ESTIMATION OF TRAVEL DISTANCE FOR LANDSLIDES IN SOIL
SLOPES
Gavan Hunter1 and Robin Fell2
Research Engineer, School of Civil and Environmental Engineering, The University of New South Wales
2
Professor, School of Civil and Environmental Engineering, The University of New South Wales, Sydney
1
ABSTRACT
Methods for prediction of the post failure travel distance for landslides from cuts, fills and natural soil slopes are
presented. The methods first require assessment of the likely mechanics of initial sliding, based on the material
properties and slope geometry with a view to identifying if the subsequent travel of the landslide will be “rapid” or
“slow”. The post failure travel distance is then estimated for “rapid” slides from consideration of the slide mechanism,
material type, slope geometry and/or slide volume; and for “slow” slides based on the residual factor of safety and
estimated surface of rupture.
1
INTRODUCTION
“Rapid” landslides in soil slopes have the potential for loss of life, destruction of property and damage to the natural
environment because the velocity of the slide mass is such that persons in the travel path do not have time to evacuate,
and the kinetic energy is such that even small landslides can destroy buildings and other structures. “Slow” landslides
do not have the same destructive capabilities, but they are capable of causing significant damage to property. To assess
the risk imposed by a landslide it is necessary to estimate the annual probability of sliding and slide volume, the travel
distance and velocity, the elements at risk (property and persons) and the vulnerability of the persons or property in the
path of the slide, considering the likelihood that persons may have time to evacuate from the travel path.
The travel distance and slide velocity, which are described in this paper, affect the areal extent impacted by landsliding
and hence the elements at risk, the likelihood that persons can evacuate and the vulnerability. Available methods for
estimating the travel distance (or travel distance angle) of “rapid” landslides are usually based on slide volume (Heim
1932; Scheidegger 1973; Hsu 1975; Smith and Hungr 1992; Corominas 1996; Finlay et al 1999; amongst others) as the
main dependent variable, with further differentiation in some methods based on the type of slope (cut, fill or natural
slope); slide classification (rock fall, debris flow, etc.); degree of confinement of the travel path; and obstructions to the
slide mass. The methods cover a very large range of volumes from several cubic metres to hundreds of billions of cubic
metres, and there is great uncertainty in the estimates of travel distance particularly at the smaller volume range
appropriate to most slides in soil slopes. Slide velocity is estimated by consideration of the potential energy and energy
losses described by the rheological model used for analysis, mainly frictional energy. The estimates can be crude and
inaccurate due to simplifying assumptions used in the modelling.
This paper reports the results of analysis of data from about 450 landslides in predominantly soil slopes and presents
methods for identifying whether a landslide will travel “rapidly” or “slowly” for a number of classes of landslide. It
then presents methods for estimating the post failure travel distance. For “rapid” landslides the travel distance angle of
the failed slide mass is calculated from assessment of the failure mechanics of the initial slide (whether contractile or
dilative on shearing), the type of slope, slide volume, geometry of the slope at and below the slide source area, and the
degree of confinement of the travel path of the landslide.
For “intact”, generally “slow” landslides estimation of the post failure travel distance is based on the residual factor of
safety allowing for strain weakening along the surface of rupture and assessment of the likely influence of inertia forces
of the sliding mass.
2
DEFINITIONS
The following terms are used in this paper.
Landslide velocity. The terms used are those defined by IUGS (1995) and are given in Table 1.
Australian Geomechanics – May 2002
65
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
Velocity
Classification
7
6
5
4
3
2
1
HUNTER & FELL
Table 1: IUGS (1995) velocity classifications for landslides.
Description of
Velocity limits
Value in mm/sec
Velocity
Extremely rapid
> 5 m/sec
> 5x103 mm/sec
Very rapid
3 m/min to 5 m/sec
50 to 5x103 mm/sec
Rapid
1.8 m/hour to 3 m/min
0.5 to 50 mm/sec
Moderate
13 m/month to 1.8 m/hour
5x10-3 to 0.5 mm/sec
Slow
1.6 m/year to 13 m/month
50x10-6 to 5x10-3 mm/sec
Very slow
16 mm/year to 1.6 m/year
0.5x10-6 to 50x10-6 mm/sec
Extremely slow
£ 16 mm/year
£ 0.5x10-6 mm/sec
“Rapid” Landslide – a landslide that has a travel velocity in the rapid, very rapid or extremely rapid class as defined by
IUGS (1995). Most of what are termed “rapid” landslides in this paper fall into the very and extremely rapid classes,
having velocities greater than 3 metres/minute and generally in the order of metres/sec.
“Slow” Landslide – a landslide that has a travel velocity in the moderate, slow or very slow class as defined by IUGS
(1995), in the range 10’s of mm/year to less than 1.8 m/hour.
Landslide Classification is according to Hutchison (1988), but a dual classification is used to describe the initiating
landslide in the source area and the subsequent slide in the travel region for “rapid” landslides. The main terms used to
describe the movement of the landslide are shown in Figure 1 and defined below.
Figure 1: Landslide classification system and main slide types.
Dilative and Contractile Slides - description of the initial tendency of the soil on the surface of rupture to increase
(dilate) or decrease (contract) in volume under drained shear, or to develop negative or positive pore pressures in
undrained shear when in a saturated (or near saturated) condition.
Flow Slide is used to describe the initiating slide in saturated (or near saturated) contractile soils, where the failure or
“rapid” acceleration of the slide mass occurs due to large loss of strength (“liquefaction”) on undrained shearing. Flow
slide is also used as a travel classification descriptor to describe landslides where the main volume of the slide mass is
bodily carried on a liquefied basal zone.
Defect Controlled Slide is used to describe landslides that are dominantly controlled by defects in the soil or weathered
rock mass.
