Hi Civiant!
I’m sure you must have read my article on how the underground structures or utilities are important and its engineering can provide cost effective examples. Let’s now bury ourselves for learning underground drainages, its actual work processes not only for residential use but for industrial unit areas. For most of the civil engineers, this is a good opportunity to be well versed in such specialized course. For non- civil engineers this will clear the myth of civil engineer meant to be only for designing building!
Underground Drainage Definition –
So, this is what any idiot would do to find about the topic. So, basically what I found is underground drainage calls majorly for pipes! I would love to clear the myths happening around and surprise you with actual reality.
Now let's understand types of Underground Drainage Channels -
A trench drain system is an elongated drainage system used to remove excess surface water from an area, by use of a drainage channel, covered with grating. Hence called open channel. They are placed just below the ground and come in several different styles and materials.
Trenches are mostly planned along roadside so that the surface water can be flowed from high level to trenches. The depth of these trenches is shallow as compared to pipes and can vary from minimum as 200 mm to 1.5 to 2 m depending upon the extend of catchment area.
They have subtypes depending on time saving and cost effectively – Brick wall trenches and RCC trenches, Precast trenches and cast in-situ trenches. You will witness wide variety of usage of brick wall trenches in Indian industrial projects, whereas RCC trenches has its own advantage of more strength, durable, and workable under vehicular movement. These RCC trenches can be metal molded with predetermined geometric dimensions, under perfectly controlled temperature and cured and then transported to site or directly performed the procedure on site as cast in-situ.
- Open Channel – Swale
It functions same as trenches but the water discharge is directly driven to underneath soil instead of additional channel way. The grass covering the side slopes and swale bottom provides a filtration surface for the water and helps to reduce the flow velocity. Swales can run alongside roads so that run-off from the road surface can drain directly to the swale, but they need more space as structure being trapezoid. You can also use them in the treatment of lightly contaminated run-off from hard standing around farmyards and farm roads.
- Closed Channel – Drainage Pipes
Well, if you’ve read this so far then this section must be self-explanatory now. It is the system or network of pipes and pipe fixtures connected altogether for easy flow of drainage to discharge location / treatment facility. They are buried under the soil with a design depth as per the surcharge loading and pipes design to carry waste water satisfying all hydraulic calculation requirements. The water can be flowed by two methods – gravity flow by sloping the pipe & pressurized pipe lines with the help of pumps.
There exist various pipe materials according to the type of flow with wide range of pipe diameters of minimum 50 mm to around 1 m. As per the pipe length and diameter, the Bill Of Materials (BOM) is issued to vendor. The vendor then issues pipe schedules 40, 60 or 80 standards with pipe joints and connections details. PVC, uPVC, cPVC, HDPE, LDPE, GRE, CS, SS, RCC, iron pipes, etc. are some pipe materials being normally used.
These pipes need to be checked under minimum velocity criteria, around 75 % pipe water depth criteria, pipe scouring effect, pipe slope to sustain in its design life.
Design Philosophy of Drainage System –
Drains are designed as a network covering total plant area & will ultimately lead to the nearest natural stream/ nullah at low lying areas. For design of drains, the total plant area is divided into several sub- catchment areas.
For paved areas, the runoff coefficient is taken as 0.9 or 1 depending upon material whether asphalt or concrete paving and for unpaved or green area the runoff coefficient is considered as 0.5 to 0.7.
Design Consideration of Drainage System -
The Drains are designed as a network covering total plant area and will ultimately lead to the nearest natural stream/ nulllah at low lying areas. For the purpose of design of drains, the total plant area will be divided into several catchment areas. Theses design considerations may vary from project to project and to site which are generalized as below.
- The storm water drains can be RCC drains with rectangular/trapezoidal cross section, capable to discharge the estimated peak runoff for the maximum rainfall intensity. Road side surface drains can be connected to nearest main plant storm water drain.
- The drains are designed to have a minimum self-cleaning velocity of 0.6 m/sec to maximum non-scouring velocity of 2.4 m/sec.
- All the storm or surface water trenches should be laid in service aisles close to the road with approx. 1:500 longitudinal slope for achieving required self-cleansing velocity to maintain gravity flow, whereas for pipe the slope can be around 1/ the pipe diameter. Crossing surface drain with other utility services should be avoided.
- Surface drain with aprons will be provided around the building/shop as garland drain. Garland drains surrounding the various building/shop/ industrial plant area will be of brick masonry or concrete as per depth and location requirement. These garland drains can be finished smooth with minimum 10mm thick cement plaster (1:4) and neat cement slurry will be applied cement for uniform flow and to attain self-cleaning velocity.
- The underground RCC hume pipes of class NP3 (socket & spigot ended with rubber ring roll on joints) pipes for drainage system minimum 600 mm diameter shall be used for crossing of road and class NP4 pipe of minimum 900 mm diameter will be used for railway track crossing wherever it is required.
- Minimum earth cushion of one meter will be provided from road level to pipe top level when underground drainage pipe crosses the road. If the earth cushion of one meter cannot be achieved, then the pipe is to be protected by RCC encasing of minimum 150 mm thickness for load of heavy and medium traffic.
- Catch pit/Inspection pit of minimum size 0.8 m x 0.8 m should be provided instead of RCC storm water trenches. These catch pits/ basins shall be connected to nearest manhole to final discharge location. Catch pits/ basins are to be provided with mild steel perforated gratings. Top of catch pit will match with the adjoining road/ground level.
I hope this have provided you some clear insights of underground drainage and how over the years it has been explored, its types and design processes. The work procedure is more hectic than designing the utility itself. They mostly require permission or approval of local authorities and government to prove that the waste water from such industries or units are properly treated and discharged. Don’t worry, I won’t let you overthink on this but will come soon on actual work processes soon…!
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