UKTC Archive

Relaying Granite Cobbles within the RPA

Subject: Relaying Granite Cobbles within the RPA
From: james hedges
Date: Aug 08 2017 09:15:24
Dear Collective,

I am looking for any advice or experience you may have with a project I am 
working on as BS5837 doesn't appear to have much detail on the actual 
relaying of existing hard surfacing.

The project will potentially involve the relaying of granite cobbles 
(potentially dating back to the early 1800's, although it is not known if 
they have been relaid in the interim - certainly not in recent memory) two 
30m x 10 wide cul-de-sacs which cover well over 20% of the circular RPAs. The 
property is listed, within a Conservation Area and a significant feature of 
the area. Traffic is limited to the parking of cars, small delivery vehicles 
and the occasional larger vehicle (no through traffic).

Over the years the surface has been significantly disrupted by collapsing 
drains, surface roots, etc. and relaying of these cobbles (all a minimum of 
150mm+ deep, some weighing over 30kg) has become unavoidable. The existing 
pointing is permeable to a degree as it has broken up over the years. Removal 
of a small section reveals them to be laid on a mixture of dense compacted 
earth and some form of  variable dry concrete amalgam.

The project is at an early stage with detailed specification pending 
arboricultural input regarding the 3 mature London Planes (typical 800mm dbh 
planted around the early 1900's) protected by TPO.

Plans are in place for a meeting with the TO and more extensive site 
investigation is planned. My specific questions at this stage relate to:

1) Should a 3D celular conifinement sub base be considered as the starting 
point in the design?

2) If, as I suspect, the levels may not allow for this then what alternatives 
are possible? The cobbles may need to be set into a dry cement mix - if so, 
would a geotextile barrier suffice or would an impermeable membrane be 

3) Would the cobbles be contributing to tree stability in any way (both 
weight and the occlusion of some at the tree base)?

4) Could any excavation of the existing base layer be acceptable particularly 
if it meant the inclusion of a 3D sub base became possible?

My apologies for the barrage of questions. Any advice would be greatly 
appreciated (I have already considered the new LTOA advice on hard surfacing 
with specific reference to the use of self biding gravel to create an 
interface between tree and cobbles).



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