Running a Trial Pit Site Investigation
A trial pit (also known as a test pit or inspection pit) investigation is a highly effective way of obtaining data on the subsurface soil and rock conditions which underlie a site. It allows for the various soils and rock types to be logged, the soils to be sampled and a preliminary assessment of the groundwater regime to be made.
However, to be effective the process must run smoothly. In a nutshell the procedure is to hire a TLB (Tractor-Loader-Backhoe) from a plant hire company, rendezvous with the machine on the agreed date, dig, log, sample and backfill as many pits as necessary to provide enough data to characterise the geology of the site and then to return to the office to process the data and write up the geotechnical report. It sounds simple in practice but there are a myriad of considerations which, if not attended to, can seriously derail the process. Set out below are some guidelines and sage advice to assist you in getting going and keeping your stress levels to a minimum.
Hiring the TLB/JCB
The plant hire companies need some lead time when it comes to booking a machine. Generally give yourself a week if you can – a phone call to the company will tell you if they have one available. It may take a dozen calls to track down a machine if there is a crunch on. Try and find a plant hire company as close to the site as possible – 30 km being probably about the limit that a TLB can economically travel and bearing in mind the time constraints imposed by a machine that travels at speeds of less than 60 km/h. When phoning around for a machine, ask for their ‘wet’ rate – this means they will keep the machine supplied with fuel. You don’t want to be involved in tracking the machine to filling stations and paying to pump diesel into someone else’s equipment.
Bear in mind that your hourly cost exceeds the hourly rate of the plant and it is important that you use your time as efficiently as possible. Confirm your booking with a fax or email, along with your contact number, agreed hire rates and order number, and it is worth while drawing a sketch with directions to the site. You might also be required at some stage to work with a 20 or 30 tonne excavator, particularly when investigating dam sites or looking for borrow materials. The procedures are the same, but you will have to pay for ‘establishment’ whereby the machine is brought to site on a low bed.
We have another 13 pages of relevant, hands on practical guidelines for running a test pit investigation. For the full document click here