RCC Pile Construction: Precast vs. Bored Piles

Professor Dr. Md. Jahangir Alam
BScEngg – Structure
MScEngg – Geotechnical
PhD – Earthquake Engineering
Post Doc – Soil-Structure Interaction
Compare between precast and bored RCC pile
Precast pile

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
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Slump value 50-100 mm
Retarder and plasticizer
is not needed in general
Compaction by vibration
is necessary
Clear cover > 50 mm
Durability should be
considered
Bore pile

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Slump value 150 – 200
mm
Retarder and plasticizer
should be used
Compaction is prohibited
Clear cover > 75 mm
Durability should be
considered
Mix ratio of cast in situ piling concrete
without plasticizer
Cement = CEM-II, FA = Coarse Sand, CA = Stone chips or Shingles
1 : 1.25 : 2.00
Minimum fc’ = 28 Mpa (4000 psi)
3
Mix ratio of cast in situ piling concrete with
plasticizer
Cement = CEM-II, FA = Coarse Sand, CA = Stone chips or Shingles
1 : 1.5 : 2.5
Minimum fc’ = 28 Mpa (4000 psi)
4
Wet method of construction
Or
Slurry method of construction
Excavation of bore hole can be done by
1. Percussion method of drilling
a. Usually done in Bangladesh
b. Good for dia 20” – 24” and L = 90’
2. Rotary method of drilling
a. Expensive
b. Now used in big projects
c. Any dia and length ok
d. Economic for large dia
Steps of cast-in-situ piling
Boring
 Insertion of rebar cage
 Concreting through tremie pipe

Boring
concreting
Use of casing
Temporary casing
 Permanent casing

Steel casing
water
soil
Permanent casing
examples
Steel casing
Thick caving soil:
Loose sand or
Loose nonplastic silt
Dense sand or
Stiff clay
Temporary casing necessary

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

when caving soil (loose sand and silt) exist
at top layer
To maintain the piling location
To maintain the slurry above the ground
level
Length of casing depends of soil layers
Temporary casing example
Temporary steel casing
Loose sand or
Loose nonplastic silt
or soft clay
Dense sand or
Stiff clay
Precautions for temporary casing
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Cleaning after each use
Must be as smooth as possible
Casing with bonded concrete should not be allowed
Slurry column should extend well above the level of the
piezometric surface so that any fluid flow is from the
excavation outward
Drilling Slurry

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Bentonite or polymer slurry may be used
Drilling fluid serves to put soil particles in
suspension and will form a membrane or a
filter cake at the walls of the borehole.
The membrane acts to prevent caving or
collapse of the borehole.
Also called as drilling mud
Dense Sand
Loose Sand
Drilling slurry is essential

For piling into caving soil

Caving soil types


Loose sand and gravel

Nonplastic Silt
Important note: avoid drilling slurry if only water is enough, because

Bentonite slurry is expensive

Reduces skin friction of pile
Slurry types

Bentonite slurry

Polymer slurry
Properties of drilling slurry
Item to be Measured
Range of Results at 20
C
Test Method
Density during drilling to
support excavation
greater than 1.05 g/ml
Mud density Balance
Density prior to concreting
less than 1.25 g/ml
Mud density Balance
Viscosity
30 - 90 seconds
Or 5 - 25 cP
Marsh Cone Method
Or Fann viscometer
9.5 to 12
pH indicator paper strips or
electrical pH meter
pH
New Slurry Intake
Pump
Slurry tank
Sediment
Solids
(to be disposed daily)
Drilling fluid circulation
New Slurry Intake
Pump
Slurry tank
Sediment
Solids on inclined vibrating sieve
Modern
Drilling fluid circulation
Problems of Cast in situ piling - 1
Solution: measure the depth of the excavation two or more times after
drilling ceases to see that sediment is not settling out and the hole is as
deep as indicated by the penetration of drilling tools.
Problems of Cast in situ piling - 2
Solution: do an
appropriate job of
sampling and testing
the slurry
Solution: ensure the location of
the bottom of tremie pipe is just
far enough above the bottom of
the excavation so that concrete
can start flowing
Problems of Cast in situ piling - 3
Solution: the casing should
not be pulled until it is filled
with concrete with good flow
characteristics
Pile reinforcement (cage)

The cage is designed to meet two
requirements
 The structural requirement for bending and for
column action (sometimes slender column)
 Stability requirements of the rebar cage during its
placing, during the placing of concrete and during
the withdrawal of the casing
Minimum longitudinal reinforcement

If the pile has sufficient axial strength using
only half the gross concrete area, Ag/2, the
longitudinal reinforcement ratio can be
reduced to 0.5 percent of the gross
concrete area, Ag.
Longitudinal bar in the cage

Maximum longitudinal reinforcement at the top if no drag load

Symmetrical arrangement of longitudinal rebar is recommended
unless there is compelling reasons

Minimum 5 or preferably 6 longitudinal bars are needed

16mm dia bar is the minimum size of longitudinal bar

Clear spacing between bars is 3-5 times maximum size of CA

If a very large amount of rebar is needed, concentric multiple cages
or bundled bars may be used.
Splices and sizing hoops


