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Soil Exploration 3:=

9.5 STANDARD PENETRATION TESTThe SPT is the most commonly used in sitz test in a bore hole in the USA, The test is nr,o.:e l_making use of a split spoon sampler shown in Fig. 9.7. The method has been standardiaed as risil.lD-1586 (1997) with periodic revision since 1958. The method of carrying out this te-ct is as follc,x,

1. The split spoon sampler is connected to a string of drill rnds and is lo''s'ercd into I'1:bottom of the bore hole which was drilled and cleaned in advtnce.

2. The sampler is driven into the soil strata to a maximum depth ol II in by rnakinc use of ;140 lb weight falling freely from a height of 30 in on to an anvil {i xctl ol1 lhc lrrF ri dri}l rex -The weight is guided to fall along a guide rod. Thc weiglrt is raiscd and allouc-r ll l)rll b1rneans of a manila rope, one end tied to the weight and the other end passing ot:: a pulL:1-

on to a hand operated winch or a motor driven cathead'3. The number of blows required to penetrate each of the successive 6 in depths is c,: -ited to

produce a total penetration of l8 in'4. Toavoidseatingerrors,theblowsrequiredforthefirst6inofpenetrationarenottaq:*inlr'i

account; those required to increase the penetration lrom 6 in to llJ in constitutc the,\-"aiuc.

As per some codes of practice if the N-value exceeds 100, it is terrned as refusal, arttd;:* tesl

is stopped even if the total penetration falls short of the last 300 rnm depth of pene$-d*t'il.Standaidization of refusal at 100 blows allows all the drilling organieation$ to standardize cws';r

that higher blows if required may be eliminated to prevent the excessivervean and te'.tr of lie

equ;p#nt. The SPT is conducted normally at 2.5 to 5 ft intervals. The irltcrvals ma;" be ilrcternE:

at greater dePths if necessarY.

Standardization of SPTThe validity of the SPT has been the subject of study and research try n'rany aulhors lbr thc lus

*uny y"urr. The basic conclusion is that the best results are diffrcult to reprocluce. St':ric of ilit

important factors that affect reproducibility are

l.Variationintheheightoffallofthedropweight(hammer)duringthetestZ. The number of turns of rope around the cathead, and the conditiort ot'thc tttaniirt ro;:e3. Length and diameter of drill rod5. Diameter of bore hole6. Overburden Pressure

There are many more factors that hamper reproducibility of results. Normally col'1ccti(-)n:

used to be applied for a quick.condition in the hole bottom due to rapid withdrawt *of the augei'

ASTM 1586 iras stipulated stdndards to avoid such a quick condition. Discreplncies in llteinpLrt

driving energy and its dissipation around the sampler into the surrouncli*g soil ari: the principalfactors for the wide range in Nvalues. The theoretical input energy may be expressed as

E'n= Wh (9 i:

where lV = weight or mass of the hammerh = height of fall

Investigation has revealed (Kovacs and Salomone, 1982) that the actual cntrgy transferred to ihe

driving head and then to the sampler ranged from about 30 to 80 percnt' It has been suggestedtha:

the SpT be standardized to some energy ratio R" keeping in mind the clata c'ollcctedso fil| Iior"n ths

existing SpT. Bowles (1996) ,rgg"rt, thut the olserved SPT value N be reduced to a sta.tl;rrtl blcr"counr corresponding to ZO perceilof standard energy. Terzaghi, et al',

( 199(r) suggcst 60 pcrce nt' Ti-''-

standard energy ratio may be expressed as


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328 Chapter 9

(9'4)

eorrections to the Observed SPT ValueThree types nf corrections are nofmally applied to the observed N values. They are:

l. Hanrmer efflciency correction?. Drillrod, salnplcr and borehole corrections3. Coffection due to overburden pressure

1. Hammer Efticiency Correction, EnDifferent types of hammers are in use for driving the drill rods. Two types are normally used inUSA, They are (Bowles, 1996)

1' Donut with two turns of manila rope on the cathead with a hammer efficiency En= 0.45.2. Sa{'ety with two turns of manila rope on the cathead with a hammer efficiency as follows:

Rope-pulley or carheacl = 0.7 to 0.8;'liip or autornatic hanrnrer = 0.8 to I.0.

2. Drill Rod, Sampler and Borehole CorrectionsComction factot'.s are'ttsed for correcting the effects of length

of drill rods, use of split spoonsampler with or without liner, and size of bore holes. The various correction factors are (Bowles,l 996).

a) I)rill rod length correction factor C,Length (m) Correction factor (Cr)

o _ Actual hammer energy to sampler,Eo a

*

>l0rn4-10 m< 4.1) nr

1.0

0.85-0,95

0.15

b)

c)

Sarnpler correction factor, C

Without liner C_ = 1.00Wirh liner,

Dense sanct, clay = 6.36l-oose sand = 0.90

Bore hole diameter correction factor, Co

Bore hole diarneter Correction fector, Co60-120 mm

150 mm

200 mm

1.0

1.05

Ll5

3. Correction Factor for OverburdenThe C* as per Liao and Whitman (19g6)

is

Pressure in Granular Soils, C,


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Soil Exploration ??fi

r -ll2- | 9s.76 |"'-L d ] 'P's'where, pi, =effective overburden pressure in kN/m2

There are a number of empirical relations proposed for Cr. I-lorver,cr. ttre nroft c()nlrlr{}ulyused relationship is the one given by Eq. (9.5). \ ,

4o, may be expressed as \Nro,=CNNEhCdCTCb (9,6)

N"o, is related to the standard energy ratio used by the design er. N,o, nray be expresseri as Nr,

or Nuo according to the designer's choice.In Eq (9.6) C, N is the corrected value for overburden pressure onty. 'fhc v&lue ol'C'u as perEq. (9.5) is applicable for granular soils only, whereas Cry = I for cohesive soils for ail eleptlrs.

