TABLE 2. RISK ASSESSMENT - CONSEQUENCES
| CONSIDERATIONS | HOUSE OR SMALL COMMERCIAL | HOSPITAL | RETAIL STORE |  |
| PEOPLE AFFECTED | Few | Depends on location - probably many | Possibly 100's of customers/staff | |
| CONSEQUENTIAL LOSS | None or very low | Could be high especially if patients have to be relocated | Very high if store has to close or customers are put off | |
| REPAIR TYPE | Cut through in situ slab or divert discharge pipes | RC slab, disturbance to upper floor, noise, hygiene | RC slab, noise, dust hygiene etc. | |
| LIKELY COST | Relatively low | High | High |
It is clear to see that the type of building and the location of the drains should be carefully considered, and the appropriate material for each given location selected. So it makes sense to take the time to carry out a simple risk analysis, which once made, should be discussed with the client and the final decision recorded in writing.
To produce an example, a consultant engineer went through such a process and recommended to the client that cast iron be used for a particular project. The Quantity Surveyor estimated that the extra cost of using cast iron over vitrified clay was £6000 on a building that cost £6.5 million. Given the inaccessibility of the underslab drainage and its importance in keeping the building open and healthy, this was obviously a relatively small price to pay for the peace of mind the modern cast iron systems give you.Out of interest the same engineer calculated the cost of the couplings on both systems. The mechanical couplings for the iron pipe cost 4 times that of the plastic sleeves on the clay pipes.
The clay system had about twice the number of joints because of the shorter lengths of pipes. Nearly half the saving on the clay system was achieved by what was regarded as a higher risk and therefore an inappropriate jointing system for such a sensitive inaccessible location. Clearly the client always has the last word on how he spends his money, but certainly a responsible designer should alert his client to the hazards and the risks, then let the client make an informed decision.
Recent research by EC Harris has given backing to the many people who think that the cost of modern cast iron drainage is not as high as previously supposed.
To reduce or remove the risks and consequences, the majority of professional drainage engineers have been specifying cast iron below the larger non-domestic buildings. Over the past 20 years this has been cast iron systems to BS437 with flexible mechanical couplings. Alternatively, the lighter cast iron pipes can be used in underground situations. These are lighter, thinner walled pipes than BS437 and are given extra protection by means of an internal and externally applied coating and importantly they are BBA certified. These are proven systems that have been in use in Europe for over 20 years.
"Inappropriate designs often produce inappropriate systems with inappropriate materials. The true assessment of an installation should include total installed cost, ease of maintenance, life cycle considerations and the consequence of getting it wrong - areas in which cast iron is a proven overall performer"
Gerald Tobias would like it known that Gerald Tobias Associates regularly specifies other materials for gravity drainage when they feel it is appropriate for the project.
Gerald Tobias Associates provide expert consultancy design, covering mechanical, electrical, drainage, infrastructure and car park services and are celebrating 25 years at the top of their profession.
Tel: 01444 871444 or e-mail: gtobias@gtagroup.co.uk