What Does Pilot Tube Method Mean?
The pilot tube method is a hybrid of different trenchless methods. It combines the features of horizontal auger boring (HAB), horizontal directional drilling (HDD), and microtunneling and allows the accurate installation of pipelines to line and grade. The pilot tube method is becoming popular because it is accurate, can be retracted and reinstalled and requires only a small workspace and jacking shaft.
Pilot tube microtunneling can be accomplished in three main steps namely, installing a pilot tube in line and grade, installing steel casing by jacking and removing the spoil using augers, and installing the product pipe. The method is highly accurate in clay, silt, sand, and fine and coarse gravel but not very suited to hard soils or dense granular material. It has limited capability in the soil below the water table unless a ground plug is maintained or a wellpoint is installed to control the influx of water.
The pilot tube method is used for the installation of small diameter pipelines that flow with gravity. It uses an LED target viewed down the center of the pilot tubes with the help of a theodolite and LCD monitor, providing a high level of accuracy. In brief, the pilot tube method involves using a jacking frame and a theodolite guidance system to install the pilot tubes in line and grade.
The diameter of the bore is increased by installing temporary augers and casings. Next, a powered reaming head is installed behind the augers and casing which are sized to match the outside diameter of the product pipe. In the final stage, the product pipe is installed while simultaneously removing each section of the auger and casing.
The pilot tube method is also known as pilot tube microtunneling, guided auger boring, or pilot pipe jacking.
Trenchlesspedia Explains Pilot Tube Method
The pilot tube method combines the benefits of HAB, HDD, and microtunneling to give a hybrid method that is accurate and reliable. The method can be used for pipes with an outside diameter between 4 to 48 inches. The alignment precision is within 1/4th inch per 500 linear feet throughout the entire drive length. This level of accuracy allows for the installation of product pipes by direct jacking without using a casing.
The Guidance System
The steering head of the pilot tube machine is equipped with a battery-powered LED illuminated target. This target is visible to the operator by means of a theodolite, camera, and monitor. The theodolite is set in line and grade and positioned such that the target can be viewed through the bore of the pilot tube.
The crosshairs of the theodolite are visible on the monitor screen along with the position of the target at the steering head. The operator is able to rotate the steering head to maintain the line and grade till he reaches the reception pit on the other side.
The Installation Process
The first step involves the pilot tubes being arranged in line and grade and being pushed into the ground from the launching pit. The process is controlled by the guidance system. The next step involves following the pilot tube with a reaming head that matches the diameter of the product pipe. The reaming head and the auger casings are attached to the pilot tubes and the jacking frame advances the reaming head and the casings into the ground.
As each section of auger and casing is added from the launch pit, a section of the pilot tube is retrieved at the reception pit. The process is repeated until all the pilot tubes have been removed. In the final step, the product pipe is installed by attaching a pipe adapter to the last section of the auger casing. As the pipes are thrust into the pit, the auger casings are pushed out at the reception pit. The process is continued until the product pipe reaches the reception pit.