5
446 CAN. GEOTECH. J. VOL. 15. 1978 sensitive muddy sediments. Canadian Geotechnical Journal, 14, pp. 582-603. CHAGNON, J.-Y. 1968. Les coulees d'argile dans la Province de -- Quebec. Naturaliste Canadienne, 95, pp. 1327-1343. CHARRON, J. E. 1975. A study of groundwater flow in Russell County, Ontario. Canada Inland Waters Directorate, Water Resources Branch, Scientific Series Number 40.25 p. DAVIS, S. N., and DEWIEST, R. J. M. 1966. Hydrogeology. Wiley, New York, NY. 496 p. DONOVAN, J. J. 1977. Sedimentology and hydrogeochemistry of Pleistocene Champlain Sea deposits, Maskinonge Valley, P.Q. M.Sc. thesis, Department of Geological Sciences, McGill University, Montreal, P.Q. 158 p. DREDGE, L. A. 1976. The Goldthwait Sea and its sediments: Godbout-Sept-Iles Region, Quebec North Shore. In Report of Activities, Part C, Geological Survey of Canada, Paper 76-lC, pp. 179-181. FLETCHER, E. B. 1972. Preliminary report on earth slope condi- tions, St.Edouard- de-Maskinonge, Quebec. Internal report, Fondex LtCe., Ottawa, Ont. File 31253. FRANSHAM, P. B., and GADD, N. R. 1977. Geological and geo- morphological controls of landslides in Ottawa Valley, On- tario. Canadian Geotechnical Journal, 14, pp. 531-539. GUSTAVSON, T. C. 1975. Sedimentation and physical limnology in proglacial Malaspina Lake, Southeast Alaska. In Glacio- fluvial and glaciolacustrine sedimentation. Edited by A. V. Joplingand B. C. McDonald. Society of Economic Paleontol- ogists and Mineralogists Special Publication 23, pp. 249-263. GUSTAVSON, T. C., ASHLEY, G. M., and BOOTHROYD, J. C. 1975. Depositional sequences in glaciolacustrine deltas. It1 Glaciofluvial and glaciolacustrine sedimentation. Edited by A. V. Jopling and B. C. McDonald. Society of Economic Paleontologists and Mineralogists Special Publication 23, pp. 264-280. HODGE, R. A. L., and FREEZE, R. A. 1977. Groundwater flow systems and slope stability. Canadian Geotechnical Journal, 14, pp. 466-476. KARROW, P. F. 1972. Earthflows in the Grondines and Trois- Rivikres area, Quebec. Canadian Journal of Earth Sciences, 9, pp. 561-573. LA ROCHELLE, P., CHAGNON, J.-Y., and LEFEBVRE, G. 1970. Regional geology and landslides in the marine clay deposits of Eastern Canada. Canadian Geotechnical Journal, 7, pp. 145-157. LASALLE, P., and ELSON, J. A. 1975. Emplacement of the St. N, - '. dlclsse Moraine as a climatic event in Eastern Canada. Quaternary Research, 5, pp. 621-626. ODENSTAD, S. 1951. The landslide at Skottorp on the Lidan River. Proceedings Royal Swedish Geotechnical Institute, 4, pp. 1-38. ROY, R. 1963. A comparison of groundwater hydrology in Pleis- tocene, Paleozoic, and Precambrian rocks of the Quebec St. Lawrence Lowlands and vicinity. Proceedings of Hydrology Symposium Number 3, Calgary, National Research Council of Canada, pp. 35-47. On the retrogression of landslides in sensitive muddy sediments:' Discussion R. J. MITCHELL Depurttnent of Civil Et~girleering, Queetl's Utiiversity, Kitlg~tot~, Ot~t., Cr~wacicr K7L 3N6 Received March 22, 1978 Accepted April 19, 1978 Can. Geotech. J., 15.446-450 (1978) The author has presented a number of photo- graphs (including two from the South Nation River, 1971 landslide) of intact ridges of sediment pro- truding from subsided terrace remnants in support of a wedge failure model (Odenstad 1951) to describe the mechanism, morphology, and long term micro-relief of earthflow craters. These ridges appear to be located in the lower portions of craters, originating within the zone of retrogres- sive flow sliding (as defined by Mitchell and Markell ( 1974) ) . Other published photographs have shown upthrust pinnacles, emerging from partially remoulded spoil closer to the mouths of craters, considered to have been formed by rota- tional slip during retrogressive flow sliding (see, for example, Eden 1956). Mitchell (1978) shows 'Paper by M. A. Carson. 1977. Canadian Geotechnical Journal, 14, pp. 582-602. photos taken near the backscarp of the South Nation River, 1971 landslide (in the earthflow zone) to demonstrate that differential settlement within the spoil, between intact slump blocks and remoulded sediments, may also contribute to the long term micro-relief in earthflow craters (an early draft of this publication is referenced by the author). A stereo view of the unweathered South Nation River, 197 1 landslide spoil is reproduced on Fig. 1 for reader examination-a view of the 'forest' instead of individual 'trees'. It is suggested that upthrust pinnacles, intact ridges, and flow intrusions can all be found in this classic and fascinatingly variable crater morphology. The au- thor notes, in his introduction, that "a variety of processes may be involved in different types of earthflow"; this writer suggests that a variety of mechanisms (processes) may also be involved in a given earthflow as it develops from initial rota- Can. Geotech. J. Downloaded from www.nrcresearchpress.com by UNIV CHICAGO on 11/10/14 For personal use only.

