Foundation Specifications 0207

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Cofferdams and Seal Courses

CALTRANS FOUNDATION MANUAL

Section 19-3.04 of the Standard Specifications states, “…that excavations shall be

completed to bottom of footings before driving piles”. As in many other areas of our work,

there are times when engineering judgement should be used depending on type of soil,

amount of excavation required, type of pile, and depth below water surface. Normally,

excavation would be by submerged clamshell, with the elevations being checked by sound-

ing. In the case of pile foundations, it is often advisable to over-excavate a predetermined

amount to compensate for heave of the material caused by pile driving displacement. This

is done to eliminate the need for excavation after driving. If excavation is needed, care is

required not to damage any of the driven piles.

Following the installation of the cofferdam, the footing can be excavated and piles driven.

Usually the footing area must also be dewatered. Depending on the volume of water present,

this can be achieved by pumping. Otherwise, a seal course may be necessary. If a seal course

is not shown on the contract plans and the Contractor elects to use one to control and

remove water from the excavation, the work shall be done in accordance with the provi-

sions of Section 19-3.04 of the Standard Specifications.

As the name implies, a properly constructed seal course seals the entire bottom of a coffer-

dam and prevents subsurface water from entering the cofferdam. In so doing, it permits

construction of footings and columns or other facilities in the dry. The seal course is a

concrete slab placed underwater and constructed thick enough so that it’s weight is

sufficient to resist uplift from hydrostatic forces. In terms of it’s importance to the designed

structure, the seal course normally has no structural significance.

Information pertaining to the seal course for a project may be found in the contract plans.

Additional information may be found in the RE Pending File. The decision as to thickness

of seal course required, or whether the seal course is to be eliminated, rests with the Struc-

ture Representative. This decision is based on conditions encountered on the jobsite. The

contract plans will also contain provisions for adjusting footing elevations if seal courses

are eliminated. In usual field practice, this decision is not a difficult one to make. In most

cases when water is not present the need for a seal course is clearly not there. Additional

information about seal courses can be found in Bridge Construction Memo 130-4.0.