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In-place Pile Foundation for a Tower-building Project

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Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.


1 Introduction

Pile foundations are used to carry a load and transfer the load of a given structure to the ground bearing, which is found below the ground at a considerable depth. The foundation consists of several piles and pile-caps. Pile foundations are generally long and lean, that transfers the structure load to the underlying soil (at a greater depth) or any rock having a great load-bearing ability.

“The main types of materials used for piles are Wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly.” (Pile Foundation Design: A Student Guide by Ascalew Abebe & Dr Ian GN Smith).

The objective of this project is to identify the design & use of a cast-in-place pile foundation, for the tower-building project.

The tower building project is called the Gemini Towers. The purpose of this construction (building) is to facilitate office spaces. This also resides on a rocky area. The building has been designed as per state-of-the-art designing concepts which are basically to attract foreign investors to invest in Oman. The Gemini Building has 1 basement, 1 ground and 19 floors.

Cast-in-place concrete piles are shafts of concrete cast in thin shell pipes, top driven in the soil, and usually closed end. Such piles can provide up to a 200-kip capacity. The chief advantage over precast piles is the ease of changing lengths by cutting or splicing the shell. The material cost of cast-in-place piles is relatively low. They are not feasible when driving through hard soils or rock.

1.1 Aim

The aim of this project is to design and propose cast in-place pile foundation for a tower-building project and study the efficiency for the same. To achieve this aim the following objective has to be achieved.

1.2 Objectives

The objectives of this project are as following:

  1. To study the field soil condition, suitability of pile and investigate the soil.
  2. To study the advantages and efficiency of using cast-in-place pile for the building.
  3. To study the guidelines for the design of cast in-place structure according to BS 8004, 8110, 8002, etc.
  4. To design the pile foundation as per the guidelines and the soil conditions (analyse the load, calculate the moment and determine the length and diameter and reinforcement).
  5. To use computer structural designing program for performing design (CAD and STAD).

1.3 Methods

The methods followed in preparing this project is by collecting the project plan and the soil investigation report. Then after that, research has been done on in-situ pile foundation type, to identify its characteristics.

The next step is to study the pile designing criteria by referring to BS 8004, 8110 & 8002 codes to understand the guidelines, which shall be followed to accomplish the pile design. For this, the structural loads have to be analysed and identified using ultimate state design method. Then the design is processed depending on the data gathered on soil conditions, design loads and BS code guidelines.

Thus, a proposal for the suitable pile will be prepared by identifying the reasons over the proposal.

The commonest function of piles is to transfer a load that cannot be adequately supported at shallow depths to a depth where adequate support becomes available, also against uplift forces which cause cracks and other damages on superstructure.

Chapter 2 - Literature Review

2 Pile Foundation

“Pile foundations are used extensively in bridges, high-rise buildings, towers and special structures. In practice, piles are generally used in groups to transmit a column load to a deeper and stronger soil stratum. Pile may respond to loading individually or as a group. In the latter case, the group and the surrounding soil will formulate a block to resist the column load. This may lead to a group capacity that is different from the total capacity of individual piles making up the group.” (Adel M. Hanna et al, 2004).

“Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity. The main types of materials used for piles are Wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly.” (Ascalew Abebe et al, 2005)

2.1 Functions of Piles

The purposes of pile foundations are:

  1. to transmit a foundation load to a solid ground.
  2. to resist vertical, lateral and uplift load.

“A structure can be founded on piles if the soil immediately beneath its base does not have adequate bearing capacity. If the results of site investigation show that the shallow soil is unstable and weak or if the magnitude of the estimated settlement is not acceptable a pile foundation may become considered. Further, a cost estimate may indicate that a pile foundation may be cheaper than any other compared ground improvement costs. Piles can also be used in normal ground conditions to resist horizontal loads. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.” (Pile Foundation Design: A Student Guide, by Ascalew Abebe & Dr Ian GN Smith, 2003).

2.2 Classification of Piles

2.2.1 Classification of pile with respect to load transmission

  • End-bearing.
  • Friction-piles.
  • Mixture of cohesion piles & friction piles. End bearing piles

This type of piles is designed to transfer the structural load to a stable soil layer which is found at a greater depth below the ground. The load bearing capacity of this stratum is found by the soil penetration resistance from the pile-toe (as in figure

The pile normally has attributes of a normal column, and should be designed as per the guidelines. The pile will not collapse in a weak soil, and this should be studied only if a part of the given pile is unsupported. (Eg: If it is erected on water / air). Load transmission occurs through cohesion / friction, into the soil. At times, the soil around the pile may stick to the pile surface and starts “negative skin friction”. This phenomenon may have an inverse effect on the pile capacity. This is mainly caused due to the soil consolidation and ground water drainage. The pile depth is determined after reviewing the results from the soil tests and site investigation reports. Friction piles (cohesion)

The bearing capacity is calculated from the soil friction in contact with the pile shaft. (as in Figure Mixture of cohesion piles & friction piles.

This is an extended end-bearing pile, when the soil underneath it is not hard, which bears the load. The pile is driven deep into the soil to create efficient frictional resistance. A modified version of the end-bearing pile is to have enlarged bearing base on the piles. This can be achieved by immediately pushing a large portion of concrete into the soft soil layer right above the firm soil layer, to have an enlarged base. Similar result is made with bored-piles by creating a bell / cone at the bottom by the means of reaming tools. Bored piles are used as tension piles as they are provided with a bell which has a high tensile-strength. (as in figure

2.3 Cast-in-Place Pile Foundation

Cast-in-place piles are installed by driving to the desired penetration a heavy-section steel tube with its end temporarily closed. A reinforcing cage is next placed in a tube which is filled with concrete. The tube is withdrawn while placing the concrete or after it has been placed. In other types of pile, thin steel shells or precast concrete shells are driven by means of an internal mandrel, and concrete, with or without reinforcement, is placed in the permanent shells after withdrawing the mandrel.

2.3.1 Advantages

  • Length of the pile can be freely altered to cater varying ground conditions. Soil removed during the boring process can be verified and further tests can be made on it.
  • Large diameter installations are possible.
  • End enlargements up to two or three diameters are possible in clays.
  • Pile materials are independent during driving / handling.
  • Can be installed to greater depths in the soil.
  • Vibration-free and noise-free while installation.
  • Can be installed in conditions of very low headroom.
  • Ground shocks are completely nil.