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Page 1: On the retrogression of landslides in sensitive muddy sediments: Discussion

446 C A N . GEOTECH. J . VOL. 15. 1978

sensitive muddy sediments. Canadian Geotechnical Journal, 14, pp. 582-603.

CHAGNON, J.-Y. 1968. Les coulees d'argile dans la Province de -- Quebec. Naturaliste Canadienne, 95, pp. 1327-1343.

CHARRON, J. E. 1975. A study of groundwater flow in Russell County, Ontario. Canada Inland Waters Directorate, Water Resources Branch, Scientific Series Number 40.25 p.

DAVIS, S. N., and DEWIEST, R. J . M. 1966. Hydrogeology. Wiley, New York, NY. 496 p.

DONOVAN, J. J. 1977. Sedimentology and hydrogeochemistry of Pleistocene Champlain Sea deposits, Maskinonge Valley, P.Q. M.Sc. thesis, Department of Geological Sciences, McGill University, Montreal, P.Q. 158 p.

DREDGE, L. A. 1976. The Goldthwait Sea and its sediments: Godbout-Sept-Iles Region, Quebec North Shore. In Report of Activities, Part C, Geological Survey of Canada, Paper 76-lC, pp. 179-181.

FLETCHER, E. B. 1972. Preliminary report on earth slope condi- tions, St.Edouard- de-Maskinonge, Quebec. Internal report, Fondex LtCe., Ottawa, Ont. File 31253.

FRANSHAM, P. B., and GADD, N. R. 1977. Geological and geo- morphological controls of landslides in Ottawa Valley, On- tario. Canadian Geotechnical Journal, 14, pp. 531-539.

GUSTAVSON, T. C. 1975. Sedimentation and physical limnology in proglacial Malaspina Lake, Southeast Alaska. In Glacio- fluvial and glaciolacustrine sedimentation. Edited by A. V. Joplingand B. C. McDonald. Society of Economic Paleontol- ogists and Mineralogists Special Publication 23, pp. 249-263.

GUSTAVSON, T. C., ASHLEY, G. M., and BOOTHROYD, J. C. 1975. Depositional sequences in glaciolacustrine deltas. It1 Glaciofluvial and glaciolacustrine sedimentation. Edited by A. V. Jopling and B. C. McDonald. Society of Economic Paleontologists and Mineralogists Special Publication 23, pp. 264-280.

