FIELD TESTS ON FDP PILES (FULL DISPLACEMENT PILE) AT THE COMPANY CERAMICA SANT’AGOSTINO
CLIENT: Ceramica Sant’Agostino
LOCATION OF INTERVENTION: Ceramica Sant’Agostino S.p.a., Sant’Agostino (FE)
SERVICES RENDERED: design of testing field, definition of load tests,
analysis of the results of static and dynamic tests on piles
START/END OF WORKS: 2013
At the Ceramica Sant’Agostino plant, a test field on pilot piles has been realized in order to refine the knowledge about the performance of FDP piles (Full Displacement Pile) in order to rationalize and optimize the design of new foundations, within the seismic improvement and reconstruction interventions that will affect several industrial buildings of the above mentioned industrial complex. In fact, following the seismic events that hit the Emilia-Romagna Region in May – June 2012, several warehouses at the Ceramica Sant’Agostino plant have been irreparably damaged, and therefore demolished, or need to be adapted to provide them with a suitable resistance to horizontal seismic actions.
1. FDP piles
It was chosen to use FDP piles because:
- They have the performance of a soil displacement pile (infixed pile) because there is no soil removal during construction but they do not need to be beaten, generating potentially damaging vibrations for existing neighboring buildings.
- They offer high performance, better than traditional drilled poles.
- Unlike the latter, they do not require complicated and costly hole support systems.
- They produce a very small amount of material to be taken to landfill.
- For the two previous reasons, the practical management of the site is greatly improved.
- They adapt very well to the lithologies found on site, silt-clay with a modest presence of not excessively thickened sands.
- They have a high speed of realization.
The FDP piles of the test field are roto-fixed piles without soil removal, made by means of a machine, similar to the one that makes drilled piles, which, however, uses a special excavation tool that, during the advancement by rotation in the soil, moves the latter laterally, compacting it and avoiding its removal.
The result is a pile that performs much better than a drilled pile, in which the removal of soil from the hole causes detensioning in the hole itself with consequent lower performance in terms of resistance and rigidity, which is similar, even conceptually, to the performance of an infixed pile, known as a “soil displacement pile”.
When the tool has reached the required depth, the concrete casting starts from the bottom of the hole, withdrawing the tool in parallel. When the tool has been completely withdrawn and the hole is filled with concrete, the reinforcement cage is inserted by crane.
2. TEST FIELD Layout
The layout of the testing field has been studied to optimize the distribution of the piles and to maximize the number of load tests that can be performed.
A planimetric scheme is shown below.
It was decided to test poles of three different diameters:
1) FDP pole diameter Ø = 320 mm length L = 20.0 m
2) FDP pole diameter Ø = 420 mm length L = 24.0 m
3) FDP pole, diameter Ø = 510 mm, length L = 16.0 m.
Three FDP contrast poles Ø = 510 mm were constructed in a triangular arrangement.
Three poles of the three different diameters were made in the middle of each side of the triangle.
This arrangement will make it possible to carry out a static load test on each of the three diameters of FDP pole, using a pair of the vertex poles for the contrast each time.
At the end of the static load tests, it will be possible to perform CASE dynamic tests on the contrasting poles, which, working in pairs, are not pushed to failure, so as to cross-reference and compare the results.
Load tests performed
In total, therefore, the following load tests were performed, by type and numerosity:
Among the dynamic tests with a high level of deformation, the most widespread is the so-called CASE test, whose purpose is the determination of the limit load of the pile-soil assembly.
The test, now defined by ASTM D4945-08 “Standard Test Method for High Strain Dynamic Testing of Deep Foundations”, is performed by acquiring the force and speed values of a mechanical wave propagating in the pile following the impact generated by the fall of a hammer on the head of the same.
3. Load Test Interpretation
The interpretation of a static load test can be carried out with many analytical methods, among which Chin’s (1970) and Davisson’s methods are the most calibrated and known in the international scientific literature.
In Chin’s method, with the values considered most significant in each load step, a yield-flexibility graph is constructed, where flexibility is defined as the yield/load ratio.
Davisson’s method (1972), starting from the load-breakdown curve measured in a static load test on a pile, allows to calculate the limit load, which is defined as the load that corresponds to a breakage that exceeds the elastic shortening of the pile by a certain value.
By way of example, an interpretation of a static load test and one of the dynamic test is given.
CHIN – Choice of straight lines
Equation of straight lines and limit load calculation
Simulation of the load-slip curve and comparison with experimental curve
The final result is obtained:
- Chin: Qu = 1000 kN
- Davisson: Qu = 1005 kN
CASE load test
Load curve simulation – failure
Lateral resistance distribution along the pile shaft