Site investigation - Practical Engineering Geology - page 162

Standard
weight falling
standard height

W

6 marks each

data recorded
- number of blows of
hammer per 75 mm
penetration
2
3
3
4
3

75 mm apart

Reference
beam

'seating blows'
- disregarded

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'N' value =
15

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3
5

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Connecting
rods

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Hole allows
expulsion of
water and air

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Borehole

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Not to scale

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Sample that can be
described and
tested

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Factors that will influence results and may require corrections to be made

Excess water pressure
may develop in silty
fine sand leading to low
SPT values

Casing

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- efficiency of
impact

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x
x x x

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Water
table

Length of rods
(their weight and
adsorption of
energy by
bending)

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Clean, dense sand may
dilate causing suction
and high temporary
resistance
Nature of soil
(also in situ stresses)

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Water flow into the borehole
may loosen ground

Efficiency of test

1 2 3

Figure 4.30 Principles and details of the SPT test.

added together as the N-value. Care must be taken in soil that the
external water table is balanced, otherwise water may flow in from

the bottom of the hole, causing softening and too low an N value.
There are various corrections suggested for tests conducted in silty
sand and for depth of overburden. Details are given in Clayton (1995).