2.3.2 Disadvantages

  • Susceptible to "necking" or "wasting" in pressing ground.
  • Concrete is not pumped under suitable conditions and cannot be inspected.
  • The cement on the pile shaft will be washed up, if there is a sudden surge of waster from any pressure caused underground.
  • Special techniques need to be used to ensure enlarged pile ends.
  • Cannot be easily prolonged above ground-level especially in river and marine structures.
  • Sandy soils may loosen due to boring methods and base grouting may be required for gravely soils to improve base resistance.
  • Sinking piles may result in ground-loss, leading to settlement of nearby structures.


3 Load Distribution

To a great extent the design and calculation (load analysis) of pile foundations is carried out using computer software. The following calculations are also performed, assuming the following conditions are met:

  1. The pile is rigid.
  2. The pile is pinned at the top and at the bottom.
  3. Each pile receives the load only vertically (i.e. axially applied).
  4. The force P acting on the pile is proportional to the displacement U due to compression.

Therefore, P = k U

Since P = E A

E A = k U

k = (E A ) / U


P = vertical load component

k = material constant

U = displacement

E = elastic module of pile material

A = cross-sectional area of pile (Figure 3 - load on single pile)

The length L should not necessarily be equal to the actual length of the pile. In a group of piles. If all piles are of the same material, have same cross-sectional area and equal length L, then the value of k is the same for all piles in the group

3.1 Pile foundations: vertical piles only

3.1.1 Neutral axis load

The pile cap is causing the vertical compression U, whose magnitude is equal for all members of the group. If Q (the vertical force acting on the pile group) is applied at the neutral axis of the pile group, then the force on a single pile will be as follows:

Pv = Q / n


Pv = vertical component of the load on any pile from the resultant load Q

n = number of vertical piles in the group (see figure 3.1.2)

Q = total vertical load on pile group

3.1.2 Eccentric Load

If the same group of piles are subjected to an eccentric load Q which is causing rotation around axis z (see fig 3.1b); then for the pile i at distance rxi from axis z:

Ui = rxi . tanθ

∴ Ui = rxi θ => Pi = k . r xi . θ

θ is a small angle ∴ tanθ ≈ θ (see figure 3.1.2).

Pi = force (load on a single pile i).

Ui = displacement caused by the eccentric force (load) Q.

rxi = distance between pile and neutral axis of pile group.

rxi positive measured the same direction as e and negative when in the opposite direction.

e = distance between point of intersection of resultant of vertical and horizontal loading with underside of pile.

(Figure 3.1.2 – Example of a pile foundation – vertical piles)

The sum of all the forces acting on the piles should be zero ⇔

Mxi = Pi . rxi = k . rxi . θ rxi = k . θ r2xi =>

=> Mxi =

From previous equation,

Mz = ΣMz

Applying the same principle, in the x direction we get equivalent equation. If we assume that the moment MX and MZ generated by the force Q are acting on a group of pile, then the sum of forces acting on a single pile will be as follows:

If we dividing each term by the cross-sectional area of the pile, A, we can establish the working stream σ:


4 Load on Pile

4.1 Introduction

“Piles can be arranged in a number of ways so that they can support load imposed on them. Vertical piles can be designed to carry vertical loads as well as lateral loads. If required, vertical piles can be combined with raking piles to support horizontal and vertical forces.” (Pile Foundation Design: A Student Guide by Ascalew Abebe & Dr Ian GN Smith)

“Often, if a pile group is subjected to vertical force, then the calculation of load distribution on single pile that is member of the group is assumed to be the total load divided by the number of piles in the group.” (Pile Foundation Design: A Student Guide by Ascalew Abebe & Dr Ian GN Smith)

However, if a given pile group is subjected to eccentric vertical load or combination of lateral & vertical load that can start moment force. Proper attention should be given during load distribution calculation.

4.2 Pile Arrangement

“Normally, pile foundations consist of pile cap and a group of piles. The pile cap distributes the applied load to the individual piles which, in turn, transfer the load to the bearing ground. The individual piles are spaced and connected to the pile cap. Or tie beams and trimmed in order to connect the pile to the structure at cut-off level, and depending on the type of structure and eccentricity of the load, they can be arranged in different patterns.” (Pile Foundation Design: A Student Guide by Ascalew Abebe & Dr Ian GN Smith)

A) Pile Group Consist Of Only Vertical Piles.

B) Pile Group Consist Of Both Vertical And Raking Piles

C) Symmetrically Arranged Vertical And Raking Piles

(Figure 2.2 - Pile Foundation Design: A Student Guide by Ascalew Abebe & Dr Ian GN Smith))

In this section, considering pile/soil interaction, calculations on the bearing capacity of single piles subjected to compressive axial load has been described. During pile design, the following factors should be taken into consideration:

  • Pile material compression and tension capacity.
  • Deformation area of pile, bending moment capacity.
  • Condition of the pile at the top and the end of the pile.
  • Eccentricity of the load applied on the pile.
  • Soil characteristics.
  • Ground water level.

4.3 The behaviour of piles under load

Piles are designed in line with the calculations based on load bearing capacity. It is based on the application of final axial-load, as per the given soil conditions at the site, within hours after the installation.

This ultimate load capacity can be determined by either:

  1. The use of empirical formula to predict capacity from soil properties determined by testing. or
  2. Load test on piles at the site.

When increasing compressive load is applied on the pile, the pile soil system reacts in a linear elastic way to point A on the above figure (load settlement). The pile head rebounds to the original level if the load realises above this point.

“When the load is increase beyond point A there is yielding at, or close to, the pile-soil interface and slippage occurs until point B is reached, when the maximum skin friction on the pile shaft will have been mobilised. If the load is realised at this stage the pile head will rebound to point C, the amount of permanent settlement being the distance OC. When the stage of full mobilisation of the base resistance is reached (point D), the pile plunges downwards without any farther increase of load, or small increases in load producing large settlements.” (Pile Foundation Design: A Student Guide).

