Engr. Imran Hafeez

Selection of Pile Selection of i) type, ii) length andiii) capacity

of pile is based on two parametersa. Soil conditionsb. Magnitude of load

Before the actual construction begins, pile load tests must be made to verify the design values and the foundation design must be revised according to the test results.

Factors effecting the selection of pile

1. Length of pile in relation to the load and type of soil

2. Character of structure3. Availability of materials4. Type of loading5. Factors causing deterioration6. Ease of maintenance7. Estimated costs of types of piles, taking into

account the initial cost, life expectancy and cost of maintenance.

8. Availability of funds

Geotechnical Design By Geotechnical Design, we mean;

– Depth below the ground.

– Dimensions in plan (dia etc.)

These are dependant upon the pile capacity.

Total ultimate load carried by and element of pile is called total ultimate capacity of

pile Qu

“Qu = Qs + Qb”

Qs = Shaft resistance

Qb = Bearing resistance

And for Allowance capacity, there are two schools of thought I.e.

Qa = Qu / FOS or

Qa = Qs/ (FOS)1 + Qb/ (FOS)2

Qs is more uncertain than Qb, so apply higher factor of safety to Qs.

METHOD FOR CALCULATING THE CAPACITY:

Three methods are used for calculating the capacity of pile,

1. Static formula (applied for driven and in-situ piles)

2. Dynamic formula (applied for driven piles)

3. Load test (Testing is done for 2 times the design load , if it is stable then o.k. It is a full scale test and is the best method. But it is the most expensive and time consuming. Load application is very difficult.)

STATIC FORMULAFor finding out Qs;

Qs = (Fs) (As) *Fs = c + q K tan

Where

Fs = unit skin friction

As = Effective Area of shaft

The length of the pile Coming in contact with soil. Below the N.S. L 1.5 to 1.8 m is generally ignored (damage due to installation and cracking)

q = Aug. Effective overburden pressure.

= r Df

Avg = (0 + rD)/ 2

 

= Adhesion factor or Cohesion Reduction factor

*This is called Alpha () Method proposed by Tomlinson (1971)Qs

Overburden Pressure

Avg. Pressure Line

r Df/2

r DfSTATIC FORMULAQs

Adhesion factor or Cohesion Reduction factor

Before 1971, it was known that depends on the consistency of soil (spt value).If the consistency is very poor, then 100% contact. But if consistency is very hard, less contact.

• So for • V. loose to loose sand & v. soft to soft clay = 1• Very hard (stiff to very stiff clay) = 0.4

After 1971, Tomlinson said that, the “” value depends upon the penetration, actual formation, sequence of layer. He related the “” value with the penetration Ratio (PR) and gave a table where

PR = Effective Length of pile driven in cohesive soil / Dia. of pile

= Le/ D

STATIC FORMULAQs

Factors

K = co-efficient of lateral earth pressure. Generally K value is from zero to 1.75

Where

Ko = pressure at rest.

= (1-sin ) √OCR• OCR = Over consolidation Ratio & for N.C. C , OCR = 1 • Safe to use the value close to K0.•  value of K are higher for driven piles as compared with cast insitu

piles.• Usually for driven piles the values are 1 and for insitu piles it

is 1

STATIC FORMULAQs

Factor

Tan ;Is the co-efficient of friction and = fraction Angle, depending upon the material of construction, for timber pile

Timber = 2/3 ’For steel pile =20Degree

For concrete pile, = ¾

Now fs can be calculated , As is also known .

STATIC FORMULAQs

TO FIND Fs USING EMPERICAL METHOED

For the empirical method, we do two tests i.e. SPT test and CPT test.

 

I) SPT TEST ( MAYORHOFF 1976):

 According to Mayorhoff

  Fs = (€m) (N) in Kpa

€m = Constant. Depending upon the method of installation.

= 2 (for large volume displacement (Driven piles))

= 1(for No volume or displaced (Cast piles))

N = Average value of SPT Resistance (Average is taken for the SPT Values at depths ‘8B’ above the tip and ‘4B’ below the tip.

Qs

Depth for SPT

8B

4B

B

Qs

This test is further of two types,

(a) By Mentle Cone:In this only a cone is attached to the Rod. This type of cone is called Mentle Cone. Here only the cone comes in contact. The resistance measured is called the cone resistance “qC “

Fs = 0.0005 qc in Kpa

(b) Friction Jacket Cone:In this case, there is a Sleeve. The Resistance offered by sleeve is qcs

Fs = (€m) (qcs)

€m = 1.5 for driven piles

€m = 1.0 for cast Insitu piles

qcs = Resistance offered by shaft

qcs = total Resistance - qc

 

CPT TEST (MAYERHOFF):

Qs

FOR CALCULATING Qb:

Qb = (Ab) (qult)

  qult = Bearing capacity of Soil.

Ab = X – Sectional area of the Pile

 

So the problem lies only in finding out of

qult. it can be founded by

i) Bearing capacity equations

ii)Empirical methods

Qb

Bearing capacity equationsqult = CNC + rDf Nq+ ½ r B Nr

This equation was for shallow footing. Modifying it for deep footing i.e. D/B ≥ 4

qult = CNC’ + rDf Nq’ + 0.5rBNr’

NC’, Nq’,Nr’, is constants different from NC and Nr.

Nc’ = 9 (instead of 5.7)

 

For pure clay) Φ=0)Nc’ = 9 , Nq’= 1, Nr’ = 0

Thus (qult)Net’ = CNC’ = 9c

 

For pure sand:

qult = 0+ q’Nq’+0

For driven piles, B is very small so the 3rd term may be negative.

qult = q’ Nq’

q’is different from q because q = Avg. & q’ = MaximumQb

EMPERICAL METHODS (SPT and CPT tests)

(a) SPT TEST (Mayerhoff)

qult = 40N Lb/ B ≤ 400N in Kpa

Lb = Length of pile in the stream of end bearing.

(b) CPT TEST

qult = qc

qc = Cone Resistance

QU = Qs+Qb

Note:

We know that

Fs= c + K Tan δ

qult = CN’C + q’ Nq’ + 0.5rBN’r

q’ = Effective Earth pressure depending on the length of the pile

q’ = rD

= r (Length of Pile)

Stratum # 01

Stratum # 02Lb

End Bearing