micro pile design

1. Case Study on Micropile
- Chinthamani Hospital
A.Suriyaprakash
Assistant Design Engineer
M/S Karthikeyan Associates

2. Introduction
• A micropile is a small-diameter (typically less than 300 mm (12 in.)),drilled
and grouted non-displacement pile that is typically reinforced.

3. Applications of Micropile
• Insitu Reinforcement
Slope Stabilization and earth Retention
Ground Strengtheing and protection
Settlement Reduction
Structural Stability
• Structural Support
Earth Retention
Foundations for New Structures
Underpinning of existing Foundation-Scour protection, Repair and
rehabilitation of existing Foundations, Arresting or prevention of movement,
Upgrading of foundation capacity.
Seismic Retrofitting

4. Advantages of Micropiles
• Micropiles are often used to underpin the existing structure where
need of minimal vibration or noise is of prime importance.
• Micropiles can be easily laid where low head room is a constraint.
• Micropiles can be easily installed at any angle below the horizontal
using the same equipment used for ground anchors and grouting
objects.
• Offer a practical & cost effective solution to costly alternative pile
systems as well as a solution to job sites with difficult access.
• Do not require large access road or drilling platforms

5. Types of Micropiles
• Type A – Gravity Grout
• Type B – Pressure through Casing
• Type C – Single Global Post Grout
• Type D – Multiple Repeatable Post Grout

6. Details of Micropile

7. Requirements
• Step-1 – Evaluate project requirements and Micropile feasibility
• Step-2 – Geotechnical Data
• Step-3 – Applicable loading Combinations
• Step-4 – Preliminary Design of Micropiles
• Step-5 – Structural Design of Micropile Cased Length
• Step-6 – Structural Design of Micropile Uncased length
• Step-7 – Geotechnical Design of Micropile
• Step-8 – Design of Micropile for lateral Loading
• Step- 9 –Estimate Micropile Settlement

8. Design based on Geotechnical data
• Allowable safe load = Skin Friction + End Bearing
• Skin Friction
Cohesionless soil = Pdi* tanɤ* Asi
Cohesive soil = α*c*As
• End Bearing
Cohesionless soil = Ap*Po*Nq
Cohesive soil = Ap*Nc*Cp

9. Pile cased length Structural Capacity
• Nominal Allowable Tensile Strength ,
Pt,allowable = 0.55*fy-steel(Abar+Acasing)
• Compression Allowable load,
Pc,allowable = 0.4*fc,grout*Agrout+0.47*fy-steel(Abar+Acasing)

10. Pile Uncased length Structural Capacity
• Nominal Allowable Tensile Strength ,
Pt,allowable = 0.55*fy-steel*Abar
• Compression Allowable load,
Pc,allowable = 0.4*fc,grout*Agrout+0.47*fy-steel*Abar

11. Design of Micropile Based on geotechnical data
• Dia of pile ,D
• Bond Nominal Strength ,α
• Bond length,L
• Allowable geotechnical
bond Strength,
= 3.14*D*α*L/S.F

12. Lateral Load Capacity of Micropile
Step-1
• Combined Axial compression and bending of Cased length
= ((fa/Fa)+(fb/(1-(fa/fe’))*Fb) <=1.0
fa = Axial stress = load/area
fb= Bending stress = Moment/Section modulus
Fa = Allowable axial stress =0.47*fy(casing)
Fb= Allowable Bending stress = 0.55*fy(casing)
Fe’=Euler’s buckling stress = π^2 * E/FS*(Kl/r)^2

13. Lateral Load Capacity of Micropile
Step-2
• Combined Stress check
= ((Pc/Pc,allowable)+(Mmax/M,allowable) <=1.0
Pc = maximum axial compression load
Pc,allowable = 0.4*fc,grout*Agrout+0.47*fy-steel(Abar+Acasing)
Mmax = Maximum bending moment in micropile
M,allowable = 0.55*fy * Section modulus

14. Check Structural capacity to resist test loads
• Verification test load = 2*Design load
• Proof test load = 1.6*Design load
• Pile cased length allowable load,
= 0.68*fc,grout *Agrout +((fy(casing)*(Abar+Acasing)/1.25)
• Pile uncased length allowable load,
= 0.68*fc,gout * Agrout + 0.8*fv,bar * Abar
• Allowable loads should be more than Verification and proof loads.

15. Estimate Micropile Settlement
• Elastic Settlement = safe load * length / Stiffness = PL/AE
 elastic stiffness = Agrout * Egrout + Asteel * Esteel
Both cased length and uncased length are calculated.
Settlement = <5mm for lateral design
= <12mm for axial design

16. CHINTHAMANI HOSPITAL

18. TRIAL PIT

19. MICROPILE MACHINE

20. SITE EXECUTION

21. SITE DIFFICULTIES

22. Thank you