EFFICIENT DESIGN OF VERTICAL MICROPILE SYSTEMS TO LATERAL LOADING Dr. Jesús Gómez, P.E. PE Andy Baxter, P.G.
Outline When are micropiles subject to lateral load? How do we analyze them? Shear Friction Concept Bending Friction Example The case of Crystal Bridges
Where are micropiles subject to lateral load? Building foundations (earthquake, wind) Basement wall foundations Retaining wall foundations Excavation support Tower and stack foundations Machine foundations Slope stabilization
Bridge and tower foundations Courtesy: Fundaciones Franki, C.A.
Bridge and tower foundations Courtesy: Precomprimidos- Venezuela
Bridge and tower foundations Courtesy: Precomprimidos- Venezuela
Building foundations P V
Why just not use battered micropiles?
Analysis of vertical micropiles subject to lateral loads A micropile is not good for lateral loading when working alone In soils, 10-20 kip is typical maximum Use pile cap-micropile system instead
Shear Friction
Shear Friction Does not consider lateral resistance of pile itself Lateral resistance offered by soil acting on pile Shear or bending resistance of pile Does not consider moment equilibrium Only translational movement
Bending Friction
Bending Friction Does not consider lateral resistance of pile itself Lateral resistance offered by soil acting on pile Shear or bending resistance of pile Considers moment equilibrium Only rotational movement The larger the lateral load, the larger the resistance Closed form solution assuming linear elastic materials
Bending Friction Failure occurs as: Bearing capacity failure of soil/rock Geotechnical or structural failure of pile in uplift Structural failure of pile cap in shear or bending In rock, capacity can be very large In soils, capacity can be larger than expected Efficient design is finding suitable pile cap dimensions
Bending Friction Shear friction, shear and bending resistance of micropile il also develop We have not combined all formulations This is not necessarily new. Tiedowns used sometimes in the heel of L- shaped retaining walls
Formulation
Formulation H a t σ l
Formulation a = Ep + 2 Ep 2 + Es Es b Ep b l' Δs = 1 2 E s b l' H a t 1 6 E s b a 2 Δ p = l' a a ( ) Δs
Example b = 5 ft Allowable bearing pressure = 5 ksf Pile cap-soil interface friction angle = 32 degrees Young s modulus of soil = 1,000 ksf Micropile : 1 # 14 bar, Fy = 75 ksi. Apparent elastic length = 10 ft. Tallow = 108 kip Lateral load 30 kip per micropile
Example 25.0 Stress (ksf f) 20.00 15.0 t = 3 ft t=45ft 4.5 t = 6 ft Maximu um Bearing 10.0 5.0 q allow 0.0 3 3.5 4 4.5 5 5.5 6 6.5 7 Width 'l' of pile cap (ft)
Example 2.50 ainst Slidin g Factor of safety ag 200 2.00 1.50 1.00 0.50 t = 3 ft t = 4.5 ft t = 6 ft 0.00 3 3.5 4 4.5 5 5.5 6 6.5 7 Width 'l' of pile cap (ft)
Example 1.40 Rotat tion of Pile Cap (deg) 1.20 1.00 080 0.80 0.60 0.40 0.20 t = 3 ft t = 4.5 ft t = 6 ft 0.00 3 3.5 4 4.5 5 5.5 6 6.5 7 Width 'l' of pile cap (ft)
Example 120 Upli ift on Microp pile (kip) 100 80 60 40 20 t = 3 ft t = 4.5 ft t = 6 ft 0 3 3.5 4 4.5 5 5.5 6 6.5 7 Width 'l' of pile cap (ft)
Crystal Bridges Museum of American Art
Site Location
Crystal Bridges
Crystal Bridges
Micropile Installation #20 Williams bar, Fy = 75 ksi, L = 13 ft 5.5-inch casing, Fy = 80 ksi, L = 3.5 ft Open hole drilling
Micropiles under wall
Lateral Load Test
Lateral Load Test 000 0.00 Load (kips) 0 40 80 120 160 200 240 280 320 360 400 Displace ement (inc ches) 0.10 0.20 0.30
Lateral Load Test Rotation (deg) 0 0.1 0-0.1-0.2-0.3-0.4-0.5 50 100 (kip) Load 150 200 250 300 350
Evidence of lateral deflection
Strain Gauges
Lateral Load Test VERIFICATION LOAD TEST - LOAD TRANSFER FROM STRAIN GAGE DATA Load (kip) -100 0 100 200 300 400 500 600 0.00 1.00 Ground Surface 2.00 3.00 400 4.00 5.00 6.00 Depth [ft] 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 50% DTL 75% DTL 100% DTL 150% DTL 200% DTL
Lateral Load Test
Conclusions Vertical micropile systems can resist significant lateral loads Twelve micropiles in rock loaded to 160+ kip without signs of failure In soils, possible to obtain large lateral capacities through efficient design of laterally loaded systems Testing of micropile systems in soils needed (research effort) Naturally, battered micropiles seem more efficient for lateral resistance, but not always practical
Acknowledgement Andy Baxter, P.G., co-author Paul Gintonio, Foundation Specialists, installed micropiles Allen Cadden, P.E.
Jesús Gómez, Ph.D., P.E., Schnabel Engineering jgomez@schnabel-eng.com eng.com