Quality Assurance Handbook for Rural Roads Volume-I - pmgsy

1200: RCC SLAB CULVERTS AND MINOR BRIDGES
General
169
Quality Assurance Handbook for Rural Roads
A culvert is a cross drainage structure having a total length of 6 m or less between the inner faces of dirt
walls or extreme ventway boundaries measured at right angles. A minor bridge is a bridge having a
total length upto 60 m. A small bridge is a bridge where the overall length of bridge between the inner
faces of dirt walls is upto 30 m and where individual span is not more than 10 m. In rural roads, individual
span lengths upto 15 m are used exceptionally.
In this section different items of RCC solid slabs, box culverts, and composite bridges have been dealt
with. The cross sections of foundations, substructures such as abutments, return walls besides design of
superstructure, drainage spouts, railings, parapets etc. and all other details shall be strictly in accordance
with contract documents.
For minor bridges having overall length of 30 m and above, a method statement for construction indicating
items (a) to (j) as per clause 1201 (xi) of MoRD Specifications for Rural Roads, shall be submitted.
A. Methodology
A1 Foundations
1. Take the minimum depth of foundation upto the stratum having specified bearing capacity shown
on drawing but not less than 2 m below the scour level where no bed protection is provided or 1.5
m below the protected bed level.
2. In case of rocky bed, ensure embedment of foundation into the rock below, the minimum depth
being 500 mm for hard rocks and 1200 mm for soft erodible rocks.
3. Provide 300 mm thick plain concrete M15 grade footing, unless otherwise specified on the drawings.
Provide a minimum offset of 150 mm for the base of substructure.
4. Set out plan dimension of the foundation at the bottom of foundation trench and check with respect
to original reference line and axis.
5. Before laying foundation concrete, clean the earth surface of all loose material and sprinkle water
to wet. Provide side formwork as per required dimensions and height.
6. Lay foundation concrete continuously to the required thickness upto the level of construction
joint proposed.
7. Finish the concrete surface smooth with a trowel and ensure curing as specified in Section 800.
8. Carryout dewatering where necessary for laying of concrete so as to keep the water level below
foundation level with adequate provisions/ precautions.
9. Remove loose sand laid on foundation before commencement of back filling. Refill all spaces
excavated and not occupied by permanent work with earth upto surface of surrounding ground.
In case of excavation in rock, fill the circular space with M15 concrete.