Kam Ng

Pile Setup in Cohesive Soil. I: Experimental Investigation

AbstractPile setup in cohesive soils has been a known phenomenon for several decades. However, a systematic field investigation to provide the needed data to develop analytical procedures and integrate pile setup into the design method rarely exists. This paper summarizes a recently completed field investigation on five fully instrumented steel H-piles embedded in cohesive soils, while a companion paper discusses the development of the pile setup method. During the field investigation, detailed soil characterization, monitoring of soil total lateral stress and pore-water pressure, collection of pile dynamic restrike data as a function of time, and vertical static load tests were completed. Restrike measurements confirm that pile setup occurs at a logarithmic rate following the end of driving, and its development correlates well with the rate of dissipation of the measured pore-water pressure. Based on the field data collected, it was concluded that the skin friction component, not the end bearing, contrib...

Resilient modulus of subgrade materials for mechanistic-empirical pavement design guide

This paper describes the test program, proposed test protocol for subgrade resilient modulus (Mr), determination of Mr values, and development of design tables and constitutive models for Mr estimations. Subgrade samples were collected from 12 locations throughout the state of Wyoming, USA for standard laboratory and Mr tests. The proposed Mr test protocol was validated using test data collected from a Round Robin Test Program. The study shows that Mr values of soils having R > 50 increase with increasing deviator stresses. In contrast, Mr values of soils having R ≤ 50 decrease with increasing deviator stresses. Design tables of Mr values were developed. Also, three constitutive models were locally calibrated for Mr estimations. The constitutive Model Type B yields a relatively better estimation of the resilient modulus than Models A and C. The proposed methods could be adopted to increase the efficiency of pavement design in the USA and other countries.

Systematic back-calculation protocol and prediction of resilient modulus for MEPDG

Abstract A research focusing on the characterisation of representative local material properties was conducted to facilitate the full implementation of the Mechanistic-Empirical Pavement Design Guide for roadway designs in Wyoming. As part of the test program, falling weight deflectometer deflection data were collected from 25 test sites in Wyoming for back-calculation of subgrade resilient modulus. Also, subgrade materials from these test sites were sampled for laboratory resilient modulus measurement in accordance with the AASHTO T 307. The back-calculation is a user-dependent procedure and produces a non-unique resilient modulus estimation. To alleviate this limitation, this paper focuses on the recent development of a systematic back-calculation protocol for subgrade resilient modulus using MODCOMP6 software. The protocol is intended for use on a flexible pavement with a crushed base. The proposed procedure discusses pre-analysis checks, seed modulus adjustment, pavement structure adjustment and program termination criteria. A correlation study was conducted to correct back-calculated resilient modulus to laboratory-equivalent values. The results conclude that a non-zero intercept linear regression model provides a better correlation than the widely used zero intercept linear regression model. Furthermore, better correlations are achieved when the back-calculated resilient modulus of a lower subgrade layer and resilient modulus measured at higher laboratory test sequences Nos. 11 to 15 are considered. The non-zero model based on Mr test sequence No. 14 and lower subgrade layer yields the best correlation. For the zero model, a C-factor of 0.645 based on Mr test sequence No. 15 and lower subgrade layer yields the best correlation.

Medium-Scale Experimental Study of Pile Setup

Research on the gain in pile resistances over time known as pile setup is often conducted in a field using a full-scale instrumented test pile driven into soil. The challenges associated with this full-scale field method include cost, duration, subsurface heterogeneity, and impractical extraction and reuse of sensors and piles. A medium-scale laboratory program for a pile setup research was established to allow a better control of soil type and layering, reuse of sensors and piles, and subsurface characterization before and after a pile installation. A large manhole was designed to hold compacted soil for the experimental study. A steel frame was constructed around the manhole to function as a base for cone penetration tests (CPT), support for a pile driver, and reaction for a static load test. The pile driver consisted of a steel pipe that housed a 45 kg (100 lb) weight used to drive the pile. Sensors and equipment used in this study, such as Pile Driving Analyzer (PDA), strain gauges, earth pressure cells, and piezometers, were similar to those used in the full-scale field experiment. Strain gauges were placed along the pile shaft and protected by steel angles. A series of dynamic load tests and a static load test were performed on each test pile to measure pile setup. Conclusions obtained from this medium-scale test program agreed with those obtained from full-scale field experiments, making this an alternative method for future deep foundation research. New findings were obtained to better explain the pile setup phenomenon which cannot be feasibly measured in past studies.