“Slide of Debris” is a general term that is used to describe landslides in colluvium, talus or other slope mantling debris.
Debris Flow is used to describe turbulent slide movements of a combination of water, air and debris. The slide mass is
a broken up mass of material that no longer retains its original structure or fabric.
Debris Slide or Intact Slide is used to describe sliding on a defined basal surface where the slide mass retains its
structure and fabric during travel. Debris slide is generally used as a descriptor for slides that travel beyond the source
area, and intact slide for slides where post failure deformation is largely confined to the source area.
Source area slope angle ( a 1 ), rupture surface inclination ( a base ), initial downslope angle ( a 2 ), distal downslope
angle ( a 3 ), defined in Figure 2, and cut slope angle ( a cut ), are used to describe the longitudinal slope geometry.
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HUNTER & FELL
Travel distance, L, travel distance angle, a , and landslide height, H, defined in Figure 2, are used to describe the
overall longitudinal geometry of the slide mass from crest of the source area to distal toe of the travel for sub-groups of
landslides that are generally classified as “rapid”.
Figure 2: Definition of travel distance, travel distance angle and slope geometry.
Degree of confinement of the travel path. Terms used to describe the degree of confinement of the travel path are;
confined, the travel path is constrained by the relatively steep side-slopes of a gully or small valley; unconfined, the
travel path is on open slopes; and partly confined; the travel path is not sharply defined by a topographic depression.
Slope failure geometry is classified based on the position of the initial failure within the slope and the longitudinal shape
of the downslope geometry. Types 1, 2 and 5 are defined in Figure 3 and Type 3 in Figure 9. For Types 1 and 2 the
slope below the cut or fill slope is near horizontal.
Figure 3: Classification of slope categories (a) Type 1 – failure at top of cut of fill slope, (b) Type 2 – toe of failure
coincident with toe of cut or fill slope, and (c) Type 5 – surface of rupture extends below and daylights beyond the toe
of the slope.
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ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
3
HUNTER & FELL
DATA USED IN THE STUDY
The database of case studies comprises some 450 landslides in predominantly soil slopes. The case studies have been
divided into the broad groupings “rapid” landslides and “slow” intact landslides, reflecting the methods used for
analysis of the post failure deformation behaviour. “Rapid” landslides generally travel significant distance beyond the
source area and often become disaggregated in travel. For “slow” landslides the travel of the slide mass is generally of
limited extent (less than about 5 to 10 m in most of the case studies, but up to 20 to 30 m), and the slide mass remains
intact during deformation, most of which is retained in the source area. It is recognised that there is some overlap
between the two broad groupings. A detailed description of the data is given in Hunter and Fell (2001, 2002).
4
IDENTIFICATION OF “RAPID” AND “SLOW” LANDSLIDES
4.1
IDENTIFICATION OF “RAPID” LANDSLIDES
There are three classes of soil slope that are likely to result in “rapid” landsliding once an initial failure has been
triggered:
(i)
Flow slides in saturated (or near saturated) granular soils that are contractile on shearing under the stress
conditions imposed by the slope geometry, and reach a flow liquefaction condition as shearing continues, e.g.
loose granular fills, mine waste stockpiles, mine tailings, hydraulic fills and submarine slopes.
(ii)
Slides in sensitive clays (or quick clays), and fine grained loess soils, where changes in pore water chemistry or
the method of deposition result in a soil structure that is contractile on shearing.
(iii)
Slides in dilative soils, including in steep cut slopes in residual soil, colluvium or completely weathered rock;
“slides of debris” from slopes with a steep source area slope angle; and defect controlled slides in soil or
completely weathered rock where the slope of the basal surface ( a base ) is greater than the friction angle of the
basal surface of rupture.
It is important to distinguish between the trigger mechanism/s for the processes leading up to slope instability (e.g.
rainfall) and factors that lead to “rapid” sliding, i.e., the slope geometry and material properties within the unstable
slope. For example, heavy rainfall and hydro-geological conditions may be a significant factor in the development of
an unstable slope condition, but do not mean that “rapid” sliding will occur, that will depend on the slope geometry and
material properties such as whether it is contractile or dilative.
4.1.1
“Rapid” Flow slides in contractile soils
The mechanics of contractile granular soils is complex and subject to ongoing research. Fell et al (2000) and Hunter
and Fell (2001) give a summary. The collapse surface or instability line concept, proposed by Sladen et al (1985),
provides a useful framework for understanding the mechanics associated with static liquefaction of contractile soils and
failure at effective stress states below the steady state line.
The structure, relative density, degree of saturation and permeability of a soil are important in assessing whether or not
a soil under the applied effective stress conditions has the potential to liquefy under static loading. Slope geometrical
factors and pore water pressure conditions determine the effective stress conditions, and together with the material
properties define the regions of the slope that may be susceptible to liquefaction (Lade 1992) and subsequent flow
sliding. It is a necessary condition of static liquefaction that the soil is saturated (or near saturated) so that load can be
transferred onto the pore fluid as the soil contracts on shearing. Permeability is also important because if sufficiently
high, pore pressures developed on shearing may be readily dissipated and therefore a liquefaction condition will not
develop.
The triggering of static liquefaction is a change in effective stress conditions and the associated strains that could
potentially result in contraction of the soil mass and development of undrained conditions. Interfaces defining different
materials in the slope can be important in the triggering of flow slides, as differences in modulus and permeability will
concentrate shear strains under changes in effective stress conditions.
Particle size distributions for soil types susceptible to liquefaction in undrained loading and within which flow slides
have occurred are given in Figure 4. Most cases were statically triggered but several of the hydraulic fill embankment
dams and mine tailings case studies were earthquake triggered. For the mine tailings, in most cases the tailings were
derived from the crushing of ore, and the finer clay and silt fractions are predominantly finely ground rock particles
rather than clay minerals. Therefore, the finer particle size distributions, representative of the middle region of the
beach, generally classify as silts of low plasticity (plasticity index typically less than about 5 to 10%).
