Standard Penetration Test

PRESENTED BY: Yama Moheb

Ultra Design and Research Company

INTRODUCTION

Especially for

cohesion less soils,

which cant be easily

sampled

Most commonly

used In-situ test

• Relative density of cohesion less soils.

• Angle of shearing resistance of cohesion less soils.

• Unconfined compressive strength of cohesive soils

USEFUL IN FINDING

OUT

Any drilling equipment is

acceptable that provides a

reasonably clean hole,

which is at least 5 mm

larger than the sampler or

sampling rods, and less

than 170 mm diameter.

INSTRUMENTS1. DRILLING EQUIPMENT FOR BOREHOLES:

It is a sampler for obtaining a disturbed sample of soil and consists of-o Driving shoe : Made of tool-steel,

about 75 mm longo Steel Tube : 450 mm long, split

longitudinally in two halveso Coupling : 150 mm long, provided

at the topo Check Valveo 4 Venting Ports : 10 mm diameter

INSTRUMENTS2. SPLIT – SPOON SAMPLER

• Hammer of 63.5 kg

• A driving Head (Anvil)

• A guide permitting a free fall of

0.76 m and over lift capability

of at least 100 mm.

INSTRUMENTS3. DRIVE – WEIGHT ASSEMBLY

• Operating at approximately

100 rpm

• Equipped with suitable rope

and overhead sheave for

lifting drive-weight

INSTRUMENTS4. CATHEAD

• Closed system

• Delivers approximately 60%

of the maximum free fall

energy

• Highly variable energy transfer

5) HAMMER

a) Safety Hammer

b) Donut Hammer

• Open system• Delivers approximately

45% of the maximum free fall energy

• Highly variable energy transfer

• Safest system

• Delivers approximately 95 -

100% of the maximum free

fall energy

• Consistent and effective

energy transfer

• Increased production

c) Automatic Hammer

PRO

CEDU

RE New Picture Effects1.DRILLING OF BOREHOLE

• Drill the borehole to the desired sampling depth and clean out all disturbed material.

• The equipment used shall provide a clean borehole, 100 to 150 mm in diameter, for insertion of the sampler to ensure that the penetration test is performed on undisturbed soil.

• Casing shall be used when drilling in sand, soft clay or other soils in which the sides of borehole are likely to cave in.

PRO

CEDU

RE 2.Driving the Casing

• Where casing is used, it shall not be driven below the level at which the test is made or soil sample is taken.

• In the case of cohesion less soils which cannot stand without casing, the advancement of the casing pipe should be such that it does not disturb the soil to be tested or sampled; the casing shall preferably be advanced by slowly turning the casing rather than by driving, as the vibration caused by driving may alter the density of such deposits immediately below the bottom of the borehole.

PRO

CEDU

RE3. ASSEMBLING EQUIPMENT

• Attach the split-spoon sampler to the drill rod and lower into the hole until it is sitting on the undisturbed material.

• Attach the drive weight assembly.• Lift the 63.5 kg hammer

approximately 0.76 m and allow it to fall on the anvil delivering one seating blow.

• Mark the drill rod in 3 successive .15 m increments to observe penetration.

PRO

CEDU

RE New Picture Effects

PRO

CEDU

RE New Picture Effects4. PENETRATION TESTING

• Raise and drop the hammer 0.76 m successively by means of the rope and cathead, using no more than two and one forth wraps around the cathead.

• The hammer should be operated between 40 and 60 blows per minute and should drop freely.

• Record the number of blows for each 0.15 m of the penetration.

• The first 0.15 m increment is the "seating" drive.

PRO

CEDU

RE• The sum of the blows for second and third increment of

0.15 m penetration is termed "penetration resistance or "N-value".

• If the split spoon sampler is driven less than 45 cm (total), then the penetration resistance shall be for the last 30 cm of penetration (if less than 30 cm is penetrated, the logs should state the number of blows and the depth penetrated).

• If the no. of blows for 15cm drive exceeds 50, it is taken as a refusal and the test is discontinued.

• Tests shall be made at every change in stratum or at intervals of not more than l-5 m whichever is less. Tests may be made at lesser intervals if specified or considered necessary.

PRO

CEDU

RE

• The intervals be increased to 3 m if in between vane shear test is performed.( as per IS:2131-1963) .

• The entire sampler may sometimes sink under its own weight when very soft sub-soil stratum is encountered.

• Under such conditions, it may not be necessary to give any blow to the split spoon sampler and SPT value should be indicated as zero.

PRO

CEDU

RE Ne4. Handling Sample

w Picture Effects• Bring the sampler to the surface and open it.

Remove any obvious contamination from the ends or sides and drain excess water. Carefully scrape or slice along one side to expose fresh material and any stratification.

• Record the length, composition, colour, stratification and condition of sample.

• Remove sample and wrap it or seal in a plastic bag to retain moisture. If the sample can be removed relatively intact, wrap it in several layers of plastic and seal ends with tape.

CORR

ECTI

ON

S DILATANCY CORRECTION

• Silty fine sands and fine sand below the water table develop pore pressure which is not easily dissipated.

• Pore pressure increases the resistance of the soil thus, Penetration Number (N) also increases

• This correction is applied when observed value of N exceeds 15

CORR

ECTI

ON

S DILATANCY CORRECTION

• Terzaghi and Peck (1967) recommended the following correction-

CORR

ECTI

ON

S OVERBURDEN PRESSURE CORRECTION

• In granular soils, overburden pressure affects the penetration resistance

• If two soils, having same relative density but different confining pressures are tested, the one with a higher confining pressure gives a higher penetration number as the confining pressure in cohesion less soils increases with the depth, the penetration number for soils at shallow depths is underestimated and that at greater depths is overestimated.

