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ADSC RESEARCH PROJECT UPDATE: ROCK SOCKETS IN THE SOUTHEASTERN U.S.
W. Robert Thompson, III, P.E., D.GEDan Brown and Associates, PC
Montgomery, Alabama
Introduction
• Research project sponsored by ADSC SE Chapter
• Rock‐socketed drilled shafts
• Aim to improve:
• Design methods
• Cost‐efficiency
• Involve designers, contractors, owners (DOTs)
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Current Practice in Southeast
• Allowable Unit Base Resistance of 60 to 120ksf (maybe up to 200ksf)
• Design for base resistance only
• Inspected by probe hole drilled in base of socket
• If seams found, excavation continues until inspector satisfied “sound rock” is below shaft
• Lawrenceville – Rock defined as Rock Auger Refusal = 2in/5min with LLDH rig
Test Program
• Two sites tested to date
• Nashville, Tennessee
• Hard Limestone (sometimes solutioned)
• Birmingham, Knoxville, Chattanooga
• Lawrenceville, Georgia (Metro Atlanta)
• Metamorphic rock (Gneiss, Schist, etc.)
• Piedmont Formation – Georgia to Virginia
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Test Program
• Thorough Subsurface Explorations
• Two test shafts at each site
• O‐Cell testing device
• Inspected by local geotechs
• Conducted field day with local ASCE/G‐I Technical Group
Nashville Site
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-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
00 10 20 30 40 50 60 70 80 90
RQD, %
Dep
th B
elo
w T
op
of
So
cket
Test Shaft 2 RQD
Test Shaft 1 RQD
Rock Core RQD ‐
Nashville
qu = 5,000 to 20,000 psi
• 48in core barrel to excavate limestone sockets (16ft)
• Mechanical cleaning only
• Inspectors consensus:
• TS 1 had 3in to 4in soil seam 19in below base
• Shaft should have been extended
• TS 2 no significant seams
• Typical conditions sought
• Both shafts needed additional cleaning
• Significant concrete overrun in TS 2
Construction ‐ Nashville
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• Test Shaft 1 best for side resistance
• Fully mobilized side resistance vs Test Shaft 2
• No overrun on concrete to complicate interpretation
• Fully mobilized at small displacement: 1 % of dia. (~0.2in)
Unit Side Resistance ‐ Nashville
Unit Side Resistance ‐ Nashville
0
5
10
15
20
25
0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2%
Un
it S
ide R
esis
tan
ce, ksf
Displacement/Diameter, %
Unit Side Resistance vs Normalized Upward O-Cell Displacement
Nominal Dia = 48" Nominal Dia = 52.5"
~ 0.2in
Test Shaft 1
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Unit Base Resistance ‐ Nashville
• Displacements about 1% of loaded area
(B = 39in for TS 1; B = 29in for TS 2)
• Inspections indicated soil seam 19in below TS 1
(10% of B approx. 0.5B below shaft base )
• Maximum unit base resistance
• TS 1 = 500 ksf
• TS 2 = 1250 ksf
Unit Base Resistance ‐ Nashville
-2
-1.75
-1.5
-1.25
-1
-0.75
-0.5
-0.25
00 250 500 750 1000 1250
Dis
pl/D
ia, %
Bearing Pressure, ksf
Unit Base Resistance vs Normalized Downward O-Cell Displacement
Test Shaft 1
Test Shaft 2
E=536ksi (1%)
E=235ksi (1%)
E=630ksi (0.5%)E=335ksi (0.5%)
~ 0.5in (D =39in)~ 0.3in (D =29in)
Soil seam beneath TS 1?
