Properties of soils
are most important to construction. If we understand main things on soil
properties we can avoid construction failure. For this reason we have to study
about soil properties.
There are residual
soil and sedimentary soils. These are different by formation process with them.
Residual soils are formed
by chemical and physical weathering from patent rock then lie on parent rock.
Sedimentary rocks formed by chemical and physical weathering from parent rock
then transported by wind and river then deposited. But sedimentary soils are
undergoing various additional processes such as erosion, hardening,
cementation, primary consolidation, secondary consolidation, etc. From this
sedimentary became complex than residual soils. When sedimentary rocks are
undergoing to various process, sorting process occurring on sedimentary rock so
sedimentary rocks became homogeneous than residual soils. When we talking about
stress history we can able to identify over consolidated stress and normally
consolidated stress on sedimentary rocks but on residual soils we cannot identify.
These two causes make complex residual oils than sedimentary rocks.
Srilanka have more
residual soils.so if we want construct on residual soils we must know about
characteristics about residual soils. Most of the countries have sedimentary
soils that’s why soil concepts develops for sedimentary soils. We do not do
experimental for residual soils widely. But in construction concept of
sedimentary soils use for residual soils.
When we applied
tezaghi’s concept for residual soils we take 20mm sample for laboratory testing
but in field residual soil characteristics vary within small areas.so this will
affect our values, we draw the settlement versus square root time graph will be
continually curve. And settlement taking place much faster than predicted.
Because of residual soils are partially saturated so consolidation settlement
happed by air dissipation. When we draw settlement versus stress (log scale) there
is virgin consolidation part is linear as sedimentary soils.so there is no
reason for use conventional method using on residual soils.
research using by oedometer when we draw compression curve plotted using log
scale and linear scale they found for some soil same got linear curve have very
close to linear except to high stress. There is thus no increse in
compressibitity with stress.the compressibility either remains constant or
decrease as stress level increase.there is no longer trace, on the linear plots,of
the prconsolidation pressure determined frome the log plots.there is simply no
significant change of slope at the pressures identified as preconsolidation
pressures according to the standard casagrande construction.
Significance of the
In Sri Lanka soil
found as residual soils.so we will have to do construction on residual soils.
Therefore we must apply proper concept for when we dealing with soil. But most
of theory developed for residual soils. And that concept also applying for
residual soils. But as previously explained there are some problems we had face
at in construction by used this conventional theory. Therefore we have to develop
theory for residual soils.