Environmental Engineering Reference
In-Depth Information
zones vertical tubes, called “channels” or “drains” may have been formed by the decom-
position of roots. These may be open or infilled with sand. The mass permeability in
these zones may be 10
2
m/sec to 10
4
m/sec. These near-vertical features are not located
readily by conventional drilling and water pressure testing and so in these cases the perme-
ability is best determined by vertical infiltration tests involving relatively large test areas.
3.11.3
Use of lateritic soils for construction
Lateritic soils usually make excellent earthfill construction material. The most notable
case of the use of lateritic soil was in Sasumua Dam, where Terzaghi (1958) showed that
despite its apparently peculiar classification properties (i.e. plotting below the “A” line),
the lateritic soil was an excellent dam building material. When used as fill, laterite soils
are characterised by high effective friction angle and medium to low density and perme-
ability. In most cases they are readily compacted despite often having high and poorly
defined water content. For example at Sirinumu Dam, lateritic clays were readily com-
pacted at water contents between 40% and 50%. However, some particularly silty lat-
erites with high halloysite contents can be difficult to compact.
The ferricrete gravels and weak rocks in the near-surface crust zone are used in lateritic
areas throughout the world as base or sub-base material in pavements for roads and
airstrips. Strongly cemented rock from this zone has also been used successfully as rock-
fill and rip-rap.
In the laterite and mottled zone soils are usually non dispersive and in the authors'
experience, very resistant to erosion
in situ
and recompacted.
3.11.4
Karstic features developed in laterite terrain
Sinkholes similar to those seen in karstic limestone are known to occur also in some lat-
erite profiles developed on non-carbonate rocks.
Twidale (1987) describes sinkholes up to 50 m diameter and 15 m deep on the Western
Stuart Plateau in Northern Territory, Australia. The plateau is underlain by a sequence of
siltstones and quartzites, ranging from 40 m to 230 m in thickness, which is underlain by
limestone. The laterite profile is developed at the top of the non-carbonate sequence and
may be 20-30 m deep. Twidale (1987) suggests that where the sequence is thin some of the
sinkholes may have formed by the lateritic materials collapsing directly into voids in the
underlying limestone. However he believes that they most probably developed as follows:
-Voids were formed within the lateritic profile by the removal of silica and silicate min-
erals by solution, these being carried down joints, deeper into the sequence, and
- The ferricrete cap collapsed into the voids.
Twidale believes that the subsurface voids developed in late Tertiary time and that col-
lapse of the ferricrete cap into the voids has continued intermittently since that time. He
provides details of the youngest sinkholes, formed in 1982, and describes how many older
ones are much modified by sidewall collapse and infilling with slopewash and alluvial soils.
Twidale (1987) refers also to karstic features in laterite developed on peridotite (ultrabasic
rock) in New Caledonia. The karstic features in that area occur in broad plateaus surrounded
by steep ridges which are remnants of relatively fresh peridotite. They include:
- internal drainage into sinkholes;
- recently active sinkholes up to 15 m wide, and
- swamps, believed to represent collapsed areas now filled with sediment and water.
