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Development and Field Testing of Multiple Deployment Model Pile (MDMP)

CHAPTER 1. INTRODUCTION

1.1 Overview

Piles are common foundation members enabling the transfer of large superstructure loads into weak compressible layers or through them to strong bearing strata. Pile foundations are traditionally designed either as end-bearing or friction piles. End-bearing piles are assumed to support the entire load at the pile's tip, while friction piles rely on load transfer along the pile shaft to develop their capacity.

The displacement required to activate the shaft resistance at a point (displacement to yield = skin quake) is estimated to be 2.5 mm (0.1 in); for example, see quake values proposed by Smith (1960) and the interfacial friction test results by Paikowsky et al. (1995b). In contrast, about 0.1B of displacement (where B is the pile tip width/diameter) is necessary to activate the tip resistance (Bowles, 1988). In reality, all piles are friction piles until some or all of their shaft resistance is mobilized (along the entire pile's length), allowing the pile end to develop resistance. As a result, piles often carry the service load in friction, even if designed as end-bearing piles. The need to accurately analyze the shaft resistance component is, therefore, important for an economical design of pile foundations. For this purpose, testing methods are required that measure and evaluate the shaft friction and its variation during the service life of the pile. The obtained information enables one to improve the understanding of the underlying mechanics and hence to develop better soil-structure interaction theoretical tools. Theories of this kind can include, for example, the consideration of capacity changes with time, thus leading to better design procedures than those currently utilized in common practice.

The installation and testing of full-scale test piles is both an expensive and an inconvenient method of obtaining information for the design of pile foundations. As an alternative method, model piles, which can be used to simulate the behavior of full-scale piles, are used to obtain this information. A model pile is a scaled-down calibrated pile equipped with instrumentation, having the capacity to monitor the pile-soil interaction over the pile history. The pile history includes the three main stages of the pile's life: the initial driving, the consolidation or pore pressure/surrounding soil equalization, and finally, the loading during service. Model piles utilize electronic sensors to measure load transfer, radial stresses, pore pressure, displacement, acceleration, temperature, and inclination Such monitoring can include: (1) during installation -- the dynamic pile response as well as the soil and water pressures; (2) during equilibrium - the excess pore pressure dissipation, variation of the soil pressure with time, and along with its influence on the pile's performance; and (3) during service -- the load displacement relations and its distribution along the pile.

The acquired data can either be applied directly in the design process (e.g., measurement of skin resistance) or extrapolated to a full-scale pile behavior (e.g., radial consolidation process) or used to develop and/or calibrate theoretical models and their parameters (e.g., bearing-capacity factors, interfacial load-displacement relations, etc.).

The presented work deals with the development, calibration, and testing of a model pile, known as the MDMP, Multiple Deployment Model Pile and its evaluation as an in situ testing tool for design and construction.

1.2 Purpose

The following goals were set for the presented research:

• Design and build a model pile capable of:
  
  
  1. Monitoring (while withstanding) dynamic measurements of stresses and accelerations during driving.
  2. Monitoring the pore pressure and radial soil stresses with time.
  3. Measuring the pile/soil interaction.
  4. Multiple deployment at various sites using standard drilling rig operation.
  5. Independent static load testing (without the need of a drill rig).
     
     
• Gather data to expand the database compiled at the UMass-Lowell concerning:
  
  
  1. Pore pressure dissipation and capacity gain with time.
  2. Static pile capacity predictions based on dynamic measurements.
  3. Pile behavior under the static cyclic load-testing procedure.
     
     
• Investigate the ability to obtain design parameters using a model pile under field conditions to be applied to full-scale pile analysis and design.

1.3 Scope

1. A literature search of existing model piles was conducted, allowing a review and determination of the most appropriate features of each model pile. Some of the important requirements were: robust design for impact driving and multiple deployments, ability to model open- and closed-ended pile conditions, monitoring pore pressure and total stresses, and versatility of use in different deposits.
   
