to 0.2 for limestone. Results show that the full-waveform inversion technique is applicable to both shallow and deep foundation design in which soil stratigraphy and variability are important. The technique is also computationally practical, because the results were all achieved in about 3 h of computer time on a standard laptop computer.

Detection of Embedded Anomalies Using 2-D Full Seismic Waveform Tomography

The paper presents an application of 2-D time-domain waveform tomography for detection of embedded voids/anomalies. The measured seismic surface wave fields were inverted using a full waveform inversion (FWI) technique, based on a finite-difference solution of 2-D elastic wave equations and Gauss-Newton inversion method. The key advantage of this approach is the ability to generate all possible wave propagation modes of seismic wavefields (body waves and Rayleigh waves) that are then compared with measured data to infer complex subsurface properties. Both the pressure wave (P-wave) and shear wave (S-wave) velocities are inverted independently and simultaneously for possible indication of soil types. The inversion results of real data set show that the waveform analysis was able to delineate a complex profile with an embedded void and highly variable bedrock both laterally and vertically. Independent invasive test (standard penetration test, SPT) was also conducted to verify the seismic test results. INTRODUCTION Sinkhole detection in a site usually begins with non-destructive testing (NDT) assessment study, as NDT data can provide general subsurface conditions over large volume of materials. At suspicious locations (anomalies), more involved invasive methods such as Cone Penetration Test (CPT) or Standard Penetration Test (SPT) are then conducted to obtain more detailed information if needed. Since the 1980s, various approaches have been developed and employed to characterize sinkholes, ranging from gravity, resistivity, Ground Penetrating Radar (GPR) and traditional seismic wave methods (FHWA, Technical Manual 2003). These existing methods 2382 Geo-Congress 2014 Technical Papers, GSP 234