Example 9.3The observed standard penetration test value in a deposit of hrlly subnrergcd satrd was 4.5 at ir rlcptltof 6.5 m. The average effective unit weight of the soil is 9.69 kN/rn3. The other cllta givcn are (n)hammer efficiency = 0.8, (b) drill rod length correction factor = 0.9, and {c) borehole coritclionfactor

=1.05. Determine the corrected SPT value for standard energy (a) R,..

=(r0 pe rce ut, irrid (b)

R",= ToEo'

SolutionPer Eq (9.6), the equation for Nuo may be written as

(i) Neo = CNN Et,C,tC,Cbwhere N = observed SPT value

CN = overburden correctionPer Eq (9.5) we have

r tll2- _l9s.76 l'uN-l-----;-l

lPolwhere 2i, = effective overburden pressure

= 6.5 x 9.69 = 63 kN/m2

Substituting for pi ,

,^, _(gs.to),/, = r.zl:" \60./Substituting the known values, the corrected N* isNrur = L233 x 45 x 0.8 x Q.9 x 1.05 = 42For 70 percent standard energy

N-.. = 41, 9 = lf,t, 0.7


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330

$.6$pTVALUESRELATEDToRELATIVEDENSITYoF

Chapter 9

froHE$lot\lLEs$ solLS

Althoughrhc

SpTisnol consiclered as a refined ancl completely reliable method of investigation,

fhe N,.,. virlues give uscl'ul inlortttation with regard to ctlnsistency ol' cohesive soils and relativc

densiiy'of'colresionless soils. 'lhe corrclation between N,,, values and relative density of granular

soils suggesteci by Peck, et al', (1974) is given in Table 9'3'Bcfore using Table 9.3 the observed N value has to be corrected for standard energy and

over.burclen prru.ui*. T|e conelations given in Table 9.3 are just a guide and may vary according to

rhe fincness of the sand.Meyerhof (1956) suggested the following approximate equations for computing the angle

of

iriction @ from ths known value of D,.

For granular ssil with fine sand and more than 5 percent silt'0 = 25 + 0' l5D,For granular soils with fine sand and less than 5 percent silt'

Pn = 30 + 0' l'5D,

wherc l),. is cxprcsscd itr perce nt'

g.7 SPT VALUES RELATED TO CONSISTENCY OF CLAY SOILpeck et al., (1g74) have given for saturated cohesive soils, correlations between N.o, value andcensistency, This correlrtion is quite useful but has to be used according to the soil conditions met

ir thc tlclil. Table 9.4 give s the correlations.The N .,. value to be used in Table 9.4 is the blow count corrected for standard energy ratio

li.*. Thc preient practice is to relate 4,, with N,,,, &S follows'

,1,, = &Nrr,. kPa(e.e)

Table 9.3 Nro, and @ Related to Relative Density

(e.1)

(9.8)

No,

Compactness Refative density, D, (o/o) 00

0-44- l0

I ()-.i{)

:3(),5()

>50

Vcry loosel-ooscMcrliunrl)errscVery Dense

0- 15

I 5-3535-6565-85

>85

41

Table 9.4 Relation Between N orand q,Consistency N Q,' kPa

Vcry softSoftMediumSriffVery StiffIlard

a-2

4-88- 15

I 5-30>30

400

where 4, is the unconfined compressive strength.


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Soil Exploration

or 1, = =4=' kpaN ,o,where, /< is the proportionality factor. A value of( r 996).

331

(9.10)

i = 12 has been rccomntt:nded by 11o,,vles

Example 9.4For the corrected N values in Ex 9.3, determine the (a) relative density, and (b) the angle of fi"ictirin.Assume the percent of fines in the deposit is less than 5Vo.

SolutionPer Thble 9.3 the relarive density and @ are

For Nuo = 42, D r= "l|Vo, 0 = 39"For Nro = 36, Dr=717o, Q = 37.5oPer Eq (9.8)

For D, = 77Vo, 0 = 30+ 0.15 x77 = 41.5oFor D, ='1 l%o, 0 = 30 + 0.15 x 7l = 40.70

Example 9.5For the corrected values of N given in Ex 9.4, determine the unconfined compressive srl.eugtli qu ina clay deposit.

Solution(a) From Table 9.4

ForN* = +21For Nr, = 36J a, > 400 kPa - The soil is of a hard consistency.

(b) Per Eq (9.9/Q,= kN",., where k= 12 (Bowles, 1996)ForNuo - 4), Qu= 12x 42 = 504 kPaFor Nro - 36, Qu= 12 x36 = 432kPa

Example 9.6

Refer to Example 9.3. Determine the corrected SpT value for rt,,. = lff) psrcent, ancl thecorresponding values of D, and @. Assume the percent of fine sand in tiie tieposit-is less rhrn S%.SolutionFrom Example9.3, Nuo- 42

Hence N,* =2* =251.0

From Table 9.3, Q= 34.5o and D,= 57.5EoFrom Eq. (9.8) for D"= 57.sEo, Q = 30 + 0.15 x 57.5 = 3g.6..

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