Lesson 25 Static Pile Load Testing, O-cell, and Statnamic Reference Manual Chapter 18
STATIC LOAD TESTING Most accurate method to determine static pile capacity Perform at design or construction stage Axial compression, axial tension, and lateral loading
TYPES of LOAD TESTS Axial compression (ASTM D1143) Axial tension (ASTM D3689) Lateral (ASTM D3966)
REASONS TO LOAD TEST Develop information for use in the design and/or construction. Confirm the suitability of the pile-soil system to support the pile design load with an appropriate factor of safety. Implement new static or dynamic analysis methods or procedures. LRFD calibration.
PREREQUISITES FOR STATIC LOAD TESTS Detailed subsurface information program Well-defined soil stratigraphy Static capacity calculations to select pile type, length, and load test locations
DEVELOPING A LOAD TEST PROGRAM Specify required capacity of loading apparatus Specify load cell & spherical bearing plate Specify dial gages with sufficient travel Require dynamic monitoring on load test piles
EFFECTIVE USE OF LOAD TESTS (Design Stage) Allows testing of several pile types or lengths to determine most-economical foundation design Confirms driveability Establishes preliminary driving criteria for production piles Release results to bidders lower costs
EFFECTIVE USE OF LOAD TESTS (Construction Phase) Confirms the pile design loads are OK and installation procedures are satisfactory Refines estimated pile lengths and establishes pile penetration requirements May be only practicable time for load testing on smaller projects
COMPRESSION LOAD TESTS Pile loaded incrementally at pile head Load and pile head movement versus time is recorded Load movement curve is plotted Failure load and corresponding movement are determined Additional instrumentation for load transfer versus depth
Static Load Test Basic Mechanism Q 1 +Q 2 +Q 3, etc. Q 1 +Q 2 +Q 3 Q 1 Q 1 +Q 2 Load (Q) Telltale A Movement Telltale B Telltale A Pile Head Telltale B
COMPRESSION TEST EQUIPMENT Load applied by jacking against reaction piles and beam or weighted platform, or by direct application Load measured with load cell and jack pressure Use spherical bearing plate Movement measured with LVDTs or dial gages with mirror and wire line as backup
Static Load Test - Test Setup Reaction Beam Load Cell Hydraulic Jack Ram Stiffeners Plate Spherical Bearing Bourdon Gage LVDT Dial Gage Wire Mirror Scale Test Pile Grade Bracket Attached to Pile
Reaction Beam Static Load Test Load Application and Monitoring Components Spherical Bearing Plates Load Cell Hydraulic Jack Reference Beam Vishay Box Jack Pressure Gage Jack Pump
Load Test Movement Monitoring Components Reference Beam Dial Gage LVDT Smooth Surface Magnetic Base
Jack Against Weighted Platform
Direct Load Application
Jack Against Reaction Beam and Piles
Load Test Hardware Reaction Beam Spacers Spherical Bearing Plate Ram Hydraulic Jack Test Pile Load Cell
FAILURE CRITERIA The commonly used failure criteria are based on the elastic pile compression plus an offset. Davisson Method The elastic compression, Δ, is calculated as follows: Δ = PL / AE Where: P = axial load in kn (kips) L = pile length in mm (in) A = pile cross sectional area in m 2 (in 2 ) E = elastic modulus of the pile material in kpa (ksi)
FAILURE CRITERIA (b < 24 in) The recommended offset is based on the pile diameter. In US Units s f = + (0.15 +b/120) In SI Units s f = + (4.0 +0.008b) Where: s f = Settlement at failure load in inches (mm) b = Pile diameter or width in inches (mm) = Elastic deformation of total pile length in inches (mm)
FAILURE CRITERIA (b > 24 in) The recommended offset is based on the pile diameter. s f = + (b / 30) Where: s f = Settlement at failure load in inches (mm) b = Pile diameter or width in inches (mm) = Elastic deformation of total pile length in inches (mm)
CalTRANS Pile Load Testing 4000-Ton Mobile Compression or Tension Test Frame
QUESTION Would you feel comfortable using a pile that has been tested to geotechnical (plunging) Failure in compression as a production pile? Why? / Why not?
LOAD TRANSFER EVALUATIONS Determine relative resistance contributions from shaft and toe. Determine load transfer behavior along shaft (shaft resistance distribution). Confirm, correlate, and calibrate static analyses, WEAP input, CAPWAP soil resistance distributions. Refine dragload magnitudes.
