DEEP FOUNDATION TESTING - QA/QC Measures Post Installation

Nandhagopal Raja is writing,

PILE INTEGRITY TEST

Pile Integrity test (PIT) is a low strain, non-destructive integrity test method for foundation piles. This test can be performed on various types of piles including concrete piles, steel piles, and timber piles. The purpose of the test is to detect any damage or defects in the pile that may affect its load-bearing capacity.

In this method, a small hammer is used to strike the top of the pile, which creates a stress wave that travels down the pile. At the same time, an accelerometer is placed on the pile to record the wave that is reflected back up the pile. The reflected wave data is then analysed to identify any issues such as cracks, voids, or changes in cross-section in the deep foundation. The amplitude and shape of the wave can also provide information on the length of the pile and the type of material it is made of. For cast in place concrete piles or concrete filled pipe piles integrity test is carried no sooner than 7 days of cast of piles or after achieving a minimum of at least 75% of the design strength or whichever occurs earlier.

Pile Integrity Tester (PIT) Consists following equipment:?

• A computer-based PIT data acquisition system for display of analog signals during data acquisition, with atleast 12-bit resolution (16 bit or higher resolution is preferred) with a sampling frequency of at least 25 000 Hz each for the motion sensor and the optional instrumented hammer, if used.?
• Accelerometer - An accelerometer is a sensing device that measures the vibration, or acceleration of motion of a pile. Obtain velocity data from integration of signals from (one or more) accelerometers, provided the acceleration signal(s) can be integrated to velocity in the analog display for reducing data. The accelerometer(s) should be placed at (or near) the pile head and shall have their sensitive axis parallel with the pile axis. Accelerometers shall be linear to at least 50 g.?
• Impact Force Application - The impact may be delivered by a hand-held hammer that will produce an input force pulse of generally less than 1ms duration and should not cause any local pile damage due to the impact. A hammer with a very hard nylon tip can induce a short input force pulse without causing local pile damage. The impact should be applied axially to the pile (normally on the pile head).

The test method is as outlined in ASTM D 5882-16 (Low strain impact integrity testing for deep foundations).

The pile top is prepared to obtain clean and hard surface by chipping/grinding. The accelerometer is firmly fixed on pile top using wax and small impacts were given using a hand held hammer. Pile top motions as a function of time was measured using the collector model Pile Integrity Tester (PIT-X or PIT-Q collector), which is a precision instrument developed by PDI Inc. USA. The record of pile top motions (together with all the information provided viz., soil profile, pile geometry, etc.) were further analysed using PIT-W Software in time domain and the integrity evaluated although the method is approximate and not exact. This method will not give information on pile bearing capacity.

It is important to ensure that the pile top is above the water level and the pile is free from the loose material and other foreign materials. The pile head is to be grinded to smooth surface for fixing of the motion sensor and for striking. The accelerometer is to be attached to the prepared pile head and ensure for its firmness and at selected locations away from the edges of the pile head. In case of larger pile diameters (> 500mm) the accelerometer shall be fixed firmly at various positions that an integrity evaluation near the pile head may be made for each section of pile. The position of the handheld hammer is to be in such a way that the impact is applied axially to the pile and the spacing between the hammer impact & accelerometer shall not be greater than 300mm. It is set up the PIT instrument for displaying and recording of data. Multiple impacts, as desired by the testing engineer, are given to capture the consistent velocity signals. The data quality is verified by the operator for all the impacts & its consistency.

The interpretation for pile length depends on the wave speed used. Generally, wave speed for a good quality concrete ranges from 3500m/sec to 4300m/sec. For piles where multiple reflections occur near toe, interpreted length is decided based on the pile length input provided and the allowable wave speed limits. Interpretation for pile length cannot be performed if clear toe reflection is not identified. Early positive reflections are evaluated for a possible defect. Change in impedance due to reduction in cross-sectional area (necking), honeycombs, inclusions, voids etc. can be termed as defects. The following figure shows the difference between good and bad pile.

The limitations of the test are,

·???The accuracy of length determination depends on the assumed wave speed which can vary from 3500m/sec to 4300m/sec for a good quality concrete, hence a variation in length up to ± 10% is expected.

·???Significant change in pile cross-sectional area (neck or bulge) and soils stiffness might cause repetitive multiple positive reflections making the interpretation inconclusive.

·???The length up to which reflections could be clearly seen depends on L/D ratio and the soil resistance. Generally, clear toe reflections can be obtained up to an L/D ratio of 40.

CROSS HOLE SONIC LOGGING

Cross hole sonic logging (CSL) is a type of non-destructive testing method used to determine the integrity and quality of concrete within drilled shafts, piles or other deep foundations. Two or more sonic tubes are installed by being tied to the reinforcement prior to casting of concrete in a bored pile. The cross hole probes / trans-receivers emit ultrasonic pulses and the other probes receive the same simultaneously while the access tubes are filled with water.

A device called the CHAMP-Q provided by PDI, USA is being used to provide this service.

