1. What is High Strain Dynamic Pile Testing ?
High Strain Dynamic Pile Testing or PDA testing is a method to determine STATIC PILE CAPACITY using a dynamic impact. The method involves measurement of force and velocity response of the pile on field under the impact of a hammer of predetermined weight falling from a known height. The test is used in lieu of conventional static load testing. The test is conducted using Pile Driving Analyzer (PDA) and with its allied strain sensors and accelerometers. The entire equipment conforms to ASTM D4945and other relevant standards worldwide. It is required that high strain dynamic test data be subjected to CAPWAP analysis to refine the assumed parameters during a field test.
2. Is CAPWAP a mandatory requirement after HSDPT ?
It is mandatory to evaluate a typical blow from HSDPT of bored piles for CAPWAP to refine the parameters assumed during a field test. All restrike test data is also mandatorily subjected to CAPWAP analysis and atleast few End of Initial Drive data is also required to be analyzed.
3. How to check reliability of high strain test data and CAPWAP?
Refer to the technical paper “
Guidelines for successful High Strain Dynamic Load Tests & Low Strain Integrity Tests for Bored Piles” in the downloads section which provides helpful hints for data and report acceptance. In general, a good match quality is a pre-requisite to accept high strain dynamic load test reports.
4. Is it possible to manipulate the HSDPT test.?
It is difficult to manipulate the test results as an expert can easily find out the manipulations in the CAPWAP printout. However, for an unsuspecting client who only looks at the end result, it may not be noticeable as it has been our experience several times that some testing companies have manipulated the PDF files and presented a manipulated report. It is recommended to take a screen shot of the field data and compare it with final report.
Is purchasing an equipment sufficient to qualify for conducting high strain dynamic load tests and low strain integrity tests.
The easiest part of this testing is to buy the equipment for testing as it can be purchased by anybody who has the financial resources. The operation of the equipments, data collection and interpretation is complex. Moreover several times knowledge is also required of design, method of piling, soil behaviour and several other inputs
which means it requires a real expert to conduct and analyze test data.
5. What are the norms for selection of testing company for these tests ?
It is important that the testing company selected for the job shows sufficient past credentials, technical papers, reliability studies and documentation to convince the client about his knowledge of the subject before the company is approved for testing. For those working in Europe or USA, an option would be to clear the “PDCA Exams” which certify the knowledge of the tester.
It is important that the client or consultant interact with the testing company, explain the requirements, evaluate the knowledge of the testing company by presentations, visits to office and check the database before a testing job is awarded. Awarding testing jobs to material or soil labs who do not have specifically trained engineers or systems to handle these complicated field tests may result in improper tests and poor output leading to confusions and disputes.
6. Is PDA testing based on any Empirical Formula?
The method is based on the Smith algorithm developed in 1960. In 1964 research was initiated in Case Western Reserve University under funding from Federal Highway Administration. By 1974, Dr. Goble and his associates had successfully demonstrated the use of this method and today it is a standardized test tool used worldwide and a part of various code specifications of various European, Asian and Americas.
The method is NOT Related with any of the empirical formulae viz. Hiley's Formulae, ENR, etc. It is based on the WAVE EQUATION approach studied by Smith in 1960. Today, such procedures based on Wave Propagation Theory are collectively called the Case Method and are conveniently applied in the field by dedicated electronic equipment called the Pile Driving Analyzer (PDA) system.
7. Can Empirical Formulae be used to determine pile capacity on field ?
Use of empirical formulae such as Hiley's or ENR or any modified formulae has been discarded worldwide due to various limitations and many assumptions involved. Such formulae were in use in 1950s before advent of computers and when not much information or Wave Equation type procedures were available. Using Hiley's or ENR formulae can lead to dangerous situations and should be strictly avoided. Refer to a graphical comparison of results between HSDPT and other dynamic formulae. The error in general may vary between 50%-200%.
8. What is CAPWAP Analysis and its need ?
CAse Pile Wave Analysis Program is an analytical method that combines measured field data with pile wave equation type procedures, to predict the pile's static bearing capacity and soil resistance distribution. Measured force and velocity data is directly input from the PDA. Based on the measured velocity data, the program computes force required to induce the imposed velocity. Both measured and computed forces are plotted as a function of time and the iterative analysis is continued till there is good agreement between both the curves. If the agreement is not satisfactory, the soil resistances at the pile point and along the pile are adjusted until a good match is obtained. This gives better estimates of the actual static pile capacity measured during field-testing, and also the friction and end bearing components.
