Ultimately, the purpose of any geotechnical investigation is to obtain information about the unknown conditions that lie below the surface. Often the geotechnical investigation is the first information on the site, often before even the sight survey. We are exploring the unknown. After exploration, we only know the conditions at the testhole locations, but that can imply a variation between the testholes. We can have tremendous variation in short distances both vertically and horizontally.
The information required is to provide input into the type of foundation, design parameters, and other issues that can be expected. Two large “unexpected” costs often encountered are unsuitable fills and poor foundation conditions, requiring expensive foundations. The more information we provide, the better the owner can estimate the cost and performance before construction is started.
With fills, additional testhole may be required to examine the extent and depth of the fills. Environmental cleanup can be dangerous due to fill settlement downdrag on piles. Redevelopment sites often have old fill over old utilities. Old fill compaction was often poor, and old fills often contain garbage.
Heavy buildings, high retaining structures, elevators, and similar are not included in this section: heavy buildings may have special requirements. This is intended for one story industrial buildings, three storey or so apartments, motels, and other buildings, but may apply to others in a general way. Areas with complex geology may require additional testholes. Thalwegs, stream erosion channels, occur in bedrock and clay till formations. Native buried peat moss formations occur in geologically recent river and delta deposits.
The testholes are intended to provide an indication of the vertical and horizontal variability of the strength and settlement characteristic of the various geological strata to sufficient depth to support the expected loads. One testhole tell the conditions only where the testhole was at: we have no clue as to the variation across site. One testhole is suitable for lift stations, or similar very small structure.
Two testholes are suitable for linear structures, 100 to 200 metre long sewer, roadways; 50 metre short height retaining walls, add one per each additional 200 metres of roadway or sewer, 1 per each 50 metres retaining wall.
Three testholes is the minimum required to describe a plain in geometry; however, the contact between strata is seldom plainer, and especially water lain strata. Three testholes is suitable for buildings to 500 square metres, add 1 per 250 m2 to 1000, 1 per 500 thereafter. Personally, I would not build a house without 4 testholes.
Should something odd or expensive to deal with be found, additional testholes for clarification may be required. Old coal mines, undocumented environmental cleanups, any industrial use, or redevelopment site can be hazardous.