ARBT05 PrelimResearch1
ARBT05 PrelimResearch1
ARBT05 PrelimResearch1
ARCH 401
ARBT05 – Prelim Research no. 1
1. Footing Systems
1. Mat Footing
Mat footings are used when the building load is so high, that spread, or strip footings
could not bear the weight, or their employment would be inefficient. Furthermore, mat
footings are helping to reduce the varying settlements caused by construction on non-
homogenous soils or uneven load distribution on the footing.
Thickness
Depending on the total load applied to the mat and underlying foundation system, the
thickness of mat foundations can vary from 1 ft (0.3 m) to more than 20 ft (7 m). The
reinforcing system in the mat can be quite substantial, with heavy reinforcing bar mats in the
bottom, top, or both locations within the mat depth.
Function
1. Transfer superstructure loads and spread it over the entire area of the building
footprint.
2. Reduce differential settlement of structures constructed over weak soil.
Usage
1. It is employed for the construction of commercial buildings. In this case, the loads are
commonly large. Mat foundations are popular in areas where basements are common.
2. Mat foundation is also used for low bearing capacity soil in order to spread the load of
a building and hence construct a stable foundation.
3. It is used to reduce differential settlement of buildings.
4. Raft or mat foundation is used when the soil layer is unstable. In this case, strip
foundation would cover more than 70% of the ground area beneath the building.
There are also situations usually in mining areas that soil layer may suffer
movements.
Fig. 1 : Mat Foundation
Requirement
As per IS 1080, a minimum depth of 50 cm shall be used for mat foundation. This is
required to ensure that the soil has a safe bearing capacity which is assumed in the design.
The depth of mat foundation must satisfy shear requirements.
Allowable Bearing Capacity
For mats on clay, the factor of safety should not be less than 3 under dead load
or maximum live load. However, under the most extreme conditions, the factor of safety
should be at least 1.75 to 2. For mats constructed over sand, a factor of safety of 3 should
normally be used.
Construction
1. Remove dirt and excavate soil to a uniform and flat level.
2. The foundation bed is then compacted by ramming.
3. Then, a waterproof plastic sheet is laid over the earth.
4. After that, pour around 7cm layer of plain cement concrete to create a perfectly flat
and level base for the foundation.
5. Lay reinforcement on spacers over the foundation bed. Reinforcements are provided
in both directions in the form of steel mesh. Two meshes are reinforced at the top and
bottom of the foundation to balance upward and downward bending forces.
6. After all the steel has been put in place, concrete is poured to the desired thickness,
which is usually in the range of 200mm to 300mm thick for small buildings: this can
be much thicker if heavy loads are to be carried. A minimum rebar cover of 50mm
should be maintained.
7. Finally, a suitable curing regime should be used to make sure that concrete achieves
the designated compression strength.
Fig. 2: Excavation for Raft or Mat Foundation Construction
2. Pile Footing
Pile foundation, a kind of deep foundation, is a slender column or long cylinder made
of materials such as concrete or steel which are used to support the structure and transfer the
load at desired depth either by end bearing or skin friction.
Pile foundations are usually used for large structures and in situations where the soil
at shallow depth is not suitable to resist excessive settlement, resist uplift, etc.
When to Use Pile Foundation
Following are the situations when using a pile foundation system can be
When the groundwater table is high.
Heavy and un-uniform loads from superstructure are imposed.
Other types of foundations are costlier or not feasible.
When the soil at shallow depth is compressible.
When there is the possibility of scouring, due to its location near the riverbed or
seashore, etc.
When there is a canal or deep drainage systems near the structure.
When soil excavation is not possible up to the desired depth due to poor soil
condition.
When it becomes impossible to keep the foundation trenches dry by pumping or by
any other measure due to heavy inflow of seepage.
Types of Pile Foundation
Classification of Pile Foundation Based on Function or Use
Sheet Piles
This type of pile is mostly used to provide lateral support. Usually, they resist lateral pressure
from loose soil, the flow of water, etc. They are usually used for cofferdams, trench sheeting,
shore protection, etc. They are not used for providing vertical support to the structure. They
are usually used to serve the following purpose-
Construction of retaining walls.
Protection from riverbank erosion.
Retain the loose soil around foundation trenches.
For isolation of foundation from adjacent soils.
For confinement of soil and thus increase the bearing capacity of the soil.
Load Bearing Piles
This type of pile foundation is mainly used to transfer the vertical loads from the structure to
the soil. These foundations transmit loads through the soil with poor supporting property onto
a layer which can bear the load. Depending on the mechanism of load transfer from pile to
the soil, load-bearing piles can be further classified as flowed.
