VERTICAL MICROPILE LATERAL LOADING. Andy Baxter, P.G.

Transcription

1 EFFICIENT DESIGN OF VERTICAL MICROPILE SYSTEMS TO LATERAL LOADING Dr. Jesús Gómez, P.E. PE Andy Baxter, P.G.

2 Outline When are micropiles subject to lateral load? How do we analyze them? Shear Friction Concept Bending Friction Example The case of Crystal Bridges

3 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

4 Bridge and tower foundations Courtesy: Fundaciones Franki, C.A.

5 Bridge and tower foundations Courtesy: Precomprimidos- Venezuela

6 Bridge and tower foundations Courtesy: Precomprimidos- Venezuela

7 Building foundations P V

8 Why just not use battered micropiles?

9 Analysis of vertical micropiles subject to lateral loads A micropile is not good for lateral loading when working alone In soils, kip is typical maximum Use pile cap-micropile system instead

10 Shear Friction

11 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

12 Bending Friction

13 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

14 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

15 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

16 Formulation

17 Formulation H a t σ l

18 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

19 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

20 Example 25.0 Stress (ksf f) t = 3 ft t=45ft 4.5 t = 6 ft Maximu um Bearing q allow Width 'l' of pile cap (ft)

21 Example 2.50 ainst Slidin g Factor of safety ag t = 3 ft t = 4.5 ft t = 6 ft Width 'l' of pile cap (ft)

22 Example 1.40 Rotat tion of Pile Cap (deg) t = 3 ft t = 4.5 ft t = 6 ft Width 'l' of pile cap (ft)

23 Example 120 Upli ift on Microp pile (kip) t = 3 ft t = 4.5 ft t = 6 ft Width 'l' of pile cap (ft)

24 Crystal Bridges Museum of American Art

25 Site Location

26 Crystal Bridges

27 Crystal Bridges

28 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

29 Micropiles under wall

30

31 Lateral Load Test

32 Lateral Load Test Load (kips) Displace ement (inc ches)

33 Lateral Load Test Rotation (deg) (kip) Load

34 Evidence of lateral deflection

35 Strain Gauges

36 Lateral Load Test VERIFICATION LOAD TEST - LOAD TRANSFER FROM STRAIN GAGE DATA Load (kip) Ground Surface Depth [ft] % DTL 75% DTL 100% DTL 150% DTL 200% DTL

37 Lateral Load Test

38 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

39 Acknowledgement Andy Baxter, P.G., co-author Paul Gintonio, Foundation Specialists, installed micropiles Allen Cadden, P.E.

40 Jesús Gómez, Ph.D., P.E., Schnabel Engineering eng.com