Pre-drilling is done with a hydraulic or pneumatic drilling rig, capable of drilling at any angle and direction, with a minimum torque of 400 Kgf.m and advance force capable of drilling holes fully coated or not. Tricone-type drill bits with a diameter of at least 4 inches are commonly used. Care must be taken to ensure that the holes are made by processes that minimize the change of the natural state of consistency or compactness of the ground. For that, the advance of the drilling tool and the pressure of injected water, as well as the procedures to guarantee the stability or containment of the walls of the hole, must be controlled. The entire drilling process must be monitored and recorded in specific reports that provide the history of the hole, to identify the rock mass traversed, as well as the observed occurrences, such as localized water losses or identified faults or voids.
At the end of the drilling, the tube is manually transported from the assembly area to the drilling location. The introduction of the tube and its installation must be done slowly, but it continues. Normally, drilling equipment is used to assist in the installation of the pipe, but with discretion, without applying excessive pull-down.
The forepoling injection aims at the correct filling of the annular space between the steel tube and the perforation performed, aiming to incorporate/solidify the steel element to the solid for the correct transfer or absorption of loads by the forepoling.
In the case of heavily fractured rock mass, grout injection under low pressure and volume can be an important component in stabilizing blocks and filling fractures, which may also reflect on the reduction of permeability of the rock mass traversed. The first stage of injection or sheath is performed with a simple shutter, through the interior of the forepoling tube, in the first sleeve (bottom).
Simple Hydraulic Shutter
Then, after an interval of time between 2 and 3 hours, the cleaning of the inside of the forepoling tube is performed, by introducing a PVC column inside each one of them, promoting the circulation of water to clean the remaining grout from the sheath, essential for the sequence of injections without locking the shutter.
The injection is then continued, sleeve by sleeve, observing a time interval between 8 and 10 hours after sheath application. The new injection phase with controlled pressure and volume, called primary, begins. The primary injection, then, sleeve by sleeve, is performed with controlled volume and pressure, through a double shutter that allows the injection of individual sleeves.
Double Hydraulic Shutter