These specifications for excavation and backfill are minimum requirements and will cover most work in general. Special requirements for unusual conditions shall be agreed upon at the time of submitting detail plans and specifications.
(a) Trench Excavation. The trench excavation for both sewer and water pipe bedding shall be true to line and grade, as shown on the plans. The banks of the trench shall be kept as nearly vertical as possible and shall be properly drained.
The bottom of the trenches shall not be more than twenty-four inches wider than the outside diameter of the pipe with a minimum clearance between the bottom of the pipe and the trench bottom of one-fourth of the pipe diameter, but not less than six inches, in order that the pipe may be bedded in No. 6 stone or slag as described in backfill subsections (g), (h) or (i) hereof. Shale, ledge rock, boulders and large stones shall be removed from the trench to give a minimum clearance from pipe or fittings of six inches.
(b) Unstable Ground. Wherever wet, unstable or other undesirable soil is encountered which does not provide proper bearing for piping, such material shall be excavated to good ground and the excavation refilled with suitable material to the required subgrade, compacting the fill in six-inch layers. If it would be more economical and desirable, soft ground may be crossed by supporting the pipe on concrete cradles, poured to cover the lower quadrant of the pipe. In no case shall any pipe or appurtenances be laid in or on topsoil.
(c) Unauthorized Excavation. Wherever excavation in good ground has been carried below the established grade for installation of the piping, such excavated space shall be refilled with suitable material, as approved by the City Engineer, to the required subgrade, compacting the fill in six-inch layers.
(d) Sheeting, Shoring and Bracing. All excavation shall be adequately protected from caving. If such caving occurs and disturbs the bedding, the grade or the alignment of the piping, the work shall be removed, suitable sheeting, shoring and bracing provided, and the work reinstalled. The sheeting and bracing, in general, shall be removed as backfilling progresses and in such a manner as to avoid caving of the trench. Voids left by the withdrawal of sheeting shall be carefully filled and tamped.
(e) Excavation Under Railroad Tracks. Excavation under railroad tracks shall be determined separately for each individual crossing and shall be shown on the plans and described in the specifications.
(f) Tunnel Excavations. Where the excavation crosses main highways or railroad tracks, tunneling may be permitted where depth of trench and soil conditions are adequate. All tunnel backfill shall be of proper moisture content to compact readily and shall be thoroughly tamped and rammed under, around and over the pipe from the floor of the tunnel to the sidewalls and top thereof. Where tunneling is to be used, the method and details shall be shown in the plans and specifications.
(g) Backfill for Water Mains. After the piping has been laid to line and grade, trenches shall be backfilled with No. 6 limestone, carefully deposited under and on both sides of the pipe, thoroughly and carefully rammed by hand tamping methods until such fill has been brought to the center line of the pipe. Then, fine, loose earth shall be placed and thoroughly and carefully rammed by hand tamping methods until such fill has been brought to six inches above the pipe. This portion of the backfill shall be carefully placed and thoroughly compacted until a firm and continuous support on the bottom and sides is secured. Where the trench is excavated through cinder fill or other material which, in the opinion of the Inspector, is not suitable for backfill, suitable material for backfilling shall be hauled to the site. The remainder of the trench may then be backfilled by hand or mechanical equipment. Water settling may be permitted if it is possible to do so without floating the pipe.
(h) Backfill for Sanitary Sewers. After sewer piping has been approved for line and grade, the trenches shall be backfilled with No. 6 limestone or slag, carefully deposited under and around the pipe and the backfill shall be thoroughly and carefully rammed by hand tamping methods until enough limestone or slag fill has been placed to provide a cover of at least six inches above the top of the pipe. This backfill shall provide a firm and continuous support on the bottom and sides of the pipe. The remainder of the backfill shall consist of material previously excavated and may be placed by hand or mechanical equipment. Water settling will be permitted, providing floating of the pipe does not occur.
(i) Backfill for Storm Sewers. After sewer piping has been approved for line and grade, trenches shall be backfilled with No. 6 limestone or slag, carefully deposited under and around the pipe and the backfill shall be thoroughly and carefully rammed by hand tamping methods until enough limestone or slag fill has been placed to extend up around the pipe, a distance from the bottom of the pipe shown in the minimum bedding depth table as follows:
Minimum Bedding Depth for Pipe | |
Internal Diameter (Inches) | Bedding Depth (Inches) |
12 | 4 |
15 | 4 |
18 | 5 |
21 | 6 |
24 | 7 |
27 | 8 |
30 | 9 |
33 | 10 |
36 | 10 |
39 | 11 |
42 | 11 |
48 | 12 |
54 | 13 |
60 | 13 |
(j) Backfill in Paved Areas. Wherever trenches occur in existing roadways and paved areas, the backfill shall consist of No. 6 stone. It shall be thoroughly compacted in six-inch layers or puddled with a hose and long nozzles, after the backfill is in place. When the area is to be paved with asphalt, the top eight inches of the backfill shall consist of crushed slag or limestone to match the road base and shall then be paved to match the existing road or paved area. For all concrete pavement, the concrete must project at least one foot beyond the trench sides.
Wherever trenches occur in future paved areas, the backfill shall be as per the foregoing paragraph or in accordance with subsections (g), (h) or (i), hereof, if left unpaved for a period of at least six months after completion of the backfill operation.
(Ord. 69-35. Passed 10-27-69.)