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Pile Instrumentation Case Studies in Southern India

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Ravikiran Vaidya

 

 

Ravikiran Vaidya
Principal, Geo Dynamics, Vadodara

 

1.0 Synopsis: Low Strain Pile testing (LSIT), Cross-hole Sonic Logging testing (CSL) and High Strain Dynamic pile testing (HSDPT) has now been routinely conducted on various infrastructure, industrial, and real estate projects across the country. In Southern India, HSDPT has been used by the author at Chennai Metro, Kochi Metro, Bangalore metro, Petronet LNG, Vallarpadam, Lulu, Trivandrum, Cochin Port, Mattanchery Bridge Project and several other projects. These pile testing methods have been valuable tools in ascertaining integrity and capacity of small and large diameter piles. This paper tries to highlight the usefulness of these methods which were used in many prestigious projects. Case studies are presented in this paper where these technologies were at forefront to provide increased QA/QC along with savings in time and cost. The methods also enabled the engineer to verify design/ workmanship and in some cases also avoid potential problems at the project site.

2.0 Introduction

LSIT,CSL and HSDPT has now become a routine for all major projects across India since these technologies provide useful information and are also quick, reliable and relatively inexpensive. Earlier in India it was common practice to perform conventional load tests only on selected piles and the amount of testing would range from 0.5% to 2%. However, now scenario has changed significantly and 100% piles are being subjected to LSIT, 15%-20% of the piles are being tested with CSL and 2%- 5% of the piles are being subjected to HSDPT. Such testing practices have resulted in improved QA as well as assessment of pile integrity which earlier was not possible due to limited information produced during conventional load testing.

The tests were developed in USA in 1975 by Dr. George Goble & his associates Frank Rausche & Garland Likins with active support from Federal Highway Administration. Simultaneously, it was also developed at the TNO Institute of Building & Construction Research, Netherlands.

LSIT is used to ascertain quality of pile shafts on site. Generally, the pulse echo method that uses only velocity measurement is widely used and recommended, although testing is also conducted by using an instrumented hammer to measure the intensity of impact. Today integrity testing is in code specifications of various countries in America, Europe, and Asia. It is also standardized as per ASTM D5882. In India, it is a part of the IS: 14893.

High Strain Dynamic test is quite often used to replace or supplement conventional static testing on construction and infrastructure projects. The test has been found to be reliable, quick and inexpensive compared to static tests. It is standardized as per ASTM D4945 and various codes internationally. Currently it is one of the most widely used Non Destructive test worldwide to ascertain pile static capacity.

CSL is also one of the widely used tests worldwide to evaluate pile integrity and is easy to interpret compared to pile integrity testing. The method is standardized as per ASTM D 6760-08. Cross-Hole Sonic Logging generally applies to drilled shafts and requires that at least two tubes be installed in a drilled shaft prior to pouring the concrete.

The ultrasonic pulses are sent from one tube to the other at 20 to 50mm vertical intervals. The arrival time of the pulse at the receiver tube indicates the quality and integrity of the concrete between the tubes. However, in general, such special advance preparations of installing the inspection tubes are not economically feasible.

The CHA determines the quality of concrete between pairs of tubes pre-installed in r.c. bored piles. The CHA is easy to use and data interpretation is straightforward. Defects in the pile are easy to recognize.

Since these tests provide information regarding pile foundation failures in term of load carrying capacity or the pile integrity, these technologies have become quite popular in short span of time. Over the years, for many projects these tests have been used and identified defective piles and/or piles unable to achieve required load. Selected case studies are presented in following sections.

3.0 Case Studies
3.1 Mall Project, Kochi

At a Mall project site in Kochi, 800mm and 1000mm diameter piles were installed. The depth of these piles was around 32m to 45m. As described in the available borelog, the soil at the site consists of alternating layers of sands with varying content of silt/clay and Fat Clay. A generalized subsurface condition along with SPT N values is presented as Table 1.