It is essential that CAPWAP be done on piles subjected to PDA test especially for bored piles subjected to Restrike. Some other methods like SIMBAT do not own CAPWAP and hence lack the ability to correctly predict pile capacity and other parameters.
9. Is High Strain Dynamic Pile Testing reliable and a standardized method?
As mentioned earlier, the method is used in more than 30 countries and is a part of code provisions of ASTM, AASHTO in USA, also part of U.K., German, Australian, Dutch, Canadian and also various Asian countries. In India, Geo Dynamics is a market leader and well known for its integrity and capabilities in this field. We have a database of more than 150 reliability studies between static and high strain dynamic tests for concrete bored piles all over India in a variety of soils. Similarly, Geo Dynamics also has a large database of correlation studies on pre-cast and steel piles justifying its expertise in this field.
10. What is the nature of data in case of High Strain Dynamic Pile Testing?
The output in case of PDA test is a measured force and velocity curve. A CW analysis that shows match between measured and computed force is also submitted as displayed below. A simulated static test curve and variation of skin friction numerically and graphically also forms part of the report. (not included here).
11.What is the nature of output from a Pile Integrity Test ?
A typical output from PIT is given below for easy reference.
Is it required to exercise the same caution in selecting a testing company for low strain integrity testing as required for high strain dynamic testing. Yes. Today every laboratory has invested into an integrity testing equipment thinking that it is an easy test which just requires data collection with few blows of hammer. Manipulation is rampant leading to either poor quality foundations or poor quality report and disputes at site. Whereas the test itself provides lots of information if done properly, the integrity of the testing company several times is questionable and difficult to determine. We currently recommend the same levels of scrutiny for selecting a testing company both for integrity and dynamic load testing.
12. Can low strain integrity testing be used for piles of 1.8m or more diameter piles ?
Whereas it is possible to theoretically test any pile diameter as long as a certain minimum L/D ratio is ensured, cross hole sonic logging test is more preferred for large diameter bored piles. It is possible that some defects maybe missed out for very large diameter bored piles of 1.8m or more and current database is limited for low strain integrity testing of such large diameter bored piles.
13. Is there a minimum or a maximum Pile Length/diameter (L/d) ratio which limits low strain integrity testing?
Old literature does suggest a limiting value of L/d ratio upto 30. However, with the development of solid state electronics it has been possible to test piles even with an L/d ratio upto 50. Geo Dynamics has conducted several tests of 50m piles with a diameter of 1m and even 75m pile lengths. Thus it largely depends on the soil and bulges. If soil has intermediate hard layers or pile has bulges it maybe difficult to obtain a conclusive result. However, in general we have not faced a difficulty testing long piles.
14. Is it possible to select piles for static or dynamic load testing based on low strain integrity tests ?
It is our recommendation that piles for load testing shall be selected on the basis of integrity tests as these tests reveal several problems like defects, soft material at pile bottom for rock socketed piles, poor concrete quality etc.
15. Is it recommended to reject the piles on the basis of low strain integrity tests ?
Piles with major defects which cannot be rectified by excavation or by grouting after drilling holes should in general be rejected as testing such piles may only lead to additional costs and eventually replacement costs too. However, this decision should be jointly taken with the engineer to the project. If several piles show defect, then a prudent way would be to load test a couple of piles before acceptance / rejection.
16. Cross Hole Sonic Logging Tests
When shall these tests be used on a project site? What is the number of tubes required for this test?
The test is applicable for large diameter piles generally of 900mm and above to evaluate concrete quality by obtaining scans between pairs of tubes. One tube shall be provided throughout the pile length for every 300mm diameter of pile.
Is it acceptable to use PVC or Galvanized Iron pipes for sonic logging. PVC and GI pipes are smooth from outside and this may lead to debonding between concrete and the tubes if proper care is not taken during installation and post installation. It is our recommendation to use MS pipes with threads or with a push fit technology.
17. Static Load Tests
What are the check lists before conducting conventional vertical, lateral and pullout tests.
The girder design should be verified to ensure that it does not bend or topple. Similarly calibration certificate for pressure gage, dial gages or LVDT should be submitted prior to testing. If rebars are welded at sides, the length of welding should be specified and checked.
What are the checks to be applied before selecting a testing company for Non Destructive Testing of Super Structures.
Calibrations of rebound hammer, frequency of probes for UPV testing are the basic checks. A sample report and a method statement prior to testing will ensure that the testing company is capable of meeting client requirements with interpretative reports rather than mere submission of values from the test.