The 1985 edition of IS 7740 outlines detailed guidelines for the proper construction, installation, and upkeep of road gullies utilized in India's stormwater drainage infrastructure. It covers aspects such as design criteria, various inlet types, suitable materials, and maintenance protocols aimed at ensuring efficient surface water runoff management and preventing blockages.
Overview
The 1985 edition of IS 7740 outlines detailed guidelines for the proper construction, installation, and upkeep of road gullies utilized in India's stormwater drainage infrastructure. It covers aspects such as design criteria, various inlet types, suitable materials, and maintenance protocols aimed at ensuring efficient surface water runoff management and preventing blockages.
Audience
Contents
Structure
IS 7740 defines the scope relating to the design, evaluation, and quality assurance of structural components and materials involved in road gully construction. It adopts the rounding protocols as per IS 2-1960 to maintain consistency in numerical data presentation. The section mainly outlines applicability limits and rounding conventions without prescribing specific formulas or tables.
Clause 2.0 elaborates on essential definitions ensuring clarity and uniform understanding of terms such as curb inlet, gully chamber, and related components. It permits certain configurations like curb inlets without gully chambers and provides schematic depictions of combination inlet gullies. The rounding instructions reinforce adherence to IS 2:1960 for numerical data precision.
Stormwater inlets are categorized into three main types: curb inlets that allow flow through curb openings, gutter inlets comprising horizontal grate-covered openings in gutters, and combination inlets that feature both vertical and horizontal gratings. Each type has specified construction details, including pipe diameters (typically 150 or 250 mm RCC pipes), slope requirements (minimum 1:30), grate slot dimensions (25 mm spacing), and grating material standards (IS: 5961-1970 for cast iron). The section includes typical cross-sectional layouts and design considerations.
Guidelines prescribe placing gullies at the ultimate curb line edges for roads wider than 6 meters, with adjustments for narrower roads starting at 3.65 meters. Cross slopes direct surface runoff towards curbs to facilitate efficient drainage. Proper alignment with curb and channel ensures effective water disposal, preventing overflow and the need for repositioning during road widening.
Though not formula-based, the code provides vital construction instructions emphasizing water-tightness, structural strength to endure traffic loading, and use of reinforced concrete or precast elements. Gullies should have easy access for maintenance, with typical dimensions for openings and chamber depths provided. The road slope should generally range from 1.5% to 2% directing runoff into gullies.
Materials recommended for gratings include cast iron (conforming to IS: 5961-1970), mild steel bars welded to steel flats or angles, and reinforced cement concrete. Gratings come in vertical or horizontal slot configurations, with a preferred slot width of 25 mm oriented perpendicular to water flow. Structural details cover pipe diameters, concrete layering, and typical grating placement about 25 mm below curb channel level. The section also addresses maintenance concerns like clogging and mosquito-proofing.
Regular inspection and cleaning are crucial to maintain unobstructed flow. Cast iron or mild steel gratings require monitoring for corrosion and damage, with cleaning recommended monthly or after heavy rainfall. The code suggests a basic flow capacity formula to ensure adequate discharge. Proper maintenance includes timely repair or replacement of components to maintain gully functionality.
Key material specifications include a minimum 20 mm clear cover for reinforcement, a 12.5 mm thick cement mortar bearing course (1:2 mix), and in-situ bed blocks of cement concrete (1:2:4). RCC slab covers use a 1:1:2 mix. All dimensions are expressed in millimeters with values rounded according to IS 2-1960 standards. Typical RCC gully grating designs and layering sequences are illustrated.
The standard specifies bearing course thicknesses, reinforcement covers, and slab dimensions. Typical grating sizes, masonry thickness, concrete layers, and pipe diameters are detailed to ensure structural integrity. The section includes notes on mild steel plate thickness and washer specifications for grating assemblies.
Though no explicit formula is given, guidelines suggest the connection pipe length between gully chambers and stormwater manholes should be sufficient (generally exceeding 1 meter) to enable smooth flow and prevent clogging. RCC pipes of 150 mm or 250 mm diameter are standard, with outlet inverts positioned 150 mm above the chamber bed to function as grit traps. Construction details include multi-layered sections from gratings to base masonry.
The code mandates mosquito-proof flaps that are lowered during cleaning to allow water passage while blocking mosquito entry. Gratings should be intact and painted with anti-corrosive coatings post-cleaning. Maintenance schedules include pre-monsoon cleaning, monthly upkeep during rains, and post-monsoon inspections. Material thicknesses and welding requirements for steel components are specified.
Systematic inspections are to be conducted before, during, and after the monsoon, focusing on cleaning, flap functionality, and grating integrity. Monthly checks during the rainy season ensure prompt removal of blockages. Preventive measures include flushing gratings and applying protective paints. The section provides a maintenance schedule and flowchart to guide routine upkeep.
Frequently Asked
IS 7740 specifies the code of practice for road gullies used in surface water drainage, covering cast iron gratings conforming to IS 5961-1970, mild steel bar gratings made from 16 mm diameter bars welded to steel flats or angles, and reinforced cement concrete (RCC) gullies including RCC gratings. It provides guidance on their construction, installation, and maintenance to ensure effective drainage and durability.
The code recommends cast iron gratings adhering to IS 5961-1970 standards for durability and strength. Alternatively, gratings fabricated from 16 mm diameter mild steel bars welded to 40x6 mm mild steel flats or angles are acceptable. Reinforced cement concrete (RCC) gratings are also permitted, especially where corrosion resistance is desired or for lighter traffic conditions. Gratings are typically placed about 25 mm below the curb channel level to prevent debris accumulation.
Road gullies should be installed at curb edges, aligned with the ultimate road width, even if the road is initially narrower to avoid relocation during future widening. For roads wider than 6 meters, gullies are placed at the edges near curbs, with slopes directing runoff towards these points. Standard spacing ranges from 18 to 36 meters depending on road conditions and drainage requirements. Double gullies are used on main roads with widths of 9 meters or more, while single gullies suffice elsewhere.
IS 7740 emphasizes regular preventive maintenance, including periodic manual cleaning before and after the rainy season to remove silt, leaves, and debris. The design incorporates features such as outlet pipes positioned 150 mm above the chamber bed to act as grit chambers and reduce sediment entry. Gratings should be inspected for damage or corrosion, with repairs or replacements done promptly. Root intrusion and joint cracks must be monitored and addressed to avoid clogging.
The connection pipe length is generally recommended to be sufficient to ensure smooth flow and prevent clogging, typically exceeding 1 meter. While IS 7740 does not provide an explicit formula, a common engineering practice uses a length greater than or equal to the pipe diameter divided by 40 (Length ≥ Diameter/40). For example, a 150 mm diameter pipe would require a minimum length of approximately 3.7 meters. Pipes are usually RCC with diameters of 150 or 250 mm, and the outlet invert is set 150 mm above the chamber base to function as a grit trap.
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