E
qBs
2179.0
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Implications ‐ Nashville
• “Sound Rock”: conditions similar to Test Shaft 2
• 1 or 2 small seams < ½ inch thick
• Allowable unit base resistance = 500ksf
• “Fair Rock”: conditions similar to Test Shaft 1
• soil‐filled seams up to 10%B, at depths > ½ B
• Allowable unit base resistance = 200 ksf
• Displacement approx. 0.5%B
(1/4 to 3/8 inch for B = 4 to 6 ft)
Implications ‐ Nashville
• Side resistance not factor in “Sound Rock” for typical designs (when socket not needed for lateral)
• Utilizing side resistance in “Fair Rock” conditions may be prudent
• When socket > 10ft to “find” base resistance
• Use lower base resistance + side resistance
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Lawrenceville Site
Construction ‐ Lawrenceville
• TS 1 – Test side and base resistance of 40.5in rock socket
• Started with core barrel, completed with rock auger
• Penetration = 4 to 6.5in/min – not “Rock Auger Refusal”
• TS 2 – Test base resistance at “Rock Auger Refusal” with 66in socket and 19in O‐cell
• Drilled with rock auger
• Two distinct zones of PWR
• Penetration = 3in/5min – not “Rock Auger Refusal”
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Rock Core RQD ‐ Lawrenceville
-25
-20
-15
-10
-5
0
5
100 10 20 30 40 50 60 70 80 90 100
Dep
th B
elo
w T
op
of
So
cket
(S
haf
t 1)
or
PW
R (
Sh
aft
2),
(ft)
RQD, %
Test Shaft 1 Test Shaft 2
qu = 7,000 to 11,000 psi
Test Shaft 1 ‐ Lawrenceville
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Test Shaft 2 ‐ Lawrenceville
0
5
10
15
20
25
30
35
40
45
50
55
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Un
it S
ide
Re
sis
tan
ce
, k
sf
Displacement/Diameter, %
Unit Side Resistance vs Normalized Upward O-Cell Displacement
PWR - Shaft 1 - Nominal Dia = 42"
ROCK - Shaf t 1 - Nominal Dia = 40.5"
PWR - Shaft 2 (19-35ft) - Nominal Dia = 66"
PWR - Shaft 2 (35-42.5) - Nominal Dia = 66"
Unit Side Resistance ‐ Lawrenceville
Shaft 1 ~0.2in
Shaft 1 ~0.6in
Shaft 2 ~0.3in
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0
5
10
15
20
25
30
35
40
45
50
55
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Un
it S
ide
Re
sis
tan
ce
, k
sf
Displacement/Diameter, %
Unit Side Resistance vs Normalized Upward O-Cell Displacement
PWR - Shaft 1 - Nominal Dia = 42"
ROCK - Shaf t 1 - Nominal Dia = 40.5"
PWR - Shaft 2 (19-35ft) - Nominal Dia = 66"
PWR - Shaft 2 (35-42.5) - Nominal Dia = 66"
Unit Side Resistance ‐ Lawrenceville
0.3in
Unit Base Resistance ‐ Lawrenceville
~ 2in
~ 1.6in
E
qBs
2179.0
TS 2 in softer rock
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Implications ‐ Lawrenceville
• Terminated in material that did not meet local criteria for rock: “rock auger refusal”.
• Nominal/ultimate unit base resistance significantly greater than current design limits.
• High nominal/ultimate unit side resistance can be achieved.
Implications ‐ Lawrenceville
• Formed committee
• Atlanta area practitioners, ADSC Southeast Chapter, and the researchers
• Reviewed results, local practice
• Identified key considerations for applying results
• Developed recommended design values
• Developed specific criteria to be met
• RQD, penetration rate, inspection criteria
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Conclusion
• Tests demonstrated:
• High nominal base and side resistance
• Higher design values than have historically been used can easily be achieved
• Less than “perfect” conditions exceed current design values
• Design guidelines are suggested to provide more economical use of drilled shaft foundations in the two markets.
Conclusion
• Site‐specific criteria are provided to apply test results.
• ALWAYS have a thorough site investigation
• Inspection program to confirm the findings of the site investigation.
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Conclusion
• Reports available:
• Expo Proceedings
• http://danbrownandassociates.com/publications
• Many thanks go out to:
• ADSC Member Firms and Suppliers
• Participating Geotechnical Firms
• Loadtest, Inc.
• Individuals
Questions?