Cross Hole Analyzer-TM (CHAMP-Q) Consists following equipment:

·???????? A computer based CSL data acquisition system for display of signals during data acquisition, with a minimum 12-bit A/D converter with a sampling frequency of at least 500,000 Hz and recording of all pulse signals for full analysis and individual inspection.

·???????? Ultrasonic transmitter and receiver probes capable of producing records at a minimum frequency of 40,000 Hz with good signal amplitude and energy through good quality concrete. The probes shall be less than 1.1 inches in diameter and shall freely descend through the full depth of properly installed access tubes in the bored piles.

·???????? Two depth sensors to independently determine transmitter and receiver probe depth.

·???????? Triggering of the recording system time base with the transmitted ultrasonic pulse

By sending ultrasonic pulses through concrete from one probe to another (probes located in parallel tubes), the CSL procedure inspects the bore piles structural integrity, extent and location of defects, if any. Both the time between pulse generation and signal reception (First Arrival Time or “FAT”) and the strength of the received signal give a relative measure of the quality of concrete between transmitter and receiver. Dividing the distance between transmitter and the receiver by the FAT value yields the approximate concrete wave speed, which also is a relative indicator of concrete quality. For equidistant tubes, uniform concrete between the test tubes yield consistent arrival time with reasonable pulse wave speed and good signal strength. Non uniformities such as contaminated or soft concrete, honeycombing, voids and inclusions exhibit delayed arrival times which reduce signal strength.

The limitations of the test are,

·???????? Proper installation of the access tubes is essential for effective testing and interpretation. The method does not give the exact type of defect (for example, inclusion, honeycombing, lack of cement particles, etc.) but rather only than a defect exists.

·???????? The method is limited primarily to testing the concrete between the access tubes and thus gives little information about the concrete outside the reinforcement cage to which the access tubes are attached to the inside of the reinforcement cage.

The results of the test are given in the mode of ratings of the pile integrity considers the increases in “first arrival time” (FAT) and the energy reduction relative to the arrival time or energy in a nearby zone of good concrete. The following criteria for evaluation of the concrete from the CSL test shall be considered.

·??(G) (Good): FAT increase 0 to 10% and Energy Reduction < 6 db

·??(M) (Minor Anomalies): FAT increase 11 to 20% and Energy Reduction of < 9 db

·??(P/F) (Poor/Flaw): FAT increase 21 to 30% and Energy Reduction of 9 to 12db

·??(P/D) (Poor/Defect): FAT increase >31% and Energy Reduction > 12 db

HIGH STRAIN DYNAMIC PILE LOAD TEST

1. High strain dynamic pile load test (HSDLT) is a method to determine the load-carrying capacity of a pile. This test involves using a high-impact hammer to strike the top of the pile, creating a compressive stress wave that travels down the pile. During the test, accelerometer and strain sensors are attached to the pile to measure the velocity and force of the stress wave as it travels down the pile and is reflected back up. These measurements can then be used to calculate the resistance of the pile in compression and tension. The signals are digitized by the Pile Driving Analyser (PDA), results are computed, and the data array of the signals for a blow is stored. As per PDI guidelines & ASTM standards, the minimum pile settlement per blow required for mobilizing ultimate pile capacity is 3 mm. If the measured pile set/blow is less than 3 mm then the capacity shall be considered as ‘Mobilized capacity’ only. A typical force velocity response is shown below.

The PDA functions based on the CASE method to compute the static pile capacity from the pile top force and velocity data using the assumed case damping factor (Jc) and uniform cross-sectional area. Hence, it is recommended to use the CAPWAP software to analyse the data in a better way and estimate the pile capacity. CAPWAP analyses also provides an approximation of the shaft friction / end bearing components, pile integrity and stress distribution. CAPWAP completes the dynamic load testing procedure and simulates a static load test. The allowable dynamic stresses such as compression stresses induced at top and bottom, tension stresses along the shaft, and the energy transferred to the pile or shaft, and the pile integrity are also monitored based on estimations with respect to the yield strength of the concrete, steel, and their respective cross-sectional areas. The strain gauges have a nominal range of 2000-2500 micro strain. The strain gauge measurements during driving are averaged and converted to a force-time history for the pile area at the gage location, based on the manufacturer supplied gage factor and Young’s Modulus for concrete. The accelerometers, have a range of 5,000 g’s. They are bolted adjacent to the strain gauges and their responses during driving are electronically averaged and integrated into velocity as well as displacement-time histories.

The PDA indicates the BETA factor when there is reduction in pile cross sectional area. Nominally the BETA factor represents the percentage of pile cross section available compared to the full pile cross section as per the design. The PDA inspects the wave up curve for local minimums and when present, signals that the pile is damaged. BETA factors are very useful for driven piles with uniform cross-sectional area, which will ease the damage evaluation during the time of testing

THERMAL INTEGRITY PROFILING

Thermal Integrity Profiling (TIP) helps in evaluating the entire cross-section and the entire length of the deep foundation element by measuring the heat generated by curing of concrete to assess the quality of bored piles. The thermal wire cables are tied up to the reinforcement cage of the pile and after the concreting is completed the measurement are made. The average temperature within a concrete shaft is dependent on its diameter, the concrete mix design and on the time of measurement relative to concrete casting. Measured temperatures at the reinforcement cage vary with the distance to the center of the shaft and with the concrete cover.

