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Investigation
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| Residential home walls separating in a home in the suburb of Green Island, Dunedin, due to ground instability on slumping hillsides.
V.E. Neall
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Investigating slope instability
Landslide
hazard zonation is usually conducted in response to the recognition
that slope instability has had an impact on human activities in
the past and is thus likely to do so in the future (Varnes 1984,
p. 16-31). Preliminary studies should be conducted to establish:
- What
are the landuses or resources that are affected? Is the problem
local or regional? Are the problems areal or linear? Do they involve
housing, agricultural, transportation, industry, or mineral or
energy resources?
- What
are the economic, social or environmental consequences of slope
failures? What types of decisions by the public or administrative
entities require what kind of information?
- Have
failures occurred in the past and what type are they?
- Should
the investigation be directed towards avoidance, prevention or
correction?
- Is
it better to cover a large area, gathering limited data, or study
one or a few smaller representative areas in more detail?
- What
basic data are already available - geologic, topographic, geomorphic,
hydrogeologic, soils, aerial photos? What more will be required?
- What
kinds of geotechnical investigations - pits, borings, in situ
or laboratory tests appear to be necessary?
- What
kinds of response to hazard delineation can be expected? What
has been the response so far on the part of the public and local
authorities?
- What
are the resources available in terms of time, funds and technical
or support personnel?
Deep-seated landsliding near Utiku, Rangitikei Valley, involving both the main trunk railway line and State Highway 1. There are a number of landslides in this photo. Can you identify them each by their curved fractured surfaces? V.E.Neall
Table 4 sets out a plan for a landslide hazard zonation investigation.
In a simplified statement, hazard zonation requires the acquisition
of relevant data through remote sensing and ground studies and compilation
of this data as maps and reports.
Table
4. A plan for a landslide hazard zonation investigation
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Phase
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Liaison
with engineers
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Desk
studies
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Field
studies
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I
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Brief
received from client, discussions with senior engineers and
engineering geologist involved.
Brief re-examined.
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Familiarisation
with project.
Examination of available literature and maps.
Air-photo interpretation.
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II
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Continuing discussions with engineer's field staff.
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Field
mapping.
- investigation of landforms, materials and processes.
- review of trial pit and borehole information (if available.
Geomorphological map compilation.
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III
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Report
with maps passed to client.
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Derivative
maps compiled.
Data additional to initial brief compiled.
Site investigation suggestions defined.
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Remote sensing and ground surveys
Remote
sensing involves examination of objects or phenomenon without being
in actual contact with them. Information about landslides can be
gathered from air photographs. In the hands of an experienced interpreter
and in combination with a good knowledge of the geology and geomorphologic
development of the region being assessed. Thermal infra-red imagery
can also be used as it can be used to evaluate moisture and drainage
conditions (Crozier 1984, p. 122-125). Satellite imagery allows
a combination of geomorphic, hydrologic and vegetation parameters
to be analysed in one image (Varnes, 1984, p. 16-17).
Remote
sensing is a powerful technique for landslide hazard zonation. However,
any conclusions based on air photo interpretation, for example,
should be tested in the field. Geological and/or geomorphological
maps may not be available and ground surveys will be required to
create these. Dense forest may cover old landslide scars and deposits,
and air photos and images may not be representative of seasonal
conditions that promote land instability. The recognition and identification
of various slope movements is the most important aspect of ground
surveys (Varnes, 1984, p.17-20). Features which are used to help
establish the style of slope movement are shown in Table 5 and Figure
6 (landslides). The paper by M. J. Crozier (1984) provides a comprehensive
study of the recognition, analysis and assessment of the field evidence
for slope instability in New Zealand.
Table
5. Ground features used to identify mass movements (from
Varnes 1984).
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Type
of motion
Kind of material
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Parts
surrounding slide
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Crown
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Main scarp
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Flanks
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Falls,
topples.
Rock
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Consists
of loose rock; probably has cracks behind scarps; has irregular
shape controlled by local joint system.
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Is usually
almost vertical, irregular, bare and fresh; usually consists
of joint or fault surfaces
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Are mostly
bare edges of rock.
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Soil
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Has cracks
behind scarp.
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Is nearly
vertical, fresh, active and spalling on surface.
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Are often
nearly vertical.
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Slides.
Rotational.
Slump.
Soil
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Has numerous
cracks that are mostly curved toward slide.
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Is steep,
bare, concave towards and commonly high; may show striae and
furrows on surface running from crown to head, may be vertical
in upper part.
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Have striae
with strong vertical component near head and strong horizontal
component near foot; have scarp height that decreases toward
foot; may be higher than original ground surface between foot
and toe; have en echelon cracks that outline slide in early
stages.
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rock
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Has cracks
that tend to follow fracture pattern in original rock.
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Is steep,
bare, concave toward slid and commonly high; may show striaeand
furrows on surface running from crown to head, may be vertical
in upper part.
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Have striae
with strong vertical component near head and strong horizontal
component near foot; have scarp height that decreases toward
foot; may be higher than original ground surface between foot
and toe; have en echelon cracks that outline slide in early
stages.
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Translational.
Block.
Rock or soil
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Has cracks
most of which are nearly vertical and tend to follow contour
of slope.
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Is nearly
vertical in upper part and nearly plane and gently to steeply
inclined in lower part.
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Have low
scarps with vertical cracks that usually diverge downhill.
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Rock.
Rock
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Contains
loose rock; has cracks between blocks.
