316691611 Standard Penetration Test PDF

Appendix A Standard Penetration Test (SPT)

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Standard Penetration Test (SPT) "SPT" is an abbreviation for the Standard Penetration Test, which is the most widely used in-situ soil characterization test in the world. Colonel Charles Gow of the Raymond Pile Company 02 first introduced this testing procedure in 1902. The purpose of this test is to obtain an indication of the relative density of sands and gravels. However it can also used to obtain an indication of the character of silts, clays and weak rock. The testing procedure was later modified to include the estimation of soil strength parameters and soil compressibility parameters.The popularity of the Standard Penetration Test can be attributed to both the ease of the test as well as the inexpensive cost. The continued popularity of the SPT is due to the many design correlations associated with the SPT - N value. Engineering applications of SPT results include: determination of settlement of granular soils, estimation of liquefaction potential, compaction control and bearing capacity of piles. The SPT equipment (ASTM D1586, Eurocode 7) comprises a split tube (sampler) with a driving head to recover disturbed soil samples. A split spoon sampler with two halves opened is shown in Figure A1.

Figure A1. Split spoon sampler.

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The head of the tube is threaded for connection (via a series of drive rods) to a hammer. The device is driven into the ground at the base of the borehole with a free falling 63.5 kg hammer dropping a distance of 760 mm. The standard is much less specific about the required design of the SPT hammer, stating only that the hammer must have a mass of 140 lbs (63.5 kg) and must drop vertically as freely as possible through 30" (0.76 m) before hitting the anvil. As a result, different hammer designs have evolved and they vary considerably. The most commonly used hammer types in North America are the donut, safety and automatic hammers, as shown in the Figure A2.

Figure A2. Different SPT hammer configurations The SPT test involves driving the split spoon sampler into the bottom of a borehole. The total blows required from a hammer, over the interval 150 to 450 mm (6 to 18 inches) are summed to give the blow count N. A typical components of a SPT set up is shown in the Figure A3.

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Figure A3. Typical SPT set up

Correction of the Field SPT N The quality of test results depends on several factors, such as actual energy delivered to the head of the drill rod, the dynamic properties (impedance) of the drill rod, the method of drilling and borehole stabilisation. The actually delivered energy can vary between 50 - 80% of the theoretical free-fall energy. The SPT can be difficult to perform in loose sands and silts below the ground water level (typical for land reclamation projects), as the borehole can collapse and disturb the soil to be tested. The following factors can affect the test results: nature of the drilling fluid in the borehole, diameter of the borehole, the configuration of the sampling spoon and the frequency of delivery of the hammer blows. Therefore, it should be noted that drilling and stabilization of the borehole must be carried out with care. The measured N-value (blows/0.3 m) is the so-called standard penetration resistance of the soil. The penetration resistance is influenced by the stress conditions at the depth of the test. Peck et al. (1974) proposed, based on settlement

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observations of footings, the following relationship for correction of confinement pressure. The measured N-value is to be multiplied by a correction factor CN to obtain a reference value, NCorrected, corresponding to an effective overburden stress of 1 t/ft2 (approximately 107 kPa), NCorrected = NField . CN where CN is a stress correction factor and p' is the effective vertical overburden pressure at the test level. CN = 0.77 . log10 (20/p') A chart to obtain the correction factor CN is given in Figure A4. There is a correlation developed, as shown in Figure A5, to obtain strength parameters from the SPT value after application of the Peck correction for sandy soils.

Figure A4. Chart to obtain the overburden correction factor as suggested by Peck et al. (1974)

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Figure A5 – Chart to obtain angle of internal friction from the corrected SPT N value for sandy soils. Uncertainty In The Performance Of The Spt

There is a large number of uncertainties involved in the performance of SPT Unfortunately most ofthese sources have not been sufficiently quantified. Table 1 summarizes a total of 27 sources of uncertainty and bias,classified in five categories.This classification aims to assist the practicing personal in taking the necessary measures to minimize the uncertainties in the performance of the test: Category A includes sources that depend on the type of soil, with emphasis in sources related to granular materials since the use of the SPT is generally not recommended for cohesive soils. For example: Vertical stress, Mineralogy, Coarse gravel and cobbles in soil, horizontal stress and geologically aged deposits. Category B lists sources of uncertainty due to the presence of water. For example Pore pressure generation, and moisture sensitive behavior of geologically aged sand. Category C includes reducible sources related to the equipment and its maintenance.

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For example hammer efficiency, borhole diameter, rod length, sampler, Lack of hammer free fall because of ungreased sheaves, new stiff rope for lifting weight, Use of bent drill rods etc. Category D includes sources that are reducible if the test procedure is performed carefully. For example Inadequate cleaning of hole, Inadequate head of water in the borehole, Careless measurement of hammer drop, Sampler driven above bottom of casing, More than two turns on cathead, Hammer strikes drill rod collar eccentrically, Incomplete release of rope in each drop, and Careless blow count. Category E lists irreducible sources in the inves-tigation procedure. For example Human factor, and Weather and site conditions.

Energy Correction method for SPT N value

Different researchers tried to correct the field SPT N values due to the errors due to reducible sources related to the equipment and its maintenance. In this method of the SPT correction, following corrections are applied. Overburden correction Overburden correction factor, CN, is obtained by the following expression.

CN 

95.76 / po

Where Po/ is the effective overburden pressure in kPa.

