11.10   EARTHWORK FOR STRUCTURES

11.11   EXCAVATION

The Excavation Classification Line (ECL) is the contractual dividing line between Class 20 and Class 21 Excavation.  The ECL does not apply to projects which do not include a bid item for Class 21 Excavation.  The ECL is a fixed value defined in the contract documents, calculated as the higher of (a) the average low water elevation and (b) the average stream bed elevation plus one foot, as recorded at the time of design.  It is understood that water elevations and stream bed elevations may be subject to change between the time of design and the time of construction, but normal fluctuations of water or streambed elevation shall not be taken as a basis for adjusting excavation pay quantities. Class 21 Excavation is measured and paid by plan quantity, unless otherwise modified by Specification 2402.05.

11.12   BACKFILLING STRUCTURES 

The contract requirements for backfill of structures are provided in Specification 2402.03.  Suitable excavated materials may be reused as backfill unless the contract specifies otherwise.  When granular backfill materials are specified, backfill material shall meet requirements of Specification 4133.  It is important to note that special requirements apply for granular backfill used under flowable mortar or flooding applications, in accordance with Specification 4133 and Specification 4134, respectively.  More restrictive gradation requirements are enforced in these applications to ensure the backfill material is drainable.

Culverts

When backfilling culverts, the backfill material used must have sufficient moisture to permit required compaction.  Loose layers must not exceed 8 inches.  Backfilling of culverts must be done simultaneously on both sides.  This is to ensure that loading from the backfilling materials does not displace the culvert due to unequal side pressures.

Backfill materials are required to be thoroughly compacted in accordance with Specification 2402.03, H.  Rollers may be used for compaction of most backfill, but mechanical tampers must be used in the areas immediately adjacent to the culvert which the roller cannot reach.  Use of mechanical or hand tampers are also required for backfill placed within 3 feet of culvert wingwalls which exceed 8 feet height, to minimize potential damage to the wingwall.  Thorough compaction, especially immediately adjacent to the culvert, is critical to prevent future settlement of the backfill material.  Poor compaction can result in settlement of the roadway embankment and pavement.

When the underlying soils below a culvert are known to be unsuitable or prone to unacceptable settlement, the contract documents will require a specific thickness of the underlying soils to be removed and replaced with a granular blanket.

The quantities for culvert excavation, and granular blanket when specified, shall be measured and paid based on the pay limits identified in the contract.  In the course of construction, the contractor may excavate material below or outside of the excavation pay limits, but the contractor will not be reimbursed for excavation or backfill outside of the contract established pay limits.  If more material is excavated than required, it must be replaced with acceptable material and restored to contract line and grade at the contractor's expense.  In the event a design revision to the dimensions or location of the culvert is needed, excavation pay quantities shall be adjusted in accordance with Specification 2402.02, B.

Bridge Abutments

For bridge abutments, all loose material must be removed prior to backfilling.  After removal of loose material, the bottom of the excavation is to be lined with a low-permeability geotextile fabric and a subdrain is to be installed in accordance with the abutment backfill details included in the design plans.  The excavation behind the abutment, following installation of geotextile fabric and subdrain, must be backfilled using approved granular materials which comply with specific gradation requirements.  In most cases, the contract documents require abutments to be backfilled with a layer of porous backfill meeting the requirements of Specification 4131, followed by a layer of floodable backfill meeting the requirements of Specification 4134.  The floodable backfill materials are to be surface flooded and compacted with vibratory equipment in accordance with plan requirements.  The purpose of this backfill system is to provide a well compacted and freely draining backfill zone behind the abutment to minimize settlement and material loss due to erosion.

The quantity of excavation for bridge abutments is paid per cubic yard of material excavated within the pay limits defined in the contract.  Material excavated beyond the contract pay limits will not be paid.  In most cases, payment for excavation at bridge abutments is for the quantity of material removed and does not include the quantity of backfill material placed.  The contract generally specifies the price of abutment backfill materials to be incidental to other contract bid items.

Measurement and Payment

When measurement and payment of backfill material is on a contract quantity basis, no weight tickets or other measurements are necessary.  A note in the field book is required indicating that approved material was properly placed as required by the specifications and plans.