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1. Bore Hole
Spacing and Depth

2. Soil Boring

3. How many Bore Holes?
The number of bore holes depends on:
(i) Type and size of the project
(ii) Budget for site investigation
(iii) Soil variability
The bore holes have to be located where the loads are expected.
SPACING OF BORINGS (Das, 1999)

Type of project Spacing (m)
Multistory buildings 10 – 30
One-story industrial plants 20 – 60
Highways 250-500
Residential subdivision 250-500
Dams and dikes 40 - 80

4. Spacing Boring
There are no hard and fast rules for the spacing of the boreholes.
The following table gives some general guidelines for borehole
spacing. These spacing can be increased or decreased, depending
on the subsoil condition. If various soil strata are more or less
uniform and predictable, the number of boreholes can be reduced.

5. Depth of Boring
The approximate required minimum depth of the
borings should be predetermined. The estimated
depths can be changed during the drilling
operation, depending on the subsoil encountered.
To determine the approximate minimum depth of
boring, engineers may use the following rule:

6. 1.Determine the net increase of stress, under a foundation with depth
as shown in the Figure.
2.Estimate the variation of the vertical effective stress, ', with depth.
3.Determine the depth, D = D1, at which the stress increase  is equal to
(1/10 ) q (q = estimated net stress on the foundation.)
4.Determine the depth, D = D2, at which /' = 0.05.
5.Unless bedrock is encountered, the smaller of the two depths, D1 and
D2, just determined is the approximate minimum depth of boring
required. Table shows the minimum depths of borings for buildings
based on the preceding rule.

7. Determination of the minimum depth of boring

8. Depth of Boring

9. For hospitals and office buildings, the following
could be use to determine boring depth

10. When deep excavations are anticipated, the depth of
boring should be at, least 1.5 times the depth of
excavation. Sometimes subsoil conditions are such
that the foundation load may have to be transmitted to
the bedrock. The minimum depth of core boring into
the bedrock is about 3m. If the bedrock is irregular or
weathered, the core borings may have to be extended
to greater depths.

11. Minimum depth of boring (ASCE, 1972)
Determine the net increase of stress, Δσ, under the foundation (as shown in Figure)
Estimate the variation of the vertical effective stress, σ'
v , with depth
Determine the depth D = D
1, at which stress increase Δσ = q/10, where q =
estimated net stress on the foundation
Determine the depth D = D
2, at which Δσ/ σ'
v = 0.05.
Unless bedrock is encountered, the smaller of the two depths, D
1 and D
2 will be the
approximate minimum depth of boring required.

12. The minimum depth of boring for a building with a width of 30.5
m (100 ft) will be as follows (Sowers and Sowers, 1970)
No of stories Boring depth
1 3.5 m
2 6.0 m
3 10 m
4 16 m
5 24 m

13. Depth of Borings (according to IS 1892-1979)
Type of foundation Depth of boring
1.Isolated spread footing or raft One and half times the width (B) of the
foundation

2. Adjacent footings with clear spacing One and half times the length (L) of
less than twice the width footing
3. Pile and well foundation To a depth of one and half times the width of
structure from the bearing level (toe of pile or bottom of well)


4. (a) road cut Equal to the bottom width of the cut
(b) Fill Two meters below ground level or equal to the
height of the fill whichever is greater.

14. D = 1.5 B for A ³ 4B and D = 1.5 L for A < 2B
D = 3 B for A >2B and <4B
D = 4.5 B for A < 2B
D = 1.5 B for A ³ 4B

15. Depth of boring according to IS 1892-1979

16. A Raft Foundation With Dimensions Of (25m X 30 M) Has Been Constructed. Find
Number And Depth Of Borehole Required
generally 5 no. of boreholes are drilled with 4 being near to the each corner of
foundation and 1 at the centre now depth of borehole is : given data: = 25m 30m..