4.4 Geotechnical design methods

In order to separate their behavioural responses to applied pile load, soils are classified as either granular / noncohesive or clays/cohesive. The generic formulae used to predict soil resistance to pile load include empirical modifying factors which can be adjusted according to previous engineering experience of the influence on the accuracy of predictions of changes in soil type and other factors such as the time delay before load testing.

From figure 4.1b, the load settlement response is composed of two separate components, the linear elastic shaft friction Rs and non-linear base resistance Rb. The concept of the separate evaluation of shaft friction and base resistance forms the bases of "static or soil mechanics" calculation of pile carrying capacity. The basic equations to be used for this are written as:

Q = Qb + Qs - Wp

Rc = Rb + Rs - Wp

Rt = Rs + Wp


Q = Rc = the ultimate compression resistance of the pile.

Qb = Rb = base resistance.

Qs = Rs = shaft resistance.

Wp = weight of the pile.

Rt = tensile resistance of pile.

In terms of soil mechanics theory, the ultimate skin friction on the pile shaft is related to the horizontal effective stress acting on the shaft and the effective remoulded angle of friction between the pile and the clay and the ultimate shaft resistance Rs can be evaluated by integration of the pile-soil shear strength πa over the surface area of the shaft.

τa = Ca + σ n tanφ a

Where: σn = Ks σv

∴ τa = Ca + KS σv tanφa


p = pile perimeter

L = pile length

φ = angle of friction between pile and soil

Ks = coefficient of lateral pressure

The ultimate bearing capacity, Rb, of the base is evaluated from the bearing capacity theory:

Ab = area of pile base.

C = undrained strength of soil at base of pile.

NC = bearing capacity factor.


5 Calculating the resistance of piles to compressive loads

5.1 Cast in Place Piles – Shaft resistance

These piles are installed by drilling through soft overburden onto a strong rock the piles can be regarded as end-bearing elements and their working load is determined by the safe working stress on the pile shaft at the point of minimum cross-section, or by code of practice requirements. Bored piles drilled down for some depth into weak or weathered rocks and terminated within these rocks act partly as friction and partly as end-bearing piles.

The author Duncan C. Wyllie, gives a detailed account of the factors governing the development of shaft friction over the depth of the rock socket. The factors which govern the bearing capacity and settlement of the pile are summarized as the following:

  1. The length to diameter ratio of the socket.
  2. The strength and elastic modulus of the rock around and beneath the socket.
  3. The condition of the side walls, that is, roughness and the presence of drill cuttings or bentonite slurry.
  4. Condition of the base of the drilled hole with respect to removal of drill cuttings and other loose debris.
  5. Layering of the rock with seams of differing strength and moduli.
  6. Settlement of the pile in relation to the elastic limit of the side-wall strength.
  7. Creep of the material at the rock/concrete interface resulting in increasing settlement with time.

The effect of the length/diameter ratio of the socket is shown in Figure 5.1a, for the condition of the rock having a higher elastic modulus than the concrete.

It will be seen that if it is desired to utilize base resistance as well as socket friction the socket length should be less than four pile diameters. The high interface stress over the upper part of the socket will be noted.

The condition of the side walls is an important factor. In a weak rock such as chalk, clayey shale, or clayey weathered marl, the action of the drilling tools is to cause softening and slurrying of the walls of the borehole and, in the most adverse case, the shaft friction corresponds to that typical of a smooth-bore hole in soft clay. In stronger and fragmented rocks the slurrying does not take place to the same extent, and there is a tendency towards the enlargement of the drill hole, resulting in better keying of the concrete to the rock. If the pile borehole is drilled through soft clay this soil may be carried down by the drilling tools to fill the cavities and smear the sides of the rock socket. This behaviour can be avoided to some extent by inserting a casing and sealing it into the rock-head before continuing the drilling to form the rock socket, but the interior of the casing is likely to be heavily smeared with clay which will be carried down by the drilling tools into the rock socket.

As mentioned in Duncan C. Wyllie, suggests that if bentonite is used as a drilling fluid the rock socket shaft friction should be reduced to 25% of that of a clean socket unless tests can be made to verify the actual friction which is developed.

It is evident that the keying of the shaft concrete to the rock and hence the strength of the concrete to rock bond is dependent on the strength of the rock. Correlations between the unconfined compression strength of the rock and rock socket bond stress have been established by Horvarth(4.50), Rosenberg and Journeaux(4.51), and Williams and Pells(4.52). The ultimate bond stress, fs, is related to the average unconfined compression strength, quc, by the equation:


α = reduction factor relating to, quc as shown in Figure 5.1b

β = correction factor associated with cut-off spacing in the mass of rock as shown in Figure 5.1c.

The curve of Williams and Pells in Figure 5.1b is higher than the other two, but the β factor is unity in all cases for the Horvarth and the Rosenberg and Journeaux curves. It should also be noted that the α factors for all three curves do not allow for smearing of the rock socket caused by dragdown of clay overburden or degradation of the rock.

The β factor is related to the mass factor, j, which is the ratio of the elastic modulus of the rock mass to that of the intact rock as shown in Figure 5.1d. If the mass factor is not known from loading tests or seismic velocity measurements, it can be obtained approximately from the relationships with the rock quality designation (RQD) or the discontinuity spacing quoted by Hobbs (4.53) as follows:

RQD (%)

Fracture Frequency Per metre

Mass Factor j

0 - 25



25 – 50

15 – 8


50 – 75

8 – 5

0.2 – 0.5

75 – 90

5 – 1

0.5 – 0.8

90 – 100


0.8 - 1

5.2 End Bearing Capacity

Sometimes piles are driven to an underlying layer of rock. In such cases, the engineer must evaluate the bearing capacity of the rock. The ultimate unit point resistance in rock (Goodman, 1980) is approximately.

N = tan2 (45 + / 2)

qu = unconfined compression strength of rock

= drained angle of friction



Type of Rock

MN / m2

lb / in2


70 – 140

10.000 – 20.000


105 – 210

15.000 – 30.000


35 – 70

5000 – 10.000


140 – 210

20.000 – 30.000


60 – 70

8500 – 10.000

Table 5.2a

Type of Rock

Angle of Friction (deg)


27 – 45


30 – 40


10 – 20


40 - 50


25 - 30

Table 5.2b

The unconfined compression strength of rock can be determined by laboratory tests on rock specimens collected during field investigation. However, extreme caution should be used in obtaining the proper value of qu, because laboratory specimens usually are small in diameter. As the diameter of the specimen increases, the unconfined compression strength decreases - a phenomenon referred to as the scale effect. For specimens larger than about 1 m (3f) in diameter, the value of qu remains approximately constant.

