Introduction to
Monolithic Preformed
Concrete Manhole Bases.
Monolithic Concrete Manhole Bases made to individual site
requirements, with preformed channels & benching and flexible
joints to suit diameter & type of pipe used.
FlexiBase The Contractors Choice
INTRODUCTION
Monolithic Preformed Concrete Manhole Bases.
Monolithic Preformed Concrete Manhole Bases (FlexiBase) can be manufactured to individual requirements,
following an accurate (take-off) from drainage plans and have numerous advantages over the traditional labour
intensive method of forming manhole bases with insitu concrete on site.
However, due to their weight for handling & fitting on site the complete Monolithic Preformed Concrete Manhole
Base, with flexible sealed pipe joints, may only be suitable for pipe sizes ranging between 150 -600 ID using
manhole rings from 1200-1500 ID, (weighing between 2-6 ton).
Where a base for larger pipes and larger manhole rings may weigh up to 20 ton, a lighter version of a precast
manhole base (Starter Base) may be individually manufactured, incorporating the more difficult formed bottom parts
in precast concrete allowing the more straight forward top parts to be completed on site.
Type 1: 10/12 KNOCKOUT MANHOLE BASE
A standard stock item, flat bottom Manhole Base to suit 1050 & 1200 standard manhole rings with “knock out” holes
at positions1800, 900 450, to accommodate maximum overall pipe diameter 430mm fitted on site, benching is then
completed on site.
Type 2 : FLEXIBASE
A Monolithic Preformed Concrete Manhole Base manufactured individually to suit on site requirements, complete
with preformed channels /benching and flexible seal to suit the required type of pipe entry.
Used for different pipe sizes ranging from 150 - 600 and top rebate to suit 1200 and 1500 standard manhole rings
or wide wall gasket sealed manhole rings with cone on top rather than using coverslab. To give a fast fit, flexible
complete watertight manhole.
Type 3: STARTER BASE
A monolithic concrete ½ base individually manufactured with preformed ½ channel to suit angle and pipe size
required. Smaller side entry pipes, toe holes and top half benching can be poured insitu when the main pipe (up
to 1200 Dia) are fitted on an expandable sealant. The top section (method varies) can be fitted to leave ready for
manhole ring.
1200 Ø
1050 Ø
10/12 knockout flat bottom base
As there may be hundreds of different arrangements of bases to cater for on
the site requirements, the following questions should be asked before quoting
for a specific project as they are made only to order.
• What diameter of pipe(s) are used in the base?
• What type of pipe ~ pvc, twinwall, concrete etc?
• What different patterns of main channel are required in the base ie 1800, 900, 450 etc? Also include side entry inlets along with their invert level.
• What diameter of manhole ring is required to fit the base.?
• What is the total depth of manhole, cover level to invert level?
All of this information can be put on a Site Summary Sheet, supplied by Tracey
Concrete Ltd, to ensure accurate quotation.
1800 Ø
1500 Ø
1200 Ø
900 Ø
top method
varies
600 Ø
1200 flexibase 2-3 ton
for 150, 225, 300 Ø pipes
1500 flexibase 4-6 ton
for 375, 450, 525, 600 Ø pipes
1800 starterbase 6 ton approx
(complete flexibase would weigh app. 12 ton)
2
FlexiBase The Contractors Choice
the ultimate in fast fit, strong, watertight manholes
(ask for more details)
A = Road or Cover Level
?
Allow
150mm
750 x 600
675 x 675
675 Diam
or 600 Diam
to suit type of
cover used
75
10
75
10
75
See Further
Details
Seating Slabs
10
10
Cover
Slab
750
x 675 wide
Clear Opening in cone
600 Cone
75
675 Max.
E2
Variable
E3 make up of manhole rings
Alternatives
•
•
340 wide double step
where cover level to
top benching 3000mm
or less
340 wide ladder rung
with side stringer
if deeper than 3000mm
900 Cone
Standard
Manhole
Rings
210
See Joint
Detail
1200
Lubricated, Flexible
Rubber seal, can
include vertical load
transfer rubber
Wide wall,
reinforced,
watertight
manhole rings
900
C - D = E1. (See Chart)
A - B = C = Cover to invert Level Difference
125
O - Internal Diam
WT
(
Crown or Soffit
of Secondary
smaller inlets to
be level with
Main Channel, that
is normally the size
of outlet pipe
600
Benching sloped
towards
main channel
Standard
Manhole Rings
May require
150 concrete
surround
)
G
BW
D
B=
Main Channel
Invert Level
See Further Detail
Joint detail to suit
Diam. and type of pipe
Used.
