4.4 Strip and trench fill foundations

4.4 - Appendix 4.4A
	Diagram 24	Foundation dimensions
	Table 10	Minimum width of strip footings

Appendix 4.4-A

Approved Document A1/2, Section 2E, specifies the size of strip foundations using Diagram 24 and Table 10.

Also see Technical booklet D of Building Regulations (N Ireland) 1990.

Strip foundations should be:

• Located centrally under the wall
• of thickness P or 150mm (whichever is greater)
• of the width shown in Table 10.

Type of ground (including engineered fill)	Condition of ground	Field test applicable	Total load of load-bearing walling not more than (kN/linear metre)
			20	30	40	50	60	70
			Minimum width of strip foundation (mm)
I Rock	Not inferior to sandstone, limestone or firm chalk	Requires at least a pneumatic or other mechanically operated pick for excavation	In each case equal to the width of the wall
II Gravel or Sand	Medium Dense	Requires pick for excavation. Wooden peg 50mm square in cross section hard to drive beyond 150mm	250	300	400	500	600	650
III Clay Sandy clay	Stiff Stiff	Can be indented slightly by thumb	250	300	400	500	600	650
IV Clay Sandy clay	Firm Firm	Thumb makes impression easily	300	350	450	600	750	850
V Sand Silty sand Clayey sand	Loose Loose Loose	Can be excavated with a spade. Wooden peg 50mm square in cross section can be easily driven	400	600	Note Foundations on soil types V and VI do not fall within the provisions of this section if the total load exceeds 30 kN/m.
VI Silt Clay Sandy clay Clay or silt	Soft Soft Soft Soft	Finger pushed in up to 10mm	450	650
VII Silt Clay Sandy clay Clay or silt	Very soft Very soft Very soft Very soft	Finger easily pushed in up to 25mm	Refer to specialist advice

This table is applicable only within the strict terms of the criteria described within it.