PCパイルの打込み実験

抄録

Recently, concrete piles are driven almost always by blow. This method of driving simplifies the process of construction but sometimes it will have destructive effect on the piles. It is necessary to investigate the process to find out the factors for the destruction of the piles and improve that method.
In this study, RC piles (φ350mm×13m) and PC piles (φ350mm×13m+φ350mm×7m), both centrifugally spun, were used as test piles, and drop hummers (1t, 2t and 3t) and diesel hummers (Delmag-12 and Delmag-22) used.
The ground into which the piles were driven has layers of sand (N-value=15∼20) at the depth of 6∼7m, gravel (N-value=30∼50) at the depth of 12∼13m and silt and sand (N-value=20∼35) deeper below.
The impact strains at the head, center and foot of the piles were measured by means of wire strain gauges, and the effects of cussion materials, eccentric blow and the characteristics of ground on the pile stress have also been investigated. Comparative studies were made of some of the theoretical equations for the impact stresses with their experimental results.
Principal conclusions obtained are as follows.
(1) Rebound of a pile increases as the ground becomes harder.
(2) Set per blow decreases as the ground becomes harder and the depth of the set is greater by drop hummers than by diesel hummers.
(3) Delmag-12 cannot drive a pile into the ground which is more than N-value=50.
(4) The impact stresses caused by pile-driving ordinarily reach their maximum at the head of the pile and their minimum at the foot of the pile.
(5) The impact stress at the foot of the pile increases as N-value of the ground increases. But the impact stress at the head of the pile is not so related with N-value.
(6) The impact stress at the head of the piles by drop hummers is greater than that by diesel hummers. The former sometimes exceeds the impact strength of RC pile concrete.
(7) The maximum impact stress caused by eccentric blows of both drop hummers and Delmag-22 sometimes exceeds the impact strength of the pile concrete. The tensile stress and bending moment due to the eccentric blows sometimes cause cracks in the pile. In these respects, the PC pile is superior to the RC pile, and the diesel hummer is superior to the drop hummer. It is very important to drive a pile as normally as possible and it is necessary to set a proper cussion at the head of the pile.
(8) The impact stress caused by pile-driving should be analyzed by the wave equation rather than by the energy equation.
Further studies are needed of the following problems.
(1) The impact strength of concrete.
(2) The correlation of the compressive strength by the normal test piece and that by the test piece made by the centrifugal force.
(3) Estimation of the impact stress in a pile caused by driving.
(4) Fundamental studies on pile-driving analysis by means of the electric computer.