Evaluation & Guidance Development for Post-Grouted Drilled Shafts

1. Evaluation & Guidance Development for Post-Grouted Drilled Shafts Dr. Antonio Marinucci, MBA PE • ADSC West Coast Chapter Annual Meeting Pismo Beach, CA 20 May 2016

2. Outline Background Current State of Practice in U.S. Evaluation and Analysis Summary and Findings Caltrans Research Interest Impacts on Industry

3. Background

4. Load Transfer – Conventional Shafts (mod. FHWA GEC-10, 2010) Mobilized (4% to 5%) * D (cohesive) (10%) * D (cohesionless) Mobilized (0.2% to 0.4%) * D Due to Strain Incompatibility – Designers typ neglect either side or tip resistance (or reduce contributions)

5. FHWA Research (2011) • Since early 2000s - increased use of PGDS • Spurred by FL DOT sponsored research • Muchard (2015) • 19 different DOTs  35 bridge projects; >1,800 shafts • Performed mostly by specialty service firms • Most in FL and TX, followed by CA and LA • FHWA and DOTs • No standardized design & evaluation guidance • Many issues remain • Actual improvement mechanisms? • Consistency and control of the grouting process? • Objective  Guidance Documents • Design methodologies • Verification (QC/QA)

6. Post-grouting of Drilled Shafts (PGDS) • Two basic types • Base (or tip) grouting • Technique used to inject, under pressure, a neat cement grout beneath the base of a drilled shaft • Better align load transfer mechanisms • Improve mobilization of shaft resistance • Enhance or improve axial load-displacement performance • Increase axial resistance – mainly at base • Side (or skin) grouting • Technique used to inject, under pressure, a neat cement grout around the perimeter (along length) of a drilled shaft • Enhance or improve side resistance

7. PGDS Generalized Process Drill borehole in soil/rock Install reinforcement, NDT tubes, instrumentation, and grout distribution system Place concrete in borehole

8. PGDS Generalized Process • Water flush grout pipes • After flushing is complete, close valve at return • After concrete has set/cured, can begin post-grouting operation • Inject grout under pressure below base of shaft

9. PGDS Generalized Process (at pump) Grout pressure (gauges, transducers) Upward displacement (visual survey, dial gauges) Volume of grout (flow meter, strokes, cement) • Injection of grout continued until criteria is achieved • Pressure • Displacement • Volume • Strain (maybe) • Bi-directional force induced at tip mobilizes • Negativeside resistance • Positivetip resistance

10. PGDS Generalized Process Axial Loading • Following post-grouting, base resistance has been mobilized (or pre-loaded) • Reversal of side resistance to resist axial loading • Positiveside resistance • Positive tip resistance

11. Purposes / Benefits of PGDS • Improve overall performance • Mobilize resistance at smaller displacements • Optimize design - better alignment of load transfer curves • Increase (usable) end bearing in design computations • Increase resistance of shaft • Improve soil below base • Increase side resistance (upward grout migration) • Increase tip area (some cases) • Reduce settlement under loading • Design verification • Verifying lower-bound resistance

12. Purposes / Benefits of PGDS • Risk mitigation • Improve reliability in tip resistance • Reduce uncertainties with bottom cleanliness • Soil loosened during construction may be restored • Tool for QC (Muchard, 2005) • Level of “proof testing” • Atypical behavior noticed • Upward displacement – low side resistance • Large grout takes - low base resistance; imperfect bottom cleaning • Cost consideration • More constructible - shorten length and/or diameter • Save time and money

13. Current State of Practice in U.S.

14. Effect of Soil Conditions (from literature) • Granular soils • Considered most suitable • Silty sands or fine-grained sands • Some improvement observed • Extremely silty or micaceous soils – more prone to hydrofracture when grouting to high pressures • Cohesive soils (low ) • Considered unsuitable • Improvements reported in international case histories • Strongly cemented soils or rock • Considered unlikely to be improved

