The RIALTO® system

Design Criteria

The RIALTO® system must be designed ad hoc, according to the specific context of the bridge, in order to guarantee full effectiveness and maximum speed of the intervention.

Everything must be detailed in project, drawn up according to the RFI Design Manual.

The main aspects to be considered in the design phase are described below.

The planimetric dimensions

It is advisable for the membrane to cover the entire width of the railway site, with the sole exclusion of the lateral bands such as, sidewalks, parapets, etc.

Before the intervention it is necessary to find out type and geometry of the extrados of the bridge and the lateral walls, through an exam of the original design documents and some excavations below the ballast.

The lateral strips of the bridge, such as walkways, curbs, veils, etc., where it is not appropriate to lay the membrane, must in any case be examined and, if necessary, renovated to avoid the percolation of rainwater.

Underground services must be located, identified for their temporary or permanent removal before the waterproofing intervention.

In the longitudinal direction, it is advisable for the membrane to extend beyond the end of the bridge deck for a sufficient length to prevent the structures from being reached by water falling on the embankment.

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THE ALTIMETRICAL CONFIGURATION

For the system to function properly, the installation surface must:

A) Be inclined everywhere towards the valley points, in order to move the water away, avoiding stagnation. Generally, it is advisable to create a slope of approximately 3-5%.
B) Remain at a sufficient depth with respect to the sleepers, in order to avoid interference with the tamping and rehabilitation operations of the track. Generally, it is advisable to keep the membrane at least 80 cm from the surface of the iron.
C) Do not be too deep, to limit the volumes of material to be removed and therefore the execution times of the intervention.
D) Adapt to the geometry of the bridge extrados. In this regard, it is necessary to carry out some localized tests, up to the expected installation depth, to verify that the hypothesized heights are actually achievable.

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THE LOCATION OF HIPS, VALLEYS AND WELLS

Generally, a transversal hip is placed in the centre of each span of the bridge and two longitudinal hips at the base of the para-ballast walls. In this way the entire surface to be waterproofed is divided by the watersheds into hydraulically independent portions.

Inside each of these portions it is necessary to place at least one valley point, with a relative hole in the membrane and a water collection well.

It is generally advisable to provide at least one valley for approximately every 100 m2 of membrane, where the slopes are of the order of 3-5%.

The removal of water can occur in three ways:

With drainpipes coming out of the side of the embankment.
With drain outlets that come out from the underside of the bridge.
With drainpipes that exit underneath or on the side of the bridge and go down to the ground.

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Transport, handling and unrolling methods

The membrane is transported from the production site to the installation site wrapped around special metal tubes. For reasons of space, each membrane is generally rolled along the short side.

Unrolling the membrane on site can take place in two ways:

Longitudinal unrolling, keeping the beam raised and perpendicular to the tracks.
Transversal unrolling, keeping the beam raised and parallel to the tracks.

The first method is generally preferable for single-span bridges, as it is possible to keep the beam raised using road-rail loaders. Transversal unrolling is instead preferable for multi-span bridges, which require the laying of a sequence of several membranes.

srotolamento-della-membrana-in-cantiere-sistema-rialto.

Scavo e formazione delle superfici di posa

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INSTALLATION IN SEVERAL PHASES

When the duration of line interruptions is not sufficient to complete the intervention, it is possible to install the system in several phases.

The overall surface can be divided into two or more longitudinal portions. For example, for a 6-span bridge, the membrane can be laid on spans 1, 2 and 3 in a first phase and on spans 4, 5 and 6 in a second phase.

The overall surface can instead be divided into two adjacent portions in a transversal direction. For example, the membrane can be laid on the entire even track in a first phase and on the entire odd track in a second phase.

Installation in phases requires some precautions to protect the edges of the membranes which are installed in each phase and which must be joined to the edges of the membranes installed in the subsequent phase.
Longitudinal unrolling, in which the membrane is unrolled while keeping the beam raised and perpendicular to the tracks.

Transversal unrolling, in which the membrane is unrolled while keeping the beam raised and parallel to the tracks.

Once the first membrane has been laid, a metal edge is placed on it which will act as a paraballast element during the laying of the next membrane. The vertical edge of the membrane is temporarily fixed to the outside of the side.

Once the ballast has been rebuilt on the first membrane and the excavation for the installation of the second membrane has been completed, the vertical edge of the first membrane is freed from the bank and is placed horizontally at the base of the same.

After the installation of the second membrane, the junction between this and the first is made, using a riveted joint, which must be along a watershed line.

Once the joint has been completed, the temporary bank is removed and the railway ballast is reconstructed on membrane 2.

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