Vitrified clay pipe v plastic Upvc pipe

[shutie]

We have a large septic tank job coming off soon:

2 no. 14,000L tanks
25 manholes
grnd conditions are good
fair height difference between highest and lowest manholes


In the drawings it is detailed as vitrified clay pipe (150 dia) for whole job but we rarely use clay so on enquiring if we can use solid plastic pipe instead (160 dia) the designers have come back saying we can use plastic pipe but we have to use a twinwall rocker outside the manhole, clay rocker incased in concrete as well as clay pipe and fittings inside the manhole, for every manhole and every leg in and out of it.

Can anyone explain why it should be done this way and is it viable?

Would plastic pipe for the whole job not be better?

Advantages - disadvantages?


any help would be much appreciated

S

 

[bimr]

Rockers are typically used when the soil may allow larger than normal settlement (in poor soil conditions where peat and soft clay and alluvium is present), but not where the structure is on virgin soil and the pipe is properly supported.

We do not use rockers in our designs. If the soil conditions are poor, then get a geotechnical evaluation and improve the soil conditions.

Each joint added using rockers is also an additional spot where leakage may occur.

I recommend using the plastic pipe.