HODGE, R. A. L., and FREEZE, R. A. 1977. Groundwater flow systems and slope stability. Canadian Geotechnical Journal, 14, pp. 466-476.

KARROW, P. F. 1972. Earthflows in the Grondines and Trois- Rivikres area, Quebec. Canadian Journal of Earth Sciences, 9, pp. 561-573.

LA ROCHELLE, P., CHAGNON, J.-Y., and LEFEBVRE, G. 1970. Regional geology and landslides in the marine clay deposits of Eastern Canada. Canadian Geotechnical Journal, 7, pp. 145-157.

LASALLE, P., and ELSON, J. A. 1975. Emplacement of the St. N, - ' . dlclsse Moraine as a climatic event in Eastern Canada. Quaternary Research, 5, pp. 621-626.

ODENSTAD, S. 1951. The landslide at Skottorp on the Lidan River. Proceedings Royal Swedish Geotechnical Institute, 4, pp. 1-38.

ROY, R. 1963. A comparison of groundwater hydrology in Pleis- tocene, Paleozoic, and Precambrian rocks of the Quebec St. Lawrence Lowlands and vicinity. Proceedings of Hydrology Symposium Number 3, Calgary, National Research Council of Canada, pp. 35-47.

On the retrogression of landslides in sensitive muddy sediments:' Discussion

R. J . MITCHELL Depurttnent of Civil Et~girleering, Queetl's Utiiversity, K i t l g ~ t o t ~ , O t ~ t . , Cr~wacicr K7L 3N6

Received March 22, 1978

Accepted April 19, 1978

Can. Geotech. J . , 15.446-450 (1978)

The author has presented a number of photo- graphs (including two from the South Nation River, 1971 landslide) of intact ridges of sediment pro- truding from subsided terrace remnants in support of a wedge failure model (Odenstad 1951) to describe the mechanism, morphology, and long term micro-relief of earthflow craters. These ridges appear to be located in the lower portions of craters, originating within the zone of retrogres- sive flow sliding (as defined by Mitchell and Markell ( 1974) ) . Other published photographs have shown upthrust pinnacles, emerging from partially remoulded spoil closer to the mouths of craters, considered to have been formed by rota- tional slip during retrogressive flow sliding (see, for example, Eden 1956). Mitchell (1978) shows

'Paper by M. A. Carson. 1977. Canadian Geotechnical Journal, 14, pp. 582-602.

photos taken near the backscarp of the South Nation River, 1971 landslide (in the earthflow zone) to demonstrate that differential settlement within the spoil, between intact slump blocks and remoulded sediments, may also contribute to the long term micro-relief in earthflow craters (an early draft of this publication is referenced by the author). A stereo view of the unweathered South Nation River, 197 1 landslide spoil is reproduced on Fig. 1 for reader examination-a view of the 'forest' instead of individual 'trees'. It is suggested that upthrust pinnacles, intact ridges, and flow intrusions can all be found in this classic and fascinatingly variable crater morphology. The au- thor notes, in his introduction, that "a variety of processes may be involved in different types of earthflow"; this writer suggests that a variety of mechanisms (processes) may also be involved in a given earthflow as it develops from initial rota-

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Page 2: On the retrogression of landslides in sensitive muddy sediments: Discussion

DISCUSSIONS

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Page 3: On the retrogression of landslides in sensitive muddy sediments: Discussion

CAN. GEOTECH. J. VOL. 15, 1978

Frc. 2. Energy dissipation in wedge failure.

tional slip, through a retrogressive flow slide stage, to the earthflow phenomenon.

