Dynamic Load Test Should not Be Carried out on Micropiles.
I also commented the above in the same seminar at a local Institution of Engineers Malaysia recently. The seminar was supported by Dr. Julian Seidel and Mr. Peter Middendorp as oversea speakers and Ir. Richard Yu and Myself Ir. Mun Kwai Peng as the speakers from the local engineers practicing stresswave measurement. At the end of the seminar, I made a report saying the above statement in the Institution's monthly bulletin. There were some feedbacks on the above statement, which were of different opinion on the above subject. I am seeking other experts' opinion worldwide on this subject as whether we should allow such practice.
But first let me explain why I made such statement.
In our micropile piling practice in this country (Malaysia), I believe this type of piling is only practise in this region and not elsewhere in the world. The pile is formed as follows: A bore hole (150mm to 300mm diameter) is formed in the ground using a boring method similar to soil investigation with a temporary steel casing; Reinforcement (continuous in length) is placed in the centre of the bored hole; The hole is then grouted with cement grout from the bottom to the top; The temporary casing is then extracted and the grout is allowed to harden to form the pile. The reinforcement can be steel bars or API pipes and the design is based on shaft friction only. No base resistance is allowed in the pile design. The typical working loads in these piles are ranging from 60 tons to 200 tons depending on the reinforcement and a factor of safety of at lease 2 is usually specified.
If we take a typical case; said a pile 200mm diameter with 4 numbers of 50mm diameter high yield steel bars will be designed with a working load of about 100 tons (test load to 200 tons minimum).
If a dynamic load test is required on the above pile, the minimum hammer weight for the dynamic load test in order to mobilized the load will be at least 2 tons. When such a hammer strike on the 200mm diameter pile with a hammer drop of said 1 meter, the stress generated by the hammer strike will be in the range of 60 to 70 N/mm2. The hammer strike will definitely damage the pile along the pile shaft.
Dynamic load test should not be carried out on such pile as the micropile has not been designed to accept the dynamic force. After the dynamic load test, the micropile may not be in good working condition. The question will arise as who will be responsible to the performance of the pile after the dynamic load test.
I would like to hear the comments from as many countries as possible to see their practices.
I agree with you that DLT is not suitable for testing micropile for simple reason that the high impact stress by the test hammer can damage the micropile to be tested.
But Mr.Mun,you must remember there are some wrong doers DLT testers in Malaysia who for the sake of monetary gains willing to claim wonders by DLT. DLT on micropiles is quite similar to situations I have come across some DLT tester claiming that a hammer slightly less than 10 tonnes in weight can mobilise 3000 tonnes with a drop height of 1.8 metres in a DLT.
As long as there are wrong doers, you continue to see wrong practices in your country.