# What is Deep Foundation?

By BYJU'S Exam Prep

Updated on: September 25th, 2023

A Deep Foundation is one of the foundation types, and it is preferred when loose soil strata are present at the top layer of the soil. Foundations can be categorized into shallow and deep foundations. Foundations are structures designed to transfer a load of superstructure to the beneath soil layers.

### Deep Foundation PDF

A deep foundation is required when the top layer of soil does not have sufficient bearing capacity to transfer the load of the superstructure to the soil layers. Deep foundations can be categorized into many types, including pile foundations. The article contains fundamental notes on the “Deep Foundation” topic of the “Geotechnical Engineering” subject.

Table of content

## What is Deep Foundation?

A deep foundation is designed to transfer a superstructure load to the below layer of soil. It is required when the top layer of the soil stratum does not have the sufficient bearing capacity to transfer the loads. The bearing capacity of soil is the strength of the soil element that it can take before the failure occurs.

The criteria for the foundations to be deep foundation depends on the various parameters. According to Dr Karl Terzaghi, a foundation is called a deep foundation if its depth is longer than its width (ie. Df/B > 1). Where Df is the depth of the foundation and B is the width of the foundation.

## Types of Deep Foundation

Deep foundations can be classified based on different parameters. It can be classified based on the type of soil present below the foundations. Based on the capacity of the soil, it can be classified into different types as below.

• Basement foundations
• Raft foundations
• Caissons
• Shaft foundations
• Cylinders
• Pile foundations

## Bearing Capacity of Piles

The ultimate bearing capacity of a pile is the maximum load it can carry without failure or excessive settlement of the ground. The bearing capacity of piles also depends on the methods of installation. Let us check out various methods used in deep foundation:

A. Analytical Method

(i) Qup = Qeb + Qsf

(ii) Qup = qbAb + qsAs

where,

• Qup = Ultimate load on pile
• Qeb = End bearing capacity
• Qsf = Skin friction
• qb = End bearing resistance of unit area.
• qs = Skin friction resistance of unit area.
• Ab = Bearing area
• As = Surface area

(iii) qb ∼ 9C

where C = Unit Cohesion at the base of the pile for clays

(iv) qs = α C‾

where,

• α C‾ = Ca = Unit adhesion between pile and soil.
• C‾ = Average Cohesion over depth of pile.

(v) Qsafe = Qup/Fswhere Fs = Factor of safety.

(vi) Qsafe = (Qeb/F1) + (Qsf/F2)

(vii) For Pure Clays Qup = 9CAb + α C‾As

B. Dynamic Approach

Dynamic methods are suitable for dense cohesionless soil only.

(i) Engineering News Records Formula

(a) Qup = WH/(S+C)

(b) Qap = Qup/FOS

where,

• Qup = Ultimate load on pile
• Qap = Allowable load on pile
• W = Weight of hammer in kg.
• H = Height of fall of the hammer in cm.
• S = Final set (Average penetration of pile per blow of hammer for last five blows in cm)
• C = Constant
• = 2.5 cm → for drop hammer
• = 0.25 cm → for steam hammer (single-acting or double acting)

(c) for drop hammer

Qap = WH/{6(S+2.5)}

(d) For single Acting Stream Hammer

Qap = WH/{6(S+0.25)}

(ii) Hiley Formula (I.S. Formula)

Qup = ηhb.WH/(S+0.5C)

Qap = Qup/Fs

• where, Fs = Factor of safety = 3
• ηh = Efficiency of hammer
• ηb = Efficiency of the blow
• ηh = 0.75 to 0.85 for single-acting steam hammer
• ηh = 0.75 to 0.80 for double-acting steam hammer
• ηh = 1 for drop hammer.

where

• w = Weight of hammer in kg.
• p = Weight of pile + pile cap
• e = Coefficient of restitutions
• = 0.25 for wooden pile and cast iron hammer
• = 0.4 for concrete pile and cast iron hammer
• = 0.55 for steel piles and cast iron hammer
• S = Final set or penetrations per blow
• C = Total elastic compression of the pile, pile cap and soil
• H = Height of fall of the hammer.

C. Field Method

(i) Use of Standard Penetrations Data

Qup = 400 NAb + 2N‾As

where,

• N = Corrected S.P.T Number
• N‾ = Average corrected S.P.T number for the entire pile length

Qap = Qup/Fs

Fs = Factor of safety = 4 → For driven pile

= 2.5 → for the bored pile.

qb = 400 N and qs = N‾

(ii) Cone penetration test

Qup = qcAb + 0.5 qc‾ As

where

• qc = static cone resistance of the base of the pile in kg/cm2
• qc = average cone resistance over depth of pile in kg/cm2

Ab = ¼ (bu)=Area of the bulb (m)2

## What is an Under-Reamed Pile?

An ‘under-reamed’ pile is one with an enlarged base or a bulb; the bulb is called ‘under-ream’. Under-reamed piles are cast-in-situ piles, which may be installed in sandy and in clayey soils. In this type of deep foundation, ratio of bulb size to the pile shaft size may be 2 to 3; usually, a value of 2.5 is used.

As1 = πbL1 and qs1 = αC; α<1

As2 = πbuL2 and qs2 = αC; α=1

where, bu = dia of bulb, Spacing = 1.5 bu.

Qup = qbAb + qs1As1 +qs2As2

## Negative Skin Friction in Piles

Negative skin friction in a pile is the frictional resistance offered by the surface of the pile. And it reduces the ultimate load capacity of the piles. Negative skin friction occurs in a pile due to a sandy layer surrounding the soil. It acts in the downward direction hence, reduces the overall bearing capacity.

(i) For Cohesive soil

Qnf = Perimeter. L1αC for Cohesive soil.

where Qnf = Total negative skin friction.

Fs = (Qup – Qnf) / Applied load
where Fs = Factor of safety.

(ii) For cohesionless Soils

Qnf = P x force per unit surface length of the pile

= ½ × P × KγD2n. tanδ

Qnf = ½ × P × KγD2n. tanδ

(friction force = μH)

Where γ = unit weight of soil.

K = Earth pressure coefficient (Ka < K < Kp)

δ = Angle of wall friction. (φ/2<δ<φ)

## Group Action of Pile

The ultimate load-carrying capacity of the pile group is finally chosen as the smaller of the

(i) Ultimate load carrying capacity of n pile (n Qup)

and (ii) the Ultimate load-carrying capacity of the single large equivalent (block) pile (Qug).

To determine the design or allowable load, apply a suitable safety factor.

(i) Group Efficiency (ηg)

ηg = Qug/n.Qup

Qug = Ultimate load capacity of pile group

Qup = Ultimate load on the single pile

For sandy soil → ηg > 1

For clay soil → ηg < 1 and ηg > 1

The minimum number of piles for the group = 3.

Qug = qbAb + qsAs

where qb = 9C for clays

Ab = B2

qs = C‾

As = 4BL

• For Square Group

Size of group, B = (n – 1) S + D

where η = Total number of piles if the size of a group is x.x

They η = x2

• Qug = η.Qup
• Qag = Qug/FOS; where Qag = Allowable load on pile group.
• Sr = Sg/Si

where, Sr = Group settlement ratio

Sg = Settlement of pile group

Si = Settlement of individual pile.

(ii) When Piles are Embedded on a Uniform Clay

(iii) In the case of Sand

Sr = Sq/Si = [(4B + 2.7)/(B + 3.6)]2

where B = Size of the pile group in meters.

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