INTERCEPTORS AND MANHOLESReported by:

Egenias, Arboy M.Ganacia, Jemica G.BSA-2B

Republic of the PhilippinesTECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES

COLLEGE OF ARCHITECTURE AND FINE ARTSAyala Boulevard cor. San Marcelino St., Ermita Manila

INTERCEPTORS

•A large sewer that receives flow from a number of trunk sewers and transports the flow to the wastewater treatment plant. These sewers do notconnect to homes, buildings or streets.

•In a storm, they allow some of the sewage to flow directly into a receiving stream, thus keeping it from overflowing onto the streets.

• The interceptor sewer has a minimum of 15 feet in diameter.

•Also used in separate systems to collect the flows from main and trunk sewers and carry them to treatment points.

Combined Sewer

Lateral Sewer

Trunk Sewer

Interceptor Sewer

Wastewater Treatment Plant

Treated Water goes to bodies

of water

Waste Water Flow

•An interceptor could be 36-48 for sanitary and 60+ for storm. In terms of flow, that is highly dependent on the pipe size, material and slope...

GREASE INTERCEPTOR

•Also known as Grease traps are plumbing devices designed to intercept most greases and solids before they enter a wastewater disposal system. 

•It is usually used in restaurants.

•They only have kitchen waste water flowing through them and are not served by any other drainage system such as toilets. They can be made from a number of different materials; e.g. stainless steel, plastics, concrete & cast iron. 

•Effectively they are boxes within the drain run that flows between the sinks in a kitchen to the sewer system.

•They range from 35 litre capacity to 45,000 litres and above capacity. They can be located above ground, below ground, inside the kitchen or outside the building.

Typical Grease Interceptor Installations:

•Grease trap sizing is based on the size of the 2 or 3 compartment sink, dishwasher, pot sinks, and mop sinks. 

Sizing for Multiple Fixtures:

1. Determine the flow rate for each fixture to be serviced by the interceptor.2. Add together 100% of the largest flow rate, 50% of the second largest, and 25% of all others.3. Result is the recommended flow rate of the interceptor.

MANHOLES

Types of Connections• How drainage fixtures and fittings are connected to

an existing system via manholes and/or inspection chambers?

• There are 4 ways to connect new drainage to an existing system:

1. - via a new branch junction or access chamber to an existing pipeline 2. - via a saddle onto an existing sewer 3. - via an existing manhole or existing inspection chamber 4. - via a new inspection chamber or new manhole

Methods 3 and 4 are the main focus.

Types of Chambers

•Access chambers are intended to provide simple access for cursory inspection and access for drain rods or other maintenance equipment. They are not intended to provide access for a maintenance operative and are generally not more than 600mm deep.

Access Chamber - 225-300mm diameter

•Inspection chambers (often abbreviated to IC) are larger than access chambers, typically a minimum 450mm diameter. Again, they provide access for maintenance equipment, but tend to have more branches/spurs feeding into them and are often up to 1000mm deep.

Manholes tend to have iron covers 600x600mm

ICs can be circular, usually 450 or 600mm diameter...

....or they can be rectangular, usually with a concrete cover

MANHOLES

•A manhole (alternatively utility hole, cable chamber, maintenance hole, inspection chamber, access chamber or confined space) is the top opening to an underground vault used to house an access point for making connections or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains and gas.

• Manholes (MH) are the largest chambers providing access to a sewer or drain for maintenance equipment, and, in some cases, for operatives to enter the system itself. The minimum internal dimensions of a manhole are 600x900mm and they can be of any depth, although most modern manholes tend to be at least 1 metre deep with inspection chambers used for shallower depths.

Access and inspection chambers are used when the depth to the drain is a meter or less; for anything deeper, something more robust is required. The most common forms of manhole construction are...

•brick-built•sectional pre-cast concrete•sectional plastic•cast in-situ concrete around a plastic

liner

•For depths up to 2.7m, the minimum internal dimensions for a rectangular manhole are 1200x750mm, although manholes with more than 3 branches may be even bigger. Anything deeper than 2.7m is a major project best left to professional drainage contractors.  

•Circular manholes are commonly used for main sewers; for depths up to 1.5m, they must have a minimum diameter of 1050mm and for anything deeper than 1.5m, the diameter has to be 1200mm.

•This cross-section shows a typical construction for a manhole in a residential setting, such as beneath a driveway. It depicts the two most common constructions, using, on the left, pcc (Plain Cement Concrete) chamber sections, and on the right, Engineering brickwork.

•The cover detail may be different for a manhole within a trafficked area, or if recess tray cover for block paving was to be used.

Plastic Manhole by Polypipe Civils Ltd.

•The step-irons should be built into the brickwork, or mortared into the pre-formed holes in pcc sections.

•Note the minimum permissible opening size of any manhole is 600x600mm.

Manhole ring section with in-built steps

•The number of branches entering a manhole will determine the length dimension. A manhole with more than 4 branches may need to be longer to fit them all in. Similarly, manholes utilising 150mm diameter channels may need to be larger.

Plan view of typical manhole with a single branch oblique junction, again illustrating two construction types.

Sectional concrete inspection chamber with crown unit and cover

Adoptable Manholes

•There are several different types of manhole to suit a variety of purposes and conditions, but an idealized manhole construction is shown here to illustrate the basic concepts and components. Not all features depicted will be found on all manholes.

