farisalmahdawi: 01/18/09

Bored Piles

Bored piles are cast in place cylindrical piles excavated either by use of rotary equipment operated augers , buckets, under static drilling fluid or large drill bit (for hard rock) with reverse circulation, with chisel grab and casing oscillator for bouldery ground, with large diameter DTH hammers and compressed air (drilled piles), among others.
Most common large diameter bored piles, are installed through an overburden of cohesive or cohesionless soil strata, with or without water tale, down to firmer ground, to achieve the design bearing capacity by skin friction, base bearing or both, to serve as foundation piles for residential, commercial, institutional buildings, industrial complexes or infrastructures.
Bored piles installed in common soil with the presence of water table, generally require the use of a short temporary steel casing and a drilling fluid as static suspension to provide support to the surrounding soil while excavating the pile and until complete backfill of the pile excavation with concrete, in order to prevent cave-in of the excavation and destabilizing the surrounding soil formation.
The preparation and handling by most effective drilling fluid, Bentonite Mud, is a sophisticated technology by itself and requires a complete set up of dedicated equipment and (basic) field laboratory.
The most common diameters of bored piles range from 0.6 meter to 2.0 m meters, likewise length can range from few meter to sixty or more meters, depending upon design loads and soil parameters.
Bored piles can be heavily reinforced if required by design, rebar cages usually are prefabricated in segments with length and weight depending upon available commercial lengths of rebars and available lifting equipment. Splicing of rebar cages can be done by lap splice, welded lap splice or mechanical threaded couplers. Casting is done by pouring concrete with the design strength and slump as required, through watertight segmental Tremie Pipes, starting from the pile bottom and letting the tremie pipe bottom end remain at least 3 meters submerged in concrete until the completion of pouring, to guarantee the pile continuity and the final good quality of the concrete cast.
Drilling fluids, if needed, can be water, a suspension of bentonite (bentonite mud), a suspension of polymers, depending upon soil type, soil conditions, presence and elevation of water table, chemical properties of water table (Ph, Salinity).
Steel casings can be temporary, in which case the wall thickness is usually big enough toallow many uses, are provided with collars for easy handling by vibro hammers and diameter slightly larger than bored piles’ nominal diameter, to allow easy passage of drilling tools. Permanent casings, if needed, are sacrificial casings and as such the wall thickness is as small as allowed by the need to drive the casing through the ground.
Bored piles are commonly employed for bridge foundations, on land and water, because the versatility of bored piles design and execution allows the construction of practically any needed diameter, including the very large diameters, and the pile reinforcement can be provided as heavy as needed by seismic design and the codes (it is not uncommon to have double wall rebar cages in order to accommodate all the needed bars, however care shall be taken to leave sufficient space between bars for concrete to low through).
Bored piles are also used to form retaining walls (see PileWall.com), as contiguous pile wall or secant pile wall or aligned pile wall, with or without post tensioned soil anchors as tie back .
Bored piles testing is usually done in two (2) stages, first stage testing to verify design assumptions and achievable design load , is done before starting the execution of the working piles: test piles are installed in the proposed construction area, as per design, and tested, first for integrity and continuity by P.I.T. (Pile Integrity Test) then for load bearing capacity , either by Static Load Test or by Dynamic Load Test PDA, (Pile Dynamic Analysis). At times the Designer might require a Pull-out test and a lateral load test.
Once the design pile capacity has been confirmed, Bored Piles construction for the working piles starts and quality control is then done on representative piles. Quality Control consists of testing the material used for the bored piles, i.e. reinforcing bars and concrete, then testing of the piles at random with PIT and PDA, and predetermined piles with static pile load test. Predetermined piles can also be tested using the cross-hole ultrasonic test, by inserting instruments through vertical pipes installed within the reinforcing steel cage all throughout the bored pile length.

- Cold Weather Concreting

WHAT is Cold Weather?
HOW to Place Concrete in Cold Weather?
Figure 1 Effect of Temperature on Set Time (1a)
Cold weather is defined as a period when the average daily
temperature falls below 40°F [4°C] for more than three successive
days. These conditions warrant special precautions
when placing, finishing, curing and protecting concrete against
the effects of cold weather. Since weather conditions can
change rapidly in the winter months, good concrete practices
and proper planning are critical.

WHY Consider Cold Weather?

Successful cold-weather concreting requires an understanding
of the various factors that affect concrete properties.
In its plastic state, concrete will freeze if its temperature falls
below about 25°F [-4°C]. If plastic concrete freezes, its potential
strength can be reduced by more than 50% and its durability
will be adversely affected. Concrete should be protected
from freezing until it attains a minimum compressive
strength of 500 psi [3.5 MPa], which is about two days after
placement for most concrete maintained at 50°F [10°C].
Low concrete temperature has a major effect on the rate of
cement hydration, which results in slower setting and rate
of strength gain. A good rule of thumb is that a drop in concrete
temperature by 20°F [10°C] will approximately double
the setting time. The slower rate of setting and strength gain
should be accounted for when scheduling construction operations,
such as form removal.
Concrete in contact with water and exposed to cycles of
freezing and thawing, even if only during construction,
should be air-entrained. Newly placed concrete is saturated
with water and should be protected from cycles of freezing
and thawing until it has attained a compressive strength of
at least 3500 psi [24.0 MPa].
Cement hydration is a chemical reaction that generates heat.
Newly placed concrete should be adequately insulated to retain
this heat and thereby maintain favorable curing temperatures.
Large temperature differences between the surface and
the interior of the concrete mass should be prevented as cracking
may result when this difference exceeds about 35°F [20°C].
Insulation or protective measures should be gradually removed
to avoid thermal shock.

Cold Weather Concreting Guidelines
1. Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
2. Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior
to placement.
3. Place and maintain concrete at the recommended temperature.
4. Place concrete at the lowest practical slump.
5. Protect plastic concrete from freezing or drying.
6. Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
7. Limit rapid temperature changes when protective measures are removed.