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Also shown in Figure 4 are the approximate bounds (the shaded region) of man made soils susceptible to static
liquefaction and flow sliding, and are representative of soils within which the initial slide occurs. The finer limit
excludes the finer grained soil types representative of the core regions of hydraulic fill embankment dams and middle
beach regions of hydraulically placed mine tailings because the initial flow slide was not in these soil types, rather these
soils liquefied after the initial slide due to instability of the retrogressive back-scarp. The coarse boundary,
representative of sandy gravels with a trace of mostly silty fines (from coal mine waste dumps), is probably an upper
bound particle size distribution due to permeability constraints. Materials with such coarse particle size distribution are
likely to be sufficiently permeable so that pore pressures developed on contraction can be dissipated. These gradation
limits are representative of the case studies analysed and should be used with caution. It is important to recognise the
influence of other material properties, such as relative density, and slope geometry on susceptibility to liquefaction and
potential for flow sliding.
Figure 4: Particle size distributions of material types susceptible to liquefaction and flow sliding, and approximate
bounds of man made soils susceptible to static liquefaction and flow sliding (initial slide).
Based on a review of the literature, an approximate guide to soil types and their relative density that are susceptible to
flow liquefaction is:
·
Clean sands of relative density less than about 15 to 30% (i.e. very loose to loose sands), but this is dependent
on the shape of the particle size distribution curve and the particle shape. Coarse and rounded sands are likely
to be less susceptible than fine and angular sands in terms of relative density.
·
Silty sands with relative densities up to 45 to 60% (very loose to medium dense).
·
Silty sands with clay contents less than about 10% to 20% derived from decomposed granite at density ratios
below 85 to 90% of Standard Maximum Dry Density (HKIE 1998). Flow slides in these materials are
typically of shallow depth (up to 3 m) and on slopes steeper than about 30 to 34 degrees.
·
Laboratory testing on coal waste and coking coal (Dawson 1994; Eckersley 1986) indicate these sandy gravels
to gravelly sands with trace to some silt fines (less than 5 to 10% passing 75 micron) are susceptible to
liquefaction at void ratios greater than about 0.3. From field studies, moisture content at placement (Eckersley
1990; Dawson et al 1998) and the method of placement have a significant effect on the initial void ratio of
these materials, with flow slides in mine waste spoil piles generally occurring where the spoil pile has been
placed by tipping from low height onto the crest of the active dump. No cases of flow slides have been
reported in spoil piles formed by dumping from a height such as by dragline. Foundation slope is also an
important factor for flow slides in mine waste spoil piles with all reported cases from coal mine waste spoil
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ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
HUNTER & FELL
piles on relatively steep hillsides (averaging 25 degrees for the flow slides in British Columbia and 17 degrees
in South Wales).
Sensitive clays (with a sensitivity usually greater than 10 to 20) are susceptible to liquefaction, but the significant risk
associated with flow slides in these soils is the potential for development of retrogressive flow sliding. The soil
properties and conditions for which retrogressive flow sliding is likely to occur are summarised by Tavenas (1984),
Leroueil et al (1996), Lefebvre (1996) and Trak and Lacasse (1996).
Laboratory and field testing methods are available for identification of soils susceptible to flow liquefaction. The
methodology for assessment based on laboratory undrained triaxial testing is in principle relatively straightforward, but
the practical application has proven difficult. Most of the difficulty is associated with obtaining “undisturbed” samples
of soils, the sensitivity of the soil behaviour to small changes in void ratio, complexity of factors such as in-situ
stratification, and the effects of stress path. If laboratory testing is to be used to assess the potential for flow
liquefaction, care must be taken to model the void ratio, fabric, initial stress conditions and stress path of the in-situ soil.
Field methods of assessing the susceptibility of sands and silty sands to flow liquefaction under earthquake and static
loading are based on Standard Penetration Tests, SPT, and Cone Penetration Tests, CPT. Hunter and Fell (2001)
provide a summary. A useful field method for assessment of susceptibility to flow liquefaction is considered to be the
recent research by Cubrinovski and Ishihara (2000) where assessment of the flow liquefaction potential, for sands and
silty sands, takes into consideration the field strength profile through SPT test results, particle shape and particle size
distribution through the maximum and minimum density of the sand.
4.1.2
“Rapid” Landslides in dilative soils
The transformation of the initial slide in dilative soil into a “rapid” debris slide or debris flow is dependent on the
material properties and slope geometry. The range of material type is very broad, ranging from high plasticity silts to
low plasticity sandy clays and clayey silty sands to dominantly coarse granular soils with low fines contents (less than
about 10% minus 75 micron). Figure 5 provides particle size distributions from the source area of landslides that
developed in “rapid” debris slides or debris flows. In general, “slides of debris” that undergo significant break up and
remoulding, transforming into debris flows range from clayey silty sands (with low plasticity fines) to coarse grained
granular soils, predominantly cobble to boulder size, with low fines contents, and travel velocities are typically in the
range very rapid to extremely rapid. “Slides of debris” that remain relatively intact during travel (i.e., debris slides)
generally comprise the higher clay content soil types (high plasticity silts and low to medium plasticity sandy clays and
clayey sands), and reach maximum slide velocities in the order of metres per minute (rapid to low end of very rapid
category).
In terms of slope geometry, “rapid” travel of the slide mass predominantly occurs where the source and immediate
downslope angle is greater than about 25 degrees, but can occur on slopes down to about 18 to 20 degrees. These slope
gradients are typical for regions around the world, and landslides occurred on open slopes as well as in areas conducive
to concentration of surface and/or subsurface water.