• For uniformity, the N- values obtained from field tests under different effective overburden pressures are corrected to a standard effective overburden pressure.

CORR

ECTI

ON

S 1. GIBBS AND HOLTZ’ CORRECTION (1957)

CORR

ECTI

ON

S AP

PLIE

D 2. PECK, HANSEN AND THORNBURN’S CORRECTION

CORR

ECTI

ON

S AP

PLIE

D3. PECK AND BAZARAA’S CORRECTION

• One of the most commonly used corrections• According to them,

FACTORS COMMENTS

Attitude of operators Blow counts for the same soil using the same rig can vary, depending on who is operating the rig, and perhaps the mood of operator and time of drilling.

Overdrive sampler Higher blow counts usually result from an overdriven sampler.

Sampler plugged by gravel Higher blow counts result when gravel plugs the sampler, resistance of loose sand could be highly overestimated.

Plugged casing High N-values may be recorded for loose sand when sampling below groundwater table. Hydrostatic pressure can cause sand to rise within the casing.

FACTORS AFFECTING ‘N-VALUES’

FACTORS COMMENTSInadequate cleaning of the borehole

SPT is only partially made in original soil. Sludge may be trapped in the sampler and compressed as the sampler is driven, increasing the blow count (This may even prevent sample recovery.)

Not seating the sampler spoon on undisturbed material

Incorrect N-values obtained.

Driving of the sample spoon above the bottom of the casing

N-values are increased in sands and reduced in cohesive soils.

Failure to maintain sufficient hydrostatic head in boring

The water table in the borehole must be at least equal to the piezometric level in the sand, otherwise the sand at the bottom of the borehole may be transformed into a loose state thereby decreasing the blow counts

FACTORS COMMENTS

Overwashing ahead ofcasing

Low blow count may result for dense sand since overwashing loosens sand.

Drilling method Drilling technique (e.g., cased holes vs. mud stabilized holes) may result in different N-values for the same soil.

Free fall of the drive weightis not attained

Using more than 1-1/2 turns of rope around the drum and or using wire cable will restrict the fall of the drive weight.

Not using correct weight Driller frequently supplies drive hammers with weights varying from the standard by as much as 10 lbs.

FACTORS COMMENTS

Weight does not strike the drive cap concentrically

Impact energy is reduced, increasing N-values.

Not using a guide rod Incorrect N-value obtained.

Not using a good tip on the sampling spoon

If the tip is damaged and reduces the opening or increases the end area the N-value can be increased.

Use of drill rods heavier than standard

With heavier rods more energy is absorbed by the rods causingan increase in the blow count.

- Relative Density- Effective Stress Friction Angle- Unconfined Compressive Strength

CORRELATIONS BETWEEN SPT AND SOIL PROPERTIES

* Some correlations require the raw N-values whereas others use the corrected N-values.

SPT N-Value Relative Density

0-4 Very loose4-10 Loose

10-30 Medium

30-50 Dense

Over 50 Very dense

Relative Density

Consistency qu (kN/m2)

Very Soft

Soft Medium Stiff Very Stiff Hard

SPT N-value <2 2-4 4-8 8-15 15-30 >30

qu <25 25-50 50-100 100-200 200-400 >400

Unconfined Compressive Strength Of Cohesive Soils

• Relatively quick and simple to perform.• Provides a representative soil sample.• Provides useful index of relative

strength and compressibility of the soil.• Able to penetrate dense layers, gravel,

and fill.• Numerous case histories of soil

liquefaction during past earthquakes are available with SPT N-values. The method based on this history can reflect actual soil behaviour during earthquakes, which cannot be simulated in the laboratory.

ADVANTAGES

• The SPT is an in situ test that reflects soil density, soil fabric, stress and strain history effects, and horizontal effective stress, all of which are known to influence the liquefaction resistance but are difficult to obtain with undisturbed samples.

The SPT equipment is rugged, and the test can beperformed in a wide range of soil conditions.

There are numerous correlations for predictingengineering properties with a good degree ofconfidence.

• continuance

• The SPT does not typically provide continuous data, therefore important data such as weak seams may be missed.

• Limited applicability to cohesive soils, gravels, cobbles boulders.

• Somewhat slower than other sample methods due to sample retrieval.

• In addition to overburden pressure and relative density the SPT N-value is also a function of soil type, particle size, and age and stress history of the deposit

DISADVANTAGES

• Due to considerable differences in apparatus and procedure, significant variability of measured penetration resistance can occur. The basic problems to consider are change in effective stress at the bottom of the borehole, dynamic energy reaching the sampler, sampler design, interval of impact, penetration resistance count.

• Samples that are obtained from the SPT are disturbed.• The greatest disadvantage to SPTs is the lack of reproducibility

of the test results• Drilling disturbance, mechanical variability, and operator

variability all can cause a significant variation in test results. • Another disadvantage to SPTs is that progress is slower than

other in place tests because of incremental drilling, testing, and sample retrieval, and SPTs may be more expensive than other in place tests.

continuance

• The drill rods should be of standard specification and should not be in bent condition.

• The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear.

• The drop hammer must be of the right weight and the fall should be free, frictionless and vertical.

• The height of fall must be exactly 750 mm. Any change from this will seriously affect the N value.

PRECAUTIONS

• The bottom of the borehole must be properly cleaned before the test is carried out. If this is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed soil.

• When a casing is used in borehole, it should be ensured that the casing is driven just short of the level at which the SPT is to be carried out. Otherwise, the test gets carried out in a soil plug enclosed at the bottom of the casing.

• When the test is carried out in a sandy soil below the water table, it must be ensured that the water level in the borehole is always maintained slightly above the ground water level. If the water level in the borehole is lower than the ground water level, ‘quick' condition may develop in the soil and very low N values may be recorded.

continuance

Thank You