Some definitions:•Shallow Manhole - a manhole that has a

constant diameter or same cross-section throughout

•Deep Manhole - a manhole with an access shaft of a smaller diameter or plan size than the main shaft

•Cover and Frame - see table for guidance on strength rating of various covers

•Seating ring - sometimes used in place of regulating between cover and cover slab

•Brickwork - only engineering brick should be used, laid to English Bond

• Corbelling - the method of projecting brickwork outwards to reduce an opening or to carry a load. Each course should not oversail by more than 50mm

• Cover slab - also known as a 'biscuit'. Best thought of as a 'lid' for the main shaft, with a single access opening, minimum 600x600mm

• Reducing slab - used to accommodate a change in chamber diameter from a larger diameter main shaft to a narrower diameter entrance shaft

• Straight back tapers - perform the same function as reducing slabs, ie to accommodate a change in shaft diameter, but without providing a landing. Also known as 'Cone Sections'

•Landing Slab - used in Deep Manholes, these limit the maximum shaft depth to 6 metres, and may be thought of as shaft dividers, providing a resting point or 'landing' at convenient intervals

•Chamber sections - the individual sections used to construct a sectional shaft. Obviously not present with brick-built shafts

•Soffit - the underside of a cover slab, arch or other structure. Opposite of Invert

•Benching - a smoothed concrete topping, usually a granolithic mortar, sloping at not less than 1:30 and neatly shaped and finished to the base of a manhole

•Invert - the lowest point on the surface of a pipe, channel or culvert

•Rocker Pipe - a short length of pipe, usually less than 1 metre, placed at the inlet/outlet of a solid structure, such as a manhole or building, to accommodate differential settlement between the structure and the drainage system

Idealized Adoptable Deep ManholeCross-section

Idealized Adoptable Manhole Plan view

PCC Manhole components

Cover Slabs

•Cover slabs are the 'lid' for many manholes, especially the larger ones. They are also known as 'Reducing Slabs', because they reduce the opening size or the chamber dimensions, and, on site, they are affectionately referred to as 'biscuits' because that's the sort of humour that gets us sent to serve in the trenches.

•The basic role of a cover slab is to provide a firm platform to both 'cap' the chamber and to carry the cover along with any regulating brickwork. They are typically manufactured in a high-strength, steel-reinforced concrete, and, for manholes, the minimum opening size of 600x600mm is created within the cover slab during the casting process. Although the vast majority of cover slabs are supplied fully-cured from specialist manufacturers, custom slabs may be cast on site and lifted into position once cured, or, in certain cases, cast in-situ atop the chamber itself.

•A secondary role for cover slabs is to reduce the apparent size of the chamber, so that, for example, a circular 1800mm diameter chamber or a rectangular 1200x750mm chamber, can be fitted with a standard 600x600mm cover at the surface.

•These are heavy items, and they are generally fitted with two or more 'lifting eyes', which are steel, loops embedded into the concrete, that should be used to sling the biscuits from a crane or excavator during lifting and placement.

•The cover slab is normally mortar-bedded onto the top of the chamber with the internal face of the joint tooled smooth. When the surround concrete is placed around the chamber, it is brought up to be level with the top of the cover slab, as shown opposite. The regulating brickwork can then be built on top of the cover slab and the cover and frame fitted to suit the required level.

Manhole covers• Covers for manholes come in a wide variety of

shapes, sizes and materials. The specification of manhole and access covers for public areas is laid down, which is summarized in the table opposite. Domestic driveways and patios often have the appallingly unattractive pre-cast concrete crown units and covers, and most block paved areas now feature recessed tray covers, fabricated from galvanized steel. Over recent years, covers manufactured from new high-strength composite materials have started to be used, primarily in pedestrian areas.

Class Test Load (tonnes)

Typical Uses

A15 1.5 Pedestrian areas only

B125 12.5 Car parks, domestic driveways,areas with occasional vehicle access

C250 25 Carriageways if <500mm from kerb face, car parks, service stations.Must be non-rocking/silent type

D400 40 Carriageways and hard shoulders.Must be non-rocking/silent type

E600 60 Loading areas, docks,commercial/industrial areas

F900 90 Exceptionally heavy loads, ports, airports

Ductile and Cast Iron covers

•are manufactured to conform to but other materials, such as galvanized steel or pre-cast concrete are not included within the scope of that standard. Manufacturers of galvanized steel covers have formed a Trade Association (FACTA) in order to impose some standards and quality into the market, but, at the time of writing, there is no nationally or internationally recognized standard.

•Most covers actually consist of 2 components; the cover itself, and a frame. The frame is typically bedded on mortar to a specified height set to suit the surrounding paving or ground level and the cover sits inside the frame, from where it can be lifted and possibly removed when access to the manhole is required.

Manhole cover and frame construction

•Many of the heavier units have the cover itself split into 2 triangles, known as 'double triangular' format. The two halves can be bolted together but are often left unattached to facilitate easier removal. Removal is achieved by the use of 'keys' which fit into the eyes on the surface of the cover, are turned through 90° and then it's down to brute force, although there are special cover-lifting contraptions now available.

• As mentioned above, many paving projects now use Recess Tray Covers rather than the types of covers illustrated above. Recess Trays can be used with any type of paving, although they are most commonly associated with Block Paving.

Opening double-triangular cover with MH keys