15. Grout Distribution Systems - Open Type • (FHWA GEC-10, • 2010) (Castelli, 2012) Sleeve-port (Tube-à-Manchette) distribution system Redundant system (multiple, independent U-tubes) Grout does not act across entire base area at same time Can use NDT tubes Well-suited for all shaft sizes, esp. larger diameter shafts Well-suited for phase grouting

16. Grout Distribution Systems - Closed Type (Applied Foundation Testing) Flat-jack distribution system Grout acts across area of plate at same time Good for smaller diameter shafts (≤ 6 ft in diameter) Requires separate grout tubes - cannot use NDT tubes No redundancy in system Not conducive to phase grouting

17. Grout Distribution Systems - Gravel • Sleeve-port distribution system • Gravel basket distribution system • (Sliwinski and Fleming, 1984) • (Bolognesi and Moretto, 1973) • Use • Min. thickness = 4 inch (level bottom of hole) • Max. thickness = 24 inch (thinner preferred) • Sufficiently clean (no additional fines added) • Must be tamped down • Purpose • Aid Grout Distribution • Contoured Shaft Bottom • Correction for over drilling

18. Grout and Grouting • Simple water-cement mix • Type I/II Portland cement; no Sand • {Admixtures - control flowability and set times} • Water/cement ratios – 0.4 to 0.6 (high as 0.75) • Important properties of grout mix • Flow, pumpability, viscosity • Compressive Strength - 2,000 to 2,500 psi (…stiffer than the soil) • Quality control (in field) • Specific gravity measured using mud balance • Fluidity (flowability) measured with a flow cone

19. Grout and Grouting • Grouting pressures • Pressure = f(stress state, self-weight of shaft, side friction) • Upper limit = 750 to 800 psi (50 to 55 bar) • Risk of clogging when > 600 psi (45 bar) • Maintained to ensure base is pressurized • Phase grouting - use when… • Desired grouting criteria not achieved • Examples • Excessive grout takes are observed • Grout pressure not achieved or sustained • Excessive movements of shaft occur

20. Measurements and QC/QA (Grouting Criteria) Grout pressure Upward displacement Volume of grout • Grout Pressure • Minimum value for some duration • Grout Volume • Minimum (net) volume - ensure system filled, no line blockage • Maximum volume • Top-of-Shaft Displacement • Maximum = ¼ to ¾-inch (typ) • Reset - each phase of grouting • Phase grouting • if criteria not achieved

21. Measurements and QC/QA (in Real Time) (Winters, 2014; Mullins, 2015)

22. Measurements and QC/QA (in Real Time) (Applied Foundation Testing) • Strain gauges – during grouting • How effectively grout distributed across base of shaft • Compared to grout pressure and shaft uplift

23. Design Methods

24. Design Methods • Tip Capacity Multiplier Approach (Mullins et al, 2006) • Component Multiplier Approach (Hu et al, 2001; Xiao et al, 2009; Guoliang et al, 2012) • Axial Capacity Multiplier Approach (Ruiz, 2005; Ruiz et al, 2005) • Truncated Cone Approach (Liu and Zhang, 2011) • Simplified Approach (McVay, 2010)

25. Tip Capacity Multiplier (TCM) Approach (Mullins et al, 2006) • Most widely used in U.S. post-grouting practice • Predicts tip resistance at a normalized displacement • Based on 26 load tests; 2-4ft diam.; 25-60ft long; Sands • GPI = Grout Pressure Index (ratio)

26. Tip Capacity Multiplier (TCM) Approach (Mullins et al, 2006)

27. Component Multiplier (CM) Approach • (Hu et al, 2001; Xiao et al, 2009; Guoliang et al, 2012) Empirical method based on data from 186 sites Does not explicitly include sustained grout pressure Ultimate shaft capacity predicted by Guoliang et al (2012)

28. Current Design Methods - Limitations • Analytic predictions of ungrouted tip resistance are themselves sources of considerable uncertainty • Predictions of tip resistance of PGDS are a product of two highly variable estimates • Require input of information / data that is difficult to define • Do not consider various factors in predictions • Effect of grouting apparatus • Grout characteristics • Other construction variables • Grouting variables that affect resistance of PGDS • Etc…