Geotechnical considerations in hydraulic modeling of bridge abutment scour

Scour-related failure of earthen abutments in bridge waterways involves interacting geotechnical and hydraulic processes. Current bridge design guides, however, inadequately account for geotechnical factors influencing scour depth. Our paper presents results of laboratory experiments investigating how soil strength affects scour depth at spill-through abutments. We focus on several difficulties faced when conducting flume experiments on scour at abutments formed of erodible soil. The difficulties include attaining scale-reduced shear strengths, controlling soil compaction, and quantifying the shear strength of model soil. We indicate the similitude considerations involved, and describe a process of soil tests relating model soil strength to soil compaction that uses hand-held devices to determine soil strengths. The correlations developed between soil compaction and shear strength helped to work around some of the difficulties in flume experiments. We suggest practical solutions to issues pertaining to simulating the shear strength of compacted soils used in flume studies of abutment scour.

Failure of Spill-Through Bridge Abutments during Scour: Flume and Field Observations

AbstractThis paper presents early findings from laboratory tests and field observations on the failure of spill-through abutments subject to abutment scour. These findings show that geotechnical and hydraulic processes interact to erode embankment soil during abutment scour, producing lesser scour depths than predicted using leading abutment scour equations. A major failure location is the flow waterline beginning at an abutment’s upstream corner where soil is exposed to the highest values of flow velocity and turbulence. Undercutting and toppling of soil blocks occurs sequentially along the face of the spill slope, eroding it back and eventually exposing the abutment column. Further erosion then may breach the embankment. The laboratory findings, based on uniform sand compacted to varying densities and thereby shear strengths, show that soil strength influences scour depth.

Verification of Recommended Load and Resistance Factor Design and Construction of Piles in Cohesive Soils

To enhance regional design and construction practices for driven piles, FHWA permitted the development of regional resistance factors for the design of foundation piles. By fitting allowable stress design safety factors to the load and resistance factor design (LRFD) framework, several state departments of transportation (DOTs), including the Iowa DOT, have adopted interim procedures. Subsequently, an LRFD procedure that incorporates setup was developed for piles in cohesive soils through comprehensive research in Iowa. The proposed LRFD procedure used an Iowa DOT in-house static analysis method and the wave equation analysis program for construction control. To verify the adequacy of the proposed procedure and investigate its economic implications, differences in pile design between the interim and the proposed LRFD procedures were evaluated on the basis of independent data collected from more than 600 production steel H-piles driven in cohesive soils. This study concluded that the proposed LRFD procedure would not significantly increase the design and construction costs. The incorporation of pile setup into the LRFD procedure was found to provide additional economic benefits. Although the current Iowa DOT policy is to drive piles to the contract length, if a suitable pile termination procedure were used once the desired resistance was achieved, a general saving of 20% in pile length would be anticipated for both procedures. Although the research and findings presented in this paper are specific to a local area, these methods could be adopted nationally to increase the efficiency of bridge foundations for all states.

