Geology for Initial Planning and Scoping
Geology for Construction Materials
Geohazard Identification
Impacts of Geology on Construction
Geohazard Identification (Part 4)
“A geohazard is a geological state that may lead to widespread
damage or risk. Geohazards are geological and environmental conditions and
involve long-term or short-term geological processes. Geohazards can be
relatively small features, but they can also attain huge dimensions (e.g.,
submarine or surface landslide) and affect local and regional socio-economy to
a large extent (e.g., tsunamis)” (Wikipedia, 2021).
As the name suggests (GEOhazard), the geological conditions
are the most important factors to look at when assessing geohazards. Geohazards
exist for every civil development completed across the world. Some of the most
obvious ones include:
These are large scale natural hazards that are generally
considered at a regional scale. They are also typically more obvious and well
known.
Of course, the Meckering earthquake kind of disproves this theory!
“For the people of Meckering in the Wheatbelt region
of Western Australia, the public holiday on 14 October 1968 probably
started like any other. But at 10.59 am, a devastating earthquake measuring 6.9
on the Richter scale took only 40 seconds to smash buildings to the ground and
reduce the main street to rubble” (Johnston, JF and White SR, 2018).
This geohazard was NOT well known before the earthquake. Australia’s
first Earthquake Code AS 2121, was published in 1979 as a direct result of the
Meckering earthquake. This meant that
updated building codes were gradually rolled out in other states in subsequent
decades, due to growing awareness of seismic hazards nationally.
This just goes to prove that there may be some geohazards
that exist that do not get identified!
This concept was also covered in my previous blog (The Lifecycle of Geology in a Civil Engineering Project (Part 1) – Geology for InitialPlanning and Scoping) about the Leaning Tower of Pisa.
The full geological conditions and associated geohazards were
not fully understood until 2001, when serios stabilisation was undertaken. This
was 828 years after construction began!
Apart from the large scale natural geohazards, there are the
smaller, more localised geohazards that are relevant to a specific development
or site.
These can include:
- Soft soils (like beneath the Leaning Tower of Pisa)
- Organic soils
- Calcareous sands
- Man made materials (landfills, reworked/built up areas)
- Reactive clays
- Collapsible soils
- High groundwater levels
- Karst/solution features/sink holes
- Erosion (coastal, river etc.)
The presence of any of these geohazards could have a
significant impact on the proposed development. The sections below provide some
further information on some of the geohazards listed above.
Karst Features
A karst feature is a topographic (ground) surface formed by
the dissolution of soluble rocks, such limestone or dolomite.
In the City of Wanneroo, karst features are everywhere!
Anyone been to Crystal Cave in Yanchep?
Although the Crystal Caves are large and obvious, there are hundreds
of others across the city.
The underlying geological formation (Tamala Limestone) is a
porous rock prone to solution by weakly acidic water such as groundwater and
rainfall. These waters circulate through cracks and pores within the limestone
aiding the carbonic acids in these waters to remove the calcium carbonate from
the limestone, by solution. This process is known as karst weathering.
Sometimes, these features can occur outside of known areas
of karst, which makes it difficult to identify the hazard!
Reactive Clays
Reactive clays are those that shrink and swell when they dry
or get wet. Clays with a high content of expansive minerals can form deep
cracks in drier seasons or years. The geological origin of these soils is very
important because clays with kaolinite,
montmorillonite and
halloysite, have the
most dramatic shrink-swell capacity.
The presence of these materials can affect the integrity of
structures.
Fortunately, the presence of these soils is relatively well
known because of their link to the geological formation from which they came
(i.e weathering of igneous rocks). They are relatively well mapped across the
state.
As a geologist, being able to identify the geohazards for a
given development on a site is vital.
Afterall, we don’t want our building falling down because we
didn’t identify a known geohazard!
References
Johnston, JF and White, SR (2018) Understanding
the Meckering earthquake: Western Australia, 14 October 1968: Geological
Survey of Western Australia, 26p.