“Rapid” sliding in cut slopes for the case studies from Hong Kong occurred for cut slopes greater than about 34 degrees
and in colluvium, residual soils and decomposed rock types derived from granitic and volcanic rocks (generally silty
sands to sandy silts with low clay contents containing varying quantities of gravel to boulder sized material). Finlay et
al (1999) report a minimum cut slope angle of 30 degrees for sliding in Hong Kong, but according to their data only a
small number of slides (in Hong Kong) occur in cut slopes this shallow, with 95% of slides occurring in cuts greater
than 45 degrees. It should also be recognised that the percentage of cut slopes in Hong Kong shallower than about 30
degrees is very low.
For cuts in high plasticity clays (case studies in London clay and Upper Lias clay), the post failure movement classified
in the rapid class for Type 1 and Type 2 slope failure geometries in cut slopes steeper than 30 to 35 degrees. For the
Type 1 slope failure geometries, the toe of the slide broke up and travelled “rapidly” down the face of the cut slope for
cut slopes steeper than about 22 degrees, but most of the slide mass travelled at a “slow” velocity and was retained in
the source area.
For the defect controlled slides in dilative soils only a limited number of slides were analysed and they were all from a
similar geological environment, making it impossible to give general conclusions with any degree of confidence.
However, the analysis indicated that “rapid” sliding occurred where the inclination of the basal slope of the surface of
rupture ( a base ) is 5 to 20 degrees greater than the estimated basic friction on the surface of rupture ( f b ) for compound
slides and 0 to 15 degrees for translational slides. The principle that basal slide surfaces needs to be steeper than the
residual strength for “rapid” sliding to occur is a logical one, and the findings can be used as a guide. These findings
are in general agreement with the findings for “rapid” slides in rock slopes (Glastonbury and Fell 2000).
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HUNTER & FELL
100
2
3
1
Percent passing (%)
80
2
1
1
1
1
4
60
5
40
20
1
0
0.001
Clay
0.01
Silt
6
7
0.1
1
Sand
10
100
Gravel
Particle size (mm)
1 - San Francisco Bay region (Ellen et al 1988; Johnson & Rodine 1984; Fleming et al 1989)
2 - La Honda, San Mateo County, San Francisco (Ellen et al 1988)
3 - Alameda County, San Francisco (Ellen et al 1988)
4 - Heath Canyon, Los Angeles (Johnson and Rodine 1984)
5 - Colluvium, Hong Kong (case studies), grading estimated from description
6 - Colluvium, Japan (Ikeya1989), grading estimated from description
7 - Vancouver & Queen Charlotte Islands, Canada (Fannin et al 1996), grading estimated from
description
Figure 5: Source area particle size distributions of dilative slides of debris in natural slopes.
4.2
IDENTIFICATION OF “SLOW INTACT” LANDSLIDES
“Slow” landslides that usually remain relatively intact during sliding occur in compacted fills (such as dams), cuts in
high plasticity clays and fills on soft ground.
4.2.1
Compacted Fills
Fills constructed of granular and cohesive fine grained soils are usually likely to fail in a “slow” and intact manner if the
fills are well compacted (i.e. density ratio greater than about 95% of Standard Maximum Dry Density). The
performance of dams confirms this. However, for several case studies “rapid” movement of the failed slide mass was
observed:
·
Where the foundation was contractile on shearing and susceptible to flow liquefaction.
·
Where brittleness was associated with the mechanics of failure. Typical examples of slide brittleness were in
the formation of the back-scarp through brittle soil type, through release on the lateral margins and in slopes
with protected concrete facings.
·
Type 1 slope failure geometries with fill slopes greater than about 27 degrees, where the toe of the slide mass
broke up and travelled “rapidly” down the face of the fill slope, but where most of the slide mass travelled at a
“slow” velocity and was retained in the source area.
4.2.2
Cut Slopes in Heavily Over-Consolidated High Plasticity Clays
Cuts in heavily over-consolidated high plasticity clays usually fail in a “slow” and intact manner confirmed by the
analysed case studies from cuts in London clay and Upper Lias clay, but this is dependent on the slope failure geometry
and cut slope angle. For cut slopes less than about 30 degrees and of Type 1 or 2 slope failure geometry the bulk of the
slide mass remained intact and travelled at a “slow” velocity. For cut slopes steeper than 30 to 35 degrees and of Type
1 or 2 slope failure geometry break up and “rapid” velocity of the slide mass is likely.
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ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
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For Type 5 slope failure geometries (all cases associated with retained cut slopes) the slide mass remained intact and 4
of 5 cases were of “slow” post failure velocity. In one Type 5 case study the post failure velocity was in the rapid
range, possibly due to development of the shear surface through the retaining wall on the lateral margins.
4.2.3
Fills on Soft Ground
The fills on soft ground case studies analysed (13 case studies) were all from well-monitored case studies that were
constructed to failure. The slide remained intact during sliding and in most cases travelled at a “slow” velocity. In
several cases the velocity of the intact slide mass was estimated to be in the rapid range, they included two case studies
with low plasticity sensitive clay foundations and one case study with ductile clay foundations where rapid movement
was possibly due to internal brittleness in the slide mechanism.
5
METHODS FOR PREDICTING TRAVEL DISTANCE ANGLE OF “RAPID”
LANDSLIDES
5.1
TRAVEL DISTANCE ANGLE VERSUS SLIDE VOLUME AND DEGREE OF CONFINEMENT
Corominas (1996) analysed data for 204 landslides in soil and rock, and found reasonable correlation of H/L to slide
volume using a power function of the form of Equation 1, for division of the data set based on landslide type,
confinement of the travel path and consideration of obstructions to travel.
