that if an earth dam is built with careful constructioncontrol and incorporates filters, then it should be safe enough, even if it is constructed with dispersive soil.. Alternatively, hy
Trang 1that if an earth dam is built with careful construction
control and incorporates filters, then it should be safe
enough, even if it is constructed with dispersive soil
Alternatively, hydrated lime, pulverised fly ash,
gyp-sum, or aluminium sulphate have been used to treat
dispersive soils used in earth dams
Humid Tropical Zone Soils
In humid tropical regions, weathering of rock is more
intense and extends to greater depth than in other parts
of the world Residual soils develop in place as a
conse-quence of weathering, primarily chemical weathering
The mineralogy of residual soils is partly inherited from
the parent rock from which they were derived and
partly produced by the processes of weathering
Hence, the mineralogy varies widely, as does grain
size and unit weight The particles and their
arrange-ment evolve gradually as weathering proceeds In
ad-dition, weathering of parent rock in situ may leave
behind relict structures that may offer weak bonding
even in extremely weathered material Low strength
along relict discontinuities may be attributable to
par-ticles being coated with low-friction iron/manganese
organic compounds
Reproducible results from some standard tests
may be difficult to obtain from residual tropical
soils Different results can be obtained depending
upon whether the soil is pre-dried prior to testing
or kept close to its natural moisture content Also,
disaggregation of the soil structure, especially in rela-tion to particle size analysis has proved problematic Consequently, conventional index tests frequently have been modified in an attempt to make them more applicable for use with tropical residual soils
Of course, it would be wrong to assume that all trop-ical soils behave differently from those found in other climatic regions For instance, alluvial clays and sands behave in the same manner and have similar geotech-nical properties, regardless of the climatic conditions
of the region of deposition
Drying brings about changes in the properties of residual clay soils in that it initiates two important effects, namely, cementation by the sesquioxides and aggregate formation on the one hand, and loss of water from hydrated clay minerals on the other In the case of halloysite, the latter causes an irreversible transformation to metahalloysite Drying can cause almost total aggregation of clay size particles into silt and sand size ranges, and a reduction or loss of plasti-city Cycles of wetting and drying may increase the stiffness of the soil fabric, which increases its shear strength and decreases its compressibility
Laterite can be regarded as a highly weathered ma-terial that forms as a result of the concentration of hydrated oxides of iron and aluminium in such a way that the character of the deposit in which they occur is affected These oxides may be present in an unhar-dened soil, as a harunhar-dened layer, as concretionary no-dules in a soil matrix or in a cemented matrix enclosing
Figure 6 Failed earth dam constructed of dispersive soil, showing piping outlets on the downstream side, near Ramsgate, South Africa.
560 ENGINEERING GEOLOGY/Problematic Soils