Knowing the sub-ground structure that your proposed borehole is to be constructed in can significantly alter the parameters, design and type of construction method used, site location, methodology used, costs and expected construction time frame needed to complete the project successfully. Having this knowledge can be extremely valuable for any project, but unfortunately geotechnical investigations are usually seen as an unnecessary cost, and something for the contractor to work out once they’re onsite.
With most trenchless projects, up to 95 per cent of its construction is undertaken out of sight, and in some cases, out of mind. When planning any trenchless project, minimising risk should always be a top priority. Drilling in unknown ground conditions can be extremely risky and in some cases detrimental to the success of the project. Yes, some projects are so small it may not be financially viable but if the project is large enough, then a budget for geotechnical investigation can be well worth the exercise. Finding a few dollars to spend during the pre-planning stage can save a lot of unwanted additional cost during and at the completion of a project. Risk should not just been seen as actual construction risk, but also financial risk. When projects have been pre-planned, and detailed feasibility and concept designs undertaken (including geotechnical), they have a much higher success rate – and minimise risk for both the client and contractor alike.
Understanding what is achievable before a project begins can be a major contributor in the project’s success or failure. Knowing what type of sub-ground formation the trenchless borehole will be in can be vital for its success. One particular area of concern when assessing a horizontal directional drilling (HDD) project is the ground condition versus the achievable bend radius. Ground conditions determine the achievable bend radius which should be used during the design. A clay type formation will allow for good steerability, but sands, silts and rock may only achieve minimum bend (less than 50 per cent of the bend radius achieved in clay conditions) – thus limiting the amount of achievable steer during the boring operation. This can impact on the required exit location, required bore depth, set up angles, bore length and ability to maintain safe clearances from existing underground services.
Knowledge of the ground conditions doesn’t just benefit the design of the borehole, but also the excavation of the entry and exit pits. Excavating your entry and exit pits can be sometimes overlooked and only a minor part of the overall project, but finding out that the ground isn’t stable enough and requires shoring or benching – or even encounters rock or cobble which can be costly and can add additional time to the construction time frame – can turn the project on its head.Article continues below…
Knowledge is all about minimising risk, and when the sub-ground conditions are unknown, this can be a major risk. When a geotechnical investigation is carried out along the proposed route, detailed in a document and included in the tender information supplied by the client, the contractor becomes more confident with what is required and can plan the job appropriately, including costs. Selection of the appropriate equipment to undertake the project can be made including any additional non-standard equipment or material to deal with the expected ground conditions, and most importantly, the correct drilling equipment can be considered and chosen. In general, the risk of unforeseen ground conditions and the potential for accidents is reduced when geotechnical data is collected along the proposed route.
Issues that can arise from changing ground conditions can be wide ranging and include:
- Borehole collapse
- Ground settlement
- Ground heaving
- Collapse of excavation pits
- Borehole deviation
- Damaging, jamming or loss of ground engaging tooling
- Damage or collapse of product pipe during installation or once in place.
To summarise, it is beneficial to both the client and contractor to understand the sub-ground formation which is likely to be encountered before a project is undertaken. Unfortunately, ground conditions can change rapidly and even the best geotechnical investigation can miss changes in ground conditions. Rock shelves can change direction and cobble layers can vary. What you find in one area may be completely different to another area several hundred metres away. Even with these uncertainties, any geotechnical information is better than none; and it is always beneficial to give contractors access to any geotechnical information before they submit their tender proposal.
This information should be viewed and taken into account when planning any project with the ultimate goal to minimise risks, cost discrepancies and safety issues for all personnel.