Section 4 Base Design and Construction
4.6 Substructure design
The majority of domestic conservatory installations can be constructed using conventional concrete foundations (see diagram in 4.6.1). However, each company will have their own preferred method based, at least in part, on knowledge of local ground conditions and local working practices.
Tip
Check whether the existing drain-
age system is a combined foul and
rainwater system
Existing manholes within the conservatory floor area may be raised and fitted with a double sealed cover, with provision for floor finish. It must be confirmed with the local water authority that this is allowed, they may insist on the manhole
Section 4 - Base Design and Construction
4.6.1 General
From the survey information and site measurement documents, it will now be possible to de- sign the actual base for the conservatory. The critical points requiring detailed attention are as follows :
· Setting Out Point (SOP). This gives a datum point from the site measure- ment survey as to where the conservatory is to be positioned on the prop- erty. It will usually be de- noted as an existing fea- ture (opening reveal, cor- ner of building etc.) or a dimension from some critical point
· The relative floor level of the conservatory in rela- tion to the house floor level.
· The relative DPC level of
the conservatory in relation to the DPC level of the property.
· d) Ensure consideration is given to level of exposure of the existing building cor- ners where the conservatory abuts, to ensure moisture penetration through brick- work does not occur. Conservatory abutments should be set in from corners sub- ject to severe weather exposure.
· e) The general ground level in relation to the house DPC level. The DPC level of the conservatory should be at least 150mm above the external ground level. Where this cannot be achieved, it will be necessary to either lower the external ground level around the outside of the conservatory or alternatively build either, a gravel filled trench 150mm wide by 150mm deep or lay a concrete gully around the perimeter wall of the conservatory to avoid damp penetration above DPC level.
· f) The general lie of the external ground level and whether it falls away from or towards the conservatory.
· g) Sloping sites can produce several possible problems. For instance, if the ground level is much lower than the proposed conservatory level, then it may not be feasible to install a ground supported floor slab in the conservatory due to the amount of in-fill required. If a suspended floor is to be constructed, this will
Section 4 - Base Design and Construction
need to build a landing/platform (generally the width of the door(s)) and with a projection equivalent to the swing of the door plus a minimum of 400mm. This will allow for the customer to safely step out before walking down the steps. Steps will be required wherever the floor level of the conservatory is more than 150mm higher than the external ground level. Approximately one step is required for every 150mm increment in height. Should the steps be higher than 600mm it will be necessary to fit a hand rail/balustrade to each side of the set of steps, and this should always conform to building regulations (Approved Document K {ADK}). Depending on which way the site is sloping, it may result in additional height to the conservatory wall below floor level.
If the external land is falling towards the conservatory then serious excavation will be neces- sary plus possibly the building of a retaining wall to give support to the remaining bank of high- level ground.
4.6.2 Foundations of plain concrete
From section 2E Approved Document A, (ADA) Conditions relating to the ground: 2E1 There should not be:
a) non-engineered fill (as described in BRE Digest 427) or wide variation in ground conditions within the loaded area; nor
b) weaker or more compressible ground at such a depth below foundation as could impair the stability of the structure
4.6.3 Strip foundations
Unless there is evidence of poor subsoil conditions on the site, a traditional concrete strip footing or trench fill foundation placed under the walls of the conservatory will normally suf- fice. The new foundation should be constructed in such a way that it does not undermine the foundations of the main building.
Section 4 - Base Design and Construction
4.6.4 Minimum width, thickness and depth of strip foundations
The minimum width of foundation should be designed to suit the wall construction, the condi- tions of the ground they are bearing on and the loads imposed on the ground by the conser- vatory.
The imposed load can be simply determined by reference to Table 4 in BS 8103-1:1995. Extracts from Table 4 – Wall load categories for floors to 4.5 m and roof to 9 m
Section 4 - Base Design and Construction
Extracts from Table 7 – Identification of ground material and minimum foundation widths for wall load categories
The minimum depth of foundations should be determined as the greatest of the following:
· A depth to the bearing stratum
· In clays subject to seasonal moisture movement, a depth not less than 1.0 metre.
Rock or soil
Simple field test
Mimimum foundation width in mm, for load category (kN per metre run)
Type Condition A (20) B (30) C (40) D (50)
Rock Hard
Requires at least a pneumatic or other mechanically operated
pick for excavation
Equal to width of wall
Gravel
Compact
Requires pick for excavation. Wooden peg
50mm square hard to drive in more than
150mm. 250 300 400 500 Sand Clay Stiff Cannot be moulded in the fingers. Requires pick or pneumatically operated spade for
excavation
250 300 400 500 Sandy clay
Clay
Firm
Can be moulded with substantial pressure with
the fingers and excavated with a spade
300 350 450 600 Sandy clay
Sand
Loose
Dry lumps may have slight cohesion but easily breaks up in fingers. Readily excavated with spade. 50mm peg can be easily
driven in. 400 600 Refer to specialist for advice and design Silty sand Clayey sand Silt Soft
Easily moulded in the fingers and readily
excavated 450 650 Clay
Sandy clay Silty clay
Silt
Very Soft Exudes between fingers when squeezed in fist 600 850 Clay
Sandy clay Silty clay
Peat ----
Refer to specialist for advice and design Made Ground ----
NOTE: In no case should the foundation width be less than the width of the wall nor should the wall over-sail the foundation.