What is a proper time for pile testing Jaroslaw Rybak, Wroclaw University of Technology

Contents Main ideas of the work Case studies Driven piles and cast in-situ piles Referring to on-site conditions Static and dynamic capacity testing - Pile setup Integrity testing - Concrete hardening (stress wave velocity) Case studies Capacity testing of driven piles Integrity testing of CFA piles

Control tests of pile load capacity As a rule (in accordance with Polish Code of Practice), load capacity testing usually covers only 1-2% of the contracted piles. The time span, which should be preserved between the end of initial pile driving (EOID) and its capacity testing, is noteworthy. Those time intervals are presented in Chart 14 in the code PN-83/B-02482.

Control tests of pile load capacity Static load testing constitutes the basic pile capacity testing due to local standards (Polish code of practice) and, as follows, contract specifications.

Static load testing takes a lot of time and money …

… but SLT provides us with relevant and important information. ■ PDA ■ SLT

And what if the test goes wrong? Improving of a bored pile capacity is difficult and costly. It is better to know early enough.

Case study 1 -Setup in sand - Pile testing The ultimate pile load capacity increase in time after 8 weeks from EOID is clearly visible (app. 26% in sand). Polish piling standard does not rely on setup effect.

Time dependent increase of pile load capacity Skov and Denver formula (1988), describe the change of the pile load capacity values in time Q/Q0 -1 = A·log10(t/t0) Q – load capacity in time (t) Q0 – the load capacity at the moment of the first test A – the empirical constant t – time elapsed from the moment of the pile installation t0 – the time of the first load capacity test The typically used (for sand) constant A = 0.20 gives the increase of 17% between 5th and 55th day.

Setup in silt - Pile test results In the period starting from the 1st day until the 13th day, the piles numbered 16 to 26 showed an increase of about 59% in the load capacity in silty clay and silt.

Second check of pile load capacity When we substitute A = 0.6 (recommended for silts) in equation Q/Q0 -1 = A·log10(t/t0) Taking into account the period from the first to the 13th day after the pile driving, we calculate a significant load capacity increase, reaching about 67%. From field test results and analysis (CAPWAP) we obtained mean gain of about 59% in the load capacity in silty clay and silt. That proved a relatively good conformity between estimations and test results.

Integrity testing Equipment (SIT, PIT, Impulse response)

Integrity testing For reinforced concrete piles made of the C50 concrete, the elastic wave velocity measured before driving reaches 4500 m/s, and after the driving – it oscillates around 4000 m/s. That is probably connected with the micro-cracking caused when driving the pile. The errors in velocity estimation may lead straight to proportional faults in the estimation of the pile's length (10%). It is even more complicated to determine the wave velocity in the process of the early concrete setting (before the 15th day of concrete embedding). Very few publications on that subject show large changeability of the estimated velocity and suggest that each time at the construction site calibration should take place.

Integrity testing - Concrete hardening Test of 132 piles Elastic wave velocity in function of time c = 2974.1 + 51.1·t [m/s] t [days]

Integrity testing - Concrete hardening Repeated test of 12 piles Average wave velocity in function of time c = 3273 + 32.5·t [m/s] t [days]

Case study 2 c = 4000 [m/s] L=10,25 m (FALSE) c = 3500 [m/s] L=9,02 m (standard) L=10,25 m (FALSE) c = 3500 [m/s] (7th day) L=9,02 m (TRUE)

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