from Load Tests in the Southeastern U.S.

Transcription

1 Evaluation of Rock Socket Design from Load Tests in the Southeastern U.S. W. Robert Thompson, III, P.E., D.GE Dan Brown and Associates, PC Montgomery, Alabama

2 Introduction Research project sponsored by ADSC SE Chapter Rock socketed drilled shafts Aim to improve: Design methods Cost efficiency Involve designers, contractors, owners (DOTs)

3 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

4 Test Program Two sites tested to date Nashville, Tennessee Hard Limestone (sometimes solutioned) Birmingham, Knoxville, Chattanooga Lawrenceville, Georgia (Metro Atlanta) Mt Metamorphic rock (Gneiss, Shit Schist, etc.) Piedmont Formation Georgia to Virginia

5 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

6 Nashville Site

7 Depth Bel ow Top of Socket RQD, % Test Shaft 2 RQD -10 Test Shaft 1 RQD Rock Core RQD Nashville qu = 5,000 to 20,000 psi -50

8 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 Shaft should have been extended TS 2 no significant seams Typical conditions sought Both shafts hf needed dadditional i cleaning Significant concrete overrun in TS 2

9 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 (0.2in)

10 Unit Side Resistance Nashville Unit Side Resistance vs Normalized Upward O-Cell Displacement 25 ~ 0.2in Unit Side Resistanc ce, ksf Test Shaft 1 Nominal Dia = 48" Nominal Dia = 52.5" 0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% Displacement/Diameter, %

11 Unit Base Resistance Nashville Displacements about 1% of loaded area (B = 39in for TS 1; B = 29in for TS 2) Inspections indicated dsoil seam 19in below TS 1 (10% of B approx. 0.5B below shaft base ) Maximum unit base resistance TS 1 = 500 kf ksf TS 2 = 1250 ksf

12 Unit Base Resistance Nashville Unit Base Resistance vs Normalized Downward O-Cell Displacement Bearing Pressure, ksf Disp pl/dia, % ~ 0.5in (D =39in) E=335ksi (0.5%) Soil seam beneath TS 1? E=235ksi (1%) Test Shaft 1 Test Shaft 2 E=630ksi (0.5%) E=536ksi (1%) ~ 0.3in (D =29in) 2 qb( 1 ν ) ρ s = 0.79 E

13 Implications Nashville Sound Rock : conditions similar to Test Shaft 2 1 or 2 small seams < ½ inch hthick 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)

14 Implications Nashville Side resistance not factor in Sound Rock for typical designs (when socket not needed for lateral) Utilizingsideresistance in Fair Rock conditionsmay be prudent Whensocket > 10ftto to find base resistance Use lower base resistance + side resistance

15 Lawrenceville Site

16 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

17 Rock Core RQD Lawrenceville RQD, % haft 1) Below Top of Socket (S or PWR (S haft 2), (ft) Depth Test Shaft 1 Test Shaft 2-25 qu = 7,000 to 11,000 psi

18 Test Shaft 1 Lawrenceville

19 Test Shaft 2 Lawrenceville

20 Unit Side Resistance Lawrenceville Unit Side Resistance vs Normalized Upward O-Cell Displacement 55 Un nit Side Resis stance, ksf Shaft 1 ~0.2in Shaft 1 ~0.6in PWR - Shaft 1 - Nominal Dia = 42" ROCK - Shaft 1 - Nominal Dia = 40.5" PWR - Shaft 2 (19-35ft) - Nominal Dia = 66" PWR - Shaft 2 ( ) - Nominal Dia = 66" Shaft 2 Shaft 2 ~0.3in Displacement/Diameter, %

21 Unit Side Resistance Lawrenceville Unit Side Resistance vs Normalized Upward O-Cell Displacement Un nit Side Resis stance, ksf in PWR - Shaft 1 - Nominal Dia = 42" ROCK - Shaft 1 - Nominal Dia = 40.5" PWR - Shaft 2 (19-35ft) - Nominal Dia = 66" PWR - Shaft 2 ( ) - Nominal Dia = 66" Displacement/Diameter, %

22 Unit Base Resistance Lawrenceville ~ 1.6in ~ 2in ρ s qb = ( 1 ν ) E TS 2 in softer rock

23 Implications Lawrenceville Terminated in material that did not meet local criteria for rock: rock auger refusal. Nominal/ultimate lti t unit base resistance it significantly ifi greater than current design limits. High nominal/ultimate unit side resistance can be achieved.

24 Implications Lawrenceville Formed committee Atlanta t area practitioners, ADSC Southeast t 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

25 Tests demonstrated: Conclusion High nominalbase and side resistance Higher design values 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.

26 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.

27 Conclusion Reports available: Expo Proceedings Many thanks go out to: ADSC Member Firms and Suppliers Participating Geotechnical Firms Loadtest, Inc. Individuals

28 Questions?