Civil Engineering Reference
In-Depth Information
ground to gain a solid bearing to support the weight of the building. Our engineer
specified the size and frequency of the piles, and the details for the construction of the
concrete strip footing that was poured on top of the piles. As you can imagine, the cost
was significant and added three days to the job.
Soil classifications
The ideal building site would have class A soil as long as there is no rock to be removed,
as this is expensive. Small rocks are fine; it is the large rocks or reefs of rock that are a
problem. This classification embraces the sandy or rocky sites that have little or no
ground movement associated with moisture fluctuations.
A building site with the S classification is good to build on. While the soil is generally
clay it has only minimal movement with the fluctuations in moisture content.
A building site with the M classification is still a good site for construction, but care
needs to be taken to carefully follow the engineers' design of the foundations. This soil is
made up of silt or clay and can have a medium level of reaction or movement from
moisture variances.
A clay site that has significant movement associated with moisture fluctuations would
be specified as class H. This is still a good building site but there will be additional cost
associated with the foundations. The engineers will take the reactive nature of the clay
soil into consideration when designing the foundations. Ensure that all the engineers'
requirements are strictly adhered to.
In Adelaide there are many building sites with a soil type of classification E. I remember
fences built at 1.65 metres high that four or five years later were less than 1.5 metres high. It
is as if the earth was eating them. The clay is so reactive to moisture variations that stumps
just sink into the ground. In central Victoria stumps are replaced because they rot out, but
in Adelaide they just sink. While doing maintenance for the housing trust in SA we regularly
had to do what is commonly termed as a jack up. It is not the replacement of stumps, rather
lifting the house off the stumps and fixing blocks to the top of the existing stump to
support the house in a level position. Obviously a good engineer can design foundations for
almost any soil type, but this is one soil type I would prefer to avoid.
When the test results come back as classification P it is not a time for celebration.
These sites are quite unstable, prone to erosion and ground movement. While it is
possible to build on these sites it is likely to be quite expensive. It brings back horrible
memories of the house in Eltham that began the slow but steady decent down the hill.
Much musing if not gnashing of teeth followed this realisation of the dilemma. The
concrete raft slab was extremely strong. Even when the house was sliding down the hill it
did not crack. Given that the slab had not cracked, we knew that if we could just stop the
house sliding down the hill it could be levelled and underpinned. The house could be
saved. It would just be in a new position: a trivial issue considering the magnitude of the
problem. As is often the case, the solution was really quite simple. A huge chain wrapped
around a mammoth gum tree. The gum tree had its roots very deep. Way past the
unstable surface soil that the house had been relying on. We have all seen images on
television of houses sliding down mountains after downpours of rain. It is likely that
these sites would have been of P classification.
