H ll B Mi il fHollow Bar Micropiles for Settlement Control in Soft Soils

Jonathan Bennett, PE, D.GE – Chief EngineerEarth Support Division

Saturday, September 25, 2010

PRESENTATION OVERVIEW

• Hollow bar micropiles exhibit outstanding bond transfer values and have the ability to stiffen soft soils thus improving the settlement behavior of foundations utilizing them.

• Over 1000 hollow bar micropiles were installed at the Spirit Aerospace facility in Kinston NC to reinforce soft coastal plain soils for settlement reduction at selectKinston, NC to reinforce soft coastal plain soils for settlement reduction at select heavily loaded equipment pads.

• Due to the number of piles required for the job, an extensive load testing program was employed to optimize the design and a mathematical model was developed to p y p g pdescribe the micropile behavior.

PRESENTATION OUTLINE• Project Background• Application• Micropile Load Testing Program• Descriptive Model• Design Process• Design Process• Production Micropile Installation• Numerical Analysis• ConclusionsConclusions

Project LocationGlobal Transpark – Lenoir Countyp y

Kinston, NC

ErieErie

PittsburgPittsburg

Harrisburg

Project LocationGlobal Transpark – Lenoir Countyp y

Kinston, NC

PittsburgPittsburg

Project LocationGlobal Transpark – Lenoir Countyp y

Kinston, NC

Project BackgroundProject Background

• Spirit Aerospace chose the Global Transpark inSpirit Aerospace chose the Global Transpark in Kinston, NC as the site for a composites manufacturing center scheduled to beginmanufacturing center, scheduled to begin operations in 2010.

• Composites manufacturing for major• Composites manufacturing for major substructures for the Airbus A350 XWB will be produced at this 585 000 SF facilityproduced at this 585,000 SF facility.

• Project was a fast track design‐build project.

ApplicationApplication

• A number of heavily loaded equipment padsA number of heavily loaded equipment pads required foundation support that would result in very low differential movement across thein very low differential movement across the foundation.

• For the purposes of this paper we will focus• For the purposes of this paper, we will focus on the trim and drill equipment pads.I l 1000 h ll b i il• In total over 1000 hollow bar micropiles were installed for pad support.

ApplicationApplication

ApplicationApplication

ApplicationApplication

ApplicationApplication

ApplicationApplication

ApplicationApplication

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Courtesy of Contech SystemsCourtesy of Contech Systems

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Courtesy of ContechSystems

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Courtesy of ContechCourtesy of ContechSystems

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Hollow Bar Micropile SolutionHollow Bar Micropile Solution

Hollow bar micropiles were recommended forHollow bar micropiles were recommended for ground reinforcement / stiffening below equipment mats:equipment mats:• Outstanding stiffness behavior exhibited in previous test programsprevious test programs

• Soil conditions were conducive to hollow bar i ll i d hi h d d iinstallation and high expected production

Design ProcessDesign Process

• Modeling and design of the equipment pads was g g q p pperformed the Prime Contractor’s Structural Engineer based on equipment requirements and expected micropile performanceexpected micropile performance.

• Performance / deflection requirements were provided by the equipment manufacturer.

• Pre‐Design Load Testing program was utilized to optimize the foundation support solution.Th S l E i i i• The Structural Engineer was using spring supports to simulate the behavior of the micropiles for foundation support and model the mat.pp

Load Testing ProgramLoad Testing Program

• Due to the anticipated size of the project, it made p p j ,a lot of sense to perform and extensive pre‐design load testing program so as to optimize the pile lengths for maximum economypile lengths for maximum economy.

• Four pairs of sacrificial hollow bar micropiles of four different lengths were installed for compression testing. Two piles of each length were installed (L=7m, 8.5m, 10m and 11.5m)

• Each test pile length had one pile with 150mm• Each test pile length had one pile with 150mm clay bit and one pile with 115mm cross bit to assess the difference due to bit size.

Load Testing ProgramLoad Testing Program

Load Testing ProgramLoad Testing Program

Load Testing ProgramLoad Testing Program

Load Testing ProgramLoad Testing Program

Load Testing ProgramLoad Testing Program

Descriptive ModelDescriptive Model• For conventional cased micropiles, the deflection of the cased length (free 

length) could be used as a starting point for estimating rough axiallength) could be used as a starting point for estimating rough axial deflection values at various magnitudes of load.

• With fully bonded micropiles or ground anchors, an assumed free length cannot be used to estimate rough deflection values because there is no f l hfree length.

• Additionally, for fully bonded micropiles, the axial deflection is tied to the stiffness of the soil making estimating rough deformation difficult. The problem is essentially a soil‐structure interaction problem.p y p

• In this particular case, the deformation performance of the fully bonded micropiles was of more interest than the structural capacity alone making estimating deformation characteristics of utmost interest. 

• A mathematical model of a load test was developed in order to assist in• A mathematical model of a load test was developed in order to assist in estimating and communicating the deformation behavior of fully bonded micropiles and for assessing overall mat deflection performance.

Descriptive ModelDescriptive Model

P

Rock Anchor

Δ

P

Soil Anchor

Δ

Descriptive ModelDescriptive Model

Descriptive ModelDescriptive Model

Descriptive ModelDescriptive Model

Descriptive ModelDescriptive Model

Parabolic Model of Axial Deformation

Descriptive ModelDescriptive Model

Stiffness FunctionStiffness Function

Descriptive ModelDescriptive Model

Fl ibilit F ti Pl tFlexibility Function Plot

Descriptive ModelDescriptive Model

Comparison of model with actual test results.

Descriptive ModelDescriptive Model

Bracketed Estimated Deformation

Production Micropile InstallationProduction Micropile Installation

Production Micropile InstallationProduction Micropile Installation

Production Micropile InstallationProduction Micropile Installation

Production Micropile InstallationProduction Micropile Installation

Production Micropile InstallationProduction Micropile Installation

Numerical AnalysisNumerical Analysis

• Some simple geotechnical finite elementSome simple geotechnical finite element analysis was performed in order to estimate the amount of settlement improvementthe amount of settlement improvement achieved by using ground reinforcement.

• Two plane strain models were created in• Two plane strain models were created in SIGMA W. One with a single micropile and one with soil only Soil parameters were adjustedwith soil only. Soil parameters were adjusted such as to match load testing results.

Numerical AnalysisNumerical Analysis

• The results of the two models were comparedThe results of the two models were compared.• The vertical settlement values for the reinforced soil were about 38% of the valuesreinforced soil were about 38% of the values for unreinfroced soil.

ConclusionsConclusions• The axial stiffness properties of hollow bar micropiles can provide 

b t ti l d ti i f ttl t i ft ilsubstantial reductions in surface settlement even in very soft soils.• High productivity rates and the ability to easily install hollow bar 

micropiles in low headroom or tight quarters applications results in a very economical approach to settlement reduction where accessa very economical approach to settlement reduction where access is restricted.

• The behavior of hollow bar micropiles can be modeled in a more precise manner than simply assuming rigid or spring supports. A model exists that can accurately describe their load‐deflection behavior and allow for direct calculation of various parameters.

• Geotechnical finite element analysis can be a useful tool to estimate the amount of settlement reduction that can be achievedestimate the amount of settlement reduction that can be achieved using hollow bar micropiles for settlement control.

THANK YOU!for Your Time and Attention

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Sho ld o ha e f rther q estion onta t jbennett@ eostr t res omShould you have further question, contact jbennett@geostructures.com.