Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)

Height Levelling of Manhole Tops with Removal

The height levelling of manhole tops with removal is always carried out when the surrounding cover layer is also damaged by the formation of cracks, deformation, etc. (Bild

As a rule, the repair is carried out by the removal of the surrounding covering and carrier layers as well as the damaged manhole top including possibly the supporting ring, if there is one, then the installing it again at the desired level.

Subsequently, the surrounding carrier and covering layers are again replaced.

Here, a differentiation is made of the variants of a circular re-plastering of the frame with bituminous jointing grout (Image (Image (Image (Image (Image (Image (Image (Image (Image (Image (Image (Image or the replacement of the bitumen covering layer up to the immediate vicinity of the manhole top.

These works can also be carried out by machine for circular manhole tops with the aid of an additional apparatus suitable for all tractor shovels, excavators, and lorry cranes [FI-SHARK]. This is a combined, hydraulically driven drilling and lifting apparatus that, after removing the manhole cover by hand, is positioned over the manhole top. A hydraulic spreader is situated in the crown of the drill and this is lowered into the manhole and is expanded into the grout joint at the point where the damage elimination is to occur. Then the manhole top is overdrilled up to the desired depth (Image (Image (Image (Image (Image After completing the drill process, the drill crown and the spreader construction are used to pull out both the manhole top and, if required, also the supporting ring as well as the drilling apparatus out of the covering layer. The renewed installation of the frame can be carried out in one of the above variants of the processes. The joint around the frame made by the drilling is only about 3 cm wide. It can be filled by pouring in a special plastics casting grout.

Other developments have the aim, instead of cement grout, of inserting equalising rings of plastics, which, because of their lower modulus of elasticity, reduce the loading peaks of the dynamic loads on manhole top and manhole caused by the traffic. The suitability of such special rings has been proven by comprehensive in-situ and laboratory tests [Stein98a].

An example of these are polyethylene equalising rings [FI-Budera] that are offered in two different types, one being parallel with 10 mm thickness and the other wedge coned shaped with 5/15 mm thickness (Image [FI-Budera].

Image  Underfilling the frame of the manhole top with the aid of polyethylene equalising rings (ASD rings) with reference to [FI-Budera] [Image: S&P GmbH]

A comparable product is the "Adapter for street runoff or entry manhole" from Sweden and patented in Europe [Patenta]. This, too, consists of profiled equalising rings of LD-PE that, according to [FI-Budera] can be used as parallel or conic, wedge shaped rings.

In order to reduce the traffic noise caused by the traversal, plastics rings are placed in the frame itself, e.g. in the region of the cover support of the ring [FI-AcoSe].

Not tested but comparable in principle with the equalising rings of plastics mentioned above, are load transfer rings made of elastomers, styropor and similar which serve the purpose of load transfer and for the equalisation of unevenness between manhole components, preferably between shaft rings. Their use for bedding of manhole tops within the scope of rehabilitation measures could be considered. In this case, compared to the load transfer between manhole components, high peak stresses resulting from traffic loads must be taken into account.

Other processes for re-installation and rehabilitation of manhole tops, in the case of a height levelling with cover removal, have the aims of a relief of the bedding of the frame of the manhole cover by transferring its loads to the bordering carrier layer of the street surface. The purpose of this is to achieve a longer useful life.

These systems are differentiated by:

  • Their constructional formation of the load transfer and
  • The size of the surface area of the carrier layer utilised for the load transfer.

In the SSU system [FI-Schwab], for which a patent application has been made, a strong bond is aimed at between manhole top and the surrounding street body by a combination of special supporting rings of reinforced concrete and specially shaped concrete edging blocks.

The special support rings are inserted into a much wider thickness than when compared to the wall thickness of the manhole so that the street surface and the throat of the manhole are utilised to transfer the load. The specially shaped edging blocks are laid in-situ concrete, whereby the renewed region of the street surface is substantially greater than the cross section of the manhole top or the manhole throat. The pointing up of the shaped blocks is carried out up to 5 cm under the upper edge with concrete and then with bituminous pointing grout (Animation

Animation  Repair of a manhole with reference to the SSU-System [FI-Schwab] [Image: S&P GmbH]
[Image: S&P GmbH]

In the application of the so-called "Teleskopschacht" [FI-Frühw], the equalising rings and corresponding jointing washers are also completely replaced by an in-situ concrete carrier layer between the manhole throat and the cover layer. The manhole top or the frame is locked into this in-situ concrete deck cover. A so-called "throat extension as sealing pipe" that is fixed with a circumferential glued seal serves as a lost shell. Changes of gradient of a maximum of 10 %% between manhole top and the level fixed by the manhole throat are possible.

The advantages of this system are:

  • There are a smaller number of cement joints;
  • Relatively easy adaptation to different heights and gradients of the cover layer;
  • Leaktightness of the whole entry manhole up to the top of the covering layer.

In the "Selflevel" manhole top [FI-GAV], the bituminous carrier layer takes on the function of the in-situ concrete, and is filled down to the manhole throat by means of a special profile of the frame of the manhole top (Image (Image (Image (Image (Image (Image (Image (Image (Image (Image (Image

The installation into the existing bituminous traffic level is carried out as follows:

  1. Removing the manhole top to be rehabilitated including the support ring and grout jointing.
  2. If necessary, bricking up the manhole taper up to approx. 20 cm below the upper level of the existing traffic lane.
  3. Installation of the frame and cover including a supporting ring bedded in grout.
  4. Replacing the bitumen carrier and covering layer and, at the same time, lifting the frame of the manhole top.
  5. Inserting a final layer of the bitumen cover layer by means of raising the manhole top above the top level of the traffic lane.
  6. Sealing the final covering layer by pressing the manhole top into the covering layer.

A comparable construction is offered by the creation of the joint intermediate layers and surrounding the manhole top with mastic asphalt (Image with the use of a metal manhole shell (Image (Image (Image (Image (Image (Image (Image (Image

Image  Underfilling the frame of the manhole top with mastic asphalt [Image: S&P GmbH]

All the processes mentioned above for including the carrier layer of the street surface structure into the load transference of traffic loads have not yet been tested as regards their long-term behaviour.

Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)