detail estimating excavation and backfill

DETAIL ESTIMATING EXCAVATION AND BACKFILL FOR STRUCTURES

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902 - EXCAVATION AND BACKFILL FOR STRUCTURES - OPSS 902 902.1 GENERAL 902.1.1 Excavation for Structures Excavation for structures deals with excavation in earth or rock for the placement of the

following structures: - Bridges - Working slabs/pads - footings for piers and abutments - wingwalls - Concrete Culverts - footings - aprons - wingwalls - Retaining Walls - footings - wingwalls with additional excavations for: - Bedding - box culverts - working slabs - Frost Tapers - bridges - concrete culverts - retaining walls 902.1.2 Backfill to Structures This is not a separate tender item. Granular backfill is included as part of the general

granular item. Concrete structures may be backfilled with rock. In certain circumstances, when granular

material is not readily available or in order to reduce high granular costs, rock available for fill may be used as backfill material. Before opting for rock backfill, a cost

comparison must be completed to determine the more economical alternative. When rock is used as structure backfill, a standard Special Provision must be included

stating the gradation of the backfill material, together with MTO Standard Drawings (Structural). The rock backfill is paid for as part of the rock

excavation from which it is obtained. 902.1.3 Tender Items The tender items associated with excavation and backfill for structures are: Earth Excavation for Structure


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Rock Excavation for Structure Unwatering Structure Excavation Clay Seal Related Tender Items Other tender items usually required with the excavation and backfilling of structures are: Granular B Type I Granular B Type II Roadway Protection Track Protection 902.1.4 Specifications The work of excavation for structures in both earth and rock, and the placing of granular

backfill, are detailed in OPSS 902. 902.1.5 Special Provisions The designer should investigate to determine whether any Standard Special Provisions

are required to be included in the contract. 902.1.6 Standard Drawings To establish the physical limits upon which to base quantities, the designer must be

familiar with: 1) Ontario Provincial Standard Drawings - 800 Series 2) Ontario Provincial Standards Drawaing - 3000, 4000 Series. 902.2 COMPUTATION Earth Excavation for Structure and Rock Excavation for Structure are Plan Quantity

Payment items. Unwatering Structure Excavation and Clay Seal are not Plan Quantity Payment items. 902.2.1 Source of Information The main sources of information are the Foundation Design Section, the Structural

Office/Section and the Regional Geotechnical Section. 902.2.2 Method of Calculation


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Earth and Rock Excavation The total excavation for structure includes the excavations required to place the concrete

structure, beddings and frost tapers. The price bid for the appropriate Structure Excavation tender item also includes the

removal of overlying asphaltic or concrete pavements, curb and gutters and sidewalks. Where however there are separate tender items for removal of pavements, curb and gutter or sidewalk which overlaps the removal of pavement, curb and gutter or sidewalk over the structure excavation, then payment for the removal of these surface features will be made under the appropriate tender items, for removal of pavement, curb and gutter and sidewalk.

The excavation required to place a concrete structure is determined by calculating the

volume in cubic metres normally based on: 1. - an upper limit of: a) original ground, or b) the designated payment surface (see 902.2.3) 2. - a lower limit of: a) bottom of footings b) bottom of bedding c) bottom of working slab/pad d) other special application 3. - the plan area of the footings In excavating for standard and rigid frame open-footing concrete culverts, the material

between the walls/footings and above the stream bed elevation shall be part of the structure excavation (see Figure 902-1).

Excavation in rock is the same as that for earth, except that the upper limit is the rock line

rather than the original ground line. Should a culvert occur in a rock cut location, the upper limit of rock is the top of shatter.

Backfill - Granular/Rock Granular backfill quantities are computed according to the dimensions shown on the OPS

Drawings or the Structure Drawings. The upper limit is the bottom of granular sub-base (except for retaining walls in cuts), and includes backfill for frost tapers, where required.

Rock backfill quantities for bridge abutments are computed according to OPSD 3505

depending on whether the structure or roadbed is constructed first. This will be determined in consultation with the Structural Section.

Clay Seal No volume calculations for clay seal are needed. If excavation is required to place the

clay seal, the volume is computed and shown only as "material available for fill". 902.2.3 Designated Payment Surface


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If using the original ground elevation results in excessive overlap between earth excavation for structure and earth excavation for grading, the designer, in consultation with the Structural Office/Section, may choose a designated payment surface at a lower elevation, with the purpose of reducing the overlap. When the original ground line is well above the subgrade line (and, in fact, above the top of proposed pavement), then a Designated Payment Surface could be struck at the subgrade elevation. This surface would be horizontal both longitudinally and transversely, (see Figures B902-2 and B902-3).

The following considerations affect the application of the Designated Payment Surface

(DPS): a) Overlapping excavations are to be tolerated unless the Highway Designer

decides that the volume obtained is unrealistic, to the extent that contract balancing would be based on erroneous quantities;

b) The DPS is to be used only in exceptional situations. 902.3 DOCUMENTATION 902.3.1 Contract Drawings Concrete Culverts New concrete culverts and existing concrete culverts requiring extension are shown on

the plans and profiles of the contract drawings. Where the design shows a need for roadway or track protection, a protection scheme will

be provided by the Regional Structural Section, and shown in full detail on the Structural Drawings. A separate tender item must be provided to cover the protection work.

Designated Payment Surface For each structure that utilizes a designated payment surface, a detailed sketch with

supporting elevation(s) must be shown on the highway grading drawings, and not on structural drawings.

Use of Specific Materials When the design depends on the use of a specific material, that material must be

identified on the drawings even when it is to be provided under the Earth Excavation (Grading) or Earth Borrow item.

Clay Seal A detailed dimensioned sketch for each location must be shown on the contract drawings.


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902.3.2 Contract Documents Excavation for Structures Excavation quantities for each structure, computed in cubic metres to the nearest whole

number, must be checked 100%.


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The quantity for each bridge, concrete culvert and retaining wall is shown separately on "Quantities - Miscellaneous 1 or 2 "or "Quantities - Structures" sheet, and the columns (Earth, Rock) totalled to form the tender quantities.

Excavated Material should be made available for full reductions by showing the quantity

on the quantities -grading sheet. Unwatering Structure Excavation Where unwatering operations are required, the appropriate column should show "100%"

against the structure entry on the quantity sheet. Granular Backfill The granular quantity for each bridge, concrete culvert and retaining wall is listed

separately on the "Quantities - 1" or "Quantities - 2" sheet. The column subtotal is transferred to the main granular item on the "Quantites - Hot Mix and Granulars" sheet and included in the tender quantity.

Clay Seal Clay seal is documented on the same quantity sheet where concrete culverts are

documented. It is identified as a separate tender item without quantity entries. Note that clay seal information for pipe culvert locations are kept separately from those for concrete culverts, and generate a separate lump sum tender item under OPSS 421.