Selection of Pile
Selection of
i) type,
ii) length and
iii) capacity
of pile is based on two parameters
a. Soil conditions
b. 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 structure
3. Availability of materials
4. Type of loading
5. Factors causing deterioration
6. Ease of maintenance
7. 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 FORMULA
For 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
= Avg. Effective overburden pressure.
 = 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 Df
Qs
STATIC FORMULA
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
Qs
STATIC FORMULA
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
Qs
STATIC FORMULA
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 .
Qs
STATIC FORMULA
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)
€m
= Constant. Depending upon the method of installation.
= 2 (for large volume displacement (Driven piles))
= 1(for No volume or displaced (Cast piles))
= Average value of SPT Resistance (Average is taken for
the SPT Values at depths ‘8B’ above the tip and
‘4B’
below the tip.
N
Qs
in Kpa
Depth for SPT
B
8B
4B
Qs
CPT TEST (MAYERHOFF):
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
€m
qcs
qcs
Qs
= 1.5 for driven piles
= 1.0 for cast Insitu piles
= Resistance offered by shaft
= total Resistance - qc
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 equations
qult
= 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 ,
Thus
Nq’= 1,
(qult)Net’ =
Nr’ = 0
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’ = Maximum
Qb
EMPERICAL METHODS
(SPT and CPT tests)
(a) SPT TEST (Mayerhoff)
qult = 40N Lb/ B ≤ 400N
in Kpa
Stratum # 01
Lb = Length of pile in the stream of end bearing.
(b) CPT TEST
qult = qc
qc = Cone Resistance
Stratum # 02
Lb
QU = Qs+Qb
Note:
End Bearing
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