Standard Penetration Test (SPT)
The Standard Penetration Test (SPT) is widely used to determine the insitu parameters of the soil. The test consists of driving a splitspoon sampler into the soil through a bore hole at the desired depth. The splitspoon sampler is driven into the soil a distance of 450 mm at the bottom of the boring. A hammer of 63.5 kg weight with a free fall of 760 mm is used to drive the sampler. The number of blows for a penetration of last 300 mm is designated as the Standard Penetration Value or Number N (ASTM D1586). The test is usually performed in three stages. The blow count is found for every 150 mm penetration. The blows for the first 150 mm are ignored as the top soil may be of disturbed nature due to advancement of borehole and hence considered as those required for the seating drive. The refusal of test when
50 blows are required for any 150 mm increment.
100 blows are obtained for required 300 mm penetration.
10 successive blows produce no advance.
The standard blow count N¢70 can be computed as (ASTM D 1586):
\[{N'_{70}}={C_N} \times N \times {\eta _1} \times {\eta _2} \times {\eta _3} \times {\eta _4}\] (31.1)
where
\[{\eta _i}\] = correction factors
N'_{70} = corrected N using the subscript for the E_{rb} and the ' to indicate it has been corrected
E_{rb} = standard energy ratio value
C_{N} = correction for effective overburden pressure p'0 (kPa) computed as [Liao and Whitman, 1986]:
\[{C_N}={\left( {{{95.76} \over {{{p'}_0}}}} \right)^{{1 \over 2}}}\] (31.2)
SPT is standardized to some energy ratio (E_{r}) as:
\[{E_r}={{Actual\;hammer\;energy\;to\;sampler,\;{E_a}}\over{Input\;energy,\;{E_{in}}}}\times100\] (31.3)
Now \[{E_{in}}={1 \over 2}m{v^2}={1 \over 2}{W \over g}{v^2}\] and \[v={(2gh)^{{1 \over 2}}}\]
Thus, \[{E_{in}}={1 \over 2}{W \over g}(2gh)=Wh\] (31.4)
where W = weight of hammer and h = height of fall
The correction factor \[{\eta _1}\] for hammer efficiency can be expressed as (Bowles, 996):
\[{\eta _1}={{{E_r}} \over {{E_{rb}}}}\] (31.5)
Different types of hammers are in use for driving the drill rods. Two types are normally used. They are (Bowles, 1996):
(i) Donut hammer with Er = 45 to 67
(ii) Safety hammer with Er as follows:
Now if E_{r} = 80 and standard energy ratio value (E_{rb}) = 70, then \[{\eta _1}\] = 80/70 = 1.14
Correction factor \[{\eta _2}\] for rod length (Bowles, 1996):
Length >10 m \[{\eta _2}\] = 1.00
6 – 10 m = 0.95
4 – 6 m = 0.85
0 – 4 m = 0.75
Note: N is too high for Length < 10 m
Correction factor \[{\eta _3}\] for sampler (Bowles, 1996):
Without liner \[{\eta _3}\] = 1.00
With liner: Dense sand, clay = 0.80
Loose sand = 0.90
Correction factor \[{\eta _4}\] for borehole diameter
Hole diameter: 60 – 120 mm \[{\eta _4}\] = 1.00
150 mm = 1.05
200 mm = 1.15
Note: \[{\eta _4}\] = 1.00 for all diameter hollowstem augers where SPT is taken through the stem
Problem 1
Given: N = 21, rod length= 13 m, hole diameter = 100 mm, p'_{0} = 200 kPa, E_{r}= 80; loose sand without liner. What are the standard N'_{70} and N'_{60}values?
Solution: For E_{rb}= 70: \[{N'_{70}}={C_N} \times N \times {\eta _1} \times {\eta _2} \times {\eta _3} \times {\eta _4}\]
Now, \[{C_N}={\left( {{{95.76} \over {200}}} \right)^{{1 \over 2}}}=0.69\] ; \[{\eta _1}\] = 80/70 = 1.14; \[{\eta _2}\] = 1.0; \[{\eta _3}\] = 1.0; \[{\eta _4}\] = 1.0
Thus, \[{N'_{70}}=0.69 \times 21 \times 1.14 \times 1.0 \times 1.0 \times 1.0=17\]
Now \[{E_{r1}} \times {N_1}={E_{r2}} \times {N_2}\] ; Thus, \[{N'_{60}}=\left( {{{70} \over {60}}} \right) \times 17=20\]
SPT Correlations in Clays (N. Sivakugan)
N'_{60}  c_{u} (kPa)  Consistency  Visual identification 
02  0  12  very soft  Thumb can penetrate > 25 mm 
24  1225  soft  Thumb can penetrate 25 mm 
48  2550  medium  Thumb penetrates with moderate effort 
815  50100  stiff  Thumb will indent 8 mm 
1530  100200  very stiff  Can indent with thumb nail; not thumb 
>30  >200  hard  Cannot indent even with thumb nail 
Note: N'_{60} is not corrected for overburden and c_{u} is the undrained cohesion of the clay.
SPT Correlations in Granular Soils (N. Sivakugan)
(N')60  Dr (%)  Consistency 

04  015  very loose  
410  1535  loose  
1030  3565  medium  
3050  6585  dense  
>50  85100  very dense 
Note: N'_{60 }is not corrected for overburden
2 Static Cone Penetration Test (SCPT)
The Static cone penetration test has been standardized by “IS: 4968 (PartIII)1976: Method for subsurface sounding for soils  Part III Static cone penetration test”. The equipment consists of a steel cone, a friction jacket, sounding rod, mantle tube, a driving mechanism and measuring equipment. The cone has an apex angle of 60° ± 15′ and overall base diameter of 35.7 mm giving a crosssectional area of 10 cm^{2}. The friction sleeve should have an area of 150 cm^{2} as per standard practice. The sounding rod is a steel rod of 15 mm diameter which can be extended with additional rods of 1 m each in length. The driving mechanism should have a capacity of 20 to 30 kN for manually operated equipment and 100 kN for the mechanically operated equipment. With help of this test, the friction and tip resistance can be determined separately which is very useful information for pile foundation.
SCPT Correlations
In Clays: \[{c_u}={{{q_c}  {\sigma _v}} \over {{N_k}}}\] ; where sv = total vertical stress and N_{k} = cone factor (1520). For Electric cone, N_{k} = 15 and for mechanical cone, N_{k} = 20.
In Sands: the modulus of elasticity can be correlated as: E = (2.53.5) q_{c} (for young normally consolidated sands), where q_{c} the tip or cone resistance.
3. Dynamic Cone Penetration Test (DCPT)
The dynamic cone penetration test is standardized by “IS: 4968 (Part I) – 1976:Method for Subsurface Sounding for SoilsPart I Dynamic method using 50 mm cone without bentonite slurry”. The equipment consists of a cone, driving rods, driving head, hoisting equipment and a hammer. The hammer used for driving the cone shall be of mild steel or castiron with a base of mild steel and the weight of the hammer shall be 640 N (65 kg). The cone shall be driven into the soil by allowing the hammer to fall freely through 750 mm each time. The number of blows for every 100 mm penetration of the cone shall be recorded and total number of blows for each 300 mm penetration is considered as DCPT N value. The process shall be repeated till the cone is driven to the required depth. DCPT is better than SPT or SCPT in hard soils such as dense gravels. In case of SPT samples are collected for testing whereas in case of SCPT or DCPT samples can not be collected. Hammer is used in case of SPT and DCPT, but for SCPT no hammer is used, the cone is pushed inside the soil.