End Bearing Piles
In this type of pile, the loads pass through the lower tip of the pile. The bottom end of the pile
rests on a strong layer of soil or rock. Usually, the pile rests at a transition layer of a weak and
strong slayer. As a result, the pile acts as a column and safely transfers the load to the strong
layer.
The total capacity of end bearing pile can be calculated by multiplying the area of the tip of
the pile and the bearing capacity of at that depth of soil at which the pile rests. Considering a
reasonable factor of safety, the diameter of the pile is calculated.
Friction Pile
Friction pile transfers the load from the structure to the soil by the frictional force between
the surface of the pile and the soil surrounding the pile such as stiff clay, sandy soil, etc.
Friction can be developed for the entire length of the pile or a definite length of the pile,
depending on the strata of the soil. In friction pile, generally, the entire surface of the pile
works to transfer the loads from the structure to the soil.
The surface area of the pile multiplied by the safe friction force developed per unit area
determines the capacity of the pile.
While designing skin friction pile, the skin friction to be developed at a pile surface should be
sincerely evaluated and a reasonable factor of safety should be considered. Besides this one
can increase the pile diameter, depth, number of piles and make pile surface rough to increase
the capacity of friction pile.
Soil Compactor Piles
Sometimes piles are driven at placed closed intervals to increase the bearing capacity of soil
by compacting.
Classification of Piles Based on Materials and Construction Method
Primarily piles can be classified into two parts. Displacement piles and Non-displacement or
Replacement piles. Piles which cause the soil to be displaced vertically and radially as they
are driven to the ground is known as Displacement piles. In case of Replacement piles, the
ground is bored, and the soil is removed and then the resulting hole is either filled
with concrete or a pre-cast concrete pile is inserted. Based on materials of pile construction
and their installation process load-bearing piles can be classified as follows:
Timber Piles
Timber piles are placed under the water level. They last for approximately about 30 years.
They can be rectangular or circular in shape. Their diameter or size can vary from 12 to 16
inches. The length of the pile is usually 20 times of the top width.
They are usually designed for 15 to 20 tons. Additional strength can be obtained by bolting
fish plates to the side of the piles.
Advantages of Timber Piles
Timber piles of regular size are available.
Economical.
Easy to install.
Low possibility of damage.
Timber piles can be cut off at any desired length after they are installed.
If necessary, timber piles can be easily pulled out.
Disadvantages of Timber Piles
Piles of longer lengths are not always available.
It is difficult to obtain straight piles if the length is short.
It is difficult to drive the pile if the soil strata are very hard.
Spicing of timber pile is difficult.
Timber or wooden piles are not suitable to be used as end-bearing piles.
For durability of timber piles, special measures have to be taken. For example-
wooden piles are often treated with preservative.
Concrete Piles
Pre-cast Concrete Pile
The precast concrete pile is cast in pile bed in the horizontal form if they are rectangular in
shape. Usually, circular piles are cast in vertical forms. Precast piles are usually reinforced
with steel to prevent breakage during its mobilization from casting bed to the location of the
foundation. After the piles are cast, curing must be performed as per specification. Generally
curing period for pre-cast piles is 21 to 28 days.
Advantages of Pre-cast Piles
Provides high resistance to chemical and biological cracks.
They are usually of high strength.
To facilitate driving, a pipe may be installed along the center of the pile.
If the piles are cast and ready to be driven before the installation phase is due, it can
increase the pace of work.
The confinement of the reinforcement can be ensured.
Quality of the pile can be controlled.
f any fault is identified, it can be replaced before driving.
Pre-cast piles can be driven under the water.
The piles can be loaded immediately after it is driven up to the required length.
Disadvantages of Pre-cast Piles
Once the length of the pile is decided, it is difficult to increase or decrease the length
of the pile afterward.
They are difficult to mobilize.
Needs heavy and expensive equipment to drive.
As they are not available for readymade purchase, it can cause a delay in the project.
There is a possibility of breakage or damage during handling and driving od piles.
Cast-in-Palace Concrete Piles
This type of pile is constructed by boring of soil up to the desired depth and then, depositing
freshly mixed concrete in that place and letting it cure there. This type of pile is constructed
either by driving a metallic shell to the ground and filling it with concrete and leave the shell
with the concrete or the shell is pulled out while concrete is poured.
Advantages of Cast-in-Place Concrete Piles
The shells are light weighted, so they are easy to handle.
Length of piles can be varied easily.
The shells may be assembled at sight.
No excess enforcement is required only to prevent damage from handling.
No possibility of breaking during installation.
Additional piles can be provided easily if required.
