Micropiles – Design and its Applications
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INTRODUCTION
A micropile is a small-diameter (typically less than 300 mm), drilled and grouted replacement pile that is typically(up to 20% As/Ac)reinforced.
A micropile is constructed by drilling a borehole, placing reinforcement, and grouting the hole.
Micropiles can withstand axial and/or lateral loads.
Micropile Classification System
Ref: FHWA-NHI-05-039, AASHTO LRFD 4th Edition, Interim 2008
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Case 2 90% of International Applications
~ 100% of North American Applications Case 1
Very Few Applications in the USA
•Based on Design Application
•Based on Grouting method
•Based on Design Application
Case 1 Micropiles
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Each Micropile is Loaded Directly
Primary Resistance is Provided by Steel
Reinforcement and Side Resistance over Bond Zone
Each Micropile Designed to Act Individually, Even When in Groups
AASHTO – Minimum spacing of 30 inches or 3 pile diameters, whichever is greater
Must check for group affects due to axial compression/tension or lateral loads
Case 1 Micropiles (After FHWA NHI-05-039) 5
Network of Micropiles
Act As Group to Reinforce The Soil Mass
Each Micropile is Lightly Reinforced
Design Procedures Not Fully Developed
Case 2 Micropiles
Case 2 Micropiles (After FHWA NHI-05-039) 7
Micropile Installation (After: FHWANHI-05-039) 8
Based on Grouting method
The method of grouting is generally the most sensitive construction control over grout/ground bond capacity . Grout-to-grout capacity varies with the grouting method.
1) Type A: Gravity Grout
2)Type B: Pressure through Casing 3)Type C: Single Global Post Grout
4)Type D: Multiple Repeatable Post Grout
Type A: Here the grout is placed under gravity head only using sand-cement motors or neat cement .
TypeB:
1) In this type neat cement grout is placed into the hole as the temporary steel casing iswith drawn.
2) Injection pressures varies from 0.5to 1.0 MPa.
The pressure is limited to avoid fracturing of the surrounding ground.
Type C:This is done in two step process:
1) As of Type A
2) Prior to hardening of the primary grout, similar groutis injected one time via a sleeve grout pipe at pressure of at least 1.0MPa.
Type D: This is done in two step process of grouting similar to Type C with modifications to step 2 where the pressure is injected at a pressure of 2.0 to 8.0MPa:
Figure 5: Based on method of grouting
CONSTRUCTION SEQUENCES
Installation process in accordance with the requirements of NBN EN1536:1999
1) Positioning and drilling of the first section of the drill casing (recoverable steel casing as temporary support during the boring process).
2) While drilling, the drill casing – inside equipped with a drilling head fixed on a rod - is oscillated into the soil.
(back and forth movement / twisting in place).
3.As the drilling process progresses, soil is removed from the borehole by the excavating and additional sections of casing are jointed (added) to protect the soil from collapsing into the borehole during drilling..
5.After reaching the design depth, clean-up of the borehole front, removal drilling tool, drilling fluid (water) pumped out from the bore Formation of the pile : insertion and lowering of the reinforcement cage, pouring of the concrete
6. During the continuous concreting process, the temporary casing elements are progressively withdrawn whereby the concrete forms the pile shaft.
