Proceedings from the Michael W. O'Neill ACIP Pile Sessions of the 83rd Annual TRB Meeting 2004: Recent Experiences & Advancements in the US and Abroad on the use of Auger Cast-in-Place Piles, (DFI)
New Concept for Monitoring the CFA Casting Phase
While installing a CFA (continuous flight auger) pile, one has to control the process in such a way that the entire volume, left behind the auger, will be fully taken by concrete. It is not easy to be done as there is no visual nor physical access. Common solution is to use a measuring system – a device which monitors the process parameters in order to guide the operator. A pressure based device monitors the concrete pressure. Keeping positive pressure means that there are no voids and no way for debris to come. Traditionally the pressure is measured at the top of the auger. This point is some 15 m (49 ft) above the bottom, so it neglects a hydrostatic pressure of about 0.36 MPa (52 psi). A volume based device measures the concrete discharge and compare it to the theoretical one. As the theoretical volume is an estimation, the quality of the results is in doubt. This article provides a different approach – a way to indirectly measure the concrete pressure at the bottom of the auger by analyzing the load on the lifting gear. As the auger, with the soil on top of it, is like a piston inside the ground, a concrete pressure at the bottom pushes the auger upward. This upwards force reduces the load in the lifting mechanism. By measuring the load, one can calculate the concrete pressure at the bottom of the auger. The concrete uplift force is called – MALI Effect.
|article #233; publication #71 (MWO-2004)|