The risks of borehole drilling to nearby structures
Can drilling a borehole crack my house?
Groundwater drilling is a critical process for accessing subterranean water reserves and involves various techniques tailored to geological conditions and the type of borehole required.
Common methods for borehole drilling include:
(1) cable tool drilling, which is one of the oldest and involves lifting and dropping a heavy sharp tool to crush rocks;
(2) direct rotary drilling, which uses a circulating fluid to remove cuttings and stabilize the borehole wall;
(3) reverse circulation rotary drilling, where cuttings are lifted out of the borehole inside the rods, reducing contamination and increasing efficiency; (4) air rotary drilling, similar to direct rotary but uses compressed air;
(5) augering, suitable for softer ground;
(6) jet drilling employing high-velocity water jets to break up soil or rock;
(7) core drilling, extracting a cylinder of material for examination (Soulsby, 2010; Department of Water and Sanitation).
Different drilling methods exert various forces on the substratum during drilling and can, in some cases, cause damage to nearby structures.
Do drilling vibrations destabilise soil?
A commonly held belief is that drilling vibrations destabilise soils underlying a structure, resulting in soil movements. However, this is rarely the case for groundwater drilling outside of clay-rich soils.
Energy from the drill rig is transferred through the rods to the drill bit, fracturing the material at the drill tip instead of travelling through soils to nearby structures (Tian et al., 2022).
As such, drilling techniques use for groundwater development rarely produce enough vibrational energy for structural risks.
Dangerous construction techniques
However, certain construction techniques such as pile driving, excavation, soil cutting, or deep-vibro compaction transfer more energy though the surrounding soil mass and have been known to pose a risk to nearby structures in certain conditions (Drabkin et al., 1996; Svinkin, 2005; Harrison, 2009; Nichols, 2009; Weng et al., 2020; Bowers & Lovenstein, 2022).
These higher energy techniques are only used during the construction of larger buildings and construction in problematic soils. They are typically not used for the development of groundwater boreholes.
Common structure damage as a result of boreholes
Damage to structures from groundwater borehole construction more commonly occurs due to borehole collapse or excess air pressures resulting from a combination of poor air return and the use of high-pressure compressors (Soulsby, 2010; Hashemi et al., 2014). Fortunately, borehole collapse and excess air pressures can be detected early in the drilling process and corrected before structural damage occurs.
How to ensure the safety of structures when drilling boreholes
By monitoring material return during borehole flushing, limiting compressor pressure to the minimum requirements of the hammer, and ensuring the annulus around rods remains clear of obstructions safety can be ensured. Soils with wet clays can present a challenge; however, with a low rate of advance, frequent flushing, careful monitoring of material return, and appropriate borehole construction uncontrolled soil movements or pressure build-up can be prevented.
Conversely, constructing a borehole at a safe distance of more than 10 meters away from any existing structures will pose minimum risks to said structures save for exceptional circumstances.
Safety for complex drilling projects
For complex drilling projects or problematic soils, a basic geotechnical investigation can provide valuable input for determining appropriate drilling plans and mitigation measures, though this input is seldom needed.
Understanding groundwater drilling methods and the associated risks is crucial for successful and safe drilling projects. With experienced and patient drillers, proper planning, careful drill monitoring, correct borehole construction methods, and adherence to best practices, groundwater can be accessed easily and safely without significant risk to surrounding areas or structures.
If you need to drill a borehole and are worried about the safety of structures around it, please get in touch with the team at GEOSS who can answer any questions you may have and ensure the safety of your groundwater drilling projects.
References:
Department of Water and Sanitation. (n.d.). Groundwater Dictionary: Drilling Techniques. Available at: https://www.dws.gov.za/Groundwater/Groundwater_Dictionary/drilling_techniques.htm Accessed at 25/04/2024
Drabkin, S., Lacy, H. and Kim, D.S., (1996) Estimating settlement of sand caused by construction vibration. Journal of geotechnical engineering, 122(11), pp.920-928.
Harrison, D., (2009). Monitoring Pile Driving Vibrations—Problem Avoidance and Case Studies. In Contemporary Topics in Deep Foundations (pp. 335-342).
Hashemi, S., (2015). Drilling and maintaining stable unsupported boreholes in poorly cemented sandy formations (Doctoral dissertation).
Nicholls, K. (2009) Estimating the Potential Impact of Cable Percussion Piling. Technical Note in Ground Engineering.
Soulsby, D., (2010). Technical review: borehole drilling and rehabilitation under field conditions. Tech. Geneva, Switzerland, ICRC.
Tian, J., Fan, C., Zhang, T. and Zhou, Y., (2022). Rock breaking mechanism in percussive drilling with the effect of high frequency torsional vibration. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 44(1), pp.2520-2534.
Svinkin, M.R., (2005). Environmental vibration problems during construction. In Proceedings of the 16th International Conference on Soil Mechanics and Geotechnical Engineering (pp. 2453-2456). IOS Press.
Weng, L.K., Yohannes, M.M. and Chong, W., (2020). Data analysis and prediction of ground vibrations due to deep vibro-techniques. Geotechnical Research, 7(4), pp.244-257.
Bowers, W & Lovenstein F. (2022) The impact of construction Vibration on Adjacent Structures. JS Held Insights.
