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Item No. 401

Structural Excavation and Backfill

401.1 Description

This item shall consist of the excavation for the placing of structures, except pipe sewers, for the disposal of such excavated material and for the backfilling around completed structures to the level of the original ground or grade indicated.  The work shall include all necessary pumping or bailing, sheathing, drainage, and the construction and removal of any required cofferdams.  Unless otherwise indicated, the work included hereunder shall provide for the removal of old structures or portions thereof (abutments, buildings, foundations, wingwalls, piers, etc.), trees and all other obstructions necessary to the proposed construction. Where excavation is not classified, it will be grouped under “Unclassified Structural Excavation”, which shall include the removal of all materials encountered regardless of their nature or the manner in which they are removed. Where excavation is classified, it shall be classed as “Common Structural Excavation” or “Rock Structural Excavation” in accordance with the following criteria: “Common Structural Excavation” shall include the removal of all materials other than rock. “Rock Structural Excavation” shall include the removal of firm and compact materials that cannot be excavated with power equipment, without first being loosened or broken by blasting, sledging or drilling.

401.2  Materials

(1) Sand 

(a)   Fine aggregate sand shall be Grade 1 conforming to Item No. 302, “Aggregates for Surface Treatments”. 

(b)   Native Sand shall be local material obtained from approved sources and subject to the approval of the Engineer. 

(2)  Flexible Base 

Flexible base shall conform to Item No. 210, “Flexible Base”. 

(3)  Lime Stabilized Base 

Lime stabilized base shall conform to, “Hydrated Lime and Lime Slurry” and, “Lime Treatment for Materials in Place”.

(4)  Concrete Base 

Concrete base shall conform to Class J Concrete Item No. 403, “Concrete for Structures”. 

(5)  Granular Material 

(a)  Coarse aggregate shall conform to Item No. 403 “Concrete for Structures”. 

(b)  Foundation Rock Foundation rock shall be well graded, hard, durable coarse aggregate ranging in size from 2 to 6 inches. 

(c)  Pea Gravel Pea gravel shall consist of hard, durable, opaque gravel, free of clay, loam, sand or other foreign substances, ranging in size from 1/4 inch to 3/8 inch conforming to ASTM C 33.

(6)  Cement Stabilized Rockfill 

Cement Stabilized base shall conform to Class J Concrete, Item No. 403, “Concrete for Structures”.

401.3  Construction Methods

(1) Excavation shall be done in accordance with the lines and depths indicated or as established by the Engineer.  Unless otherwise indicated or permitted by the Engineer no excavation shall be made outside a vertical plane 3 feet from the footing lines and parallel thereto. 

(2)  Excavation shall conform to elevations indicated or raised or lowered by written order of the Engineer, when such alterations are judged proper.  When deemed necessary to increase or decrease the plan depth of footings, the alterations in the details of the structure shall be as directed by the Engineer.  The Engineer shall have the right to substitute revised details resulting from consideration of changes in the design conditions. 

(3)  When a structure is to rest on an excavated surface other than rock, special care shall be taken not to disturb the bottom of the excavation and the final excavation to grade shall not be performed until just before the footing is placed.  Equipment selected and used by the Contractor for excavation which disturbs what was otherwise stable subgrade material, as shown by laboratory tests, will not be used as a justification for payment for excavating to extra depth or for payment for stabilizing materials which may be ordered by the Engineer. 

(4)  Excavated material required to be used for backfill may be deposited by the Contractor in storage piles as indicated or at points convenient for its rehandling during the backfilling operations, subject to the approval of the Engineer, who may require that the survey center line of the structure and the transverse or hub line of any unit of the structure be kept free of any obstruction.  The Contractor shall adjust any stockpiles, to facilitate surveying and the work of other Contractors working in the immediate proximity, as directed by the Engineer. 

(5)  Excavated material required to be wasted shall be disposed of as directed by the Engineer, in a manner which will not obstruct the stream or otherwise impair the efficiency or appearance of the structure or other part of the work.

401.4 CofferdamsThe term cofferdams, whenever used in this specification, designates any temporary or removable structure constructed to hold the surrounding earth, water or both, out of the excavation, whether the structure is formed of earth, timber, steel, concrete or a combination of these.  It includes earthen dikes, timber cribs, any type of sheet piling, removable steel shells and the like and all necessary bracing and it shall be understood also to include the use of pumping wells or well points for the same purpose.  The cost of cofferdams, when required, shall be included as a part of the bid price for excavation. It is the intent of this specification to require that a suitable cofferdam which will provide a safe work area be provided for all excavation when necessary in order to control water so that the foundation may be placed in a dry condition, as to preclude sliding and caving of the walls of the excavation.  Where no ground or surface water is encountered, the cofferdam need be sufficient only to protect the workmen and to avoid cave-ins or slides beyond the excavation limits. The type, strength and clearance of cofferdams, insofar as such details affect the character of the finished work and the safety of laborers and inspectors working therein, will be subject to review by the Engineer, but the Engineer’s review shall in no way relieve the Contractor of responsibility for the adequacy and safety of the cofferdam design.  Other details or design will be left to the choice of the Contractor, who will be responsible for the successful completion of the work.  Approval of the drawings by the Engineer will not relieve the Contractor of responsibility in any manner.  The interior dimensions of cofferdams shall provide sufficient clearance for the construction and removal of any required forms and the inspection of their exteriors and to permit pumping outside of the forms. Unless otherwise indicated, cofferdams shall be removed by the Contractor after the completion of the substructure without disturbing or marring the structure.

401.5 Pumping or BailingThe manner of pumping or bailing from the interior of any foundation enclosure shall preclude the possibility of the movement of water through or alongside any concrete being placed.  No pumping or bailing will be permitted during the placing of concrete or for a period of at least 24 hours thereafter, unless from a suitable sump separated from the concrete work by a water-tight wall.

401.6 Backfilling

(1)  General 

As soon as practicable, all portions of excavation not occupied by the permanent structure shall be backfilled.  Back-fill material shall be free from large or frozen lumps, wood or other extraneous material. That portion of backfill which will not support any portion of completed roadbed or embankment shall be placed in layers not more than 10 inches in depth (loose measurement) and shall be compacted to a minimum of 95 percent of maximum density as determined by SDHPT Test Method Tex-114-E and the re-excavated to the proper grade and dimensions. If the excavation has been made through a hard material resistant to erosion, the backfill around piers and in front of abutments and wings may be ordered by the Engineer to be of stone or lean concrete.  Unless otherwise indicated, such backfill shall be paid for as extra work. No backfill shall be placed against any abutment or retaining wall until such structure has been in place at least 7 days.  No backfill shall be placed adjacent to or over single and multiple boxes until the top slab has attained 500 psi flexural strength.  Backfill placed around abutments and piers shall be deposited on both sides to approximately the same elevation at the same time. Care shall be taken to prevent any wedging action of backfill against the structure and the slopes bounding the excavation shall be stepped or serrated to prevent such action. 

(2)  Cement Stabilized Backfill 

When indicated, trenches shall be backfilled to the elevations shown with Cement Stabilized Backfill. Special mixing equipment will not be required to transport the cement stabilized backfill unless otherwise indicated. Hand-operated mechanical tampers may be used with approval of the Engineer for compacting this backfill. 

401.7 Measurement and Payment

No direct measurement or payment will be made for the work to be done or the equipment to be furnished under this item, but shall be considered subsidiary to the particular items required by the bid.