Nondestructive Testing of Drilled Shaft Foundations

Transcription

1 Nondestructive Testing of Drilled Shaft Foundations Magued Iskander, PhD, PE, F.ASCE

2 Why NDT for Integrity? Drilled shafts cannot be visually inspected after construction Load tests are expensive - only a small number of shafts can be tested To investigate existing Deep Foundations forensic upgrade

3 NDT vs Load Testing NDT is much cheaper than load testing

4 Drilled Shaft NDE Needs Construction techniques rarely yield a straight, homogeneous shaft Use NDE to assess shaft length, cross-section, concrete quality, and presence of anomalies An ideal integrity test would reveal the nature and extent of all defects. Bulging Necking Voids Drilled Shaft Cave-In Length Stiffness

5 What is a defective drilled shaft? Geotechnical problems Structural problems Material (durability) problems The last two problems can be addressed using NDT&E

6 Drilled Shaft Defects

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9 What NDT can do An ideal integrity test would reveal the nature and extent of all defects. Minor defects are unlikely to be detected. Some defects which go undetected could be significant (location in depth and section). Some minor defects may be revealed that do not impair shaft performance (generating unnecessary concern).

10 Contractual Aspects in New Shaft Construction Until recently, NDT was rarely specified. In Europe, more weight given to this testing. DOTs now specifying cross-hole testing. To be avoided: testing company employed by contractor lack of planning in event of discovery of defective shafts (delays in checking and fixing) Problems of interpretation in marginal cases.

11 Available NDT Methods Today Shaft Head Impact Tests (Group A) Down Hole Methods (Group B)

12 Group A - Shaft Head Impact Tests Sonic Echo Impulse Response Analytical Derivatives Velocity Reflector Impedance Log

13 Surface Reflection - Principles Impulse Hammer Ground Surface Sensor Impact generates propagating waves Waves reflect from changes in impedance (shaft toe, void, neck, bulge, etc) Drilled Shaft

14 Sonic Echo - Principles A small impact on the pile head generates a stress wave down the pile (bar wave). The time taken for the wave to return from the pile tip is measured. If the pile length is known, then the concrete velocity can be calculated. c b = 2L/dt

15 Sonic Echo - continued Conversely, if the velocity is known, the pile length can be calculated. Since c b depends on modulus & density, information can be obtained about concrete quality. Early arrivals are from reflectors such as neck-ins or bulges. Limiting length/diameter ratio because of soil damping.

16 Velocity (Volts) Ideal Sonic Echo Response t L = v c t 2v c : f (E,r) Impact Toe Reflection Time (msec)

17 Velocity (Volts) Ideal Sonic Echo Response A small impact on the pile head generates a stress wave. The time taken for the wave to return from the pile tip is measured. If the length is known, then the concrete velocity can be calculated. Conversely, if the velocity is known, the length/thickness can be calculated. Early arrivals are from defects. Since Vc depends on modulus & density, information can be obtained about concrete quality t v c t L = Impact Toe Reflection v c : f (E,r) Time (msec) Vc = 2L/ T 17

18 Typical Sonic Echo Response Neck-in Soft Toe Bulge

19 Sonic Echo: Amplification

20 Impulse Response The shaft head velocity and the input force give the shaft mobility, v/f vs. frequency. At low frequencies, shaft dynamic stiffness. Higher frequencies represent resonance of the shaft, and shaft length is measured, as well as the degree of soil damping. N = 1/(density.area.velocity) = inverse of impedance

21 Early Impulse Response Vibrator

22 Impulse Response - Principles or Shaft

23 Mobility ( V / F ) Impulse Response: Mobility Plot P/Q = Resol. Soft Base f f L = v c 2 f N = PQ Rigid Base Q N P N = 1 r c v c A f m Frequency ( Hz ) 2pf m K = V/F m

24 Sound Shaft Bulge in Body of Shaft

25 Result for Enlarged Shaft Head

26 IR Velocity Reflector Change in section/soil False Toe Pile Tip

27 New Development: Impedance Log A combination of both Sonic Echo (SE) and Impulse Response (IR). Amplified SE time domain response is combined with the characteristic impedance response from the IR test.

