Research at Northwestern University: End-bearing Micropiles in Dolomite

Outline Introduction Test section details Axial load test results Axial load distributions Design implications Conclusions

Participants: TCDI-Hayward Baker, Lincolnshire, IL Vulcan Quarry, McCook, IL Northwestern University

Objective To evaluate the axial load transfer characteristics of micropiles embedded in dolomite so that rational design procedures can be developed

Overview: Axial load tests in Vulcan Quarry Four test piles with lengths of 0.6, 1.2, 1.8 and 2.4 m Piles consist of 178-mm-diameter, 13 mm wall thickness, 550 MPa steel casings filled with 38 MPa grout. Roller bit is welded to bottom. Axial load distribution determined by vibrating wire strain gages on steel, embedment gages in grout and telltale readings Two piles were extracted to examine grout-steel and grout-rock interfaces

Installation procedures Production piles ~ 30 m long Assembled pile with roller bit attached used to drill hole Left in place and grouted under high pressure Test piles ~ 1 m Hole cored Pile assembled and placed in hole Pile grouted under low pressure

Allowable stress design: Pallow = α  f 'c  Agrout + β  fy x A steel Method fy max (MPa) α β Allowable Load (kN) AASHTO (Service load design) 550 0.4 0.47 2000 Chicago Building Code 200 0.4(1) 800 Massachusetts Building Code 410 .33(3) 0.4(2) 1400

Vulcan Quarry, McCook, IL.

On site preparation of micropiles

Micropile 1 Micropile 3 Micropile 2

Top - 2 Bottom Top - 3 Top -4 Top - 1 Rock Conditions

Load test frame Reaction anchor transfer beams transfer girder hydraulic jack test pile

Axial load test results

Axial Load vs. Deflection of Micropile 1

Axial Load vs. Deflection for Micropile 2

Axial Load vs. Deflection for Micropile 3

Axial Load vs. Deflection for Micropile 4

Summary of load test results Pile 1 failed at 2000 KN and 4000 KN on second loading, cumulative tip movement = 10 mm (RQD = 22) Pile 2 failed 800 KN on first loading and 2000 KN on second loading, cumulative tip movement =25 mm (RQD = 0) Pile 3 did not fail at 4450 KN, tip movement = 2 mm (RQD = 87) Pile 4 with soft bottom exhibited a plunging failure at 2000 KN

Axial load distributions Determining moduli for composite pile – Fellenius (1989) method Data

Strain Gage Data from Micropile 3

Axial Load Distributions for Micropiles 1 and 3

Axial Load Distribution of Micropile 2

Axial Load Distribution for Micropile 4

Mobilized Side Resistance vs Axial Head Deflection

Summary of load transfer data No load transfer in upper 1 m – due to low confinement and poor rock quality Critical interface was steel/grout; verified from visual observations of extracted piles Shorter piles (1 and 2) were end-bearing; capacity a function of RQD Pile 4 with soft bottom had an average unit side resistance approximately equal to that of a smooth bar pulled from concrete (3500 kPa)

Allowable structural load (kN)     Computed Observed No. Allowable structural load (kN) Davisson allowable load with FS = 2 (kN) Allowable load for 13 mm movement (kN) (1) (2) (3) 1 1630 880 1380 2000 3800 2 1560 800 1320 400 1200 3 >2225 >4450 4 1000 not applicable   (1) – AASHTO (2) – Chicago Building Code (3) – Massachusetts Building Code

Example: Production pile Typical length in Chicago: 25 to 30 m When pile tip moves 2 mm under 4450 KN (like pile 3), design for movements For 27.5 m long pile: 12.5 mm deformation – 1350 KN capacity 25 mm deformation – 2600 KN capacity Both greater than 800 KN based on Chicago code

Conclusions Stresses in piles were in excess of those specified in codes without detrimental effects on performance Steel-grout interface governed axial load transfer behavior along side No side resistance mobilized in top 1 m of test piles due to low stresses and grout pressures and poor quality rock Due to relatively high compressibility, allowable axial loads of full-scale piles, founded on competent rock, are determined more rationally from allowable deformation considerations, rather than code-specified allowable stresses.