Energy correction factor (1)

Several investigators have measured the hammer energy in various SPT systems and found considerable variability. Some investigations

showed experimentally that the

measured blow count was inversely proportional to the energy delivered to the drill rods for blow counts less than 50. Because of the variable energy input from different SPT hammer/rod systems, it was suggested that measured blow counts (N-value) be corrected

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to reference value of 70% of the potential energy of the SPT hammer. The 70% reference energy was adopted because energy measurements carried out on different hammer configurations have indicated an average of about 70% of the theoretical potential energy of the SPT If the energy ratio of the SPT set up used is Er then,

1 

Er 70

Similarly, other correction factors due to the rod length, sampler, and borehole diameter 2, 2, and 3 respectively can be obtained from Figure A6.

Figure A6. SPT correction factors

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The corrected SPT N value is referred to as N70/ since the energy ratio is converted to that corresponding to 70%.

N70  CN1234 /

N70/ can be used to obtain the strength parameters from Figures A7

Figure A7 – Strength parameters from the N70/ from the SPT.

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Example A.1 Five boreholes are driven in a proposed building site to investigate the subsurface condition for a 20-storey building. The subsurface at the site consists of loose silty sand, stiff clay, completely weathered rock, and fractured rock. A typical subsurface condition in a borehole together with the SPT blow counts and rock properties are given in the following Figure. If the SPT setup used has an energy ratio of 60% and the borehole diameter is 75 mm, estimate the soil strength parameters upto the weathered rock layer. (Assume weathered rock layer consists of coarse grained soil and noliner is used in the SPT sampler). Depth (m)

SPT

0.00 W. T

1.00

Loose silty sand

5.00

Stiff clay

12.00 Completely weathered rock (Coarse sand)

17.00

Fractured rock

x

1.0

3

2.0

4

3.0

4

4.0

5

5.0

12

6.0

12

7.0

14

8.0

16

9.0

15

10.0

17

11.0

14

12.0

40

13.0

42

14.0

40

15.0

> 50

16.0

> 50

17.0

> 50

Answer 1

P/o (kPa) CN

Depth (m) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

16.0 22.2 28.4 34.6 41.8 49.0 56.1 63.3 70.5 77.7 84.9 93.1 101.3 109.5 117.7 125.9 134.0

2.4 2.1 1.8 1.7 1.5 1.4 1.3 1.2 1.2 1.1 1.1 1.0 1.0 0.9 0.9 0.9 0.8

2 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79

0.75 0.75 0.85 0.95 0.95 0.95 0.95 0.95 1 1 1 1 1 1 1 1 1

3

4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

N/70 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

6 5 5 6 14 13 14 15 14 15 12 32 32 29



c (kPa)

30 30 30 30 83 75 83 92 83 92 67 43 43 42

Example A2 The subsurface condition at a building site consists of a 3m thick fine sand layer underlain by a 5m thick normally consolidated clay layer followed by a hard weathered rock layer. The subsurface condition and the SPT blow counts obtained from the site investigation program are given in Figure A2. Water table is located at 2.0m below the ground surface. Estimate the soil strength parameters of the subsurface using the energy method. (Assume an energy ratio of 55%, borehole diameter of 90mm and noliner)

CN 

95.76

Where Po/ =

Po

/

Effective overburden pressure in kPa.

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Fine sand layer with wet= 15.5 kN/m3 sat = 16.5 kN/m

W. T

3

Clay layer with sat = 16 kN/m3

Weathered rock layer with sat = 16.5 kN/m3

Depth (m) 1

4

SPT

2

3

3

5

4

3

5

4

6

3

7

3

8

20

9

22

10

24

12

28

13

25

14

50 <

Bedrock Figure A2 Answer Depth (m) 1 2 3 4 5 6 7 8 9 10 12 13 14

SPT (N) 4 3 5 3 4 3 3 20 22 24 28 25 50

P/o (kPa) 15.5 31 37.7 43.9 50.1 52.3 62.5 68.6 75.3 80 95.4 102.1 108.8

N2 0.75 0.75 0.85 0.85 0.95 0.95 0.95 0.95 1 1 1 1 1

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N/70 6 3 5 3 4 3 3 17 19 20 22 19 36

/ 30 28 29

C (kPa)

12.5 18 12.5 12.5 36 37 38 39 37 45

Question A1 The average subsurface condition of a site is shown in Figure QA.1, together with the SPT blow counts obtained. As shown in the Figure QA.1, the subsurface consists of a 2m thick coarse sand layer underlain by a 8.5 m thick normally consolidated clay layer followed by a weathered rock layer. The bedrock is found at about 18m depth below the ground surface beneath the weathered rock layer. The water table is located at 2m below the ground surface. Estimate the soil strength parameters of the subsurface soil layers using the energy method for the estimation of the ultimate carrying capacity of the piles. (Assume an energy ratio of 55%, borehole diameter of 90mm and noliner)

CN 

95.76

Where Po/ =

Po

/

Effective overburden pressure in kPa.

Coarse sand layer with wet= 16.0 kN/m3 W. T

Clay layer with sat = 16 kN/m3

Weathered rock layer with sat = 16.5 kN/m3

Bedrock RQD=60%, CR=95% and UCS =12 Mpa Figure QA.1

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Depth (m) 1

4

SPT

2

5

3

6

4.5

4

6

5

7.5

5

9

6

10.5

24

12

22

13.5

25

15

28

16.5

50<

18

50 <