Evaluation of Rock Socket Design Evaluation of Rock Socket Design from Load Tests in the Southeastern U.S. W. Robert Thompson, III, P.E., D.GE W. Robert Thompson, III, P.E., D.GE Dan Brown and Associates, PC Dan Brown and Associates, PC M t Al b M t Al b Montgomery, Alabama Montgomery, Alabama
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Evaluation of Rock Socket DesignEvaluation of Rock Socket Design from Load Tests in the Southeastern U.S.
W. Robert Thompson, III, P.E., D.GEW. Robert Thompson, III, P.E., D.GEDan Brown and Associates, PCDan Brown and Associates, PC
M t Al bM t Al bMontgomery, AlabamaMontgomery, 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)
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 rig2in/5min with LLDH rig
Test Program
• Two sites tested to date
• Nashville, Tennessee• Hard Limestone (sometimes solutioned)
• Birmingham, Knoxville, Chattanooga
• Lawrenceville, Georgia (Metro Atlanta)M t hi k (G i S hi t t )• Metamorphic rock (Gneiss, Schist, etc.)
• Piedmont Formation – Georgia to Virginia
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 GroupGroup
Nashville Site
00 10 20 30 40 50 60 70 80 90
RQD, % Rock Core RQD
-15
-10
-5
Sock
et
Test Shaft 2 RQD
Test Shaft 1 RQD
RQD ‐Nashville
30
-25
-20
ow T
op o
f S
-40
-35
-30
Dep
th B
el
qu = 5,000 to 20,000 psi
-50
-45, p
b l l k ( f )Construction ‐ Nashville
• 48in core barrel to excavate limestone sockets (16ft)
• Mechanical cleaning only
• Inspectors consensus:
• TS 1 had 3in to 4in soil seam 19in below base• TS 1 had 3in to 4in soil seam 19in below base
• Shaft should have been extended
• TS 2 no significant seams• TS 2 no significant seams
• Typical conditions sought
B h h f d d ddi i l l i• Both shafts needed additional cleaning
• Significant concrete overrun in TS 2
Unit Side Resistance ‐ Nashville
• 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)( 0.2in)
Unit Side Resistance ‐ Nashville
25
Unit Side Resistance vs Normalized Upward O-Cell Displacement
~ 0.2in
20
ce, k
sf
Test Shaft 1
10
15
e R
esis
tanc Nominal Dia = 48" Nominal Dia = 52.5"
5
Uni
t Sid
e
00.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2%
Displacement/Diameter, %
Unit Base Resistance ‐ Nashville
• Displacements about 1% of loaded area(B 39i f TS 1 B 29i f TS 2)(B = 39in for TS 1; B = 29in for TS 2)
I ti i di t d il 19i b l TS 1• Inspections indicated soil seam 19in below TS 1(10% of B approx. 0.5B below shaft base )
• Maximum unit base resistanceTS 1 500 k f• TS 1 = 500 ksf
• TS 2 = 1250 ksf
Unit Base Resistance ‐ Nashville
Bearing Pressure, ksf
Unit Base Resistance vs Normalized Downward O-Cell Displacement
-0.25
00 250 500 750 1000 1250
Test Shaft 1
Test Shaft 2
1
-0.75
-0.5
pl/D
ia, %
Test Shaft 2
E=630ksi (0.5%)E=335ksi (0.5%)
-1.5
-1.25
-1
Dis
p
E=536ksi (1%)
E 235k i (1%)~ 0.5in (D =39in) ~ 0.3in (D =29in)
-2
-1.75E=235ksi (1%) 0.5in (D 39in)
( )qBs
2179.0 νρ −⋅=
Soil seam beneath TS 1?