There appears to be fourfold to fivefold reduction of the magnitude of qu in the process. The scale effect in rock is caused primarily by randomly distributed large and small fractures and also by progressive ruptures along the slip lines. Hence, we always recommend that:

The above table (Table 5.2a) lists some representative values of (laboratory) unconfined compression strengths of rock. Representative values of the rock friction angle are given in the above table (Table 5.2b).

A factor of safety of at least 3 should be used to determine the allowable point bearing capacity of piles. Thus:


6 Pile Load Test (Vesic’s Method)

A number of settlement analysis methods for single piles are available. These methods may be broadly classified into three categories:

  1. Elastic continuum methods
  2. Load–transfer methods
  3. Numerical methods

Examples of such methods are the elastic methods proposed by Vesic (1977) and Poulos and Davis (1980), the simplified elastic methods proposed by Randolph and Wroth (1978) and Fleming et al. (1992), the nonlinear load–transfer methods proposed by Coyle and Reese (1966) and McVay et al. (1989), and the numerical methods based on advanced constitutive models of soil behaviour proposed by Jardine et al. (1986). In this paper, three representative methods are adopted for the calibration exercise: the elastic method proposed by Vesic (1977), the simplified analysis method proposed by Fleming et al. (1992), and a nonlinear load–transfer method (McVay et al. 1989) implemented in program FB-Pier (BSI 2003).

In Vesic’s method, the settlement of a pile under vertical loading, S, includes three components:

S = S1 + S2 + S3


  • S1 is the elastic pile compression.
  • S2 is the pile settlement caused by the load at the pile toe.
  • S3 is the pile settlement caused by the load transmitted along the pile shaft.

If the pile material is assumed to be elastic, the elastic pile compression can be calculated by:

S1 = (Qb + ξQs)L / (ApEp)

Where Qb and Qs are the loads carried by the pile toe and pile shaft, respectively; Ap is the pile cross-section area; L is the pile length; Ep is the modulus of elasticity of the pile material; and ξ is a coefficient depending on the nature of unit friction resistance distribution along the pile shaft. In this work, the distribution is assumed to be uniform and hence ξ = 0.5. Settlement S2 may be expressed in a form similar to that for a shallow foundation.

S2 = (qbD / Esb) (1-v2)Ib


  • D is the pile width or diameter
  • qb is the load per unit area at the pile toe qb = Qb /Ab
  • Ab is the pile base area
  • Esb is the modulus of elasticity of the soil at the pile toe
  • ѵ is Poisson’s ratio
  • Ib is an influence factor, generally Ib = 0.85

S3 = (Qs / pL) (D / Ess) (1 – ѵ2) Is


  • p is the pile perimeter.
  • Ess is the modulus of elasticity of the soil along the pile shaft.
  • Is is an influence factor.
  • The influence factor Is can be calculated by an empirical relation (Vesic 1977).
  • Is = 2 + 0.35 √(L/D)

With Vesic’s method, both Qb and Qs are required. In this report, Qb and Qs are obtained using two methods. In the first method (Vesic’s method I), these two loads are determined from a nonlinear load–transfer method, which will be introduced later.

In the second method (Vesic’s method II), these two loads are determined using empirical ratios of Qb to the total load applied on pile Q based on field test data. Shek (2005) reported load–transfer in 14 test piles, including 11 piles founded in soil and 3 piles founded on rock. The mean ratios of Qb /Q for the piles founded in soil and the piles founded on rock are summarized in Table 3 and applied in this calibration exercise. The mean values of Qb /Q at twice the design load and the failure load are very similar. Hence, the average of the mean values is adopted for calibration at both twice the design load and the failure load.

In the Fleming et al. method, the settlement of a pile is given by the following approximate closed-form solution (Fleming et al. 1992):


n = rb / r0, r0 and rb are the radii of the pile shaft and pile toe, respectively (for H-piles, πro2 = πrb2 = Dh, h is the depth of the pile cross-section)

ξG = GL/Gb, GL is the shear modulus of the soil at depth L, and Gb is the shear modulus of the soil beneath the pile toe.

ρ = Gave/GL, Gave is the average shear modulus of the soil along the pile shaft

p is the pile stiffness ratio

p = Ep / GL;

ζ = ln{[0.25 +(2.5ρ(1 – v) –0.25) ξG] L/r0};

É¥L = (2/)1/2(L/r0). If the slenderness ratio L/r0 is less than 0.5p1/2 (L/r0) the pile may be treated as effectively rigid and eq. [7] then reduces to:

If the slenderness ratio L/r0 is larger than 3πp1/2, the pile may be treated as infinitely long, and eq. [7] then reduces to:

In this case, GL’ is the soil shear modulus at the bottom of the active pile length Lac, where Lac = 3r0p1/2.

In the nonlinear load–transfer method implemented in FB-Pier, the axial –Z curve for modelling the pile–soil interaction along the pile is given as (McVay et al. 1989)


At the intended design load

At twice the

design load or

the failure load

Piles founded in soil



Piles founded on rock



(Table 5a)


β = r0 0 / f, 0 is the shear stress being transferred to the soil for a given settlement Z.

f is the ultimate shear stress transferred to the soil.

rm is the radius out from the pile shaft where axial loading effects on soil are negligible, assumed to be the pile length times (one minus the soil’s Poisson’s ratio). Times the ratio of the soil’s shear modulus at the pile center to the value at the pile toe.

Gis is the initial shear modulus of the soil along the pile shaft. The nonlinear Q–Z relationship for the pile toe is given as (McVay et al. 1989).

Where Qf is the ultimate toe resistance and Gib is the initial shear modulus of the soil at the pile toe.


