Environmental Engineering Reference
In-Depth Information
6.2.1.
The collapse of loess
Collapse is a typical feature of unsaturated, rather loose and low-plasticity soils,
which are typical features of loess. Collapse corresponds to a significant reduction
in volume observed when wetting an unsaturated sample under load. This
phenomenon has been described in arid regions over many years. In 1957, Jennings
and Knight [JEN 57] proposed the double oedometer method to estimate the
amplitude of possible collapse. In 1962, Gibbs and Bara [GIB 62] proposed a simple
collapse criterion based on the dry density and liquid limit.
As commented on by Houston [HOU 95], there are numerous potential sources
of water that can cause soil wetting:
- broken water lines, canals and landscape irrigation [WAL 93];
- roof run-off and poor surface drainage [WAL 93];
- intentional and unintentional recharge [SHM 95];
- rising ground water table [ELN 95];
- damming due to cut/fill construction [KRO 94, NOO 90];
- moisture migration due to capillarity and protection from the sun [JIM 95].
Li [LI 95] also mentioned collapse problems during the impoundment of earth
dams made up of poorly compacted loessial soils in China. Similar problems have
been encountered by Lefebvre
et al.
[LEF 89] in an earth dam made up of
compacted non-plastic till in the Baie James project in Canada.
Barden [BAR 73] gave three conditions in which you can see collapse in a soil:
- an open, potentially unstable, partly saturated structure;
- a high enough applied stress component to develop a metastable condition;
- a high enough suction (or other bonding or cementing agent) to stabilize
intergranular contacts, and whose reduction on wetting will lead to collapse.
Simple capillary forces have often been mentioned as possible binding agents.
However, as stated by Barden, the majority of collapsing soils involve the action of
clay plates in the bonds between the bulky sand and silt grains. Possible effects of
other chemical cementing agents, such as iron oxide or calcium carbonate, are also
mentioned.
[JEN 57] proposed a simple test, called the double oedometer test, to
characterize the susceptibility of soils to collapse. The principle of the test is
described in Figure 6.5. It consists of running two oedometer compression tests:
