The Standard Penetration Test (SPT)

The Standard Penetration Test is an in-situ test that is useful in site exploration and foundation design.  It produces an N-value, which represents the number of blows of a standardized sampler driven into the soil a standardized distance.

How the Test Works

The sampler is 51 mm O.D. (outside diameter) and it is driven into the soil with a 63.5 kg weight having a free fall of 760 mm.  The first 150 mm of soil is neglected.  The next 300 mm of soil constitutes the test.  The number of blows for that 300 mm becomes the N-value.

It can be done in the bottom of a borehole or at the surface.

In the United States, the standard test procedure is ASTM D1586.

Typical values are 0-10 for sand, 5-10 in loose clay and 10-30 in compacted clay.

Pros and Cons

Although it is widely used, there are some important limitations that I have to mention.  The SPT is a field test which should be used as a general guide only.  The repeatability of the test is highly questionable and the relationship between N-value and soil density should be used with caution.  That being said, it is a good qualitative indicator of the compactness of the soil, or in a comparison of subsoil stratification.

Also, it is highly affected by drilling and sampling operations.  Many small issues can greatly affect the results, including inadequate cleaning of the borehole, failure to maintain the hydrostatic pressure of the borehole, variations in the driving of the hammer, etc.  You get the idea.

As you can imagine, problems with drilling and sampling will generally result in low N-values.

Also, there are considerable variations in the test throughout the world.  If you are not in the United States, you might want to consider everything I say in here as a guide because the procedure might be slightly different.

Corrections

Although the field-tested N-value should be reported on the borehole logs, many corrections have been suggested that are in common use today.

1. For actual energy delivered to the drill rod.
2. For influence of overburden stress on N-value.
3. To account for the length of the drill rod.
4. To account for the absence of presence of a liner inside the split spoon sampler.
5. To account for the influence of the diameter of the borehole.

For Actual Energy Delivered to the Drill Rod

The procedure is to determine N₆₀, the N-value for an energy level of 60%, as follows:

$$N_{60} = N\frac{ER_r}{60}$$

Where ER_r is determined as per the following table.  Please be cautious as this is just a compilation of the world’s hammer usage (taken from Seed et al., 1984, Seed & Harder et al., 1984, Skempton, 1986):

Country	Hammer	Release	ERr (%)	ERr/60
North and South America	Donut Safety Automatic	2 turns of rope 2 turns of rope Trip	45 55 55 to 83	0.75 0.92 0.92 to 1.38
Japan	Donut Donut	2 turns of rope Auto-trigger	65 78	1.08 1.3
China	Donut Automatic	2 turns of rope Trip	50 60	0.83 1.0
U.K.	Safety Automatic	2 turns of rope Trip	50 60	0.83 1.0
Italy	Donut	Trip	65	1.08

For Influence of Overburden Stress

As you can imagine, the soil deep in the ground might have a different N-value than at the surface, even at the same compaction, due the the overburden stress.  A common correction factor for overburden stress is:

$$C_N = 0.77log_{10}\frac{1920}{\sigma_v’}$$
Where σ_v‘ = effective overburden stress at the level of the N-value, in kPa.

To Account for the Length of the Drill Rod

Use the following table to find C_r:

Rod Length		Correction Factor Value
m	ft
> 10	> 33	1.0
6 – 10	20 – 33	0.95
4 – 6	13 – 20	0.85
3 – 4	10 – 13	0.70

To Account for a Liner

For a standard sampler, use Cs = 1.0.  For a U.S. sampler without liners, use Cs = 1.2.

To Account for the Borehole Diameter

Use the following table to determine C_d:

Borehole Diameter		Correction Factor Value
mm	inches
65 – 115	2.6	1.0
150	5.9	1.05
200	7.9	1.15

How to Apply the Correction Factors

There is not a overall consensus of how to apply the correction factors.  You should check with the design guidelines in your area, but I will provide the following procedure just so it is not left as a vacuum:

1. Adjust for the energy delivered to the rod by calculating N₆₀.
2. Apply the other correction factors to N₆₀.