7 Analysis of Pile Loads and Pile Caps

7.1 Rigid Pile Cap

N = combined vertical load on pile cap - unfactored

Mx = combined moment about x - x - unfactored

My = combined moment about y - y - unfactored

(Figure 6.1a – Loads and eccentricity on pile cap)

(Figure 6.1b – Plan views of pile cap and eccentricity on pile cap)

Hx = combined horizontal load on pile cap - unfactored in the x - x direction

Hy = combined horizontal load on pile cap - unfactored in y - y direction

ex = eccentricity of N from CG of pile group in x - x direction

ey = eccentricity of N from CG of pile group in y - y direction

ehx = eccentricity of Hy from CG of pile group in x - x direction

ehy = eccentricity of Hx from CG of pile group in y - y direction

h = depth of pile cap

7.1.1 Loads on pile group

P = vertical load on pile group

= N + weight of pile cap + weight of backfill on pile cap + surcharge on backfill.

Mxx = moment about x - x on pile grup

= Mx + Ney + Hyh + Mx

Myy = moment about y - y on pile group

= My + Nex + Hxh + Mx

(Figure 6.1c - Typical pile foundation showing CG of groups and coordinates of piles)

Where Mx and My are moments with respect to CG of pile group due to eccentric surcharge on backfill or pile cap.

T = torsion on pile group

= Hxehy + Hyehx

Ixx = Σy2 about x - x axis passing through CG of pile group.

Iyy = Σx2 about y - y axis passing through CG of pile group.

Iz = Ixx + Iyy

R = number of piles in group.

Vertical load on a pile =

Horizontal load on any pile = resultant of and

7.1.2 Sign convention

Vertical loads: downwards positive

Torsion on pile group: clockwise positive

Moments on pile group: clockwise positive

+ve Mxx produced compression in piles which have +ve y ordinates.

+ve Myy produces compressiion in piles which have +ve x ordinates.

Hx is positive in direction of increasing x in positive direction.

Hy is positive in direction of increasing y in positive direction.

Eccentricities are +ve for +ve x and +ve for +ve y.

(Figure 6.1.2 - Critical sections for bending moment in a pile cap)

7.1.3 Reinforcement area in pile cap

M = bending moment as found in 6.1.1 at ultimate limit state

K =

Where :

fcu = concrete characteristic cube strength at 28 days

b = width of section over which moment acts

d = effective depth to tension reinforcement

If K is greater than 0.156, increase depth of pile cap.

Ast =

Distribute this area of reinforcement uniformly across the section.

Note: The effective depth to tension reinforcement will be different in the two orthogonal directions.

7.1.4 Shear Stress in Pile Cap

(Figure 6.1.4a – Critical section for checking shear stress in pile cap)

The critical section for checking shear stress in a pile cap is /5 into the pile. All piles with centres outside this line should be considered for calculating shear across this section in pile cap. For shear enhancement, av is from face to column to this critical section. No enhancement of shear 3 then enhancement of shear should be applied only on strips of width 3. The rest of the section will be limited to unenhanced shear stress.

or enhanced vc1 if applicable.


ΣP = sum of all pile reactions at ultimate loading on left of section.

B = width of pile cap at critical section.

d = average effective depth at critical section.

(Figure 6.1.4b – zones of enhanced shear stress on critical section)

Shear capacity of section should be greater than or equal to applied shear. Ultimate limit state analysis result should be used for checking shear capacity.

7.1.5 Punching shear stress in pile cap

(Figure 6.1.5a - Perimeters for punching shear checks)

When the spacing of piles is greater than 3 times the diameter of a pile then the punching shear plane for column should be considered. For rectangular piles the plane can be considered at face of pile. The stress on this punching shear plane should not exceed vc depending on the percentage of tensile reinforcement in pile cap.

Check of punching shear stress is also required at perimeter at face of column or pile. This shear stress should not exceed 0.8√Ê„cu or 5N/mm2.

(Figure 6.1.5b - Further perimeters for punching shear checks in a pile cap)

The punching shear planes for piles will depend on location of pile with respect to edge of pile cap.


P = Ultimate vertical column load or ultimate vertical pile reaction.

vc = design concrete shear stress obtained from the above diagram.

Percentage area of tensile reinforcement for computation of design concrete shear stress will be average percentage across punching shear planes.

7.1.6 Minimum tension reinforcement in pile cap

As ≥ 0.0013bh in both directions

7.1.7 Curtailment of bars in pile cap

A minimum anchorage of 12 times diameter of bar should be provided at ends by bending bar up vertically. Additionally check that full tension anchorage bond length is provided from critical section for bending in a pile cap where design for flexure and requirement for flexural steel in tension is determined. In finding anchorage bond length beyond that section, actual area of steel provided may be taken into account.

7.1.8 Spacing of bars in pile cap

Clear spacing of bars should not exceed 3d or 750 mm.


8 Pile load test

The following reasons describe the need for pile load tests:

  • To obtain back-figured soil data which is required to design other piles.
  • To determine the length of Pile, similarly drafting the contact estimate value.
  • To verify the results obtained theoretically (equations, forumals, etc).
  • To analyse the pile’s load-settlement property.
  • To ensure structural reliability of the pile.

Test loading: There are four types of test loading:

  • Compression test
  • Uplift test
  • Lateral-load test
  • Torsion-load test

Constant rate of penetration (CRP) test and the maintained load test (MLT) are the most widely used test types.

8.1 CRP (Constant Rate of Penetration)

In the CRP (constant rate of penetration) method, the test pile is jacked into the soil. The load is adjusted to give a continual rate of downward movement. This is maintained until failure point is met.

A pile failure is described in 2 ways:

  1. The load at which the pile continues to move downward without further increase in load.
  2. As per the British Standards, the load which the penetration reaches a value equal to 1/10th of the diameter of the pile at the base.

Situations where compression tests are carried out, the following methods are normally performed to downward force or apply the load on the pile:

A platform is constructed on the pile-head. A heavy material, known as "kentledge" is kept. Or a bridge is loaded with kentledge which is balanced on temporary supports constructed over the pile.

8.2 MLT, the maintained increment load test

From Figure 7.2, the continued increment load-test, soil-anchors, adjacent tension piles or kentledge are used to deliver a reaction for the test-load applied by jacking positioned on the test-piles. A gradual increase on the load is performed, and is sustained at each level of loading until all settlements has stopped or does not exceed a specified amount within a given time.

