One of the most important aspect of soil investigation processing is sampling i.e. making test holes at desired intervals for observation and to obtain soils for laboratory analysis. The depth of these test boreholes is often vague but these should be predetermined before starting the investigation activities, though, they may change depending on the subsoil encountered in the process. To determine the approximate minimum depth of boring for foundations, engineers may use the rules established by the American Society of Civil Engineers (1972) as described in the steps below:
Borehole Depth
Step 1 – Determine the net increase of vertical stress, ∆σ, under a foundation with depth, D as shown in Figure 1.
Note: the general equations for estimating increase in vertical stress in soils varies depending on the type of load coming on the soil. There are two common approaches by Boussinesq and Westergaard with their associated equations. The equations vary for;
- Point loads;
- Line loads;
- Line loads of infinite length;
- Strip load of finite width and infinite length;
- Uniformly loaded circular area;
- Rectangular loaded area;
(specialist literature should be consulted here to know the one that applies in each case)
Step 2. Estimate the variation of the vertical effective stress, σo1 with the depth.
See: How to Determine increase in Vertical Effective Stress
Step 3. Determine the depth, D = D1, at which the stress increase ∆σ is equal to (1/10) q (= estimated net stress on the foundation).
Step 4. Determine the depth, D = D2, at which ∆σ/σo1ˈ = 0.05.
Step 5. Unless bedrock is encountered, the smaller of the two depths, D1 and D2, just determined is the approximate minimum depth of boring required (Note: if bedrock is encountered, also determine the depth, D = D3 which is the distance from the lower face of the foundation to the bedrock and in that case, choose the smaller of the three depths, D1, D2 and D3).
Step 6. After determining the value of D as explained in the first 5 steps, the final depth of boring (from ground surface to the calculated depth), Dboring = Df + D. This is so because drilling starts from the surface of the surface of the ground.
Conversely, Sowers and Sowers (1970) gives approximate depth of borings for buildings with a width of 30 m as shown in Table 1.
For hospitals and office buildings, the following expressions could apply to determine approximate depth of borings;
Db = 3S0.7 (for light steel or narrow concrete buildings)
Db = 6S0.7 (for heavy steel or wide concrete buildings)
Where,
Db = depth of boring (m)
S = number of storeys
When deep excavations are anticipated, the depth of boring should be at least 1.5 times the depth of excavation. Sometimes subsoil conditions require that the foundation load be transmitted to bedrock. The minimum depth of core boring into the bedrock is about 3 m. If the bedrock is irregular or weathered, the core borings may have to be deeper.
Borehole Spacing
With regard to borehole spacing, there is no hard and fast rule. The spacing can be increased or decreased, depending on the subsoil condition. If various soil strata are more or less uniform and predictable, fewer boreholes are needed than in nonhomogeneous soil strata. Table 2 below shows typical borehole spacing.
