Geotechnical Investigations

‘Without a site investigation the ground is a hazard’. How true! A good way to look at this problem is to consider the cost of one day’s delay once construction has started and then consider the cost of a ground investigation.

Excluding regulatory controls there are so many things that can disturb the smooth running of installing foundations, such as:

• — water-bearing gravel lense in clay soil;
• — presence of Peat;
• — isolated soft spots and groundwater; and/or
• — irregular bedrock surface.

Piling contractors are able to design piled foundations to satisfy most foundation requirements. Without a suitable ground investigation their design will be conservative and, consequently, more expensive. In fact, reputable piling contractors will not quote unless they have the ground investigation. Some will offer to do it for you – at their price and limited to their requirements only.

The simplest way to derive geotechnical information is to dig trialpits to see what the ground is like at the site. But in poor ground the trialpit sidewalls can be unstable and will not reveal how deep the foundation should be and, after reinstatement, will become a soft spot that would be a hazard for over-running vehicles.

Borehole investigations, although more expensive, are more thorough and allow the efficient design of sound foundations. And even then there is the selection of the most suitable boring method and correct drilling rig. For instance, the window sampler method is unsuitable for boring through wet running sand because every time the sampler is withdrawn from the hole the sand will run in and block the hole, usually to the top of the groundwater level. Casing needs to be installed, to seal off the groundwater, but a typical window sampler cannot do this.