According to a paper written by Bjerrum, the primary pore

pressures gradients occur in a centrifugal manner, hence determining that the

process for consolidating occurs via pore water flowing radially away from the

pile, and the soil particles will make movement towards the pile via axial symmetry

and plane strain conditions. Therefore, this determines that the majority of soil

particles will endure the unloading in shear process and result in a

deformation of the skeleton of the soil during the process of consolidation.

As previously mentioned, development area 3 is covered by

zone C where bored piles will be designed which will contain soft clay

underneath at a depth below ground level. Significantly large excess pore

pressures are created nearby the driven pile in cohesive soils. Due to the

excessive pore pressures being produced down the shaft of the pile, there will

be a reduction in effective stresses and result in enabling the required penetration

for the pile driving process. After the completion of the installation,

dissipation of the pore pressures commences to allow the soil around the pile

to consolidate. This is the process of improving the strength of the soil and

the capacity of the pile bearing increases accordingly.

3.1

Consolidation

Determine an appropriate arrangement of ground improvement

techniques to deal with the need for ground raising on site above the soft

alluvial deposits and hence provide recommendations on how to engineer the

roads, services and drains so that they remain in a serviceable state (focus on

the geotechnical processes occurring in the soft alluvial deposits and hence

provide detailed calculations to support your solution to deal with any

expected ground movements).

3

Task 2

Development area 3 is covered by zone C where bored piles

will be designed and these problems are likely to occur. To understand the behaviour

of soft clays requires a wide study of the geotechnical parameters, hence extracted

samples can be used during laboratory tests from either remoulded or cored

specimens.

·

Ground improvement techniques

·

Soil characterisation

·

Behaviour modelling

·

Geotechnical structure stability

·

Field investigation

There are a lot of well-known issues regarding the use of

soft clays. The problems can occur at any point on site and these problems can be

faced with:

With consideration to the building of a 2

to 3 storey residential housing estate, the structure load can be estimated to

be between 715kN and 750kN, therefore:

Hence the following has

been calculated:

Concrete Density = 25kN/m3

D = Pile Cap Depth = 0.4m

B = Pile Cap Breadth = 0.6m/pile

H = Pile Cap Height = 0.6m/pile

Where,

The formula to calculate a pile cap is as

follows:

Pile

Cap Design

To determine the value for Qb, the

following calculation was made:

Base

resistance Qb

Using the same formula, the following has

been calculated:

Stiff

Clay

Lower Layer

With the assumption that d = 0.25m, the following is given:

The following equation has been used to determine the value

for Qs:

Upper layer

Shaft Resistance (Qs)

In this area, a low rise 2 to 3 storey residential housing

estate with associated access and site infrastructure is to be built. This area

covers a majority of zone C and although the buildings may have light loads and

a typical option of a shallow foundation for this structure type, the upper

strata can be used to determine a pile foundation as calculated below:

2.3

Development Area 3

Hence from this it can be

determined that for a point load, P, acting on a rectangular pad foundation,

length (L), width (B), and the bearing pressure (Q) is calculated as:

Made ground in zone B is

mostly granular, hence C = 0

Bearing Capacity

Nc = 35

Nq = 4.5

Ny = 2.5

L = 3.5m

B = 3m

H = 1.531m

The Meyerhof deep foundation chart can be used to obtain the

values for the bearing capacity factors

Nc, Nq, and Ny, considering the value

of ? is given. Therefore:

? =10?

Due to the fact that development area 2 includes

construction of both zones A and B, the weak strata must be considered solely

for the purpose of construction, in other words, for zone B. Due to the fact

that granular soil in zone B is unable to hold piles, the solution for this

type of strata would be determined with the deep foundation calculation as

shown below:

