(A) (1) (a) The excavation for sewers shall be made by open trenching, except where tunneling is necessary. Existing pavement shall be saw cut and replaced in accordance with §§ 33.205 through 33.208. The walls of the sewer trench shall be vertical in the region between the bottom of the trench and the top of the sewer pipe.
(b) In this region:
1. At least six inches clearance on each side shall be maintained for sewers eight inches and smaller;
2. Nine inches clearance on each side shall be maintained for sewers less than 18 inches; or
3. As approved by the Engineer for sewers 18 inches and larger.
(2) A minimum of six-inch and not more than 12-inch clearance shall be maintained between the outer wall of the pipe barrel and the embankment or shoring unless otherwise approved by the Engineer.
(3) The trench shall be excavated to a level section and to such elevation as will give a uniform bearing and true flow line elevation when the sewer pipe is laid. All loose dirt in the bottom of the trench must be removed.
(B) (1) Mud or other soft or spongy material.
(a) Where mud or other soft or spongy material incapable of proper pipe support is encountered, it shall be excavated to a minimum depth of 12 inches below sewer subgrade. Limits of the material to be removed shall be designated by the Engineer in the field. However, this does not relieve the contractor of the requirements of these specifications, including, but not limited to, allowable deviation of alignment and grade.
(b) When so directed by the Engineer, the trench shall be extended below the pipeline grades to permit the placement of ballast for the pipe foundation. All areas of over excavation, to remove unsuitable material or for any other reason, shall be brought to grade with approved ballast material and compacted.
(2) Rock. Where rock is encountered, the trench shall be excavated to a minimum depth of three inches below the sewer subgrade and backfilled to sewer subgrade with approved import material, thoroughly compacted to grade before the sewer pipe is laid.
(3) Over-excavated areas. Over-excavated areas in the trench bottom where approved trench foundation material will be placed shall be restored to sewer subgrade with imported backfill materials, thoroughly compacted before the sewer pipe is laid.
(4) Compaction. In all the above situations, compaction shall be achieved by mechanical means. Water settling, flooding, jetting and other water consolidation methods are expressly prohibited.
(5) Bracing and shoring. The contractor shall, at all times, furnish, install and maintain sufficient bracing and shoring in trenches to ensure safety of the workers and to protect and facilitate the work. When practical, all such bracing and shoring shall be removed from the trench as the backfilling proceeds. Whenever an on-site designee of the District warrants sufficient cause to suspect that an unsafe condition exists or that existing shoring is inadequate, the District reserves the right to direct all workers involved to terminate work until the condition is remedied. Any direction given to a contractor or the employees of such for unsafe conditions shall be at the sole expense of the contractor. The District will not reimburse expenses incurred if a District employee issues a stop work command for unsafe conditions.
(6) Blasting. In the event that blasting is necessary in excavation, special permission, in writing, must be obtained from the agency having jurisdiction over the issuance of blasting permits before any blasting will be permitted. Such permission shall in no way relieve the contractor of the responsibility for obtaining any permits or licenses required by state law or local ordinance.
(7) Sewer subgrade. The sewer subgrade shall be kept dry at all times and precautions shall be taken that no storm water is allowed to enter the excavation prior to backfilling. The contractor shall, at all times, have on the job, sufficient pumping machinery for immediate use. Water shall be disposed of in accordance with the requirements of the agency having local jurisdiction, and in such a manner as to cause no damage to public health or safety, or to public or private property. The contractor shall control surface runoff to prevent entry or collection of water in excavations.
(8) Excavation material. Material excavated within streets and roadways may only be reused per approval of the city or county. Any such material shall be laid alongside the trench and kept trimmed up to minimize inconvenience to public travel.
(a) Free access must be provided to all fire hydrants, water gates, meters and private drives, and means shall be provided so that water can flow in the gutters uninterruptedly.
(b) All materials excavated in streets and roadways not permitted for backfill shall be immediately removed and properly disposed of at an approved facility by the contractor. No surplus material shall be dumped on private property.
(9) Open trench permitted.
(a) The Engineer will specify the amount of open trench permitted at any one time. In general, the length of trench open at any time shall be limited to 100 feet in business area; 250 feet or one block (whichever is less) in residential area; and 1,000 feet in undeveloped areas.
(b) No trench shall be allowed to remain open outside of work hours and/or outside of immediate work area. At the end of a work day, the trench shall be backfilled or secured with steel plates.
(10) Contractor’s responsibility. It shall be the responsibility of the contractor to conform to all the requirements of all permits obtained from all agencies, and to make the construction site safe against injury to people and/or livestock by erection of adequate posted barricades and/or temporary fences.
(11) Groundwater. The contractor shall keep excavations free from water during construction. Groundwater shall be controlled to prevent sloughing or erosion of trench walls, softening of the bottom of excavations or formation of “quick” conditions where effective stress is reduced due to upward movement of flow of water. The static water level shall be drawn down to a sufficient depth below the bottom of excavations to maintain the undisturbed state of natural soils and allow the placement of any fill to the specified density. Dewatering systems shall not remove natural soils.
(a) Dewatering and shoring plans should be approved by a licensed geotechnical engineer, as necessary, to verify subsurface soil and groundwater conditions and adequate handling of field conditions to facilitate construction.
(b) The contractor shall obtain any and all permits required in conjunction with the installation and removal of the dewatering system and shall meet all permit requirements.
(c) The contractor shall contact the local drainage authority for discharge requirements.
(d) The contractor shall dispose of water from dewatering operations so as not to cause injury or damage to adjacent property and shall, at all times, remain in compliance with the requirements of the local drainage authority.
(Ord. 2019-02, passed 4-25-2019)