LRFD Resistance Factors including the Influence of Pile Setup for Design of Steel H-Pile Using WEAP

American Association of State Highway and Transportation Officials (AASHTO) allows individual states to use regionally calibrated resistance factors based on the local soil conditions to improve the cost effectiveness of pile foundations as long as the development of these factors comply with the Load and Resistance Factor Design (LRFD) concept. This paper presents preliminary resistance factors developed for the Wave Equation Analysis Program (WEAP) using the PIle LOad Test database of the Iowa Department of Transportation (PILOT-IA). Using the pile penetration rate (blows/m) at the end of driving as the main variable, the LRFD calibrations for WEAP were conducted using 32 sites comprising the following four soil categories: clay (8 sites), mixed soil (11 sites), sand (13 sites), and all soil types (32 sites). Three different procedures, namely the Iowa DOT steel pile design chart, the DRIVEN program, and the GRLWEAP TM SPT N-value based method (SA), were used to estimate the soil driving resistance. When compared to the AASHTO (2007) recommended LRFD resistance factors, the developed resistance factors for clay increased by 125%, while the resistance factors for the mixed, sand and all soil types improved by 50%. Among the procedures used for estimating the soil driving resistance, the resistance factor based on the SA method yielded the highest efficiency factor for mixed, sand and all soil types, while the Iowa DOT design chart led to the best efficiency factors for clay soil. In addition to using 32 data sets from PILOT-IA, data from three recently completed load tests were used to examine the effect of the pile setup on the resistance factors. Pile driving records and restrike data were used to predict the pile capacity as a function of time and establish a modified empirical pile setup equation incorporating a soil property using the Standard Penetration Test (SPT) N-value. In addition to incorporating this information, the paper demonstrates the benefits of incorporating the soil setup in resistance factor calculations.

Current Limitations and Challenges of Driven Piles in Rock as Demonstrated Using Three Case Studies in Wyoming

There are currently no static analysis methods available for estimating the geotechnical resistance of a driven pile in rock. According to the American Association of State Highway and Transportation Officials (AASHTO), the limiting factor for a pile in rock is its structural capacity. The estimation of its structural axial capacity depends strongly on an effective length factor (K) and its unbraced length (L). These parameters significantly depend on its soil confinement along the pile and rock support at its toe, which lead to a large discrepancy between estimated and measured resistances. In this paper three case studies of completed pile projects in Wyoming are presented to highlight the limited knowledge and challenges pertaining to present design and construction practices of driven piles in rock. Steel H-piles were installed at Burns South, Casper and Torrington sites in Wyoming. Static analysis method were used to estimate the geotechnical resistances of these piles. Wave Equation Analysis Program (WEAP) and Case Pile Wave Analysis Program (CAPWAP) were used to verify their performances during construction. Structural capacities of these piles were also calculated. The results of the studies show that the static analysis methods and structural analyses yield inconsistent pile resistance estimations. Several recommendations are proposed: 1) develop an analytical method for piles in rock; 2) conduct full-scale load test of piles in rock; 3) characterize rock properties; and 4) develop a relationship of percent pile bracing, soil properties and embedded pile length.

Laboratory simulation to understand translational soil slides and establish movement criteria using wireless IMU sensors

Landslide monitoring and warning using inertial measurement units (IMUs) has shown the potential for remote and real-time applications. However, the studies conducted using the IMU sensors are limited to rainfall-induced landslide detection using soil moisture sensors and accelerometers for predicting slide and measuring tilt, respectively. The tilting of the slope might not occur during a slow-moving translational slide, and it may not always be possible to accurately record the soil moisture condition. The use of raw acceleration data, which is the combination of linear and gravitational accelerations, for calculating tilt or motion is another drawback of the existing studies. Hence, there is a need for a better approach to monitor slides. This paper presents two methods to define movement thresholds and criteria to identify the translational soil slides based on our understanding of the sensor data recorded during the two laboratory experiments. BNO055 sensor devices (IMU sensors) with 3-axis accelerometers and 3-axis gyroscopes were selected for this study. The linear accelerations, gravitational accelerations, and angular velocities were utilized to understand the translational soil slides by correlating the sensor behavior to that of the slope. The interpretation of the movements during the failure at each sensor location was further verified by referring to the videos recorded by two pi-cameras. The outcomes of this study confirm the applicability of the proposed IMU sensor system and the movement thresholds for effective and reliable monitoring and warning of translational soil slides.