To install the pipes, the following procedure can be used, as a function of the type of joint and of the pipe diameter:
a) for any type of joint and diameter: lay and align the pipe bars on the bed that has been previously prepared, and perform the junctions inside the trench.
b) for pipes with joints that guarantee the axial continuity:
- lay on the bed two or three bars, previously joined outside the trench, in order to reduce the number of junctions to be performed inside the trench.
- align and join the pipe bars alongside the trench or above it, by using ties; lower the jointed pipeline into the trench by using several hoisting equipments being careful not to cause excessive deformations; this method can be used for small diameters only.
In case of bell/spigot or socket joints with o-rings, please verify that the angles that have been given do not exceed those allowed. Refer Fig. 5
TYPICAL COMPACTION METHODS:
Compaction can be made by using a pulse compactor or other suitable systems. In case the backfill is composed by sand, the compaction can be obtained by saturation. If this method is used, it is necessary to verify the draining capacities of both the bed of the trench and of the native soil; if necessary provide suitable drainage systems and use an adequate quantity of water, in order to prevent the pipe from floating.
The laying trench should be filled up as soon as possible, as far as ground level or for a height of 1.3 diameters, in order to prevent the pipe from floating, in case of laying in presence of a water table or in little draining soils.
MODULUS OF SOIL REACTION:
Pipes are generally checked in the actual working conditions, by using a certain Modulus of Soil Reaction, that becomes one of the design’s mandatory prescriptions, together with:
- material to be used for embedment
- compaction degree (soil density) and
- trench width.
If the installer is given the possibility to change one of the above parameters, in the following table is shown the Modulus of Soil Reaction that can be achieved with different materials and soil densities.
Deflection checks must be carried out when the first installed pipes have been backfilled. Further periodical checks must be done throughout the entire project.
Where it is practical, a measurement has to be taken of the density of the pipe zone material primarily compacted to ensure compliance with the design assumptions.
CONCRETE BLOCK CONNECTION:
When connecting with concrete blocks and walls, it is necessary to follow some prescriptions, in order to avoid damages to the pipes, due to these two phenomena;
- high differential settlements between manufactured product and pipe, due to the considerable weight of the concrete works in comparison with the relatively light PRFV pipes; this phenomenon is particularly clear if the concrete work, or a part of it, is made after having already laid and buried the pipeline, and it can cause high longitudinal shear and flexural stresses to the pipe;
- Very sudden passage of the transversal section of the pipe from a deformed configuration due to the normal deflection, to an un-deformed circular configuration in the area filled up with concrete; this phenomenon only occurs to a short section, at the end of the concrete block, due to a complex stress condition.
In both cases the larger is the pipe diameter, the more evident is the phenomenon, and it is necessary to take care that the deflection is minimum near the manufactured product.
It is suggested to take the following precautions, according to the actual working conditions
- in correspondence to the connections with the concrete, always wrap the pipe with a rubber sheet, (Fig. 6) 100/200 mm large and 10/30 mm thick, in order to reduce strain concentrations:
- design and verify with accuracy the concrete works and thrust blocks, with particular care to the settlement check
- improve the stiffness of the backfilling near the manufactured products, by widening and deepening the trench (Fig. 6) and filling it up with well compacted granular materials; the variation of the trench section should take place gradually, over a length of about two diameters.
As an alternative, backfill the pipe with lean concrete (70 kg/mc) for a length of about one diameter (Fig. 6), near the connection without using form works, this allows the lean concrete to flow, following the natural angle of friction
- create a strong PRFV ribbing on the pipe close to the connection with the concrete manufactured product;
- If you can’t keep the differential settlement within an acceptable limit, one should use a flexible joint (bell/spigot or socket joint), which allows a rotation of 1-2° just out of the concrete manufacture (generally it is not necessary for diameters up to 300 mm).