Splices should be
staggered
Lapping at the same
location is economical
in piling

Spiral
Longitudinal bar
Sizing hoop
Sizing hoops simply provide
guides for the fabrication of
cage
Centering Devices
Two purposes:
1. Clear cover 3”
2. To flow concrete
Rebars should not
be used for
centering devices
unless they are
epoxy-coated
Better solution is to
use concrete roller
Installation of concrete rollers
Pile Cap Casting
Break 3 ft weak concrete at top of drilled shaft / bored pile
before casting pile cap
 Use top, bottom and side reinforcements in pile cap even it is
not needed by calculation
 Use thick pile cap to ensure rigid action of pile cap to
distribute column load to individual piles uniformly
 Take care of basement construction at pile cap level

Basement floor
Placement of concrete in drilled shaft

Basic characteristics of concrete for
drilled shafts
 Excellent fluidity
 SCC (Self Compacting Concrete)
Compaction under self-weight
 Resistance to segregation
 Controlled setting
Measurement of Workability
Slump Test
4”
12”
8”
1. 3 layers
2. 25 tamping per layer
3. Tamping rod dia 16mm
34
Measurement of Workability
Slump Test
Upto 125 mm
•True Slump
In case of shear slump,
repeat the slump test
25-50 mm
Upto 150 mm
•Shear Slump
Disadvantage of Slump Test
• Lean Mix > Shear type or
collapse at low w/c ratio
150-250 mm
•Collapse
•Zero Slump
35
Use of retarder in tremie concrete

If the required time of pouring concrete is more
than initial setting time, retarder must be used
Initial setting time >= 45 minute
Final setting time <= 420 minute
Usually initial setting time = 2 hours
Precautions for tremie concrete
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the concrete shall contain at least 10 per cent more cement than
that required for the same mix placed in the dry.
Slump = 150 mm to 200 mm
Successful placement of concrete under water requires preventing
flow of water across or through the concrete.
The first concrete placed is physically separated from the water by
using a “rabbit” or go-devil or plug in the pipe, or by having the pipe
mouth capped or sealed and the pipe dewatered.
Once filled with concrete, the pipe is raised slightly to allow the
“rabbit” to escape or to break the end seal. Concrete will then flow
out and develop a mound around the mouth of the pipe. This is
termed as “establishing a seal”.
Once the seal is established, fresh concrete is injected into the
mass of existing concrete.
Precautions during concreting

In the “rabbit” plug approach, open tremie pipe should be set on the
bottom, the “rabbit” plug inserted at the top and then concrete should
be added to the tremie slowly to force the “rabbit” downward separating
the concrete from the water. Once the tremie pipe is fully charged and
the “rabbit” reaches the mouth of the tremie, the tremie pipe should be
lifted a maximum of 150 mm (6 inch) off the bottom to allow the “rabbit”
to escape and to start the concrete flowing. After this, a tremie pipe
should not be lifted again until a sufficient mound is established around
the mouth of the tremie.
Precautions during concreting
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Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3
to 5 ft) and maintained at that depth throughout concreting to prevent entry of
water into the pipe.
Rapid raising or lowering of the tremie pipe should not be allowed. All vertical
movements of the tremie pipe must be done slowly and carefully to prevent “loss
of seal”.
Underwater concrete shall be placed continuously for the whole of a pour to its full
depth approved by the Engineer, without interruption by meal breaks, change of shift,
movements of placing positions, and the like. Delays in placement may allow the
concrete to stiffen and resist flow once placement resumes.
The volume of concrete in place should be monitored throughout the placement. Under
runs are indicative of loss of tremie seal since the washed and segregated aggregates
will occupy a greater volume. Over runs are indicative of loss of concrete.
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Tremie pipe dia > 6 times of maximum size of CA
Plug of cement paste is recommended as low
cost solution of initial charge
Minimum
c/c spacing of piles
Drilling and extraction of casing during
construction of a pile can cause upward
directed shear stresses to develop on the
perimeter of adjacent piles that were installed
earlier, possibly damaging those shafts
 So minimum c/c spacing = 3D
 If closer spacing is required, sequence of shaft
installation must be mentioned

Load distribution in pile
Initial loads are taken almost completely by
skin friction at very small displacement
 as loading continues, some load is transferred
to the base of the drilled shaft
 At the ultimate load, a sizeable portion of load
is carried by end bearing but at significant
amount of downward displacement

Influence of construction on soil
properties of clay

Stress released during excavation. The placing of
concrete will reimpose a stress in the clay surrounding the
drilled shaft that can be greater than the in situ stress.
The magnitude of the concrete stress is dependent on the
slump of the concrete, and high-slump concrete is highly
recommended.

Some chemical bond occur between clay and cement, so
skin friction increases. So, Shear failure does not occur at
interface but a short distance from interface
Influence of construction on soil
properties of sand
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A membrane of bentonite is created at the wall of the borehole
Stress released during excavation. The placing of concrete will
reimpose a stress in the sand surrounding the drilled shaft that
can be greater than the in situ stress. The magnitude of the
concrete stress is dependent on the slump of the concrete, and
high-slump concrete is highly recommended.
Some chemical bond occur between sand and cement, so skin
friction increases. So, Shear failure does not occur at interface
but a short distance far from interface
Quality Assurance
Pile integrity test – length and integrity
 Pile load test on test piles and service
piles

 Static load test – takes time
 Dynamic load test (PDA) - rapid
Summary of quality control of pile
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Ensuring length
Cross checking the soil test bore log with drilling bored
pile
Slump and mix ratio of concrete
Outlet of tremie pipe into concrete all the time
Maintaining clear cover
Prevention of caving by using bentonite slurry or any
other mud
Cleaning the bore hole properly to ensure end bearing