e.g. PVC, Twin Wall, Concrete
50mm
Secondary
Main Channel
Inlet
CD
CD
Flexi Base
(See further details)
Alternative
BH
Alternative
Tongue and
Rebate
Groove joint
Joint with
standard
Manholes Rings,
Cover slabs
and mastic Sealant on joints
F at Centre
BD
3
FlexiBase The Contractors Choice
The attached photographs show the typical brief sequence of
fitting the FlexiBase using 225 Spigot & Socket Pipes and 1200
Diameter Wide Wall Manhole Rings.
Irrespective of pipe or manhole sizes, the fitting sequence
remains similar.
• Pipes are normally laid spigot into socket against the flow.
• A double spigot rocker is fitted next to base and a rubber gasket is put on the end of the pipe, that will connect to the FlexiBase.
• The level foundation for FlexiBase is prepared at lower level to pipe invert (see dimensions) and foundation maybe hardcore or concrete depending on ground conditions, as per design consultants specification.
• Using the correct individually marked FlexiBase, a self lubricating rubber gasket can be placed on the top profile of the FlexiBase, ready to receive the manhole ring on top.
Note: A sealant would be placed if standard manhole ring is used.
Monolithic Concrete Manhole Bases made to individual site requirements, with preformed channels & benching
and flexible joints to suit diameter & type of pipe used.
4
FlexiBase The Contractors Choice
• Screw in lifting eyes into socket in top of FlexiBase to form lifting hooks and transport base close to required position.
• Line up FlexiBase so that outlet position is in line with the pipe to which it will be connected.
• Keeping FlexiBase suspended slightly above final resting place, push the base onto the rubber joint gasket of receiving pipe until it is pushed home.
• Then keeping pressure against outlet pipes, let the FlexiBase down to final position on foundation and check across top rebate for level.
• Unscrew the lifting eyes when satisfied FlexiBase is in correct position.
Monolithic Concrete Manhole Bases made to individual site requirements, with preformed channels & benching
and flexible joints to suit diameter & type of pipe used.
5
FlexiBase The Contractors Choice
Using the appropriate number & depth of Manhole Rings (to
suit cover to invert level) place the manhole rings on top of the
FlexiBase, ensuring steps line up.
Note: The rebate on top of individually manufactured FlexiBase
can be made to suit standard manhole rings used with standard
coverslab on top.
or as shown in photographs
Wide wall manhole rings jointed with rubber gaskets, joints and
tapered cone on top, rather than using a coverslab.
The type of manhole ring used is customers preference,
and should be decided at the time of ordering individually
manufactured base, as top rebate on base will be different.
Lifting eyes are screwed into top of tapered cone. As with
manhole rings, the bottom rebate should be inspected and
cleaned if necessary, especially where the rubber gasket joint is
to be used, before slotting manhole ring into position one on top
of each other.
Monolithic Concrete Manhole Bases made to individual site requirements, with preformed channels & benching
and flexible joints to suit diameter & type of pipe used.
6
FlexiBase The Contractors Choice
• Place the Tapered Cone down over the manhole ring rebate with gasket seal to form watertight joint, ensure steps line up with those below.
• The clear opening in the tapered cone is 750x675mm, seating slabs as required are bedded in Mortar on top of the Tapered Cone to allow the steel cover to be bedded at the correct level.
Note: These photographs do not show mortar bedding.
• Reinforced concrete seating slabs are manufactured with rectangular, square or round openings to suit size of steel lid to be used
• Whether the manhole rings are placed on FlexiBase before the remaining inlet pipes are connected to base or (vice versa) is really a choice for the contractor on site.
Monolithic Concrete Manhole Bases made to individual site requirements, with preformed channels & benching
and flexible joints to suit diameter & type of pipe used.
7
FlexiBase The Contractors Choice
The attached photographs show the top, inside and bottom of
complete preformed benched FlexiBase, designed to be fast fit,
flexible and watertight without the need for concrete surround.
This method has many advantages over the traditional labour
intensive method of using in-situ concrete, in terms of speed,
Health & Safety on site, cost and overall long term performance.
