Deviation in context of trenchless construction can be defined as the difference between the planned bore path and the actual bore hole. Deviation can occur due to problems with the drill, operator error or due to unforeseen sub-surface conditions. Unchecked deviation from planned the bore path can have serious consequences, especially in places where several underground utilities like gas pipelines, electric and cable connections and sewer pipes crisscross in and around the planned path. Boreholes are designed after a thorough geotechnical investigation to prevent problems like sinkholes and cross boring through existing pipelines.
Trenchlesspedia Explains Deviation
Technically, borehole deviation defines the geometric difference between planned and actual bore paths. Based on the nearest point on the planned path, there are two types of differences – lineal and angular. Lineal differences are horizontal and vertical deviations, while angular differences are azimuthal and inclinational differences. It can be affected by factors such as soil stratification and drilling characteristics. Some factors that influence deviation include:
- Rocks encountered during drilling
- Loose and old drill rods
- Loose fitting core barrels
- Improperly placed stabilizers
- Improper drilling force
- Wrong bit design
- Buckled drill rods as a result of excess force while drilling
- Drilling too fast or too slow
Deviation can also be caused initially by error during the set up process. Measurement while drilling (MWD) utilizes a gyroscope, a magnetometer and an accelerometer, which are installed in the drill head, to transmits real time drilling parameters to the operator. This helps in controlling deviation and in keeping the bore path in the planned route.