Data collected by the TIP system is downloaded to a computer for analysis by the TIP Reporter Software. The TIP Reporter Software displays measured temperatures versus depth and mapped on cross sections of the shaft to provide profiling data. The Thermal Integrity Profile software helps identify areas of concern such as potential over-pour bulges, necking, or cage alignment irregularities. TIP Reporter also estimates the concrete cover along the entire length of the shaft. In addition to TIP temperature measurements, this analysis requires the total concrete volume as an additional input. The estimated effective shaft radius, reinforcement cage location and the concrete cover of the reinforcement bars can then be determined.

The TIP? conforms to ASTM Standard?D7949 – Standard Test Methods for Thermal Integrity Profiling of Concrete Deep Foundations.

The advantages of this method is that,

·? Evaluates concrete quality inside and outside the reinforcing cage

·??Accelerates construction with tests conducted during concrete curing

·??Reveals necking or inclusions, bulges, variations in concrete cover, shape of shaft and cage alignment

·?? Thermal Wire?Cables can replace access tubes

·?? Thermal Aggregator Units (TAG) allow for real-time data review via the Cloud

BIDIRECTIONAL PILE LOAD TEST

The objective of the Bidirectional pile load test is to establish load-settlement relationships and ultimate capacity of the test pile. The bidirectional load test will be carried out when the test pile is obtaining the required concrete strength and bearing capacity, subjected to engineer approval. The testing SOP follows conventional top-loaded pile load test (kentledge or anchors), except no reaction system available above the pile head.

The Bi-directional pile load test (BDPLT) will be carried out in accordance to US ASTM/D8169-2018 (Bi-Directional Static Axial Compressive Load Test of Piles).

The cell system shall be fabricated based on the project requirements such as,

·???????? Pile size

·???????? Pile type

·???????? Working load (WL)

·???????? Test Load (‘n’ times of WL)

?

The positioning of jack assembly is determined based on static analysis on soil data from soil investigation (SI) test report. The soil investigation borehole results are used as the basis to compute the expected skin friction and base bearing capacities of the test pile. The jack assembly location is ideally be located at the middle of total pile capacity (= skin friction + base bearing). The main aim of positioning the assembly will be to “equalize” the bi-directional forces in the pile so that failure in one direction does not occur prematurely. The detailed location requirements will be based on the pile design and the soil conditions. It must be stressed that all calculations are based on empirical formulae which does not imply that they are fail-proof but at the time represents the most prudent and accurate location based on the available information.

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Once the positioning is finalised, the jack is welded to the reinforcement cage at the estimated location. The contractor shall ready an upper rebar cage and a flat platform prior to the pre-fabricated bidirectional jack arrive at the project site. Once arrive, the jack orientation and position will be fixed on a flat ground, horizontally. Subsequently, the upper rebar cage will put on top of the jack, vertically (i.e. perpendicular) and weld together to fabricate as a single piece of jack connected with the rebar cage. Finally, the hydraulic hoses and telltale cables will be connected from jack.

A calibrated electronic pressure gauge will be used to monitor the hydraulic jack pressure. The loading is derived by applying the calibration factor of the hydraulic jack to the pressure. For measurement of the displacement of the jack (up and down), telltale cables will be used. The end of telltale cables will be connected to LVDTs (linear vertical displacement transducer) to measure the displacements.

The jack will be internally pressurized using a common hydraulic system, creating an upward force on the shaft in upper friction and an equal, but downward force in lower shaft friction plus base bearing. The hydraulic jack load is determined by relating the applied hydraulic pressure to load calibration. A high-range calibrated pressure gauge will be used to read the pressure on the pump line. The load will be removed and testing finished when either one of the following criteria occur:

i. A pre-determined test load has been achieved prior to the reach ultimate capacity (i.e. pass condition)

ii.?The test pile reaches its ultimate capacity exceeds the limit (i.e. fail condition)

iii. The smaller value of 5% of pile diameter or jack maximum ram stroke is reached

iv. The ultimate jack capacity is reached

Load-displacement readings will be recorded (by data logger) in accordance to the Testing Schedule Table, in which generally adopting conventional top-loaded maintained load test procedure with the sample plots as shown below for

(i) Load-Displacement, (ii) Load-Time, and (iii) Displacement-Time.

Test certificate and test report will be issued with top-loaded load-settlement Q~s plot. In bi-directional testing, the hydraulic jack is pushing the upper pile section in upward direction. In other words, the upper pile section is subjected to tension load behaviour. In order to simulate the compressive load behaviour, the following skin friction correction factor, ? at the upper section of the pile section shall be taken into consideration depending on the soil stratigraphy.

Based on the Compression and Tension loads recorded, the ultimate loads are estimated and based on it the acceptance of the pile shall also be decided.

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