Use of Micropiles for Foundations of Transportation Structures Final Report

In pile design, piles must be able to sustain axial loads from the superstructure without bearing capacity failure or structural damage. In addition, piles must not settle or deflect excessively in order for the serviceability of the superstructures to be maintained. In general, settlement controls the design of piles in most cases because, by the time a pile has failed in terms of bearing capacity, it is very likely that serviceability will have already been compromised. Therefore, realistic estimation of settlement for a given load is very important in design of axially loaded piles. This notwithstanding, pile design has relied on calculations of ultimate resistances reduced by factors of safety that would indirectly prevent settlement-based limit states. This is in part due to the lack of accessible realistic analysis tools for estimation of settlement, especially for piles installed in layered soil. Micropiles have been increasingly used, not only as underpinning foundation elements but also as foundations of new structures. Prevalent design methods for micropiles are adaptations of methods originally developed for drilled shafts. However, the installation of micropiles differs considerably from that of drilled shafts, and micropiles have higher pile length to diameter ratios than those of drilled shafts. Improved understanding of the load-transfer characteristics of micropiles and the development of pile settlement estimation tools consistent with the load-transfer response of these foundation elements are the main goals of the proposed research. A rigorous analysis tool for assessment of the load-settlement response of an axially loaded pile was developed in this study. We obtained explicit analytical solutions for an axially loaded pile in a multilayered soil or rock. The soil was assumed to behave as a linear elastic material. The governing differential equations were derived based on energy principles and calculus of variations. In addition, solutions for a pile embedded in a multilayered soil with the base resting on a rigid material were obtained by changing the boundary conditions of the problem. We also obtained solutions for a pile embedded in a multilayered soil subjected to tensile loading. We then compared our solutions with the results from FEA and also with other solutions available in the literature. Finally, we compared the results of a pile load test from the literature with the results obtained using the solutions proposed in this study. Using the obtained elastic solutions, we also performed extensive parametric studies on the load-transfer and load-settlement response of rock-socketed piles. The effects of geometry of rock socket, rock mass deformation modulus, and in situ rock mass quality were investigated. To facilitate the use of our analysis, a user-friendly spreadsheet program ALPAXL was developed. This program is based on the elastic solution obtained in this study and uses built-in functions of Microsoft Excel. ALPAXL provides the results of the analysis, the deformed configuration of the pile-soil system and the load-settlement curve in seconds. It can be downloaded at http://cobweb.ecn.purdue.edu/~mprezzi. In the context of an INDOT project, a fully instrumented load test was performed on a rock-socketed micropile. The results of this micropile load test, on a pile with high slenderness ratio and high stiffness of surrounding rock, confirmed that most of the applied load was carried by the pile shaft. The shaft capacity of hard limestone obtained from the load test at the final loading step was 1.4 times larger than the shaft capacity that is obtained using the highest value of limit unit shaft resistance suggested by FHWA (the limit unit shaft resistance qsL from the load test was 2950 kPa, while the suggested values from FHWA were 1035 – 2070 kPa). Using pile and soil properties, predictions were also made using ALPAXL. The results from ALPAXL were in good agreement with the measured data at the design load level

Settlement Analysis of Axially Loaded Piles

In pile design, settlement controls the design in most cases because, by the time a pile has failed in terms of bearing capacity, it is very likely that serviceability will have already been compromised. This notwithstanding, pile foundations are often designed based on the calculations of ultimate resistances reduced by factors of safety. This is in part due to the lack of accessible realistic analyses for estimation of settlement, especially for piles installed in layered soil. This paper presents a new settlement analysis method for axially loaded piles in multilayered soil and analyzes two case histories for which load tests were performed on nondisplacement piles. The analysis follows from the solution of the differential equations governing the displacements of the pile-soil system obtained using variational principles. The input parameters needed for the analysis are only the pile geometry and the elastic constants of the soil and pile. A user-friendly spreadsheet program (ALPAXL) was developed to facilitate the use of the analysis

Pile Driving Analysis for Pile Design and Quality Assurance

Driven piles are commonly used in foundation engineering. The most accurate measurement of pile capacity is achieved from measurements made during static load tests. Static load tests, however, may be too expensive for certain projects. In these cases, indirect estimates of the pile capacity can be made through dynamic measurements. These estimates can be performed either through pile driving formulae or through analytical methods, such as the Case method. Pile driving formulae, which relate the pile set per blow to the capacity of the pile, are frequently used to determine whether the pile has achieved its design capacity. However, existing formulae have numerous shortcomings. These formulae are based on empirical observations and lack scientific validation. This report details the development of more accurate and reliable pile driving formulae developed from advanced one-dimensional FE simulations. These formulae are derived for piles installed in five typical soil profiles: a floating pile in sand, an end‐bearing pile in sand, a floating pile in clay, an end‐bearing pile in clay and a pile crossing a normally consolidated clay layer and resting on a dense sand layer. The proposed driving formulae are validated through well-documented case histories of full-scale instrumented driven piles. The proposed formulae are more accurate and reliable on average than other existing methods for the case histories considered in this study. This report also discusses the development of a pile driving control system, a fully integrated system developed by Purdue that can be used to collect, process, and analyze data to estimate the capacities of piles using the Case method and the pile driving formulae developed at Purdue