(Figure 7.2 - Test load arrangement using kentledge)

8.3 Pile Integrity Test

The Transient Dynamic Response (TDR) test is a rapid method of assessing the integrity of both pre-cast and cast in situ concrete piles. It is a natural evolution of the Steady State Vibration test first developed and applied to foundation testing by J Paquet in 1966. At that time a heavy (25Kg) vibrator was used to excite the pile at a range of frequencies.

(Figure 7.3a - Transient Dynamic Response test)

Since that time there have been dramatic improvements and miniaturisation of the equipment, the most significant single step coming in 1982 when it was found that identical results could be obtained using a transient impulse on the pile top, using a small hand held hammer acting through a load cell in place of the heavy vibrator. Advances in micro processing meant that the time domain signal could be readily converted to frequency using the Fast Fourier Transform.

This technique is now known as the Transient Dynamic Response and testing now only takes about 30 seconds per pile compared to about 15 minutes in 1965. It is now considered by many engineers to be the most appropriate test method for checking bored cast foundations.

Equipment used is lightweight and portable and is very rapid in-operation. Analysis of results can be carried out instantly on site to confirm the length of the foundation and depth of any defects if they exist. The TDR system also has a powerful software analysis program, to enable more detailed analysis of changes in pile section and the influence of soil. It can also be used to predict the expected test result before even visiting site! The required preparation is minimal and in normal conditions up to 60 piles per day can easily be tested, increasing to 200 where access is very good.

8.3.1 Principle

The method is based on measuring the frequency and amplitude response of a pile known as impulse. This response, known as Mechanical Admittance (or mobility), contains all the information necessary to check pile integrity and to analyse soil influences. At higher frequencies the resonating harmonics of the pile are detected, whereas at low frequency the response is generally linear allowing measurement of pile-head stiffness.

The TDR method of assessing piles is able to analyse acoustic anomalies corresponding to the following:

  • Pile Toe Level
  • Shaft restraints
  • Overbreak
  • Cracks
  • Reductions in section
  • Zones of poor quality concrete

8.3.2 Working Methodology

After ensuring that the concrete in the pile head is visually free of loose material and contaminants, a geophone sensor is placed in contact with the pile head, which is struck axially using the force response hammer. The response of both transducers is measured simultaneously, and these signals, velocity and force, are digitally processed and displayed on the test unit.

When a pile top is struck with the hammer a longitudinal wave travels down the shaft it can be likened to a snake swallowing an egg. When the wave reaches the base of the pile it is reflected back up to the top. By assuming a wave speed velocity it is possible to calculate the pile length. Reflections can also be obtained from acoustic anomalies within the pile shaft. At low frequency the response is generally linear allowing measurement of the dynamic pile head stiffness.

8.3.3 Length Measurement

Length measurements are calculated from the distance between resonating peaks, produced by the pile toe or acoustic anomalies along the shaft. Lateral soil restraints, overbreak, changes in shaft section, cracks and zones of poor quality concrete can all produce various types of acoustic anomaly which can be detected.

Length, L = C/2df


C = velocity of longitudinal waves in concrete

df = distance between two resonating peaks

8.3.4 Dynamic Pile Head Stiffness

The dynamic pile head stiffness is measured at low frequencies, when the pile head and surrounding soil are moving as one unit and is the reciprocal of the slope of the initial part of the curve.

Stiffness, E’ = 2 pi fm/(V/fm)


Fm = frequency at point of measurement

V = Velocity

8.3.5 Mobility (inverse of impedance)

Concrete density or conversely the cross-sectional area of the pile (if concrete strength is known) can be calculated from the mean Mobility (N) of the resonating part of the curve using the following formula.

Mobility, N = 1 / pCA


p = concrete density.

C = velocity of longitudinal waves in concrete.

A = pile cross sectional area.

8.3.6 Pile Head Preparation

In order to obtain the very best data possible when testing a pile, it is essential that the pile head is prepared properly prior to testing. Without good data any interpretation carried out will be meaningless. It is essential that the measurement transducers are mounted in the correct position and on sound concrete. The essentials of pile head preparation for integrity testing are given below:

  1. Piles should if possible be tested at the cut-off level and trimmed to sound concrete. Any weak, broken concrete that sounds hollow should be removed and the pile top left roughly horizontal over the complete cross section.
  2. Reinforcing bars should be bent slightly away if practicable and the helical removed to allow for a good swing of the test hammer.
  3. Two areas should be prepared for the transducers, one for the hammer in the centre of the pile and the other for the geophone close to the pile perimeter. The areas should be approximately 100 mm in diameter and prepared as flat and level as possible using a scabbler, scutch hammer or a hammer and chisel, then brushed free of debris with a wire brush. If at first you are unable to obtain a valid result, it is always advisable to re-prepare the pile and carry out a re-test, as cracking in the pile head is not always apparent but can affect the test result significantly.

8.3.7 Simulation of test results

The Simulation software is a finite element programme that simulates the frequency response of a real concrete pile by defining it and the surrounding soil in up to 10 segments. For each segment, the following information can be input: length, diameter, concrete wave propagation velocity, concrete density, soil shear wave velocity, soil density and base soil details.

With the TPAP simulation it is possible to super impose the simulated result onto a real frequency response curve. Soil and Concrete parameters can be changed using sliders and the simulation alters instantaneously in response.

The operator is able to carry out curve matching to simulate the probable cause of any anomalies. Simulations are generally carried out on pile test results that have shown an intermediate response and enables a high degree of confidence in the interpretation.


9 General Design Related Information

9.1 Piles

The pile foundation suggested by the consultant, Ref.1, consists of 1200 mm, 1000 mm, 900 mm and 750 mm diameter piles. The pile cut-off levels are varying and listed below. The pile loads given by the consultant are cited in the following list, table1. Based on expected structural behaviour horizontal loads are taken as 10% of the vertical working loads account for any inclination in the applied loads or any possible lateral movements. The vertical working compression loads are based on Ref. 1


Pile Type


No. of Piles





(Qw x 10%)







































































D: diameter, COL: cut-off level, L: length of pile, Qw: vertical compression working load, Q: vertical working load on pile, PBM: Project Bench Mark, H: lateral working load expected on pile (10%Q),

9.2 Materials

Concrete mix is suggested to have high durability. The minimum cube strength (fcu) is suggested to be 50 N/mm2 for all piles. These concrete grades are chosen to match the requirements of BS8004 for limiting the average stress on concrete piles to 0.25xfcu due to the high loads on the piles. Some BS8110 equations are followed in this report and the cube strength is required for design.