28 New Development: Multiple Geophone Approach Measure vibrations at 4 locations with 1 impact Develop composite array for signal processing Compare to numerical simulations Detect and quantify anomalies Approach yields identification of resonances from deep reflections without trial and error geophone locations

29 Surface NDT Methods - Summary Advantages: Relatively fast test with portable equipment Pre-placed borehole tubes not required Determine length (±5%) and concrete quality Determine cross-sectional area and dynamic stiffness (IR) Disadvantages: Not as effective for long shafts (L/D>30) and shafts in very stiff soil Length (or wave velocity) must be known/assumed Identifying multiple defective zones at different depths is delicate Limited by 1-D wave propagation (cave-in vs. neck) Anomaly detection f(depth/position in shaft)

30 Down-Hole & Cross-Hole Tests Nuclear Cross-Hole Gamma-Gamma (Down-Hole) Ultrasonic Sonic Logging (cross-hole)

31 Down-Hole Gamma-Gamma

32 Traces from Gamma-Gamma

33 Gamma-Gamma: Steel v. Plastic

34 CROSS -HOLE SONIC LOGGING (CSL)

35 CSL Equipment Probes Winch

36 Cross-Hole Sonic Logging - Principles Measures the stress wave between two pre-placed access tubes or cored holes filled with water Ultrasonic pulses are transmitted from one tube and received at the same level in a parallel tube Measure the transit time and signal amplitude Tube Location

37 Cross Hole Sonic Logging, CSL Receiver PVC or Steel Access Tubes Full of water Big Defect Out of Cage Defect T = 174 us No Signal T = 230 us Small Defect Source T = 174 us

38 CSL Single Shots

39 Example CSL Results Shaft Base Defect

40 CSL - circa 1968

41 CSL Summary Advantages: Relatively fast test Effective detection of defects (voids, necking, honeycombing) between tubes Not limited by depth or soil characteristics Disadvantages: Pre-placed tubes or coring required May not detect defects at edge of shaft Lateral location of defect not determined Down-hole sonic logging test available

42 Cross-Hole Tomography Uses similar equipment to CSL, but with multiple sources and receivers CSL Tomography Generates a 2-D image of the anomaly Advantage: identify the lateral location and shape of anomalies between tubes Disadvantage: time consuming 42

43 Example CSL and CT Results Data from Olson Engineering Pier 2 Tubes 3-4 Soft Bottom Condition 43

44 Case History Tomography in a Concrete Dam Average 17 source locations 13 receiver locations About 220 raypaths per station Compressional and shear arrivals Data from Olson Engineering 44

45 Example Tomogram with Anomalous Areas

46 Impulse Response on Alpine Reservoir Dam Spillway Reasons for NDT: Observed water seepage at joints and concrete spalling Dimensions are 156 ft long x 52 ft wide at top, tapering to 32 ft wide at bottom 6-14 inch thick concrete, reinforced with one mat at nominally 12 inches 46

47 Ultrasonic/Sonic Pulse Velocity (UPV/SPV) for Concrete Integrity and Strength Prediction Measures speed of sound through concrete Faster velocities = stronger concrete Two Accessible Sides required for interior tests Measurement of Arrival time and Signal Amplitude ASTM Standard C for UPV Testing Leslie and Cheeseman, 1949 Condition Excellent Good Questionable Poor Very Poor Pulse Velocity (feet/second) Above 15,000 12,000 15,000 10,000 12,000 7,000 10,000 Below 7,000

48 Comparison Between CSL & Gamma-Gamma 1. Principles Sonic Logging Gamma Gamma Logging Measures the stress wave between two pre-placed access tubes or cored probe holes. Ultrasonic pulses are transmitted in an access tube filled with water and received at the same level in a parallel tube, while measuring the transit time and the signal amplitude. Measures radiation count in a single access tube (backscatter) or between two parallel tubes or cored probe holes. Radiation source and detector are lowered in the access tubes. Low density zones increase photon counts on detector as the radiation is absorbed.