Figure 7: Micropile construction squences using
Basic Design Process
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Step 1 >>>Prepare preliminary design
Step 2 >>>Evaluate geotechnical capacity
Step 3 >>> Load Testing
Step 1>>> Prepare Preliminary Design
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Select Micropile Spacing
Min 30 inches or 3 diameters, whichever is greater
Based on situation (e.g., existing footing, clearances, etc)
Select Micropile Length
Consider compression, uplift, lateral loads, scour, downdrag, group affects
Max length using common track-drilling equipment is > 300 feet but most are on order of 100 feet
Select Micropile Cross Section
Allow use of common casing sizes for material availability;
Use casing vs rebar reinforcement >>better lateral and axial capacity
Step 2 >>Evaluate Geotechnical Capacity
Establish Stratum for Bond Zone
Certain soils not generally suitable (e.g., organics, cohesive soils having LL>50, PI>20); (if must be used, include comprehensive testing, increased FS)
Select Ultimate Bond Strength
PG-Allowable = PUltimate/FS
RR = φ Rn = φqp Rp + φqs Rs = φqp (qp Ap) + φqs (πds άb Lb)
Consider end bearing in high quality rock only with adequate verification of rock quality and construction methods to obtain good contact;
Contd…
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Evaluate Micropile Group Compression Capacity
Cohesive or Cohesionless Soils (Block & Punching Failures)
Evaluate Micropile Group Uplift Capacity
Cohesive or Cohesionless Soils (Block Failures)
Evaluate Micropile Group Lateral Capacity
Refer to procedures for driven piles and drilled shafts
(FHWA-NHI-05-42 and FHWA-IF-99-025;AASHTO LRFD Int 2008,Section 10.7)
Evaluate structural capacity of pile(s)
Evaluate Soil-Structure Interaction (e.g. LPILE)
Consider Battered Piles, Buckling and/or Seismic Effects
Step 3 >>Load Testing
Compression Load Test ,
Tension Load Test ;
Lateral Load Test;
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Compression Load Test
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Tension Load Test
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Lateral Load Test
Drilling Techniques
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Single Tube Advancement
Rotary Duplex
Hollow Stem Auger Casing
Drill Rod Ground Surface
Drill Bit
Casing Rotary Drill Bit
Drilling Fluid
Rotary Duplex
Drilling Machines
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Rotary Eccentric Percussive Duplex Duplex Casing and Roller Bit
Composite Reinforced Micropile
Lp
Lb
db
Possible Applications of Micropiles
As Alternate Deep Foundation Type, Especially Where Piles Penetrate Rock;
Where Spread Footings Are Feasible but There Is Potential For Erosion or Scour
Support System Close to Existing Structure;
Risk of Liquefaction From Pile Driving;
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Ref: FHWA NHI-05-39, Table 3-1
Slope Stabilization And Earth Retention [Case 1 and Case 2]
Ground Strengthening [Case 1 and Case 2]
Settlement Reduction
[Case 2]
Structural Stability
[Case 2]
In-Situ Reinforcement [Case 1 and Case 2 Micropiles]
Earth Retaining Structure Foundations
Foundations For New Structures
Underpinning Existing Foundations
Seismic Retrofitting
Scour Protection
Repair/Replace Existing Foundations
Stop/Prevent Movement
Upgrade Foundation
Capacity Structural Support
[Case 1 Micropiles]
(Est 0-5% of world applications)
(Est 95% of world applications)
Overview of Micropile Applications
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Limitations for Micropiles
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Vertical micropiles may be limited in lateral capacity;
Cost effectiveness;
Potential buckling under seismic loading and liquefaction
But need to consider methods available to quantify and/or deal with these limitations
Micropile Installation
Williamsburg Bridge , New York Seismic Retrofit,Los Angeles, California
Design and Construction of Micropiles
by S.S. Liew and C.C. Fong
Gue & Partners Sdn Bhd, Kaula Lampur, Malaysia
(29-30 Sept,2003)
Abstract: “This paper discusses the micropile classification, design concept, problem associated with the common installation methods, construction control and performance of this pilling system”
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Overview of Research work on Micropiles
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CONCLUSIONS
Micropiles can be used as a normal foundation piles and compensation piles for remedial works, especially with areas with site constraints. It can be costly option to support lateral loads and huge bending moments.
Acknowledgement
With sincere thanks to
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REFERENCES
Bruce, D. A. (1988),“Developments in Geotechnical Construction Processes for Urban Engineering”,Civil Engineering Practice, Vol. 3, No. 1, Spring, pp. 49-97.
Weltman, A.(1981).A review of micropiles types. Ground engineering, May, 43-47
FHWA (1997),“Drilled and Grouted Micropiles, State-of-Practice Review”, Bruce, D. A.
and Juran, I., Reports No. FHWA-RD-96-016, 017, 018, and 019.
AASHTO LRFD Bridge Design Specifications 4thEdition, 2007, Interim 2008, Section 10.9;
FHWA (2000), “Micropile Design and Construction Guidelines Implementation Manual”,Armour, T., Groneck, P., Keeley, J., and Sharma, S., Report No. FHWA-SA-97- 070.