Steel reinforcement is chosen as deformed bars having a yield strength fy of 460 N/mm2 and confirming with BS 4449: 1997.

The cube strengths have been chosen such that any stresses in the concrete will not exceed 50% of the ultimate strength, according to ICE 1996, Ref. 6, during proof tests and to satisfy structural and durability requirements.

9.3 Soil and Rock Properties for Design.

The strata in the vicinity of piles may be taken from site investigation borehole logs presented by GEOSCIENCE, Ref 2 shown in figure 2 below. The top layers are loose to very dense fine sands. These sand layers end at about -6.50 PBM where rocky layers start. The rocks are very weak to weak layers of sandstones and conglomerates. The Rock Quality Designation (RQD) of these rocks on average is very low and may be assumed as close as to zero for design purposes. From 6 nos. of boreholes for soil investigation, only 4 nos. of boreholes have core sample and with very low percentage recovery.

9.3.1 Rock Quality Designation (RQD):

It is calculated by dividing (the length of the rock of each sample whose it length > 10 cm) by (the length of the Bore hole) in percentage.

The value of Rock Quality Designation (RQD) will give us the value of j where it is j=0.2 for strength, as the RQD value is less than 50%. From j value we get the value of β=0.65 (Ref. 5, Figure 4.34, page 206).

9.3.2 Unconfined Compressive Strength (quc):

The distribution of unconfined compressive strength (quc) with depth for the rocks is shown in figure below (from soil test report).

The design quc values are chosen as 0.45 N/mm2, 1.04 N/mm2 and 0.55 N/mm2 for top of layers -6.50 PBM, -18.0 PBM and -28.0 PBM respectively for resistance calculations and 1.5 N/mm2 as an average for settlement calculations to estimate the modulus of deformation.

(Figure 9.4 – Soil Test Report Fiqure2 - 1 Mpa (Mega Pascal) = 1 N/mm2)

The unconfined compressive strength (quc) gives us the value of α (Ref. 5, Figure 4.33, and page 206) according to Williams and Pell Method.

Hence it’s used to calculate the shaft resistance in rocks fs ult.

9.4 Determination Of Depth Of Penetration:

The ultimate carrying capacity of a bored pile embedded in rock will be found through the skin resistance of the pile while end bearing will not be participating significantly. Generally,

Qult= Ab * qb ult + As * fs ult [Ref.5, Ref.4]

9.4.1 For shaft resistance in rocks:

fs ult = α x β x quc according to Williams and Pell Method. (Ref. 5, Equation 4.44, page 206).


α =function of quc and may be found through a graph given in Ref.5.

β = function of the mass factor j found through a graph given in Ref.5.

- For rocks with RQD less than 50% a value for j is adopted as 0.2 and corresponding β =0.65 (Ref. 5, Figure 4.34, page 207).

- For the value of quc=0.5 N/mm2 a value for α is adopted α =0.72 (Ref. 5, Figure 4.33, page 207).

9.4.2 For end bearing in fractured rock:

qb ult= (N+1) quc/5, (Ref.7, Page 553, Equation 11.60)

The value of N= tan2 (45+/2) is usually between 3 and 5.83 assuming = between 30 and 45 degrees (Ref.7, Page 553, Table 11.90), i.e.

qb ult =0.8-1.37 x quc where, (quc=0.45, 1.04, 0.55 as mentioned earlier)

For a safe end bearing it is assumed equal to qb ult = quc


Qs ult= Σ fs ult * Asi where Asi=π x D x L (surface area of the pile)
Qb ult= qb ult * Ab where Ab=(π/4)xd² (base area of the pile)

Qb all= {(N+1) quc xAb} / Fs (Ref.7, Page 553, Equation 11.61).

I.e. (D=1m)

Qb ult= factor qb x quc x Ab x1000= 0.45 x 1 x 3.14/4x 1² x 1000= 353.4 Kn.

Qb factor is between 1-4.5, in our design we take it as 1 for more safety, because it will affect the required length of the pile, however it increase the required length of the pile will decrees.

The length above the rock in the design and test level is during depending on the site condition excavation, and the pile cut off level from the site Bench Mark (The above calculation is regarded as Appendix – 1)

9.5 The settlement calculation (by Vesic's method cited in Ref.7)

Modulus of deformation of the rock mass (Em),

Em = j Mr quc = 0.4x225x1.5=135 Ref. (4)

- Modular ratio Mr = 225 (values of 150 and 300 are recommended by BS 8004 for rock group III and II respectively).

Pile shaft deformation = QL/AE

- =0.34 or 0.5 for triangular or uniform friction at maximum test load based on distribution of frictional resistance mobilized.

- E for concrete may be used as 470 √fc=470x√40=2972.5 N/mm2 provided that 85-90% of design fcu is reached. The reinforcement of 1% increases the average pile Young's modulus by ~5%.

- E av.= Ec (1+(n-1) ρ) where ρ is the reinforcement percentage and n = Es/Ec

Shaft surrounding settlement (Δs)

- Δs = Q(1-μ²)I / (π L Es ) (Vesic's method, Ref.(7))

μ: Poison's ratio ~ 0.3

L: Pile length

Es: surrounding mass modulus of deformation

An average at any level is found from the harmonic average of all Es values of the above layers.

- I = 2+0.35 √ (L/D)

The modulus of deformation for the rock is based on BS8004, Ref.4, adopting a higher j value of 0.3.

The settlement under working load: i.e. (D=1m)

-wf= ( x Qw x length above rock-Design) / Area of pile / E pile

wf= (0.34x5372x2)/0.785/31212= 0.149 ~0.15mm

-wp (pile)= ( x Qs + Qb) / Socket Length / Area of pile / E pile

wf= (0.34x735.1+4636.9)/1/0.785/31212= 0.199 ~0.2mm

-ws (shaft)= Qs/( πxSocket Length) Em av. For skin x(1- μ²) x (2+0.35 √ (Socket Length /D))

ws=735.1(3.14x1)/135x(1-0.3²)x(2+0.35√1/1))= 3.71mm

-wb(base)= Qb/Ab xD/Eav. For base x (1- μ²)x0.79

wb= 4636.9/0.785x1/135x(1-0.3²)x0.79= 31.44mm

The above calculation is regarded as Appendix – 1 Pile Length & Settlement which are used for the expected settlement for each length are shown for the two cases:

  • At working load
  • At 1.5 times the working load.