   
2. Considering the aforementioned review, a model pile was developed to measure skin and tip resistance, pore pressure, radial pressure, local displacement, and dynamic response during driving. The MDMP was designed to meet the demanding requirement of an in situ tool used during site investigation to estimate the pile performance.
   
   
3. The model pile was calibrated at UMass-Lowell to verify the performance of the instrumentation. A data acquisition system was designed to be operator-friendly and flexible to monitor the pile's instrumentation throughout the entire pile history (dynamic and quasi-static).
   
   
4. A site was selected in Newbury, Massachusetts. The subsurface conditions at the site consist of a cohesive soil layer with a depth of about 3 to 15 m (10 to 50 ft) below ground surface. The Multiple Deployment Model Pile (MDMP) was installed at the same location at two different depths, 9.27 and 12.31 m (30.4 and 40.4 ft) to the pile tip. Data were collected continuously until the excess pore pressure developed during driving dissipated.
   
   
5. Based on the tests at Newbury, the MDMP proved to be an effective in situ tool even in the harsh New England winter environment. The entire test was completed with the use of a standard drill rig typically used for site investigations. The drill rig can be used elsewhere after the pile installation and only needs to be returned to the test location at the completion of the testing sequence in order to remove the pile and the casing.
   
   
6. The data recovered were analyzed to determine: (a) the quality, significance, and predictive capabilities of the dynamic measurements; (b) pore pressure dissipation and capacity gain and their relationship; and (c) radial stress variations and their relationship to pore pressure dissipation and capacity gain.

1.4 Manuscript Layout

The following are short descriptions for each of the upcoming chapters:

Chapter 2 - Review of existing model piles used for field testing.

Chapter 3 - Description of the requirements, specifications, design, and calibration of the Multiple Deployment Model Pile (MDMP).

Chapter 4 - Design and capabilities of the peripheral accessories required to perform the MDMP field testing.

Chapter 5 - Overview of the subsurface investigation and testing at the Newbury Massachusetts Model Pile Test Site.

Chapter 6 - Presentation of the results of the model pile tests at the Newbury site.

Chapter 7 - Analysis and discussion of the model pile test results at the Newbury site.

Chapter 8 - Conclusions and recommendations.

1.5 Contributions

This research has been carried in cooperation with the Massachusetts Highway Department (MHD) and the University of Massachusetts-Lowell (UMass-Lowell).

The support of the MHD and the collaboration and assistance of Nabil Hourani, the head of the geotechnical section, and John Pettis, geotechnical engineer, are greatly appreciated.

The Federal Highway Administration (FHWA) supported the development and testing of the MDMP, along with other related work performed at the Newbury site. Thanks are extednded to Al DiMillio and Carl Ealy for their help, interest, and advice, and for providing the additional Pil Driving Analyzer (PDA) required for the multiple-source dynamic measurements.

The help provided by the laboratory director of the Civil Engineering Department at UMass-Lowell, Gary Howe, and the College of Engineering machine shop director, David Rondequ, is greatly appreciated. Mr. Howe developed the control boxes, allowing both dynamic and static measurements, and helped in all stages of laboratory calibration of the field testing.

Special thanks go to John Chen and Edward Hajduk, graduate research assistants in the Geotechnical Re search Laboratory at UMass-Lowell, which was associated with this research project. Their help and collaboration during the field and laboratory testing was appreciated.

George Saliby, a graduate student in the Geotechnical Research Laboratory, assisted during the file testing and his help is acknowledged.

Les Chernauskas of Geosciences Testing and Research (GTR) assisted with the preliminary WEAP analysis, dynamic measurements, and filed testing.

Ron Boggess of Geocognetics Inc. of Houston, Texas was in charge of the construciton of the MDMP based on an early version of the 76.2-mm (3-in) model pile and required modifications. Final adaptation, calibration, and changes were carried out at UMass-Lowell, Geotechnical Research Laboratory.