The Contractor may elect to
use whatever materials or methods he considers necessary to accomplish this
item unless specific details are required by the plans. Many times when sheeting is installed into
streambeds, the streambeds consist of sand or gravel. Sand and gravel are pervious materials and
will allow water to flow through them.
If this condition exists, water can flow under the sheets and come up
through the bottom of the cofferdam.
This can loosen the soil in the bottom of the cofferdam and cause it to
be very soft and unstable. It can also
result in water coming up through any freshly-placed concrete. If this situation exists, the Contractor
should take measures to prevent the flow of water up through the bottom of the
cofferdam. These measures can consist of
driving the sheet piling deep enough to cut off the flow of water or placing a
concrete seal in the bottom of the cofferdam prior to pumping out the
water. If additional measures are
required, they are considered to be part of the cofferdams and excavation
bracing item and no additional compensation should be allowed for these items.
In order to qualify as
cofferdams and excavation bracing for a particular substructure unit, the
Contractor must perform work to protect and maintain the excavation at that
particular substructure unit. This work can include pumping out water,
installing cribs or sheeting, sloping the sides of the
excavation, or building an earthen cofferdam.
This item may include bedrock
and may require the removal of all materials necessary for the construction of
structures according to plan. It also
includes subsequent backfill and disposal of excavated material.
Sides of excavation should be
protected from caving. If side failure
occurs, the disturbed soil should be removed and replaced with properly
compacted soil. The sides must not be
laid back to the extent where the slope will endanger the stability of adjacent
foundations. The stability of the slopes
needs to be determined by a competent person on the Contractor’s staff.
When footings are not on
piling, any material undercut, disturbed below plan, or authorized elevation
must be replaced with concrete at the Contractor’s expense. If the excavation is allowed to remain
exposed for a considerable period of time, and the material becomes unsuitable,
it must be removed and replaced with concrete at the Contractor’s expense. The additional concrete may be placed with
the footing concrete; however, the footing reinforcing steel must be located at
the elevation indicated on the plans.
Many Contractors will place
gravel in the bottom of the excavation to assist in dewatering and to provide a
better work surface for the workers.
This is unacceptable as any over excavation of spread footing subgrades
must be replaced with concrete, not gravel.
When footings are supported
on piling, any material undercut or disturbed must be replaced with properly
compacted material. If the bottom of the
excavation becomes muddy, the Contractor may remove the muddy soil and replace
it with suitable granular material.
When the cofferdam and
excavation bracing item is not provided, drainage outside the forms and pumping
necessary to keep the surface suitable for placement of concrete are included
in the excavation item.
This item includes removal
and disposal of material that, in the opinion of the Engineer, is rock or
durable shale. Shale that is removed by
the same methods and comparative effort as soil should be classified as
nondurable shale.
Rock or durable shale may be
removed by whatever methods the Contractor chooses. These usually are blasting, jack hammering,
or ripping. Note the option to excavate
by blasting may be excluded due to the close proximity of existing
facilities. It is desirable to have rock
excavation below the tops of footings and as near to the sides of the footings
as practical.
To qualify for payment as
rock excavation, the Engineer must determine that the excavated material is
indeed rock or durable shale. In
addition, all of the rock excavation below the footing top must be filled with
concrete. Rock excavation performed
above the top of footing may be to any width; however, payment above, as well
as below the top of footing, is to the plan dimensions of the footing only.
In the event bedrock is
encountered over 1 foot (0.3 m) higher than indicated by the borings, or
bedrock is not encountered at plan elevation, report the findings to the
District Construction Administrator for consideration of a change in elevation
of the footing. A plan note will usually
be provided indicating when raising the footing can begin. When bedrock is not encountered at footing
elevation, an investigation of the soil should be made as deep as
practical. Hand augers or probes are
recommended for initial investigation.
Generally, when bedrock is
found less than 1 foot (0.3 m) lower than plan elevation, the additional height
of pier or abutment can be provided by additional footing concrete; however,
reinforcement should be placed at plan elevation.
When bedrock is found 1 foot
(0.3 m) or more below plan elevation, consideration should be given to
lengthening the pier or abutment above the footing.
Relative costs should be
investigated in either case, and if the cost difference is significant, should
be reported to the District Construction Administrator for review.
When the foundations for a
bridge are spread footings, they are designed to be supported on soil or
bedrock as indicated by the soil borings.
The Engineer must examine the soil or bedrock encountered at plan
elevation for agreement with soil boring data and to ensure that it will
provide the intended bearing capacity.
This bearing capacity will be listed in tons per square foot (tonnes per sq. meter) in the plan notes. Consult the District Geotechnical Engineer or
Office
of Geotechnical Engineering with any questions.
The District Geotechnical
Engineer or Office
of Geotechnical Engineering should be consulted whenever there is doubt
that the material encountered at plan elevation will provide the necessary
bearing capacity. Whenever the material
encountered is different, and of lesser quality than indicated by the borings,
an investigation similar to that described in the section titled, Elevation
Changes, should be made and the findings reported to the District Geotechnical
Engineer for review.
Footings placed on pile
foundations that were exposed to temperatures below freezing sometimes settle
during the setting of the concrete and result in unsatisfactory footings. Therefore, it is imperative that soil in such
cases be free from frost, and if disturbed by freezing, compacted to proper
density.
When excavation for footings
is performed, and freezing temperatures are expected during the time it is
exposed, insulation such as an adequate thickness of straw is recommended for
protection from frost.
When the excavated area has
become frozen, and the area is heated in an enclosure, the effect of the
supplied heat on the frozen soil is slight, and a thorough examination for
complete removal of frost is required.
Satisfactory temperatures found in spot checks of soil where frost may
have penetrated are an indication of frost removal.
When frozen soil is thawed
out, it requires re-compaction since frost heaving has lessened the
density. If reinforcing steel has been
placed in a footing area at the time the soil was frozen, it will be necessary
for the Contractor to remove the reinforcing steel prior to re-compacting
the soil.
The backfill material behind
the abutments and beneath the approach slabs shall conform to Item 203
Granular Material Type B. The use of this material should facilitate
compaction and help alleviate the settlement of the approach slab.
When the plans do not require
the original ground line to be altered by removal of the embankment, and when
structural excavation is performed prior to building an embankment, elevations
or measurements that establish the elevation of the original ground must be
made. Measurements made and recorded from
the Contractor’s footing grade stakes can be used to establish the elevation of
the original ground.
When the original ground is
altered by removal or construction of an embankment prior to excavation, use
the plan line of the excavation or embankment items for top boundary of
excavation.
The bottom elevation of the
footing is to be as shown in the plans.
This elevation is to be verified by subtracting the total verified
height of the substructure unit below the beam seat from the beam seat
elevation.
1. Verify existing ground elevations.
2. Verify that Contractor excavated to plan dimensions.
3. Make sure that the volume of the existing structure is
deducted from the pay quantity for unclassified excavation.
4. Compaction of backfill documented on CA-EW-5
Form.
Dispose of excavated material
not needed or unsuitable per 105.16 and 105.17.
Note: In recent years this item has been bid as a lump sum. Numbers one and two still apply.