Indirect Methods - Soil Exploration, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering PDF Download

Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is widely used to determine the in-situ parameters of the soil. The test consists of driving a split-spoon sampler into the soil through a bore hole at the desired depth. The split-spoon 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:

• Rope-pulley or cathead = 70 to 80
• Trip or automatic hammer = 80 to 100

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 hollow-stem 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'₆₀values?

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)

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)

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 (Part-III)-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 cross-sectional area of 10 cm². The friction sleeve should have an area of 150 cm² 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 (15-20). For Electric cone, N_k = 15 and for mechanical cone, N_k = 20.

In Sands: the modulus of elasticity can be correlated as: E = (2.5-3.5) 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 Soils-Part 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 cast-iron 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.