The pile lengths given in Table 1 are based on the Excel sheets.

9.6 Elastic Analysis Of Lateral Behaviour Of The Piles:

The method suggested by Reese and Matlock, cited in [Ref.5, Ref. 7] is adopted. The value for the stiffness parameter nh is taken as 45MN/m3. In fact, this value is an average value for the surrounding sands overlaying rocks. This is usually conservative if compared to the values for the stiffness Khi suggested usually for sandstones in local area. Note that Khi may be assumed as nh*z/B.

The stiffness factor (pile&soil) T=(EI/nh)1/5 = 1.593038m where; i.e.D=750mm, nh=45 (Ref.5)

The depth factor Z= z/T where z is the depth in meters

H is the service lateral load applied

E = Young's modulus for the pile concrete=0.2 + 0.2xFcu

- Fcu=48.625 Hence E=29.725 KN//mm2

I = D4 π /64 the moment of inertia i.e. (D=750mm) I=0.01553m^4.

L the length of the pile should be > 4 T (to be considered as a long pile Ref.5) i.e. L= 4x1.593038=6.4m (ok but too short for a pile).

At any depth, for the fixed head piles,

-Lateral deflection: (Ref.5, Page: 336, Eq. 6.27)

Y(mm)=fy H T3 /(EI)x1000 (as per the consultant decision or between 10mm-5mm)

-Bending moment: (Ref.5, Page: 336, Eq. 6.28)

M(Kn.m)=fm H Tx1000 (which will sets the area of steel bars required for pile)

-Soil pressure: (Ref.5, Page: 336, Eq. 6.29)

P(Kn/m) = fp H/ Tx1000 (for Rocky soil ~600 KN//mm2)

Z(m)=depth x T

Pressure Average:

Pav.(Kn)= p(av.) x z(difference)

Shear Force:

V(Kn)=H X 1000 – Pav.

Lateral Bearing Pressure:

q(Kn/m²)=p/fcu (not exceeding 1000 Kn/m² depending on soil characteristics).

Coefficient of lateral subgrade reaction

K(MPa/m)=q/Y (may be compared to Khi if available).

Subgrade reaction

k (MN/m³/m)= p/y x100 (based on line load definition) Stiffness factor back calculated nh = k/z

Modules of elasticity of concrtet:

E(Mn/m²)= 4700√fcu

Second Moment of inertia:

I(m^4)= Πd^4/64

Stiffness factor:

T(m)= 5√EI/nh (for pile and soil)

Hw(MN)= kN/1000

Bending Moment Diagram:

Draw the bending moment diagram by the values of Moment M and Depth z.

Note: it’s used to calculate the required amount of steel in the pile.

Lateral Deflection:

Draw the lateral deflection diagram by the values of Lateral Deflection Y and Depth z.

Note: it’s shows how much the pile is lateraly moved (allowed between 10mm-25mm).

Lateral Bearing Pressure:

Draw the lateral bearing pressure diagram by the values of Lateral Bearing Pressure q and Depth z.

Note: it’s indicate how much the pile is laterally pushing the surrounding soil, in our case it’s a rock soil so the value of lateral bending pressure can be up to 600 Kn/m².

The values for fy, fm, fp are taken from graphs in (Ref.5. Figure: 6.28).

Coefficients for fixed headed pile with lateral load in soil with linearly increasing modulus (after Reese and Matlock) (a) Coefficients for deflection (b) Coefficients for bending moment (c) Coefficients for soil resistance.

The above calculation is regarded as Appendix – 2 Pile Plastic Analysis for Lateral Loading, after applying the lateral load, the results of the analysis will be as shown in the sample EXCEL sheets in appendix 2.

At each depth the value of nh is recalculated from the soil pressure and the deflection and listed at the last column of the tables. It may be seen that the assumed value for the nh is accepted.

9.7 Check the Bending Moment Capacity and Shear Strength

The results of design for bending moment are given in table 3. Design is based on BS 8110 with an average load factor of 1.5 and using short column analysis with no magnification for buckling as buckling is not possible in bored piles under normal loads and medium to stiff soils, Ref.4.

The design is made using ACECOMS GEAR 2003 software shown in Appendix 3 by providing the values of Qu, Mu, fy, fcu and clear cover (not less than 75mm) will automatically calculate the Number & Diameter of Bars, and manual verification in table4. Available capacities of the sections are given for each pile. Capacities are more than the applied factored moments.

Bending moments are approximately nil when the depth is about 4*T as shown in the Excel sheets for elastic analysis given in the appendix2. So, after this point there is theoretically no need for reinforcement but the reinforcement may be extended beyond this point with the same or a lower percentage. This will ensure all moments are vanished and will practically assist execution.

The above calculation is regarded as Appendix – 3

9.8 Helical Links:

BS 8004 specifies a minimum spacing of 150 mm and a maximum spacing of D/2. No need to increase volume percentage of spirals as no driving stresses will take place.

The spiral will be taken as [email protected] pitches for 750mm diameter piles while T12 @ 150mm pitches for the rest of the piles.

For shear capacity the calculations are based on ACI 318 taking the effect of the axial forces. The circular section is substituted by a square section having equal area. Effective depth is based on this square side length. All piles checked and the calculations are shown in the (appendix 4).

Here we put the value of Ultimate Load Qu, Lateral Load Hw, Diameter of the pile, Cover (not less than 75mm), Spiral Diameter, Bar Diameter, fcu, fy and the pitch distance, then the calculation in the Appendix4 will show us if the amount of shear reinforcement and the pitch distance are safe or not, also will calculate the minimum area of steel and maximum shear, as per the following formulas:

i.e.: Qu=4412KN, Hw=333.31KN, D=750mm, Cover c=75mm, Spiral Diameter (db)=10mm, Bar Diameter= 20mm, fcu=40MPa, fy=460N/mm², Pitch=150mm, phi=0.85


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