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Is usually
stepped according to spacing of joints or bedding planes;
has irregular surface in upper part and is gently to steeply
inclined in lower part; may be nearly planar or composed of
rock chutes.
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Are irregular.
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Flows.
Dry.
Rock
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Consists
of loose rock; probably has cracks behind scarps; has irregular
shape controlled by local joint system.
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Is usually
almost vertical, irregular, bare and fresh; usually consists
of joint or fault surfaces.
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Are mostly
bare of edges of rock.
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Soil
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Has no
cracks.
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Is funnel
shaped at angle of repose.
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Have continuous
curve into main scarp.
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Wet.
Debris avalanche.
Debris flow.
Soil
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Has few
cracks.
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Typically
has serrated or v-shaped upper part; is long and narrow, bare
and commonly striated.
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Are steep
and irregular in upper part; may have levees built. up in
lower parts.
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Earth
flow.
Soil
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May have
a few cracks.
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Is concave
toward slide; in some types is nearly circular and slide issues
through narrow orifice.
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Are curved;
have steep sides.
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Sand flow.
silt flow.
Soil
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Has few
cracks.
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Is steep
and concave toward slide; may have variety of shapes in outline
nearly straight, gentle arc, circular or bottle-shaped.
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Commonly
diverge in direction of movement.
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Table 5 continued
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Type
of motion.
Kind of material
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Parts
that have moved
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Head
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Body
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Foot
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Toe
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Falls,
topples.
Rock
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Is usually
not well defined; consists of fallen material that forms heaps
of rock next to scarp.
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Falls:
has irregular surface of jumbled rock that slopes away from
scarp and that, if tree or material of contrasting colours
are included, may show direction of movement radial from scarp;
may contain depressions.
Topples:
Consists of unit or units tilted away from crown.
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Is commonly
buried; if visible generally shows evidence of reason for
failure such as prominent joint or bedding surface; underlying
weak rock or banks undercut by water.
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Is irregular
pile of debris or talus if slide is small, may have rounded
outline and consist of broad, curved transverse ridge if slide
is large.
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Soil
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Is usually
not well defined; consists of fallen material that forms heap
of rock next to scarp.
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Is irregular.
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Is commonly
buried; if visible generally shows reason for failure, such
as prominent joint or bedding surface; underlying weak rock
or banks undercut by water.
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Is irregular.
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Slides.
Rotational.
Slump.
Soil
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Has remnants
of land surface flatter than original slope or even tilted
into hill, creating at base of main scarp depressions in which
perimeter ponds form; has transverse cracks, minor scarps,
grabens, fault blocks, bedding attitude different from surrounding
area and trees that lean uphill.
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Consists
of original slump blocks generally broken into smaller masses;
has longitudinal cracks, pressure ridges and occasional overthrusting;
commonly develops small pond just above foot.
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Commonly
has transverse cracks developing over foot line and transverse
pressure ridges developing below foot line, has zone of uplift,
no large individual blocks and trees that lean downhill.
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Is often
a zone of earth flow of lobate form in which material is rolled
over and buried, has trees that lie flat or at various angles
and are mixed into toe material.
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Rock
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Has remnants
of land surface flatter than original slope or even tilted
into hill, creating at base of main scarp depressions in which
perimeter ponds form; has transverse cracks, minor scarps,
grabens, fault blocks, bedding attitude different from surrounding
area and trees that lean uphill.
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Consists
of original slump blocks somewhat broken up; has little plastic
deformation; has longitudinal cracks, pressure ridges and
occasional overthrusting; commonly develops small ponds just
above foot.
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Commonly
has transverse cracks developing over foot line and transverse
pressure ridges developing below foot line, has zone of uplift,
no large individual blocks and trees that lean downhill.
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Has little
or no earth flow; is often nearly straight and close to foot;
may have steep front.
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Translational.
Block.
Rock or soil
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Is relatively
undisturbed and has no rotation.
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Is usually
composed of single or few units, is undisturbed except for
common tension cracks that show little or no vertical displacement.
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Has none
and no zone of uplift.
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Ploughs
or overrides ground surface.
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Rock.
Rock
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Has many
blocks of rock.
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Has rough
surface of many blocks some of which may be in approximately
their original attitude but lower if movement was slow translation.
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Usually
has none.
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Consists
of accumulation of rock fragments.
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Flows.
Dry.
Rock
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Has none.
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Has irregular
surface of jumbled rock fragments sloping down from source
region and generally extending far out on valley floor; shows
lobate transverse ridges and valleys.
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Has none.
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Composed
of tongues; may override low ridges in valley.
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Soil
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Usually
has none.
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Is conical
heap of soil, equal in volume to head region.
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Has none.
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Wet.
Debris avalanche.
Debris flow.
Soil
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May have
none.
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Consists
of large locks pushed along in a matrix of finer material;
has flow lines; follows drainage ways and can make sharp turns;
is ver long compared to breadth.
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Is absent
or buried in debris.
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Spreads
laterally in lobes; if dry may have a steep front about a
metre high.
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Earth
flow.
Soil
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Commonly
consists of a slump block.
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Is broken
into many small pieces; shows flow structure.
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Has none.
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Is spreading
and lobate, consists of material rolled over and buried; has
trees that lie flat or at various angles and are mixed into
toe material.
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Sand flow.
Silt flow.
Soil
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Is generally
under water.
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Spreads
out on underwater floor.
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Has none.
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Is spreading
and lobate.
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