A little reminder on the proper selection of micropile drilling technique Dr. Donald A. Bruce

1. A little reminder on the proper selection of micropile drilling techniqueDr. Donald A. Bruce “Primum, non nocere” (Lizzi, 1982)

2. Recent Case History • Micropiles installed as new foundation system for existing major line railway bridge • On Abutment No. 2, micropiles installed through loose to medium fine sands, below water table into rock • On Abutment No. 1, micropiles installed through existing masonry buttress wall overlying dense clay till.

4. Contractor used air flush rotary (and occasional percussive) in sands, and air flush TUBEX (ODEX) in the wall • Results?

5. Abutment No. 2, major sinkholes and loosened zones extending up to surface and under tracks • Abutment No. 1, severe structural damage to wall, settlement, and sinkholes under track and under road • Consequence?

6. Project completely suspended since April 2003. • This bridge upgrading itself is a relatively inexpensive project, BUT… • This bridge upgrading is on the critical path (literally) of the whole North Eastern U.S. high speed rail program. Conclusions: Knowledge of overburden drilling systems as used in urban/transportation applications is not universally at a high level AND/OR complacency has set in locally.

7. 1. Introduction • Scope • Typically 75 to 300 mm diameter • Typically ≤ 60 m depth • Typically within 30° of vertical or horizontal • Restrictions • Deviation • Damage to soil and/or structure • Federal Regulations (e.g., USACOE 1997 for embankment drilling) • Generally contractor-driven (“performance” specification)

8. 2.1 Commonalities • Continuous, straight penetration • Constant diameter, stable and clean bore • Consistent with the purpose of the drill hole (e.g., grout hole vs. anchor hole) • Appropriate combinations of thrust, torque, rotation, percussion, flush • Cost effective • Dictated by ground, not historical bias • Environmentally compatible

9. 2.2 Rock Drilling Methods 2.2.1 Rotary • High rpm, low torque, low thrust (blind or core) • Low rpm, high torque, high thrust 2.2.2 Rotary Percussive • Top Hammer • Down-the-hole Hammer • Direct circulation • Reverse circulation • Dual fluid drilling • Water hammers 2.2.3 Rotary Vibratory (Sonic)

10. 2.2.3 Sonic Drilling: Advantages • Can provide continuous, relatively undisturbed cores in soil (75-250 mm diameter) and rock • Very high penetration rates • Readily penetrates obstructions • Depths to 150 m • Can easily convert to other types of drilling • No flush in overburden, minor amounts in rock

11. 2.3 Overburden Drilling 2.3.1 For stable soils (i.e., “open-holing”) • SSCFA • Rock drilling methods 2.3.2 For unstable soils • HSCFA • Combination methods • Slurry supported • Cased GEOSYSTEMS, L.P.

12. Cased Methods for Unstable Soils • Sonic • Single tube • Rotary • Percussed • Rotary duplex • Rotary percussive duplex, concentric • Rotary percussive duplex, eccentric • Double head duplex • Rotary • Rotary percussive

14. 3. Drilling Equipment Key factors in drill rig selection • Power • Thrust/pull back • Maneuverability • Stability • Accessibility • Noise emission • User friendliness

15. 4. Circulation Type and Application • Up-hole velocity (UHV) > “sinking velocity” • UHV (m/min) = 1274 x Flush Pump Rate (Liters/min) D2 – d2 (mm) where D = drill hole diameter (in mm) d = drill string diameter (in mm) • Typical UHV • Air, or air/water “mist”: 1500 m/s (max 2100 m/s • Water: 36 m/s (max 120 m/s) • Low to medium viscosity mud: 30 m/s • Very thick mud: 18 m/s • Foam: 12 m/s

16. Air vs. Water – Rotary vs. Rotary Percussion • Guideline for selection • Provide clean hole • Enhance penetration rate • Minimize tool wear • Consistent with purpose of hole • Minimal damage to formation and/or structures • Environmentally compatible • Reconsider options if “lost flush” occurs

17. 5. Recording of Drilling Progress and Parameters • Value of real time continuous monitoring for design purposes (manual vs. automatic) • Look for “exceptions and unexpecteds” [Weaver, 1991] • Indication of progressive improvement (e.g., denser, less permeable conditions) • Concept of specific energy • Several generations/evolutions

18. Calculation of Specific Energy e = F + 2 π N T A AR where e = specific energy (kJ/m3) F = thrust (kN) A = cross sectional area of hole (m2) N = rotational speed (revolutions/second) T = torque (kN-m) R = penetration rate (m/sec)

19. 6. Specifications • Must be tailored to project at hand and to the objectives to be accomplished” [Weaver, 1991] • Generally “Performance” • Flexibility needed as project unfolds • Items which should be specified • Hole location, length, orientation, and inclination • Minimum hole diameter • What is not permissible • Deviation and straightness (measurement and tolerances) • Actions required in “extreme conditions” • Recording and presentation of data • Specifications must contain all available data

20. 7. Final Comments “Profile of a Driller” “Drillers are as diverse a group of people as the industry in which they work. Every drilling operation is different and requires a highly skilled person to ensure that the drilling process is successful.” Australian Drilling Industry Technical Training Committee Ltd. (1997)