H / L = A ×V B
Equation 1
where H and L are in metres and defined in Figure 2, V is the slide volume in cubic metres, and A and B are constants
dependent on the type of landslide.
Figure 6 presents the ratio of H/L (tangent of the travel distance angle) versus volume for all slides from the database
and includes the approximate limits of the data from Finlay et al (1999) for small volume landslides in Hong Kong. A
general trend of reducing H/L with increase in volume is evident, as there is for the Corominas data, but there is
considerable scatter. Significant departure outside the Corominas (1996) confidence limits occurs for the small volume
landslides from Hong Kong (Finlay et al 1999) and for the strongly retrogressive landslides (hydraulic fills, sensitive
clays and sub-aqueous slopes). For other sub-groups some generally plot below the Corominas mean (e.g. flows slides
in loose fills from Hong Kong and confined debris flows) and some above (e.g. cut slopes), indicating slope geometry,
material type, failure mechanics of the initial slide and degree of travel path confinement, in addition to slide volume,
all influence the travel distance angle.
The Corominas (1996) data, which was used in the analysis, was re-analysed considering only the smaller volume
landslides (less than 106 m3) typical of most slides in soil slopes. It was found that the correlations of H/L to slide
volume for the smaller volume slide groups were statistically much weaker, in-fact no correlation was statistically
evident for the confined debris flows and for the unconfined debris flows a different predictive equation was
appropriate. The findings indicate that much of the apparently good correlation by Corominas is because of the
inclusion of data from very large volume landslides.
The effect of degree of confinement of the travel path, coupled with the landslide volume, was assessed for some of the
sub-groups of landslides within the data set; mainly sub-aerial, non-retrogressive slide groups. The results indicated a
general trend of decreasing travel distance angle with slide volume for both the unconfined and confined travel paths.
Statistical correlations of H/L to slide volume for data sorted based on degree of confinement (Hunter and Fell 2002)
were generally weak or no correlation was evident.
It was evident that the flow slides in loose contractile fills have significantly lower travel distance angles than dilatant
failures in fill and also cut slopes, even though the soils were from similar origins (all cases from Hong Kong),
indicating the importance of the mechanics of failure on travel distance.
5.2
IMPROVED METHODS FOR PREDICTION OF TRAVEL DISTANCE ANGLE
For classes of slopes where there was sufficient detailed information it has been possible to derive more reliable
methods for estimating the travel distance angle. Much of the improvement comes from including the downslope angle
a 2 (as defined in Figure 2). In some cases it is apparent that there is no statistically significant relationship of the
travel distance angle with other factors, and it is recommended that the mean and standard deviation of the ratio H/L
from the database for that slope class be used with an appropriate degree of conservatism.
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In the following sub-sections only the findings from analysis of “rapid” landslides in cuts, fills of loose silty sands to
sandy silts and mine waste spoil piles, and natural soil slopes are discussed. Details of the analysis on which these
methods are based are given in Hunter and Fell (2001).
1.E+01
Finlay et al (1999) data for cuts, fills
and retained slopes from Hong
Kong
Corominas (1996), 95%
confidence limits of data
Ratio H/L
1.E+00
Corominas (1996)
data mean
1.E-01
1.E-02
1.E-03
1.E+00
1.E+02
1.E+04
1.E+06
Volume (cu.m.)
1.E+08
1.E+10
1.E+12
Cut Slopes (Hong Kong)
Unconfined debris flows in soil and rock (initial failure in dilative soil/rock mass)
Chalk talus (Hutchison, 1988)
Confined debris flows in soil and rock (initial failure in dilative soil/rock mass)
Flow slides in loose fills (Hong Kong)
Flow slides in coal waste spoil piles (South Wales and British Columbia)
Flow slides in coking coal (Hay Point)
Flow slides, chalk cliffs (Hutchison, 1988)
Strongly retrogressive flow slides (sensitive clays, hydraulic fills, tailings dams, sub-aqueous slopes)
Figure 6: Ratio H/L versus volume for all slides from database.
5.2.1
“Rapid” Slides in Cut slopes in dilative soil and weathered rock
(a) Cut slopes failing onto near horizontal ground
Finlay et al (1999) showed that the travel distance L was strongly correlated (R2 = 0.85) to the landslide height H and
the cut slope angle, a cut , for cut slopes where the slope below the cut is near horizontal and slide volume is small (<
500 m3). The Finlay et al (1999) correlation can be expressed as
H / L = 0.78 * (tan a cut ) 0.5
Equation 2
and is considered as the best prediction for slide volumes up to 500 m3.
For slide volumes up to 20,000 m3 better correlations are obtained for the H/L ratio normalised against tan a cut versus
slide volume as shown in Figure 7. For Type 1 slide geometries where the slide originates at the top of the cut slope
(Figure 3a) the solid line is the best method of prediction, while for Type 2 slides geometries (Figure 3b), the lower
bound line is better.
Obstructions to the travel of the slide mass did have some influence on H/L. For the Type 1 failure geometries the
slides obstructed by buildings tended to plot close to or above the trendline. However, scale effects are an important
factor in consideration of obstructions. What may be a significant obstruction for a slide volume of 100 m3, e.g. a
retaining wall, may not be significant for a much larger volume slide mass of several thousand cubic metres.
Australian Geomechanics – May 2002
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ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
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1.0
(H/L) / (tan (cut slope angle))
0.42
Type 2 failures in cut slopes
Type 1 failures in cut slopes
0.8
Y = 1.09 X
-0.109
2
R = 0.62
0.6
0.4
Lower bound for Type 2 cut
slopes (refer Figure 3b)
0.2
0.0
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
Volume (cu.m.)
