Method of Measurement for Structural Excavations

Measurement of Structural excavation will be limited to excavation for foundation for bridges, retaining walls, box culverts or cross sectional area exceeding 1 sq. mtr, box culvert wing walls, head walls, toe walls and aprons, manholes, pump houses and sub-stations.

For box culverts 1 sq. mtr or less, pipe culverts, storm sewers and ducts, measurement of structural excavation in rock will be made. The cross sectional area of the box culverts will be that of the net opening of the culvert. For pipe culverts and box culverts of net opening are of 1 sq. mtr or less, no measurement will be made for excavation of abutments, wing walls, headwalls, toe walls and aprons etc, unless the excavation is declared ‘in rock’.

The pay volume of structural excavations will be that within a plane prism bounded by a lower horizontal plane, vertical planes and an upper horizontal plane. Unless otherwise shown on the Drawings, the limiting planes will be defined as follows:

• Bridge Foundations: The lower plane will be the underside of the blinding concrete and the vertical planes are planes 500 mm outside the foundation dimensions in plan. The upper plane will be the existing mean ground level over the plan area of the foundation at the time when fill possession of the site is given.
• Box Culverts: The lower plane will be the underside of the blinding concrete and the vertical planes are planes 500 mm outside the floor slab dimensions in plan, including apron slabs and toe walls. The upper plane will be the mean of the existing ground level over the plane area of the culvert floor slab, including apron slabs and toe walls. Where there is a significant change in ground level over the culvert length, the volume of excavation will be calculated by dividing the culvert into several smaller lengths. Where the culvert is located in an embankment, the upper plane will be as described above irrespective of whether or not the embankment has been constructed in advance of the culvert. Excavations for toe walls below the general level of the floor slab will be measured as the actual original volume of material excavated.
• Miscellaneous Structures: Excavation measurement for retaining walls, wing walls, head walls, toe walls and other miscellaneous minor structures will be according to the methods described for box culverts.
• Box Culverts within Existing Road: Where structural excavation is carried out for installing a new box culvert within existing road, excavation below existing bituminous layers will be classified as structural excavation for the length of that section of culvert which lie within existing road / embankment. The vertical plane will be the minimum excavation limits as shown on the drawings or 30 cm from the outer face of the culvert. Removal of existing bituminous layers will be paid under relevant pay items appearing in the BOQ.
• Where structural excavation is out for replacing an existing box culvert (with net opening area exceeding 1.0 m2) at the same location, only that volume of soil which lie between the outer face of the existing culvert wall and the limiting vertical planes as described under section 206.33 in General Specifications for Roads will be measured for payment.
• Excavation for Irish Crossing cut off walls will be measured net, without any additional space along both faces of the cutoff wall. Any excavation below natural ground within Irish crossing walls for road construction as shown on relevant drawings will be classified as roadway excavation and measured under relevant pay items of Clause 203 in General Specifications for Roads.

When ground or tidal water is encountered during structural excavation, only the original volume of material excavated below water level shall be measured by cubic meter for payment under ‘Extra Over for Structural Excavation Under Water’

Excavation, bedding, backfilling for pipe culverts and ducts and box culverts of cross sectional area 1 m2 or less will not be paid directly but will be considered as subsidiary work to other related items in the Bill of Quantities except for excavation in rock.

The amount of completed and accepted work, measured as provided for above will be paid for at the unit rate per cubic meter for Structural Excavation in Soils and Structural Excavation in Rock in the BOQ which rate will be full compensation for execution, clearing, compaction of bottom of trench, side support, removal of any portion of existing structures below ground necessitated by construction of the new structure, trimming, cleaning, backfilling, compaction, disposal of surplus materials and supplying equipment, tools, labor, and other items necessary for proper completion of the works.

The unit rate per cubic meter in the BOQ for ‘Extra Over for Structural Excavation Under Water’ will be full compensation for sheet piling, pumping, dewatering, materials, equipment, labor and other items necessary for the proper execution of the works related only to the presence of water in the excavation.

In a mixed soil and rock strata excavation, when the soil layers are less than 50% of the total excavated thickness, the entire volume excavated will be paid for as ‘Structural Excavation in Rock’. Where the percentage of the strata classified as ‘Structural Excavation in Soil’ is greater than 50 % of the total thickness below the stripped overburden, the soil and rock layers will be measured separately. Overburden will be paid for as ‘Structural Excavation in Soil.’