ORIGINS AND GEOTECHNICAL ISSUES
Robert Bartus s2721227 | 4 104ENG Geotechnical Engineering Practice | August 19, 2012
The origins of soil deposits on the earth’s crust can reveal significant findings in the chemical and mechanical characteristics of the soils themselves. It is important to uncover such origins to find how and where such deposits may have formed over tens of thousands of years ago. Marine and Lacustrine deposits are two soil types that are commonly encountered during soil exploration, both of which are primarily traced back to either glacial o r more broadly, alluvial origins (Hunt 2005 ). They are somewhat alike in there fine sedimentary composition and similarly can be prone to presenting engineers with problematic conditions such as h igh secondary consolidation characteristics and subsidence, potential acid sulfate soils and quick clay landslides (Das 2007).
The occurrences of marine soils are found in offshore and coastal plain deposits. It is important to note however the time scale of which marine deposition has occurred, such soils have become exposed from residing sea levels, glacial withdrawal (isostasy) and continental/tectonic plate shifts, meaning previous offshore deposits from past glaciation periods, h ave risen and are now exposed to human activity and land use. The origin of marine soils can be traced
back to glacial or alluvial transportation
from land runoff through river outlets
(Hunt 2005). Marine deposition also
occurs due to aeolian transportation
through wind relocation of fine grained
sediments. Though more commonly
encountered are Glacial-Marine clays,
formed from glacial runoff which settle
in marine estuaries along the coastline in
calm depositional environments. These
conditions result in a non-stratified soil
formation packed with fine grained
Figure 1 : Glacial runoff transporting fine
sediments (Mattingly 2008 )
sediments (SoilWebUBC and Grand 2011 ).
Marine and Lacustrine Deposits
Robert Bartus s2721227
The nature and chemical composition of these marine clays can implicate some problematic and concerning issues to both environmental and engineering infrastructure. This is due to the properties of unique marine clays having extremely high sensitivity. Sensitivity being the cause o f quick clay landslides on relatively gentle sloping ground and also issues arising from marine depositions creating potential acid sulfate soil environments (PASS), which have h igh iron sulfide content forming low pH. conditions.
Quick clay also known as Leda clay has been found to be the direct cause of almost all clay landslides in Sweden, Norway and Canada in the past sixty years (Viberg 1984). One of the most iconic and well documented occurred in Rissa, Norway 1978, where a small excavation caused the disturbance and displacement of up to 5-6 million cubic meters of gently sloping farm and coastal land.
The cause was due to the mechanical properties of the marine clay b eing altered after the soil was remolded, or in the case of Rissa, d isturbed by a shallow excavation. The changes in the ground conditions caused the collapse of the soil structure, p roducing an almost complete loss of shear strength , allowing the soil to form a liquid mass. This phenomena is described by clays having extremely high sensitivity, the degree of sensitivity may be defined as the ratio of the unconfined compression strength in an undisturbed state to that in a
remolded state. The range of
which , considered as high as 50 or
more in Sweden (Lundström,
Larsson et al. 2009), also Das
describes the sensitivity range of
most clays from 1 (Insensitive) to 8
(Very Sensitive) any clay classified
over 8 deemed quick, also noting
that highly flocculent marine clay
deposits exhibit sensitivity ranging
from 10 to 80 (Das 2007).
Marine and Lacustrine Deposits
Figure 2 : Montreal Quick Clay Land Slide, destroying a