Figure 7: H/L normalised against tangent of the cut slope angle versus volume for cut slopes in Hong Kong failing on
to a near horizontal slope at the toe.
For the cases analysed the initial failure mechanism (defect controlled or slide through the soil mass), geological origin
and material type were not significant which is consistent with what Finlay et al (1999) found for the larger, but poorer
quality data set from Hong Kong. Given this, it is concluded that the data here can be used for cuts in residual soil and
weathered rock derived from similar granitic and volcanic (fine to coarse tuffs and rhyolite). It is probably reasonably
applicable to cuts in other residual soils and substantially weathered rocks which have silty sand / clayey sand
characteristics. They should not be applied to finer grained and highly plastic soils without verification against local
failed slopes. Local verification is highly desirable in all applications.
The data for Hong Kong was for cuts steeper than 33 degrees and up to 70 degrees, failing onto near horizontal slopes at
the toe of the cut slope, and may be applicable to cuts as flat as 25 degrees.
(b) Slopes failing on to sloping ground
Where the slope below toe of the cut is not near horizontal, Figure 7 can be used provided that the difference in slope
angle between the cut slope and slope below the toe of the cut is used in place of the cut slope angle (i.e., a cut - a 3 for
Type 1 failure geometries, Figure 3a) and where this difference in angle is greater than about 20 degrees.
For Type 2 failure geometries the important difference in slope angle is between the basal angle of the surface of
rupture near the toe region of the initial slide ( a base , see Figure 3b) and the slope below the cut. Figure 7 is considered
applicable for Type 2 geometries where this difference in angle ( a base - a 2 ) is greater than 15 to 20 degrees.
For cut slopes of Type 1 and Type 2 failure geometries that do not comply with the above slope difference criteria use
of the correlations derived for natural slopes (refer Section 0) based on degree of confinement of the travel path and
downslope angle below the source area a 2 , would be more appropriate for prediction of travel distance angle. In these
cases there is limited loss in energy due to the slope angle transition, the basic premise on which Figure 7 is based.
5.2.2
“Rapid” Slides on Steep Natural Slopes in Dilative Soils
The travel distance angle is reasonably to well correlated to the downslope angle a 2 , and the degree of confinement of
the travel path. Equations 3 to 5 give the correlation between H/L and tana 2 for the three classes of degree of
confinement. The results are summarised in Figure 8.
H / L = 0.77 * (tan a 2 ) + 0.087
(Std. Error = 0.095, r2 = 0.71)
Equation 3
Partly confined travel path H / L = 0.69 * (tan a 2 ) + 0.086
(Std. Error = 0.110, r2 = 0.52)
Equation 4
H / L = 0.54 * (tan a 2 ) + 0.147
(Std. Error = 0.027, r2 = 0.85)
Equation 5
Unconfined travel path
Confined travel path
74
Australian Geomechanics – May 2002
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
HUNTER & FELL
1.0
Unconfined travel path
Y = 0.77X + 0.087
Unconfined travel path
0.9
Partly confined travel path
2
R = 0.71
Confined travel path
0.8
Vol. Range = 20 to 26,000 cu.m.
H/L Ratio .
0.7
0.6
0.5
Partly confined
Y = 0.69X + 0.085
0.4
2
0.3
Confined travel path
Y = 0.54X + 0.147
0.2
R = 0.52
2
R = 0.85
0.1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Tan (down-slope angle)
Figure 8: “Rapid” slides on steep natural slopes in dilative soils (Hong Kong), H/L versus tangent of the downslope
angle a 2 , for all types of travel path.
The downslope angle below the source area, a 2 , is defined in Figure 9 and is the average angle of the portion of the
downslope from below the landslide source area to the point where the slope begins to flatten out. This definition is
somewhat subjective; however, for the case studies analysed it typically represents at least 50% of the length of travel
distance beyond the toe of the source area (for confined travel paths) and in a number of cases up to 100% of this length
(for a number of unconfined and partly confined travel paths). Thus, a 2 incorporates short steeper and flatter sections
(e.g., terraces) on the upper portion of the travel path.
For estimation of H/L it is recommended that a first estimate be obtained from the mean travel distance angle for the
appropriate degree of confinement of the travel path (from Table 3). Then, by transposing this travel distance angle
onto a long section of the potential slide and travel path, estimation of the downslope angle below the landslide source
area, a 2 , is possible. Better prediction of the travel distance angle can then be made from Figure 8 or Equations 3 to 5
based on assessment of the degree of confinement of the travel path.
From the study it is apparent that the downslope angle below the source area, a 2 , presents a useful method for
prediction of the travel distance angle for “rapid” slides in natural slopes. These relationships are also applicable to cut
slope failures where the difference in angle between the cut slope (Type 1 slope failure geometry) or basal angle of the
surface of rupture (Type 2 slope failure geometry) and the slope below the cut slope is less than about 10 to 15 degrees.
Figure 9: Definition of downslope angle below source area a 2 for slides on steep natural slopes.
Australian Geomechanics – May 2002
75
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
5.2.3
HUNTER & FELL
Flow slides in Fills constructed of loose silty sand
Analysis of flow slides in loose fill slopes of silty sands to silty sands with low clay content from Hong Kong indicated
that a minor correlation exists between travel distance angle and slide volume (Figure 10), but the behaviour is just as
well described by using the mean and standard deviation of H/L (Table 3).
For estimation of H/L of flow slides in loose silty sand to sandy silt fills with low clay content (less than 10 to 20%) two
methods should be used:
· For slide volumes between 50 m3 and 500 m3 use the mean and standard deviation from Table 3.
· For slide volumes between 500 m3 and 10,000 m3 use Figure 10 with an appropriate degree of conservatism.