2.2.4

Development Area 2: Rectangular Deep

Foundation

Net Safe Bearing

Capacity at depth < D
From the surface, the water table level at
1.8m has D = 3m
Water Table at depth
< D
Net
Safe Bearing Capacity at depth D to (D+B)
Water
Table at depth D to (D+B)
Net Safe Bearing
Capacity at depth D ? (B+D)
Water Table at depth
D ? (B+D)
2.2.3
Effect of Water Table Level
Therefore this determines that the long term drained
capacity of 488kN/m2 is greater than the short term undrained
capacity which was calculated previously to be 179.66kN/m2.
Net Safe Bearing
Capacity
Y
is applied when the soil is above the water table (partially saturated)
YSat is applied when the soil is below the water table (fully
saturated)
Shape Factor
?
= 26° with bearing capacities of:
NC = 22.3
NQ = 11.19
NY = 7.9
Effective strength parameters apply due
to fully drained conditions.
40
years after construction
Net Safe Bearing Capacity
Ultimate Bearing Capacity
With the use of
Skempton's chart, the value of Nc is determined as 7.4
Therefore,
Cu = 55kN/m2
C = 3.0
fu = 0
B = 4.5m
L = 4.5m
D = 3m
Y = 22kN/m3
Immediately after
construction
2.2.2
Development Area 2: Shallow Foundation
The calculations determine that the pile
foundation with 6 piles for zone A, development area 2 is able to carry the
proposed load of 3500kN.
Hence the following has
been calculated:
Concrete Density = 25kN/m3
D = Pile Cap Depth = 0.5m
B = Pile Cap Breadth = 1m/pile
H = Pile Cap Height = 1m/pile
Where,
The formula to calculate a pile cap is as
follows:
Pile
Cap Design
To determine the value for Qb, the
following calculation was made:
Base
resistance Qb
Very Stiff Clay
Using the same formula, the following has
been calculated:
Stiff
Clay
Lower Layer
And with the values input into the equation, with the
assumption that d = 0.5m, the following is given:
The following equation has been used to determine the value
for Qs:
Upper layer
Shaft Resistance (Qs)
2.2.1
Development Area 2: Pile Foundation
Approximately half of Development Area 2 is covered by zone
A as displayed in Figure 1, and 40% of it is covered by zone B. Pored piles and
shallow foundation could be applied to zone A.
2.2
Development Area 2
Therefore this determines that the long term drained
capacity of 391kN/m2 is greater than the short term undrained
capacity which was calculated previously to be 178.8N/m2.
Net Safe Bearing
Capacity
Y
is applied when the soil is above the water table (partially saturated)
YSat is applied when the soil is below the water table (fully
saturated)
Shape Factor
?
= 26° with bearing capacities of:
NC = 22.3
NQ = 11.19
NY = 7.9
Effective strength parameters apply due
to fully drained conditions.
40
years after construction
Net Safe Bearing Capacity
Ultimate Bearing Capacity
With the use of
Skempton's chart, the value of Nc is determined as 7.8
Therefore,
Cu = 55kN/m2
C = 3.0kN/m2
fu = 0
B = 2.5m
L = 2.5m
D = 2.5m
Y = 21.5kN/m3
Immediately after
construction
In this area of the development, the loads are considered to
be light and likewise the upper layer of zone A involves cohesive soil with a
layer of clay underneath. Over time, the clay will gradually become stiffer and
stiffer which will provide sufficient strength to the soil and result in a
shallow foundation which can therefore be designed to construct the mixed
retail, with consideration to the practicability and any economical issues.
2.1.2
Development Area 1: Shallow Foundation
The calculations determine that the pile
foundation with 6 piles for zone A, development area 1 is able to carry the
proposed load of 1000kN.
Hence the following has
been calculated:
Concrete Density = 25kN/m3
D = Pile Cap Depth = 0.5m
B = Pile Cap Breadth = 0.5m/pile
H = Pile Cap Height = 0.5m/pile
Where,
The formula to calculate a pile cap is as
follows:
Pile
Cap Design
To determine the value for Qb, the
following calculation was made:
Base
resistance Qb
Using the same formula, the following has
been calculated:
Stiff
Clay
Lower Layer
And with the values input into the equation, with the
assumption that d =0.3, the following is given:
The following equation has been used to determine the value
for Qs:
Upper layer
Shaft Resistance (Qs)
2.1.1
Development Area 1: Pile Foundation
A major portion of the development area is under zone A (as
shown in Figure 1) and this will be considered as the ground strata. The mixed
retail can be developed towards the North and North-East location of the site
and this is where the design of bored piles can be placed.
·
Strip loading
·
Point loading
There are 2 types of loading involved in this development
area:
2.1
Development Area 1
·
Soil Zone C – Green
·
Soil Zone B – Turquoise
·
Soil Zone A – Yellow
Figure 1: Site Model (from analysis of
ground investigation report and development proposals)
Carry out bearing capacity calculations for shallow
foundations for the scheme, alongside calculations for deep and piled
foundations, so that a comparison can be made of the relative dimensions of the
shallow and piled foundation solutions.
2
Task 1
There are three areas of development, each with a foundation
of which will be designed dependent upon the soil conditions. In the case of
the site being unable to bear the structure load, typical treatment schemes for
this situation will include piling, shallow and deep foundations.
The primary objective for this report includes evaluating
the issues regarding ground engineering and to propose solutions for these
issues, which include the design of shallow, deep and piled foundations,
slopes, retaining walls and schemes for ground improvement.
1.1
Objectives
1
Introduction