Disadvantages of Cast-in-Place Concrete Piles
Installation requires careful supervision and quality control.
Needs sufficient place on site for storage of the materials used for construction.
It is difficult to construct cast in situ piles where the underground water flow is heavy.
Bottom of the pile may not be symmetrical.
If the pile is un-reinforced and uncased, the pile can fail in tension if there acts and
uplifting force.
Steel Piles
Steel piles may be of I-section or hollow pipe. They are filled with concrete. The size may
vary from 10 inches to 24 inches in diameter and thickness is usually ¾ inches. Because of
the small sectional area, the piles are easy to drive. They are mostly used as end-bearing
piles.
Advantages of Steel Piles
They are easy to install.
They can reach a greater depth comparing to any other type of pile.
Can penetrate through the hard layer of soil due to the less cross-sectional area.
It is easy to splice steel piles
Can carry heavy loads.
Disadvantage of Steel Piles
Prone to corrosion.
Has a possibility of deviating while driving.
Comparatively expensive.
3. Drilled Shafts / Caisson
Drilled shafts, also known as drilled piers, caissons, bored piles, or cast-in-drilled-
hole piles (CIDH), are high-capacity deep foundation systems.
Common uses
Structural support
Slope stabilization
Earth retention for retaining walls and sound barriers
Process
Drilled shafts for structural support can be installed by the dry (open hole), stabilizing
polymer slurry, or steel-cased methods. The permanent casing is typically only specified in
corrosive environments, voided (open cavity) conditions, or for shafts drilled through the
water. For open hole shaft installation, the temporary casing may also be required if adverse
subsurface conditions are present (i.e., groundwater, caving soils, granular soils, sidewall
loss, etc.).
A hole of the required diameter is augured to the required bearing stratum or design
depth, cleaned out and inspected. Inspection may be manual for shallow holes or with a Mini-
SID (shaft inspection device) or down-hole camera. Large boulders encountered during
auguring may prevent shaft continuity or required bearing capacity and must be removed,
typically by drilling out. A core barrel or rooting tool may also be used. If reinforcement is
required and the client elects to remove this from their scope, Keller will purchase and
fabricate the cage(s) on-site. Following insertion of the steel cage, concrete is placed either by
freefall or tremie methods. The casing, if temporary, is then withdrawn. The completed shaft
can resist compressive and lateral loads and uplift forces.
Access conditions required for drilled shaft construction are as variable as the
diameters and depths to which they can be drilled. Drilled shafts can be constructed in low
headroom and limited access and provide effective support for most structures, including
buildings, tanks, towers, and bridges. Keller owns and maintains a diverse and up-to-date
drilled shaft equipment fleet outfitted with the highest quality, state-of-the-art tooling.
Experience combined with specialty proprietary drilling equipment and tooling gives Keller
the ability to meet specific site constraints such as limited access and low overhead
construction. For a variety of subsurface and access conditions, drilled shafts may be the
answer for your project.
Advantages
Variety of equipment and tooling for virtually any condition
Experienced at both dry and wet shaft construction
Limited access capability
Ability to construct drilled shafts in diameters ranging from 12 to 240 inches
Manufacturing facility to design and build, repair, maintain, and modify equipment
and tools needed to complete the work
Capability of completing alternate foundation systems if required by changed
conditions
Wide variety of applications
Quality assurance
Non-destructive test methods help determine the quality of the concrete throughout
the length of the shafts. Crosshole sonic logging (CSL) and/or Gamma-Gamma logging
(GGL) can be conducted by placing test pipes in the shaft reinforcement and subsequently
testing the integrity of the pile concrete. Load testing can be conducted on drilled shaft
foundations to verify the load-carrying capacity of the foundation elements and/or the quality
of the subsurface materials. This testing can be completed on production or sacrificial drilled
shafts. Load testing is often completed by one of the following methods: Osterberg Cell (O-
Cell), direct static testing, and statnamic testing, for both compressive and lateral testing.
Testing is often utilized to refine designs and can result in significant savings to projects by
removing some of the uncertainties inherent in the typical foundation design process.
4. Isolated Footing
Isolated footings (also known as Pad or Spread footings) are commonly used for
shallow foundations to carry and spread concentrated loads, caused for example by columns
or pillars. Isolated footings can consist either of reinforced or non-reinforced material. For the
non-reinforced footing however, the height of the footing has to be bigger in order to provide
the necessary spreading of load.
Isolated footings should only be used when it is certain, that no varying settlements
will occur under the entire building. Spread footings are unsuitable for the bearing of
widespread loads. In this case, either strip (continuous) footings or mat footings are used.