Table 1 Generalized Subsurface Profile Mall Kochi
Table 1 Generalized Subsurface Profile Mall Kochi

 

 

 

 

 

The safe design load on the piles was 360 tons. After completion of piling, Geo Dynamics was engaged to perform LSIT on these piles before proceeding with further work. More than 300 piles were tested using PIT. From the data it was apparent that few of the piles had defects. Data for LSIT performed on typical piles is presented as Figure 1a and 1b. It was suggested that suitable review/remedial action shall be taken for these defective piles.
Before adopting PDA dynamic pile testing, a correlation was also developed by comparing the results of static load test and HSDPT. HSDPT was performed on a routine pile for load carrying capacity assessment and verification of integrity. A 5 ton hammer was used for the testing since the test load was around 540tons.
The static load test was performed on adjoining pile. A good match was obtained between both the tests and the same is presented as Figure 2. Eventually HSDPT was approved for use at the project site.

 

Based on PIT and HSDPT results, satisfactory piles were accepted and suitable action was taken for defective piles. For this project, PIT and HSDPT was very useful to assess pile integrity and capacity.

3.2 A project site in Vallarpadam

For a project site located in Vallarpadam, it was proposed to construct a Container Terminal. The subsurface conditions consist of layers of sand and clay. A generalized subsurface condition is presented as Table 2. R.C. bored piles having diameter of 800mm, and 1200mm were installed at the site. The lengths of the piles were ranging from 48m to 75m depending on design loads. The design loads were ranging from 500tons to 1100tons.

Table 2 Generalized Subsurface Profile A project site in Vallarpadam
Table 2 Generalized Subsurface Profile A project site in Vallarpadam

 

 

 

 

 

 

 

LSIT was performed on around 500 piles. LSIT for a 75m long pile is presented as Figure 3. Generally, no major problems were reported with the PIT tests. Although L/D ratio for these long piles was quite high (i.e. around 62), satisfactory data was collected using PIT. Here it is important to note that it is possible to test long piles with PIT even though old literature suggests that testing is possible upto an L/d ratio of 25-30. There is no rule of thumb for L/d ratio and it depends on the soil and pile profile. Soils with high resistance and/or piles with major bulges are difficult to evaluate even for low L/d ratio of 20, whereas uniform piles with low resistance for significant depth can be evaluated even for longer lengths as seen in the current case.

Figure 3 LSIT data collected at a site in Vallarpadam
Figure 3 LSIT data collected at a site in Vallarpadam

 

 

 

 

HSDPT was also performed on 15 piles having 1200mm diameter after establishing the correlation with static load test. The correlation was obtained by testing the same pile statically first and then with HSDPT. The correlation is presented as Figure 4. HSDPT was conducted using 10.5 tons, 14tons & 22tons hammers depending on test loads and pile lengths. A photo showing HSDPT in progress is presented in Figure 5. The graphical output of CAPWAP performed on the one of the routine pile is presented as Figure 6.

 

All the tested piles were able to achieve the required test load. This was the first time in India that such long piles were cast and it was required to quickly evaluate the integrity and capacity of these piles to remove any doubts about the applicability of such long piles. The methods and the expertise of the analysts was useful in evaluating so many piles in quick time and also helped the project execution time.

3.3 HSDPT and LSIT at Marine Terminal Project – Southern India

At a Marine Terminal project in Southern India, it was planned to expand the existing marine facilities for which long pile foundations were designed. The site consists of mostly clayey soil with varying consistency. A typical subsurface profile is presented in Table 3.

Table 3 Subsurface Conditions at Terminal Site in Southern India
Table 3 Subsurface Conditions at Terminal Site in Southern India

 

 

 

 

 

 

R.C. bored piles having diameter of 1000mm were installed at the site. The lengths of piles were varying from 46m to 58m. The design loads were ranging from 306tons to 571tons.

NCR were issued to some piles due to suspected issues during concreting. Hence Geo Dynamics was engaged to perform the integrity and HSDPTs. LSIT was performed on 50 piles and at least one pile seemed to have defect. Representative data for one good pile and one defective pile is presented as Figure 7a and 7b. For defective piles it was suggested that HSDPT shall be conducted and subsequent decision may be taken by the engineer to the project. HSDPT confirmed PIT findings and although the pile took reasonable load, it also showed defect. The CAPWAP output for the pile with the defect is presented as Figure 8. At the project site, HSDPT was performed on 11 piles after establishing the correlation with static load test (Refer to Figure 9).