49 Comparison Between CSL & Gamma-Gamma 2. Advantages CSL Relatively fast. Detection of defects between tubes is more accurate than in Surface Reflection tests. Performance is not limited by depth. Gama-Gama Relatively fast. In twin tube mode, detection of defects between tubes is more accurate than in Surface Reflection methods. Performance is not limited by depth.

50 Comparison Between CSL & Gamma-Gamma 3. Dissadvantages CSL Pre-placed tubes or coring required. May not detect defects at edge of shaft. Gamma-Gamma Pre-placed coring or tubes required. Possible radiation contamination. Maximum distance in back-scatter: 4 inches.

51 RESULTS OF NDT TESTING IN AMHERST GRL

52 Amherst Shaft 1 Correlation Shaft 1 As Built GRL CHA Depth Desc. Legend m (ft) AT ECR NE SE SW 1.5 (5) 16" dia pail void NE, SE, EW (inside) 3.0 (10) fiberglass outside 4.6 (15) 12" dia pail inside void fiberglass outside NE 6.1 (20) NS NE Dia change ######## 7.7 (25) 4" tube coil SW, SE,, NE, EW in & outside (necking)(minor NW, NS) 9.2 (30) 10.7 (35) 12.2 (40) cardboard tube 16" dia pail inside void fiberglass outside NE 13.7 (45) 16" dia pail inside soil NE SE 15.2 (50) SW, NW, EW

53 Shaft 3 As Built GRL CHA Depth No. Legend m (ft) AT ECR Shaft 3 NE SE NS 1.5 (5) 3.0 (10) NS 4.6 (15) 16" dia pail void (inside) fiberglass outside 6.1 (20) NS ######## 7.7 (25) 5 gal fuel tank void NS 9.2 (30) 5 gal fuel tank void 10.7 (35) 12.2 (40) 5 gal fuel tank void (2 tanks) NE (minor) 13.7 (45) 15.2 (50) NS, SE (minor NE)

54 t 4 As Built GRL CHA th No. Legend t) AT ECR Shaft 4 SE SW NW 5) NS 0) bag of wood chip outside necking NW, NS (minor NE, SE) 5) SE cardboard tube 16" dia pail inside void 0) fiberglass outside NW, NS 5) & NE, SE, EW diameter change ####### SE, NS, SW, NE 0) 5 gal plastic pail void SE NW, EW 35) defect not specified 40) 4 in tube coil outside necking SE, NW, NS, EW cardboard tube void 9" dia pail inside NW 45) NE, NW 50) 16" dia pail inside soil NW, SE, EW,SE

55 Shaft 5 As Built GRL CHA Depth No. Legend m (ft) AT ECR Shaft 5 NE SW NW 1.5 (5) 3.0 (10) 4.6 (15) 6.1 (20) ####### 7.7 (25) cardboard tube void 12" dia pail inside EW, NW 9.2 (30) 0.7 (35) 5 gal plastic pail void NW (EW?) 2.2 (40) 16" dia pail inside soil NE, NS 3.7 (45) 5.2 (50) cardboard tube 16" dia pail inside void fiberglass outside NE

56 CSL Testing requires access tube preparation not sensitive to surrounding soils or pile length can not find diameter changes or bulges finds multiple defects, depth and quadrant better at finding defects inside cage (than outside) finds soft bottoms if tubes go to bottom finds voids better than soil inclusions finds larger defects easier than small defects

57 Concluding Remarks NDE tests are essential to economically assess the integrity of drilled shafts For effective use: Must select appropriate test Must corroborate NDE data with Design drawings Subsurface stratigraphy Construction records Visual and destructive tests Compare to numerical models/simulations

58 Acknowledgments Dr. Alan Davis, CTL Prof. Sarah Gassman, U. South Carolina ADSC