This data should only be used for similar materials as that of the case studies represented in Figure 10, i.e. loose silty
sands and sandy silts with fines contents (passing 75 micron) of 10 to 50% and clay content (passing 2 micron) less than
about 10% to 20% (Figure 4) derived from predominantly completely weathered granitic rocks but also volcanic rocks.
It should be applicable to other silty sandy soils, e.g. those from alluvium, colluvium or derived from sandstone.
0.7
Flow slides in loose silty sand fill (Hong
Kong) - detailed analysis
0.6
.
Additional flow slides in loose fill from
Hong Kong (Wong and Ho 1996; Sun
1998)
Flow slides in loose silty sand fill
H/L = 0.67 V
0.5
H/L Ratio
-0.082
2
R = 0.34
Vol Range = 50 to 10,000 cu.m.
0.4
0.3
0.2
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
Volume (cu.m.)
Figure 10: H/L versus slide volume for flow slides in loose silty sand to sandy silt fills.
5.2.4
Flow Slides in Coarse-grained mine waste spoil piles or Similar fills on hillsides
Figure 11 presents correlations of H/L with the tangent of the downslope angle ( tana 2 ) for flow slides in coal mine
waste spoil piles in British Columbia and South Wales in materials derived from sedimentary rocks. a 2 is taken as the
average slope from the toe of the spoil pile for a distance of at least 30 to 40% of the total distance travelled beyond the
toe. The group of high mobility slides (Figure 11), which are from British Columbia, were associated with liquefaction
of the materials mantling the downslope Golder Assoc (1995).
The following method is recommended for estimation of H/L for waste spoil piles constructed of materials similar to
those at the British Columbia and South Wales, dumped by low height tipping methods at the crest of the spoil pile onto
hillsides steeper than about 10 to 15 degrees and of similar slide volume:
76
·
For longitudinally curved downslopes, where estimation of the angle a 2 can be difficult, use the mean and
standard deviations of H/L given in Table 3. Take into consideration the potential for liquefaction of the
material mantling the travel path where the travel path is likely to be confined. Otherwise use the values for
normal mobility events disregarding the degree of confinement of the travel path.
·
Where the downslope angle is relatively consistent below the toe of the spoil pile use the downslope angle
correlations given in Figure 11, which include consideration of the degree of confinement of the travel path.
Australian Geomechanics – May 2002
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
HUNTER & FELL
0.7
Confined - high mobility
0.6
0.5
Confined - normal mobility
2
R = 0.37
7
Volume range = 3000 to 10 m
Ratio H/L
Partly confined - normal mobility
Unconfined - normal mobility
Confined to unconfined travel paths
Y = 0.54 X + 0.16
3
0.4
0.3
0.2
Confined travel path (normal and high mobility)
Y = 0.57 X + 0.13
2
0.1
R = 0.49
7
Volume range = 3000 to 10 m
0.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
3
0.40
0.45
0.50
Tan (down-slope angle)
Figure 11: H/L versus the tangent of the downslope angle below the toe of the spoil pile, a 2 ; flow slides in coarsegrained coal mine waste spoil piles.
6
POST FAILURE DEFORMATIONS OF “SLOW” INTACT LANDSLIDES
For “slow” landslides, which generally remain essentially intact on sliding, it is recommended that the Khalili et al
(1996) method be used for prediction of the post failure travel distance.
The Khalili et al (1996) method considers two modes of failure defining the upper and lower bounds of post-failure
deformation for intact slides:
·
A “rapid” model based on the principle of conservation of energy where the potential energy of the slide mass is
resisted by the frictional forces along the surface of rupture approximated using the residual shear strength.
·
A “slow” model based on equations of equilibrium between the driving force of the slide mass and resisting forces
along the surface of rupture approximated using the residual shear strength, i.e., assuming the rate of deformation
would be so slow that the effect of inertia forces of the sliding mass are negligible.
The failure model, Figure 12, is based on the assumptions of a circular slide surface, the failure is plane strain, the
failing mass moves as a rigid body and energy losses during failure is only due to the frictional forces acting along the
surface of rupture. Approximate solutions for the “rapid” model are calculated using Equation 6 and for the “slow”
model using Equation 7, where FSresidual is the factor of safety calculated using residual strengths along the surface of
rupture ( f r¢ for over-consolidated clays and Sur for soft clays), and q i and q f are the initial and final positions of the
centre of gravity as defined in Figure 12.
Rapid Model:
Slow Model:
q f = 2q i ( FS residual - 0.5)
Equation 6
sin q f = FS residual sin q i
Equation 7
Table 2 gives guidance as to whether the “slow” model or “rapid” model should be used. Note the terms “slow” and
“rapid” in this table are specific to the definitions given by Khalili et al and summarised above.
Australian Geomechanics – May 2002
77
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
HUNTER & FELL
Figure 12: Failure model for post failure deformation (Khalili et al 1996)
Table 2: Guidelines for post-failure deformation prediction using the Khalili et al (1996) model.
Slope Type
Khalili et al (1996) “Slow” Model
Khalili et al (1996) “Rapid” Model
Cut slopes in heavily overconsolidated clays
Type 1 slope failure geometry with cut slope
less than about 22 degrees.
Type 2 slope failure geometry with cut slope
less than about 25 to 30 degrees.
Type 5 retained cut slopes.
Fills on ductile soft clay foundations.
Type 2 slope failure geometry with cut slope
greater than about 25 to 30 degrees.
Fills on Soft Ground
Embankment
Dams
(excluding hydraulic fills)
Large volume (> 100000 m3) slides with
potential peak velocities in the slow range.
Smaller volume (< 100000 m3) with potential
for peak velocities in the slow to moderate
range.
Fills on strain weakening sensitive clay
foundations.