 

Again, in this case too, contractors, client and consultants could save substantial time and costs by adopting the state of art technology. Instead of discussions and wrangling over questionable piles, the methods helped resolve quality issues and also time and costs related with conventional tests was saved.

3.4 CSL testing for a Bridge site in Kochi

Geo Dynamics was engaged for a pilot study for Kochi Metro Project to conduct Crosshole Sonic Logging tests on the 2000mm diameter test pile. The method was introduced based on the suggestion of the author. 6 tubes were installed during pile casting which were used for the CSL testing. The length of the pile was 48m.

CSL results on the test pile indicated that the pile has major impedance reduction (defect) at bottom of the pile. The defect was apparent in all scans. Typical CSL scan is presented as Figure 14. It was decided to conduct static load test on the pile inspite of the defect to check whether the pile can take the required load. The static load test results showed that the pile was able to take the required test load even with defect at bottom possibly due to conservative design approach. The results were extremely useful to take future decisions based on such a pilot study. The results also formed a basis for introducing this technology during the actual project execution. It is now a part of the contract specifications.

Figure 14 CSL output for Bridge site Defect at the bottom
Figure 14 CSL output for Bridge site Defect at the bottom

 

 

 

 

 

 

 

 

4.0 Concluding Remarks

Pile instrumentation by means of LSIT, CSL and HSDPT are proven tools when it comes to QA practices for pile foundations. It is essential that these tools are used properly and judiciously for the benefit of the industry. Use of these methods at the start of the project may provide confirmation of design and avoid huge rectification costs later if the workmanship does not match with geotechnical design parameters. A specific example would be to avoid wash boring methods with truck mounted rigs. Care needs to be taken and piling contractors need to be trained to handle piling projects as a piling job is not merely boring and putting concrete inside the bore but is a complex process which requires experience and dedication. In the current scenario, the general tendency is to get the cheapest sub-contractor, cheapest testing company and cut costs without thinking about the end result. This can lead to dangerous situations either during the project time or at a later date. Use of PDA, PIT and Cross Hole Sonic Tests with adequate checks and balances should be encouraged. Malpractices should be specifically reported and highlighted with action taken if the overall construction quality has to improve in the country.

As far as such modern testing practices are concerned, it is very important that the personnel who is using it has a good understanding of the technology and one should discourage its use as “black box”. An expert agency of national repute shall be engaged to conduct the test. The testing agency shall be specialist having minimum of 15 years of experience in PIT, HSDPT etc. They should have sound knowledge of Wave Theory used for analyzing and interpretation of test results and atleast performed minimum 5 nos of correlation studies of Dynamic Pile Load Testing with Conventional Load Test in various strata’s covering friction piles, end bearing piles etc.

It is definitely preferable if the results of correlation studies have been published as documented literature. Also raw data shall be made available when desired by the consultant/client may be for third party review. Such stringent criteria for pile testing will result in appropriate reporting of results and will help the entire deep foundation fraternity across India.

5.0 References

1. Rausche, F., Likins, G.E., and Hussain, M., “Pile Integrity by Low and High Strain Impacts”, Proceedings of the Third International Conference on Stress Wave Theory on Piles, Ottawa, Canada, May 1988.
2. Berger J.A. and Cotton D.M., “Low Strain Integrity Testing of Deep Foundations”, Proceedings of Deep Foundation Institute Annual Meeting, Seattle, October 1990.
3. Rausche F., and Goble G.G., “Determination of Pile Damage by Top Measurements.” Behaviour of Deep Foundations, ASTM Symposium, Boston, 1978.
4. PIT & PDA Users Manual, May 1998, Pile Dynamics, Inc.
5. Rausche, F., Hussein, M., Likins, G., and Thendean, G., 1994. Static pile load movement from dynamic measurements. ASCE, Geotechnical Publication No.40, Proceedings of Settlement ’94 Conference, College Station, Texas, Vol. 1
6. Vaidya, R., 2006. Introduction to High Strain Dynamic Pile Testing and Reliability Studies in Southern India. IGC 2006, Chennai, India.
7. Vaidya, R., 2004. Low Strain Integrity and High Strain Dynamic Testing of Piles– an Indian Overview. 7th International Conference on the Application of Stresswave Theory to Piles, Kuala Lumpur, Malaysia.

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