VWSG with welded anchor blocks and protective channel VWSG sister bars for concrete embedment
Elevation (ft) Elevation, feet (USGS Datum) 605 Calculated Calculated Axial Axial Compression Load Load in Pile, in tons Pile (tons) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 600 595 590 585 580 575 570 565 560 555 550 545 540 535 37.6, 537.4 53.9, 549.7 16.3 tons / [ 12.3 ft (3.67 ft) ] = 0.36 tsf
T-Z CURVES 101 Based on the previous data/graph, we have an idea of load versus depth T-Z Curves show load transfer from one point to another (i.e., pt. 2 to pt. 3) Unit Load Transfer, t t = (Q 2 - Q 3 ) / (π DL) Displacement, z z = (Displ. at top) Σ(ε*ΔL)
T-Z CURVES 101 We can now relate the unit skin friction/load transfer to the displacement This allows us to determine loadsettlement relationships
Q-Z (or Q-W) CURVES 101 Based on the previous data/graph, we can also address end bearing Q-Z Curves show load transfer at the pile toe End Bearing Pressure, q q = (Q 4 ) / (π r 2 ) Toe Displacement, z z = (Displ. at top) Σ(ε*ΔL)
T-Z CURVES 101 Residual Stresses
TENSION TESTING (UPLIFT CAPACITY) SHIP IMPACT SEISMIC EVENT LATERAL LOADING
19-22
STUDENT EXERCISE #9 (FYI ) An axial compression static load test has been performed and the results must be interpreted to determine if the pile has an ultimate capacity in excess of the required ultimate capacity. The load - movement curve from the static load on a 16 inch square prestressed concrete pile is presented on the following page. The pile has a cross sectional area, A, of 256 in 2 and a length, L, of 100 ft. The concrete compression strength, f'c is 6.0 ksi. The pile has a required ultimate pile capacity of 800 kips.
Osterberg Cell Method O-Cell Reference Manual Chapter 19
Comparison Between Standard Load Test and Osterberg Cell Test
Osterberg Cell PORTABLE COMPUTER BOTTOM PLATE TELLTALE INDICATOR TOP OF PILE INDICATOR REFERENCE BEAM Cast in Pile DATA LOGGER PILE COMPRESSION INDICATOR PRESSURE TRANSDUCER HYDRAULIC PUMP WITH PRESSURE GAUGE PILE SHAFT RESISTANCE TELLTALE CASINGS HYDRAULIC RETURN LINE HYDRAULIC SUPPLY LINE OSTERBERG CELL CAST INTO PILE PILE TOE RESISTANCE
Osterberg Load Cell Test Socket Load Cell Reaction Socket 14-17
Aucilla River Subsurface Profile 2.1m 3.2m Dense Silty Sand Silty Clay with Sand Lenses 21.3m 15.3m 0.7m Limerock
Aucilla River Test Results
O-CELL ADVANTAGES Reduced Cost & Improved Safety No External Reaction System is Required End Bearing & Shaft Friction Measured Test Over Water High Capacity
O-CELL DISADVANTAGES Limited to displacement piles Pile selection in advance O-Cell is expendable Specialists are required Patented Ultimate capacity is NOT determined Void is created after unloading
Statnamic Method Reference Manual Chapter 20
Statnamic Concept
1000 Kip Device 9000 Kip Device
Retention Structure Schematic
Accelerometers attached to the pile top surface are now more commonly used to obtain displacement instead of laser.
Load Movement Plot From Load Cell & Laser Data
Displacement (mm) Derived Statnamic Load Displacement Curve (UPM) Load (MN)
Loading Rate Reduction Factors NCHRP 21-08 (2002) recommended a loading rate reduction factor also be applied to account for over-predictions associated with loading rate. Recommended reduction factors are: Rock = 0.96 Sand = 0.91 Silt = 0.69 Clay = 0.65
STATNAMIC TEST RESULT 1067 mm O.D. Open End Pipe Pile Derived Static with Rate Factor FHWA Failure Line
STATNAMIC ADVANTAGES Cost & Speed External Reaction System Not Required Works on Battered Piles
STATNAMIC DISADVANTAGES Dynamic Test Proprietary Specially Trained Personnel
Any Questions?