Drilled Shaft Foundations in Limestone Dan Brown, P.E., Ph.D. Dan Brown and Associates
Foundation Engineering How we teach our students Fundamental understanding of soil and rock behavior (good!) Focus on technical solutions to defined problems; calculations of static capacity, etc. Less materials = more efficient solution How the real world works What soil or rock is there & where is it? The biggest problem is defining the problem Engineering is intertwined with business issues and risks/responsibilities Schedule & constructability are critical
Limestone (and potential karst) in the U.S.
Characteristics of Limestone Wide range of strength properties High variability over small distance
Characteristics of Limestone Wide range of strength properties High variability over small distance Uncertainty related to stratigraphy, potential voids
Resistance Drilled Shafts Nominal resistance; strength limit R n Overburden 4% D Displacement Rock D
Resistance Some Basic Definitions / Terms LRFD: Factored resistance Q R n φr n 4% D Displacement λq φr n φ accounts for uncertainty in resistance; λ accounts for uncertainty in forces D Overburden Rock
Resistance Some Basic Definitions / Terms ASD: Allowable capacity Q R n /FS R n FS (factor of safety) accounts for uncertainty in both force and resistance Overburden 4% D Displacement Rock Q (R n /FS) D
Axial Resistance in Rock Question: how can we add side + base resistance? Isn t the concrete/rock bond strength brittle? Answer: No, largely due to ductility resulting from dilation associated with sidewall roughness
Example Tampa, FL 48in diameter, 52ft deep Sand Clay Soft Limerock 12ft Displacement (inches) 0.0-0.5-1.0-1.5-2.0-2.5 0 1000 2000 3000 Load (kips)
Displacement (inches) Side & Base Resistance 0.0-0.5-1.0-1.5-2.0-2.5 0 1000 2000 3000 Load (kips) Segment Displacement (inches) Toe Displacement (inches) 0.0-0.5-1.0-1.5-2.0 0-0.5-1 -1.5-2 0 5 10 15 20 Side Shear (ksf) 0 200 400 600 800 1000 1200 Load (kips)
Another Tampa Example
Strong Limestone Rock may have q u > f c ; foundation strength limited only by the structural strength of the shaft Socket may be required to develop flexural strength
Keys for Drilled Shafts in Limestone Address reliability at each specific drilled shaft location Boring (or probe) at each drilled shaft Inspection verification and geotechnical engineering involvement through construction Tolerant design with provision for field adjustments Case Histories provide examples
Nashville ADSC SE Chapter Load Test Shafts Depth Below Top of Socket 0-5 -10-15 -20-25 -30-35 -40 RQD, % 0 10 20 30 40 50 60 70 80 90 Test Shaft 2 RQD Test Shaft 1 RQD -45-50
Nashville ADSC SE Chapter Load Test Shafts 48in core barrel to excavate limestone sockets (16ft) Mechanical cleaning only Inspectors consensus: TS 1: 3in to 4in soil seam 19in below base TS 2 no significant seams Both shafts needed additional cleaning Significant concrete overrun in TS 2
Nashville ADSC Research: Test Results Unit Side Resistance, ksf 25 20 15 10 5 0 Nominal Dia = 48" Nominal Dia = 52.5" 0 0.2 0.4 0.6 Displacement, inches Displ/Dia, % 0-0.25-0.5-0.75-1 -1.25-1.5-1.75-2 Bearing Pressure, ksf 0 250 500 750 1000 1250 test shaft 1 test shaft 2 E=335ksi E=630ksi E=536ksi E=235ksi For Test Shaft 1: avg qu = 8300psi range = 1660 16,110 %rec = 74% 100% RQD = 9% - 65% Back-calculated C=0.4 f s C p a q p u a 0.79 s 2 qb 1 E E = 30 to 50 (q u )
Stan Musial / Veterans Memorial Bridge, St. Louis
Pylon Foundation Conditions Elev (ft) +410 water +370 +300 Loose to medium dense sand Limestone Marine construction Deep scour Hard limestone bedrock Large lateral & overturning loads (VC, seismic, wind)
Core Borings at Each Drilled Shaft Location Hard, competent limestone with relatively few seams
