Planted Foundations to PD 6547 https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760644 Sun, 7 Jun 2026 17:13:51 GMT Wed, 29 Nov 2023 14:54:53 GMT Copyright © 2023 Institute of Highway Engineers Planted Foundations to PD 6547 https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760644 https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760644 I have found the IHE Sign Structure Guide to be extremely illuminating and an grateful to the authors for it's creation and clarity.  Of particular interest to me is some of the technical background given on the design of planted foundations based on the PD 6547 method.

Whilst I'm not involved in the design of road signs, I do design a lot of site hoardings. The Temporary Works forum have a guide document on this which advocates the use of the PD 6547 foundation design method. The guide can be obtained here.  

I have 2 queries in relation to the PD foundation design method:

1) The TWf guide includes foundation sizing tables that do not satisfy the minimum foundation depth (i.e. the 1.5 x footing diameter recommendation mentioned p.24 of the highways guide). Whilst I intuitively agree with this minimum depth limit, I have not found any reference to support it, either in PD 6547 or anywhere else (and I've really looked!).

2) PD 6547 Cl.6.3.2 suggests a foundation FoS of 1.25, whereas the TWf guide has increased this to 1.5. I'm wondering if this is necessary - I can't imagine a hoarding failing having consequences any more or less serious than a road sign or lampost falling onto a highway.

I would be very keen and grateful to hear from anyone who can help deepen my understanding of the PD method.

Yours gratefully,

Martin
martin@smartbuild.uk.com

]]> Tue, 28 Nov 2023 23:40:43 GMT https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760697 https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760697 The SSG uses National Highways (previously Highways England/Agency) document to adapt the PD 6547 method to traffic signs. Some of these adaptation may be applicable also to hoardings. The original NH documents were BD 94/07, revised to BD 94/17 and then to CD 354 (now on version 1 but currently being revised again).

To answer your questions:
1) The original NH BD 94 documents didn't mention a minimum height/diameter ratio, but the SSG has always recommended 2.0 for this (although it concedes in the 2021 edition that 1.5 should be the absolute minimum). I believe CD 354 and the proposed revision of PD 6547 both insist upon 2.0. I'm not aware of any research on this - it's just geotechnical engineers' judgment as to what depth is necessary to achieve the passive resistance assumed from the soil, rather than the whole footing simply rotating in place.

2) PD 6547 has always had a safety factor of 1.25 for planted foundations, and this value was kept unchanged in the transition to sign foundations. I don't think it's ever been queried, but safety factors should always have some recognition of the consequence of failure, so a higher FoS could be appropriate for hoardings.

Hope that helps, Simon]]> Wed, 29 Nov 2023 15:16:45 GMT https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760710 https://members.theihe.org/forums/posts.aspx?group=214541&topic=1760710 Wed, 29 Nov 2023 15:54:53 GMT