Foundations and plinth - The Hempcrete Topic: Designing and Building with Hemp-lime - page 237

In many respects the plinth has more in common, both conceptually and in its materials,
with the foundation than with the hempcrete above it. For this reason these two elements
are discussed together in this chapter, which explains their roles in providing a sound basis
for the hempcrete structure above them.
Foundations
The foundations for your building will usually be specified by an architect and/or structural
engineer or geotechnical engineer, and built to that specification by you or your contractor.
The building control inspector will check the ground conditions on-site once the excava-
tion stage is complete, and advise if further depth is required.
The foundation exists for the purpose of transferring the load (weight) of the building
safely down to stable ground. Foundations for a hempcrete building follow broadly the
same principles as for any other building, although the low density of hempcrete as a
walling material does allow for other possibilities in the type of foundation that can be
used. This may translate into a lower embodied energy, since a foundation usually demands
high fossil-fuel usage, as it involves casting several tonnes of concrete.
Concrete foundations
The most common type of foundation is called a 'strip foundation'. This means digging
a trench along the footprint of the load-bearing walls until solid ground (e.g. firm clay or
rock) is reached, and then filling this with concrete to create a solid footing.
If this approach is used for a hempcrete building, then the foundation will be no different
from that for any other building, because the strip trench is dug as deep as necessary until
solid ground is reached. However, when building in an area in which the ground condi-
tions do not include an easily accessible solid-ground layer (e.g. where soft clay, gravel,
loose silt or landfill is found for some distance below the surface), a strip foundation is not
suitable. In this case the engineer will usually specify either concrete piles reaching deep
into the ground to make contact with firm ground, or a raft foundation - a concrete slab
that simply 'floats' on the poor-quality soil.
In this situation, a hempcrete building has the advantage that, being of low density, it is
lighter than a building of conventional masonry construction, and therefore the piles can be
smaller (in diameter) or the raft slab thinner than for a masonry building. This means a sig-