Elevation (ft) Elevation (ft) Limestone Bedrock Compressive Strength Data 320 305 315 300 310 305 22ft Socket 295 16.5ft Socket 290 300 295 290 Post-Award Pre-Award Post-Award Mean 285 280 Post-Award Pre-Award Post-Award Mean (below elev 300) Pre-Award Mean 285 Pre-Award Mean 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 275 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 Unconfined Compressive Strength (psi) Unconfined Compressive Strength (psi) Pier 11 Pier 12
Drilled Shaft Construction
Drilled Shaft Construction Core Boring Hole
Load Test Results Over 40ksf avg mobilized unit side resistance 450ksf base resistance mobilized at < ¼ inch displ 2,000 ksf/in 2 qb 1 s 0.79 E For rigid circular footing on elastic halfspace with ν = ¼ E = 191,000ksf = 1330ksi E 100q u for q u 13ksi
Selmon Expressway, Tampa Reversable Elevated Lanes project, 2003-2005 Single column piers in median, supported on single 6ft dia. drilled shaft
Pier 97
Site Stratigraphy Loose Fine Sand, Often Silty SP or SM Color Varies 15 to 35 Typ. Clay with Sand CH or SC Green Gray (Hawthorn Group) 5 to 15 Typ. Highly Weathered Limestone Light Brown or Light Gray (Tampa Formation) (Figure after Kuhns et al, 2003)
Seismic Tomography Results at Pier 97 P97-8 P97-9 P97-5 Three-dimensional S-wave velocity images of the ground around Pier 97. Note: Boreholes marked by red dots (sources) and blue dots (receivers). P97-6 P97-8 P97-7 P97-5 P97-9 P97-7 P97-6 P97-5 P97-8 P97-9 P97-6 P97-5 P97-7 0 SC CH Geology at Pier 97 from core drilling -10 SC Weak ground zone CH -20 SM CL Weathered limestone? -30 Weathered Limestone Softer, less competent ground - irregular -40-50 Limestone Shaft Tip -60 Soft, weak ground - irregular -70-80 Limestone? -90 View from SE View from SW View from NW View from NE Color code 3,500 Seismic velocity, ft/s 8,000 4800 5000 6200 Velocities matching color in the ground image Geotechnical Services, Inc. Ground Imaging Technolo gy Figure 5 Project: Ground imaging around Pier 97 CTE, Tampa, FL For: Ardaman & Associates, Inc. 2 Project engineer: J. M. Descour Surveyed: August 18, 2004
Sister Shaft Remediation REL Remediation Micropile Remediation 3 ft. Clear 2 ft. Clear Face to Face Revised Pile Locations (typ) PR-10 (typ) Non-Structural Cap Structural P ier & P ier Head Ideal PR-10 PR-8 (typ) Micropile Layout 9-5/8" Dia. Pipe-Pile 218 Shafts Total 50 required no remediation 87 required Micropile remediation 67 required Sister Shaft Remediation 14 new shafts, with revised design procedure
Micropile Remediation
Sister Shaft Remediation
Sister Shaft Remediation
Lessons Learned from Selmon REL Investigation: Borings at every drilled shaft location
Lessons Learned from Selmon REL Investigation More rigorous inspection by trained Geotech staff
Lessons Learned from Selmon REL Investigation Rigorous inspection Robust foundation design correlations for soft limestone; account for variability in more severely weathered rock
Lessons Learned from Selmon REL Investigation Rigorous inspection Robust foundation design Reduced resistance factor for single column piers due to consequences of failure
Selmon Widening Project, 2012 Existing SPT, and new boring at each drilled shaft location Designed for side resistance only; ignored end bearing Design using correlations from load testing based on SPT 5 Statnamic load tests to confirm
Selmon Widening Project, 2012
Load Testing During Widening Project Note: base isolation used to eliminate end bearing from measurements
Apply Common Sense and Judgment
What to do when a void is encountered? Backfill with lean mix, redrill Extend permanent casing through void, drill deeper Straddle with multiple shafts or micropiles
Lake Barkley Bridge, Kentucky Piers 4 and 5 redesigned to straddle void in limestone bedrock
Lake Barkley Bridge, Kentucky These borings encountered big void in limestone bedrock
Summary Limestone can provide excellent performance for drilled shaft foundations Design of drilled shafts in limestone requires: Understanding of geology design flexibility, robustness, quality control / assurance during construction