Es 79.0ρ
Implications ‐ Nashville
• “Sound Rock”: conditions similar to Test Shaft 21 2 ll ½ i h thi k• 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 Bsoil 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)designs (when socket not needed for lateral)
• Utilizing side resistance in “Fair Rock” conditions may• Utilizing side resistance in Fair Rock conditions may be prudent • When socket > 10ft to “find” base resistanceWhen socket > 10ft to find base resistance
• Use lower base resistance + side resistance
Lawrenceville Site
Construction ‐ Lawrenceville
• TS 1 – Test side and base resistance of 40.5in rock socketsocket• Started with core barrel, completed with rock auger
• Penetration = 4 to 6 5in/min – not “Rock Auger Refusal”Penetration = 4 to 6.5in/min not Rock Auger Refusal
• TS 2 – Test base resistance at “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”
Rock Core RQD ‐ LawrencevilleRQD %
5
100 10 20 30 40 50 60 70 80 90 100
haft
1)
RQD, %
Test Shaft 1 Test Shaft 2
5
0
5
of S
ocke
t (S
haft
2), (
ft)
15
-10
-5
Belo
w T
op o
or P
WR
(S
25
-20
-15
Dept
h
-25
qu = 7,000 to 11,000 psi
Test Shaft 1 ‐ Lawrenceville
Test Shaft 2 ‐ Lawrenceville
Unit Side Resistance ‐ Lawrenceville
50
55
Unit Side Resistance vs Normalized Upward O-Cell Displacement
Sh ft 1
35
40
45
stan
ce, k
sf
PWR - Shaf t 1 - Nominal Dia = 42"
Shaft 1 ~0.2in
Shaft 1 ~0.6in
20
25
30
nit S
ide
Res
is ROCK - Shaft 1 - Nominal Dia = 40.5"
PWR - Shaf t 2 (19-35f t) - Nominal Dia = 66"
PWR - Shaf t 2 (35-42.5) - Nominal Dia = 66"
5
10
15
20
Un
0
5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Displacement/Diameter, %Shaft 2Shaft 2 ~0.3in
U it Sid R i t N li d U d O C ll Di l t
Unit Side Resistance ‐ Lawrenceville
50
55
Unit Side Resistance vs Normalized Upward O-Cell Displacement
35
40
45
stan
ce, k
sf
PWR - Shaf t 1 - Nominal Dia = 42"
ROCK Shaft 1 Nominal Dia = 40 5"
20
25
30
nit S
ide
Res
is ROCK - Shaft 1 - Nominal Dia = 40.5
PWR - Shaf t 2 (19-35f t) - Nominal Dia = 66"
PWR - Shaf t 2 (35-42.5) - Nominal Dia = 66"
5
10
15
Un
0.3in
0
5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Displacement/Diameter, %
Unit Base Resistance ‐ Lawrenceville
~ 1.6in
~ 2in( )E
qBs
2179.0 νρ −⋅=
TS 2 in softer rock
Implications ‐ Lawrenceville
• Terminated in material that did not meet local criteria for rock: “rock auger refusal”criteria for rock: rock auger refusal .
N i l/ lti t it b i t i ifi tl• Nominal/ultimate unit base resistance significantly greater than current design limits.
• High nominal/ultimate unit side resistance can be achievedachieved.
Implications ‐ Lawrenceville
• Formed committeeAtl t titi ADSC S th t Ch t d• Atlanta area practitioners, ADSC Southeast Chapter, and the researchers
• Reviewed results local practiceReviewed results, local practice
• Identified key considerations for applying results
• Developed recommended design values• Developed recommended design values
• Developed specific criteria to be met• RQD penetration rate inspection criteria• RQD, penetration rate, inspection criteria
Conclusion
• Tests demonstrated:
• High nominal base and side resistance• High nominal base and side resistance
• Higher design values can easily be achieved
L th “ f t” diti d t• Less than “perfect” conditions exceed current design values
• Design guidelines are suggested to provide more economical use of drilled shaft foundations in theeconomical 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.
Conclusion
• Reports available:E P di• Expo Proceedings
• http://danbrownandassociates.com/publications
• Many thanks go out to:• ADSC Member Firms and Suppliers• ADSC Member Firms and Suppliers