A Continuum-Based Model for Analysis of Laterally Loaded Piles in Layered Soils

An analysis is developed to calculate the response of laterally loaded piles in multilayered elastic media. The displacement fields in the analysis are taken to be the products of independent functions that vary in the vertical, radial and circumferential directions. The governing differential equations for the pile deflections in different soil layers are obtained using the principle of minimum potential energy. Solutions for pile deflection are obtained analytically, whereas those for soil displacements are obtained using the one-dimensional finite difference method. The input parameters needed for the analysis are the pile geometry, the soil profile, and the elastic constants of the soil and pile. The method produces results with accuracy comparable with that of a three-dimensional finite element analysis but requires much less computation time. The analysis can be extended to account for soil non-linearity

Assessment of Site Variability from Analysis of Cone Penetration Test Data

Soil property values for use in geotechnical design are often estimated from a limited number of in situ or laboratory tests. The uncertainty involved in estimating soil properties from a limited number of tests can be addressed by quantifying the variability within individual soundings and of the collection of soundings at a site. It has been proposed that factors of safety or resistance factors used in design be linked to site variability. Site variability can be assessed by studying the correlation structure of in situ test data. The cone penetration test (CPT), which is a reliable and widely-accepted in situ test, can be used for this purpose. Soil behavior type (SBT) charts are often used to obtain the subsurface soil profile from CPT parameters such as the cone resistance and the sleeve friction. A soil profile generation algorithm was developed in this research to generate a soil profile from an individual CPT sounding using two modified SBT charts. Soils are variable in both the vertical and horizontal directions. A vertical variability index (VVI) was defined to quantify variability in a CPT sounding. The average of the VVIs for all CPT soundings performed at a site is the site VVI. A site horizontal variability index (site HVI) was also developed, based on cross-correlation between cone resistances, the cone resistance trend differences and the spacing between every pair of CPTs considered, to quantify the soil variability of a site in the horizontal direction. A site variability rating (SVR) system, integrating the vertical and horizontal site variability, was developed to assess the overall site variability. Depending on the SBT chart selected, the soil profile generated using the soil profile generation algorithm may be slightly different; however, the SBT chart effect on the variability indices that compose the SVR index is small. Close agreement was found between the SVRs obtained using the two SBT charts selected for this research. In order to illustrate the use of the algorithms for VVI and HVI calculations and SVR of sites, CPTs from across the state of Indiana were analyzed. CPT data were obtained from Purdue\u27s own database, INDOT’s data repository and the U.S. Geological Survey (USGS) website. Site variability is calculated for specific depths of interest. For example, that depth of interest will be shallower for shallow foundations than for deep foundations. Site variability rating maps (SVR maps) for various depths of interest were constructed for the state of Indiana, illustrating the potential use of the site variability assessment methodology. An optimal sounding spacing calculation methodology was also developed to make the site investigation process more efficient, cost-effective and reliable

Analysis of Laterally Loaded Piles in Multilayered Soil Deposits

This report focuses on the development of a new method of analysis of laterally loaded piles embedded in a multi-layered soil deposit treated as a three-dimensional continuum. Assuming that soil behaves as a linear elastic material, the governing differential equations for the deflection of laterally loaded piles were obtained using energy principles and calculus of variations. The differential equations were solved using both the method of initial parameters and numerical techniques. Soil resistance, pile deflection, slope of the deflected pile, bending moment and shear force can be easily obtained at any depth along the entire pile length. The results of the analysis were in very good agreement with three-dimensional finite element analysis results. The analysis was further extended to account for soil nonlinearity. A few simple constitutive relationships that allow for modulus degradation with increasing strain were incorporated into the analysis. The interaction of piles in groups was also studied

Performance Assessment of MSE Abutment Walls in Indiana

This report presents a numerical investigation of the behavior of steel strip-reinforced mechanically stabilized earth (MSE) direct bridge abutments under static loading. Finite element simulations were performed using an advanced two-surface bounding plasticity model based on critical state soil mechanics. Results of the simulations were found to be in good agreement with published laboratory and field measurements, including horizontal facing displacements and tensile forces in the reinforcement. A parametric study was then conducted to investigate the behavior of a full-scale direct MSE bridge abutment. The parameters considered were the horizontal distance of the footing behind the wall facing, backfill compaction, reinforcement length and spacing, and magnitude of bridge load. Results indicate that the aforesaid parameters have a significant influence on the horizontal facing displacements, bridge footing settlements, and axial strains in the reinforcements. A survey questionnaire on the current state-of-practice of direct and mixed MSE abutments was prepared and distributed to all the Departments of Transportation (DOTs) in the United States. Results obtained from the survey shed light on the percentage of use of direct and mixed MSE abutments by various DOTs, abutment height, type and dimensions of the facing element, type of reinforcement, proportioning of footing and pile in direct and mixed MSE abutments, respectively, and common problems experienced by DOTs with respect to construction and performance of MSE abutments in the field

INDOT/Purdue Pile Driving Method for Estimation of Axial Capacity

This presentation discusses the new pile driving analysis method. Models for both base and shaft dynamic resistances that account for soil nonlinearity, both radiation and hysteric damping, and rate effects on soil strength will be presented. The analysis is validated through well-documented field tests on instrumented piles. The predictions from the proposed formulas will be compared with the results from static load tests and dynamic load test. Driven pile capacity results from two projects, one on SR 55 in Lake County and the other on US 31 in Marshall County, will be discussed