Environmental Engineering Reference
In-Depth Information
3.4.5 Soil Penetration Tests
Standard Penetration Test (SPT) (ASTM D1586)
Purpose
SPT values are correlated with the compactness of granular soils, from which
φ
values are
estimated and correlated with the consistency of cohesive soils.
Test Method
The split-barrel sampler (see Section 2.4.2), 2-in. O.D. and 1-in. I.D., is driven into the ground
by a 140-lb hammer dropping in free fall from a height of 30 in. (energy
4200 in. lb). The
number of blows required to advance the sampler each 6 in. of penetration is recorded.
Standard penetration resistance, N is taken normally as the penetration of the second plus
the third 6 in. to provide “blows per foot of penetration.” The first 6 in. is disregarded
because of the possibility of cuttings in the hole or disturbance during washing. The pene-
tration resistance is composed of end resistance plus shaft friction. End resistance is the
larger component in granular soils, and shaft resistance is the larger component in clay soils.
Operational Factors Affecting N Values
Driving energy: Standard hammer weight and drop should always be used. The weight is
lifted by various methods: rope or cable on a drum, block and tackle, or by automatic ham-
mers. Common weights include a mass with a “stinger” rod that fits into the drill rod, or
a “doughnut”-shaped weight that slides over the drill rod. A quick-release grab is shown
in Figure 3.58. The weight is “grabbed,” raised 30 in., and automatically released. To
ensure that the fall is always completely free of any resistance requires care by the opera-
tor, especially with the rope-and-drum method.
In recent years, automatic drop hammers have been developed. They are activated by a
hydraulically powered chain lift device.
Sampler: The sampler is illustrated in Figure 2.61. Diameters larger than the standard
2 in. sampler require correction. The adjusted blow count B for the measured blow count
B ' can be determined by the equation (Burmester, 1962a).
B
B
(4200/ WH ) [( D 2
D i 2 )/(2.0 2
1.375 2 )]
B
B
(4200/ WH ) [( D 2
D i 2 )/2.11] (3.44)
where W is the hammer weight variation, H the drop height variation and D o and D i are
the sampler diameter variations.
The damaged or blunt cutting edge on a sampler will increase resistance. Liner-designed
tubes will produce lower N values when used without liners (Schmertmann, 1979).
Drill Rods and Casing: One study indicates that the difference in weight between N and
A rods has negligible effect on N values (Brown, 1977), although there are differences of
opinion in the literature regarding the effects. Drill rods that are loosely connected or bent,
or casing that is out of plumb, will affect penetration resistance because of energy absorp-
tion and friction along casing walls. The rod length, i.e., weight, also affects the N value.
Hole bottom conditions: (see Section 2.4.2 and Figure 2.60): Soil remaining in casing, even
as little as 2 in. of sand, will cause a substantial increase in N (plugged casing). The boil-
ing and rising of fine sands in casing when the tools are removed, which causes a decrease
in N , is common below the water table and is prevented by keeping the casing filled with
water unless conditions are artesian. Gravel particles remaining in the mud-cased hole
will increase N values by 30 to 50, and can result in a loose sand that should only be ten
blows. Overwashing or washing with a bottom-discharge bit with high pressure will
loosen the soils to be sampled.
 
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