Soil Investigation ~ before a decision is made as to the type of
foundation which should be used on any particular site a soil
investigation should be carried out to establish existing ground
conditions and soil properties. The methods which can be employed
together with other sources of information such as local
knowledge, ordnance survey and geological maps, mining records
and aerial photography should be familiar to students at this level.
If such an investigation reveals a naturally poor subsoil or
extensive filling the designer has several options:-
- Not to Build † unless a new and suitable site can be found
building is only possible if the poor ground is localised and
the proposed foundations can be designed around these areas
with the remainder of the structure bridging over these
positions. - Remove and Replace † the poor ground can be excavated,
removed and replaced by compacted fills. Using this method
there is a risk of differential settlement and generally for
depths over 4„000 it is uneconomic. - Surcharging † this involves preloading the poor ground with a
surcharge of aggregate or similar material to speed up
settlement and thereby improve the soil's bearing capacity.
Generally this method is uneconomic due to the time delay
before actual building operations can commence which can
vary from a few weeks to two or more years. - Vibration † this is a method of strengthening ground by
vibrating a granular soil into compacted stone columns either
by using the natural coarse granular soil or by replacement †
see pages 314 and 315. - Dynamic Compaction † this is a method of soil improvement
which consists of dropping a heavy weight through a
considerable vertical distance to compact the soil and thus
improve its bearing capacity and is especially suitable for
granular soils † see page 316. - Jet Grouting † this method of consolidating ground can be
used in all types of subsoil and consists of lowering a monitor
probe into a 150 mm diameter prebored guide hole. The probe
has two jets the upper of which blasts water, concentrated by
compressed air to force any loose material up the guide to
ground level. The lower jet fills the void with a cement slurry
which sets into a solid mass † see page 317.
Soil Stabilisation and Improvement