Traditional methods have many hidden extras such as
workmen spending time sealing leaking manholes after they are
constructed and before handover.
This may not be surprising given the fact that it is virtually
impossible to seal around a pipe with vibrated concrete
especially when it is inevitable that there will be differential
movement/settlement on completed manhole leading to future
leaks around the rigid joints into the manhole base.
FlexiBase, individually manufactured manhole base has flexible
sealed joints to suit the type and diameter of pipe being used.
Monolithic Concrete Manhole Bases made to individual site requirements, with preformed channels & benching
and flexible joints to suit diameter & type of pipe used.
8
FlexiBase The Contractors Choice
• Whether the remaining inlet pipes are fitted after FlexiBase has been placed and before manhole rings are placed on top (or vice versa) is largely the contractors choice.
• It may be best to fit the remaining inlet pipes immediately after the FlexiBase is fitted as this will ensure all is lined up correctly and allow some backfill to commence around base,
leaving a safer footing for personnel placing the manhole rings on top of the FlexiBase.
• Using a precast manhole base prevents the build up of ground water in trench because it can seep through the backfill surround around the base.
• Whereas, in-situ concrete base blocks the trench causing a
build up of ground water behind the in-situ concrete and is vulnerable to leaks at rigid pipe inlets, this relies completely on skilled labour as does forming channels and benching inside the insitu base.
• The base and complete manhole in the photograph below was assembled with semi skilled labourers in just over one hour.
Monolithic Concrete Manhole Bases made to individual site requirements, with preformed channels & benching
and flexible joints to suit diameter & type of pipe used.
9
FlexiBase The Contractors Choice
Where 100 or 150 pvc service inlets are to enter the main pipe
such as gullies or house sewer connections, a rubber saddle
(Forsheda 910 Seal) can be pushed into pre-cored pipe ready to
receive the 100 or 150 pvc connection.
Pipes can be cored on site or preferably pipe supplied to site
cored to the correct size for the inlet pipe requested and rubber
saddles fitted.
Cored pipe can be turned to point the direction of the incoming
service pipe that is usually in the top third of the pipe, allowing
the incoming service pipe to drop into main pipe.
The completed joint remains watertight and flexible up to 30 or 40
in any direction.
The use of straight collar, bend etc tight to the rubber collar
ensures the pvc pipe which enters the main pipe can be cut
to the required length (varies depending on wall thickness of
main pipe) and cannot push further into main pipe past desired
dimension.
Lubricate the rubber saddle and pvc pipe
and push pvc pipe into collar.
rubber collar
forsheda 910 seal
the length of this stub pipe
(cut on site) dictates how far
p.v.c. pipe projects into main
pipe and cannot be pushed
further as the wider bend hits
the rubber collar
top 1/3
straight collar or
bend as required
flow
flexible
hole cored to correct
diam for collar size
for 100Ø p.v.c. = 138mm
main concrete for 150Ø p.v.c. = 186mm
pipe
wall thickness
varies with pipe diam
10
FlexiBase The Contractors Choice
The Forsheda 146 pipe seal is basically made up of the bulky
part of rubber that is tapered sliding seal to correct dimension
for joint, then covered with a thin flap which contains lubricant in
the cavity.
The bulky part of the rubber seal is placed against the shoulder
ledge approx 60mm (varies on pipe diameters) from end of pipe
on Spigot with the flap towards the end of pipe.
As the socket of the pipe touches the rubber flap on the spigot
end, the lubricant in cavity will allow the socket to be pushed up
and over to compress the bulky part of rubber and seal joint.
The flap will be pushed by the socket, over the bulky rubber
and the flap will end up to the opposite side (as shown below)
thus preventing concrete to concrete contact between Spigot &
Socket if joint is deflected.
No site lubricant is required for this seal.
Place the seal, positioned correct direction, on the bottom of
Spigot and stretch upwards to fit over Spigot, the tension holds
rubber in place.
Then position seal against shoulder all around the pipe.
1. place rubber ring on spigot
lubricant inside flap
flap towards end of pipe
2. centre the spigot to socket
3. push joint home
seal is compressed and lubricated flap
moves overseal to opposite side
the lubricant is in the flap to
limit friction when pushing joint
together
gap seal compresses 30 - 35%
spigot
spigot
shoulder position varies
from small to larger
pipes 50 - 80mm
socket
spigot
socket
seal size also varies with pipe diam
11