USE OF MICROPILES IN TEXAS BRIDGES by John G. … OF MICROPILES IN TEXAS BRIDGES by ... TxDOT Bridge...
Transcript of USE OF MICROPILES IN TEXAS BRIDGES by John G. … OF MICROPILES IN TEXAS BRIDGES by ... TxDOT Bridge...
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USE OF MICROPILES IN TEXAS BRIDGES
by
John G. Delphia, P.E. TxDOT Bridge Division
Geotechnical Branch
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DEFINITION OF A MICROPILE
A micropile is a small diameter (typically less than 12 in.), drilled and grouted non-displacement pile that is typically reinforced
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MICROPILE APPLICATIONS
FOUNDATION STRUCTURAL
SUPPORT
IN-SITU EARTH REINFORCEMENT
~ 95% of the applications world wide
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MICROPILE APPLICATIONS
FOUNDATION STRUCTURAL
SUPPORT
Repair/Replacement of Existing
Foundations
Upgrading of Foundation
Capacity
Foundations for New Structures
Underpinning of Existing Structures
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MICROPILES
• used in restricted overhead access areas
• used in close proximity to existing structures
• used in difficult ground conditions (karst areas, uncontrolled fill, etc.)
• may be placed on an incline (i.e. battered)
• vertical micropiles may be limited in lateral capacity
• due to their slenderness ratio micropiles are limited in axial capacity
• cost
ADVANTAGES
DISADVANTAGES
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MICROPILE COST FOUNDATION TYPE COST ($/LF) MICROPILES 150 DRILL SHAFT (18) 41 DRILL SHAFT (24) 85 DRILL SHAFT (30) 106 DRILL SHAFT (36) 112 DRILL SHAFT (42) 131 DRILL SHAFT (48) 203
MICROPILES COST = ~ 42 TO 48 inch DRILLED SHAFTS
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EXAMPLE OF THE USE OF MICROPILES
- Underpinning of a bridge
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BRIDGE LAYOUT
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Phased Construction EAST WEST
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• Bent 5 showed signs of distress shortly after pouring the slab. The outer column of bent 5 settled 4” the day after the slab pour.
• No cracking of the bridge slab was observed, which indicates the settlement occurred while the deck concrete was curing.
PROBLEMS OBSERVED DURING CONSTRUCTION
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Bent 5 Bent 4
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Column – Bent 5
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Column – Bent 5
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What to do? • Obtain additional borings along the
structure
• Evaluate the foundations using the new borings
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Additional Boreholes - Location
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Additional Boreholes - Location
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B-4A
B-11
B-3A
B-3B
B-7
B-8
Location of Additional Boreholes
Bent 4 Bent 5
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Bore Logs - Bent 5
358.8’ 354.0’
B4A B11
DS Tip = 360.0’ DS Tip = 360.0’
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Bore Logs - Bent 5
342.8’
B3A B3B
DS Tip = 360.0’
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Bore Logs - Bent 4
363.5’
364.5’
DS Tip = 366.0’
DS Tip = 366.0’
B8 B7
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• Based on the new borings the drilled shafts for Bents 4 and 5 were of insufficient length
• Bents 4 and 5 needed to be repaired to restore the capacity of the foundation elements
• Revised drilled shaft tip elevations were provided for Phase 2 construction
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Repair Options • Option 1 - Underpin Bents 4 and 5 by placing drilled shafts
along the bent line. Shafts would be connected with a tie beam incorporating the existing columns.
• Option 2 – Underpin bents 4 and 5 by placing drilled shafts perpendicular to the bent line, and installing two shafts per installed drilled shaft. Shafts would be connected with a footing tied into the existing column.
• Option 3 – Underpin bents 4 and 5 by placing micropiles. A footing would be placed on the micropile group and tied into the existing column.
• Option 4 – Remove the superstructure over Bents 4 and 5 and associated bent caps to allow installation of drilled shafts to proper design length. Rebuild bent and superstructure.
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REVIEW OF SITE CONDITIONS
WEST EAST
5’
~25’
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Repair Options • Option 1 - Underpin Bents 4 and 5 by placing drilled shafts
along the bent line. Shafts would be connected with a tie beam incorporating the existing columns.
• Option 2 – Underpin bents 4 and 5 by placing drilled shafts perpendicular to the bent line, and installing two shafts per installed drilled shaft. Shafts would be connected with a footing tied into the existing column.
• Option 3 – Underpin bents 4 and 5 by placing micropiles. A footing would be placed on the micropile group and tied into the existing column.
• Option 4 – Remove the superstructure over Bents 4 and 5 and associated bent caps to allow installation of drilled shafts to proper design length. Rebuild bent and superstructure.
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Micropile Repair Option • The design of micropiles was conducted under TxDOT
supervision.
• The allowable design load was 60 Tons/micropile.
• The total length of each micropile was estimated as 70.0’ with a bond zone of 30.0’ min. into stiff strata.
• The reinforcing bar consisted of 1 ¾” (#14) dia, grade 75, equipped with post grout tubing and centralizers.
• Micropiles were 8” diameter (grout filled, post grouted and cased with N80 casing 9’ from surface).
• Grout Strength = 4000 psi.
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Micropiles Repair Option
• Four micropiles were installed per drilled shaft and tied to the existing column with a pile footing. The pile footing was designed by TxDOT.
• Prior to installation of production piling, one sacrificial micropile was tested under tension load, in accordance with ASTM D3689 “Quick Test”.
• The test pile performed adequately with a deflection of 0.026” at 60 Ton (design load) and 0.386” at 120 Ton (maximum test load).
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Micropile Repair Option
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Footing Retrofit (Bent 4)
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LOAD TEST
Trucks
Bent Line
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Load Test Results
• Test Load ~ 90% of Design Load
• Bent 4: No appreciable movement was measured under a
20 min. hold at the test load.
• Bent 5: A maximum measured settlement of 0.007” was
observed for the sustained test load. Once the load was taken off of the bent, a recovery of 0.003”of elastic settlement was noticed.
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SUPERSTRUCTURE REPAIR
Micropiles and the associated footings stabilized the foundations for Bents 4 and 5
The superstructure was restored by jacking
it up to the required elevations.
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Bridge Deck Prior to Foundation Stabilization and Jacking Up of the Beams
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Bridge Deck After Foundation Stabilization and Jacking Up of the Beams
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CONCLUSIONS • Micropiles can be used for foundation support on new
bridges and to underpin existing structures
• Micropiles are commonly used : - in restricted overhead access areas - in close proximity to existing structures • When used in appropriate conditions micropiles can be
cost effective
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Questions?
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