The basic program for design and analysis of isolated footings is GEO5 Spread
Footing. It can design the entire footing and to compute settlement, rotation and bearing
capacity of the footing. Also, it determines the required longitudinal and shear reinforcement
(punching).
5. Combined Footing
Combined footings are constructed for two or more columns when they are close to
each other, and their foundations overlap. Design of combined footings with example is
discussed. The function of a footing or a foundation is to transmit the load form the structure
to the underlying soil. The choice of suitable type of footing depends on the depth at which
the bearing strata lies, the soil condition and the type of superstructure.
Whenever two or more columns in a straight line are carried on a single spread
footing, it is called a combined footing. Isolated footings for each column are generally the
economical. Combined footings are provided only when it is absolutely necessary, as
1. When two columns are close together, causing overlap of adjacent isolated footings
2. Where soil bearing capacity is low, causing overlap of adjacent isolated footings
3. Proximity of building line or existing building or sewer, adjacent to a building
column.
It is a cantilever retaining wall but strengthened with counter forts monolithic with the
back of the wall slab and base slab.
Counter fort spacing is equal or slightly larger than half of the counter-fort height.
Counter-fort wall height ranges from 8-12m.
2. Freestanding
3. Hybrid
Retaining walls that use both mass and reinforcement for stability are termed
as Hybrid or Composite retaining wall systems.
4. Gravity
Gravity retaining wall depends on its self-weight only to resist lateral earth pressure.
Commonly, gravity retaining wall is massive because it requires significant gravity load
to counter act soil pressure.
Sliding, overturning, and bearing forces shall be taken into consideration while this type
of retaining wall structure is designed.
It can be constructed from different materials such as concrete, stone, and masonry units.
It is economical for a height up to 3m.
Crib retaining wall, gabions, and bin retaining wall are also type of gravity retaining walls
5. Cantilevered
Pile retaining wall are constructed by driving reinforced concrete piles adjacent to each
other as shown in the Fig.
Piles are forced into a depth that is sufficient to counter the force which tries to push over
the wall.
It is employed in both temporary and permanent works.
Piled walls offer high stiffness retaining elements which are able to hold lateral pressure
in large excavation depths with almost no disturbance to surrounding structures or
properties.
Sheet pile walls are built using steel sheets into a slope or excavations up to a required
depth, but it cannot withstand very high pressure
Sheet pile retaining wall economical till height of 6m
7. Gabion Wall
Gabion retaining wall walls are multi-celled, rectangular wire mesh boxes, which are
filled with rocks or other suitable materials.
It is employed for construction of erosion control structures.
It is also used to stabilize steep slopes.
8. Riprap
Riprap is a layer of stones on an embankment slope used to prevent erosion and to protect
the structure from the effects of stream flow. Riprap is used in various situations on ditch
banks, channel bottoms, berm slopes, or any area where erosion is to be prevented. The most
common types of riprap used are gabion baskets, machine placed riprap, wire enclosed riprap,
or hand placed riprap. The type of erosion protection used can come from the Engineer's
recommendations in the field, Hydraulic Section recommendations, Geology Program
recommendations, or at the discretion of the Squad Team Leader.
A riprap detail sheet does not need to be included if the geometry and elevations of the
riprap can be shown on the General Plan & Elevation sheet. However, the fabric under the
entire length of riprap must then be shown on the elevation or noted on the GP&E or as a
General Note.
Generally, the top of the riprap is located 1'-0" above design highwater.
Machine Placed Riprap
consists of stones that are machine placed on an earth embankment or gravel bedding.
Larger stones are placed in the toe course and on the outside surface of the slope protection.
Bulldozers or other suitable equipment may be used to dump or spread the layers of stone.
Wire Enclosed Riprap
as the name implies, consists of wire enclosed segments that are fabricated on site, laced
together, and filled with stones to provide an area of dense protection against erosion. The
size of the enclosure segments is formed to the dimensions shown on the plans.
Hand Placed Riprap
consists of hand placed stones on an earth embankment or gravel bedding. Larger stones
are placed first with close joints. Smaller stones are then placed to fill the voids as best as
possible.
The Contractor may use wire enclosed riprap instead of gabions, in compliance with the
Standard Specifications. The Standard Plan (Wire Enclosed Riprap and Gabions) should be
included in the References on the General Notes sheet of the plans.
Wrap-Around Riprap
is used primarily in large fill areas or in areas where soil erosion is likely to occur at the
abutments. This technique is used at the discretion of the Squad Team Leader. The pivot
points for wrap-around riprap are usually at the end of the wingwall at rear face for elephant
ear wingwalls or the end of the abutment cap at rear face for sweptback wingwalls.