29. Evaluation and Analysis of Published Load Test Results

30. Data Reported in Literature

31. Mechanisms for Improving Performance • Identify and separate improvement in performance Pre-Mobilization of load– (occurs in any type of ground) • Improved mobilization of resistance due to “pre-loading” of shaft • At a given load - results in stiffer response with less settlement Ground Improvement at Tip – (increase in nominal resistance) • Improved tip resistance – densification and permeation • Improved tip resistance - formation of an enlarged tip area • Improved side resistance - upward flow of grout around perimeter of the shaft

32. Improvement Mechanisms - Analysis • Considering each mechanism separately would lead to improved understanding and predictions • Challenge • Make sense of data, given variations in • Grouting methods • Ground conditions • Method of load testing • Quality of test documentation… • Analytical work • Performed by graduate students at Univ of Missouri • Under direction of Dr. Erik Loehr

33. Analytical Modeling Analytical model of load transfer behavior of ungrouted shaft (modified: Loehr et al, 2014) Generalized load transfer behavior of an ungrouted shaft (modified: FHWA GEC-10, 2010)

34. Analytical Modeling - Comprehensive Method Considering pre-mobilization (only) (mod. Loehr et al, 2014)

35. Analytical Modeling – Simple Method Considering pre-mobilization (only) (mod. Loehr et al, 2014)

36. Evaluation of Results - trends?? • GIR = Ground Improvement Ratio (pre-mobilization only) • GIR=1.0 - pre-mob (only) • GIR>1.0 - both mechanisms • TIR = Total Improvement Ratio • (accounts for both effects) • TIR>1.0 – improvement from post-grouting

37. Evaluation of Results - trends??

38. Summary of Findings

39. Summary of Findings • Improvement in shaft performance from at least four different “improvement mechanisms” • Pre-mobilization • Improvement beneath shaft tip • Enlarged tip area • Upward flow of grout around shaft perimeter • Design Methods (based on load tests considered) • TCM Method overestimated tip resistance • CM Method slightly underestimated shaft capacity • Soil conditions • Improvement in performance similar for shafts in sand and clay • Improvement may be realized for shafts tipped in rock

40. Summary of Findings • Pre-mobilization Effect • Does not result in an increase of ultimate axial resistance • Function of bi-directional load induced during grouting • Greater the load attained  more enhanced performance of PGDS • Redistribution of load-transfer behavior • Stiffer load-displacement response at intermediate loads • Ground improvement effect • Improvement is ground-dependent • Greater improvement expected in clean, loose, granular materials • Less improvement expected in cohesive soils or intact rock • Enhancement will increase ultimate axial resistance of PGDS

41. Caltrans Research Interest

42. Caltrans RFP – Research Interest • Issue - use of tip PGDS in Caltrans very limited due to • Concerns with reliability and repeatability • Lack of Standards and Specifications, QC/QA protocol • Substructure Cmt - use of PGDS as “risky, unreliable, unverifiable” • Objective – develop guidance documents / specifications • Estimate PGDS capacity using instrumentation • Recommended instrumentation • Strain gauges near shaft tip, telltales, measurement of top of shaft displacement • Detailed protocols for instrumentation • Amount of each type, placement, monitoring procedures • Estimates of uncertainty in inferred capacity • QC/QA measures for non-instrumented production shafts

43. ConclusionsImpacts on Industry

44. Take away – Impacts on Industry • Pre-mobilization = readily estimated or predicted • Independent of material type or stress-strain behavior of ground • More predictable than ground improvement mechanism • Maximizing induced load (not grout pressure) is key to performance • Enhanced load-deformation response of PGDS • Greater reliability in drilled shaft construction • Greater tip resistance and stiffer response at intermediate loads • Cost efficient designs  reduced foundation costs • Use side and tip resistance in designs due to strain compatibility PGDS- not to be used in lieu of good workmanship in construction of a drilled shaft

45. Thank you foryour attention!! Questions??