Large volume (> 100000 m3) slides with
potential peak velocities in the moderate to
rapid range.
Smaller volume (< 100000 m3) with potential
for peak velocities in the rapid range.
Analysis shows that the “slow” model often over-estimates the amount of initial deformation for the materials and slope
conditions identified in Table 2, and it can therefore be regarded as a reasonable upper bound for these cases. The
reason for this is that in the case of cut slopes in heavily over-consolidated clays (that fall into the “slow” model class)
the typical deformation behaviour is for incremental deformation over a number of years, i.e. first time failure followed
by subsequent reactivations of movement.
For those case studies under the “rapid” model, deformations of the smaller volume slides (including cut slopes and fills
on soft ground) usually occurred within a matter of minutes to hours. For the larger volume embankment dam case
studies, the deformation of the slide took from days to months, but typically involved a phase of large acceleration in
the post-failure deformation behaviour. Hence in the terms of this report all are defined as “slow” slides.
For the slides under the “rapid model” it was also observed, as for the “slow” model slides, that the “rapid” model tends
to over-estimate the amount of deformation. The reason for this is likely to be in the formulation of the “rapid” model,
which is based on the assumption that all available potential energy of the slide mass is resisted by frictional forces
along the surface of rupture approximated by the residual soil strength. Therefore, energy losses associated with
disaggregation of the slide mass, internal shearing or energy used in deformation to reach residual strength conditions is
not considered in the model.
78
Australian Geomechanics – May 2002
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
7
HUNTER & FELL
SUMMARY AND CONCLUSIONS
“Rapid” slides in soil slopes have the potential for loss of life, destruction to property and damage to the natural
environment. Landslides of slower velocity do not have the same destructive capabilities of “rapid” landslides, but they
are capable of causing significant damage to property.
Methods for identifying whether a landslide will be “rapid” or “slow” are discussed in Section 0. The guidelines are
approximate, and where there is doubt, it is recommended that the travel distance be determined using both the “rapid”
and “slow” methods, and a weighting then be applied to the more likely slide velocity (e.g. 0.6), and the residual (e.g.
1 – 0.6) applied to the other when assessing risk.
For “rapid” landslides it is apparent that the use of landslide volume alone does not allow reliable predictions of travel
distance angle. Consideration of material type, mechanics of failure, slope geometry, travel path confinement as well as
volume, improves the predictions. Tables 3 and 4 summarise the results of the analysis. Caution is recommended in
using the data due to the considerable element of uncertainty in the methods, but more particularly to only apply them to
conditions similar to those from which they were derived. In all cases, it is best to collect case studies from the area
under study to calibrate the empirical methods to the data in this paper, so that local geological and climatic factors can
be allowed for.
For “intact” landslide the Khalili et al (1996) method provides a means for estimation of the distance of travel.
Guidelines for its application are summarised in Table 2. An appropriate degree of caution should be used when
applying the methods, taking account of the consequences of failure and the state of knowledge of the soil properties.
Table 3: Mean and standard deviation of H/L for several types of slopes giving “rapid” slides.
Initial
Slide
Material Type /
Slope Type
Flow Slides
in
Contractile
Soils
Coal mine waste
spoil piles (sandy
gravels) (2)
Loose silty sand
fills (Hong Kong)
Failures in
Dilative
Soils (1)
Notes:
Natural Slopes
(incl. Corominas
debris flow data)
Degree of
Confinement
No.
of
H/L
Range
H/L
Mean
H/L
S. Dev.
Confined, high
mobility
7
110,000
to 5.6 x
106
0.18 to
0.28
0.208
0.035
No
confinement
condition,
normal
mobility
All unconfined
47
3000 to
8 x 106
0.22 to
0.49
0.359
0.076
16
50 to
10,400
50 to
13,000
0.28 to
0.60
0.22 to
0.67
0.405
0.094
Confined
19
0.426
0.110
Partly
confined
10
80 to
3000
0.28 to
0.62
0.470
0.114
Unconfined
52
20 to
140,000
0.22 to
0.75
0.547
0.137
Volum
e
Range
(cu.m.)
Comments
High mobility
associated with
liquefaction
susceptible materials
mantling downslope.
Similar mean and std.
dev. for all travel path
types.
For preliminary
estimate of travel
distance angle
For preliminary
estimate of travel
distance angle
For preliminary
estimate of travel
distance angle
(1) Inclusive of defect controlled slides, slides of debris and slides through the soil mass.
(2) High mobility events associated with confined travel path and liquefaction susceptible materials mantling downslope
travel path.
S. Dev. = standard deviation.
Australian Geomechanics – May 2002
79
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
80
Australian Geomechanics – May 2002
HUNTER & FELL
ESTIMATION OF LANDSLIDE TRAVEL DISTANCE
8
HUNTER & FELL
ACKNOWLEDGEMENTS
This work forms part of a research project on the pre and post failure deformation behaviour of soil slopes, being
undertaken within the School of Civil and Environmental Engineering at the University of New South Wales, Australia.
The support of the Australian Research Council and industry sponsors of the project: New South Wales Department of
Land and Water Conservation; Snowy Mountains Engineering Corporation; Goulburn Murray Water; Australian Water
Technologies; United States Department of the Interior, Bureau of Reclamation; Dam Safety Committee of New South
Wales; Australian Capital Territory Electricity and Water Corporation; Queensland Department of Natural Resources;
Snowy Mountains Hydro-Electric Authority; South Australian Water Corporation; Water Corporation of Western
Australia; Pells Sullivan Meynink; Roads and Traffic Authority, New South Wales; New South Wales Department of
Public Works and Services; Queensland Department of Main Roads; is acknowledged. Also acknowledged is the
Geotechnical Engineering Office of the Hong Kong Government for their assistance in supplying case study data.
9
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