Analysis of the retrogressive flow slide stage, using conventional circular arc techniques, is quite plausible in homogeneous soil profiles; the Oden- stad mechanism appears to provide a more realistic analysis when the soil profile is non-homogeneous (layered). Since both analyses are static upper bound limit solutions, however, some similarity in the results is to be expected: for example, the author's equations [7] and [8] yield values of -yH/c,, comparable to values from circular arc analysis and the stability factor intercept on his Fig. 16 (for no tension cracks) is identical to the classical upper bound value of the critical height of a vertical slope, H, = 4 ~ , , / ~ . No matter how one 'slices' it, these answers are all embedded in the charts due to Taylor (1937).

The more general problem in analyzing retro- gression is that static analyses are not directly applicable to dynamic phenomena. The author, for example, states (p. 558) that retrogression cannot, because of the lateral earth pressure in the spoil, be simply a function of the stability factor -yH/c,,. Since the entire system is in motion at this stage it is extremely difficult to assess the lateral earth pressure-it is likely very low during the acceleration stage of an earthflow and would likely increase to a value in excess of the passive pressure if the spoil decelerates and acts to stabilize the backscarp. Energy considerations must be invoked in attempting to provide an analytical solution. As noted by Mitchell and Markell ( 1974), the amount by which the parameter -yH/c,, exceeds that required for static failure can be considered, without refer- ence to strains (or velocities), as a dimensionless measure of the energy per unit volume of soil avail- able to cause flow of spoil, hence continued earth- flow. The parameter yH/c,, was proposed, by

Mitchell and Markell, mainly as a correlative parameter for field observations and not as a static stability factor.

The author's analysis relates subsidence ratio to the stability number but does not relate either to the size of the earthflow. Indeed, the subsidence ratio is not a,constant in most earthflows: at the South Nation River earthflow, for example, the subsidence ratio varies from about 0.66 at the mouth of the crater to 0.38 at the backscarp. His analysis closely predicts the backscarp value. In simplicity, this analysis is arranged to predict the vertical subsidence at the backscarp but makes no attempt to predict how far this backscarp might retrogress from the original unstable bank. This writer suggests that the retrogressive distance R is of greater engineering significance.

The Odenstad model can, however, be extended beyond that analysis presented by the author. In extending the analysis it will be assumed that wedge failure occurs, as shown schematically on Fig. 2, with tension cracks extending through the stiffer overlying layer (this stiff layer can be sand and silty sediments or a weathered crust, the latter often extending to depths up to 8 m in the Champlain Sea deposits). Consider simultaneous failure over a retrogressive distance R, as shown on Fig. 2: the energy input (at constant velocity) is given, per unit width and time, as Ei = -yhlVoR whereas the energy dissipation is calculated as

[ I ] E,, = c,, RVo/(sin2 a cot a)

where c,, = undrained undisturbed strength of the soft clay layer and (c,,), = the remoulded shear- ing resistance on the basal sliding plane.

The calculation assumes that the weight of an inverted prism (such as that shown by shaded area

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Page 4: On the retrogression of landslides in sensitive muddy sediments: Discussion

DISCUSSIONS 449

on Fig. 2 ) is just sufficient to cause remoulding as required to allow the mechanism to develop. The critical failure angle af for simultaneous ground subsidence depends on the ratio (c,),/c, and in- creases with increased retrogressive distance R. Independent of (c,,),/c,,, af = 45" when R < 2h2 cot a (i.e., one terrace retrogression). If one con- siders the development of the landslide as an acceleration-deceleration phenomenon (rather than as constant velocity subsidence) the energy ratio, E,/Ed, would be high during acceleration and the failure angle af would approach 45"; during de- celeration, for (c,,), = cJ8, the critical angle would approach af = 75". A similar value, aP = 75", is obtained for constant velocity sub- sidence with (c,,), = c,/8 and R = 56h2 (con- sidered, from field evidence, to be the practical upper limit of retrogression). The author's assump- tion of af = 60" appears to be a reasonable aver- age value and, as noted in the paper, there is some correlation between the widths w of individual intact grassed blocks shown on Fig. 1 and the pre- dictions of 0.5h2 < w < 2h2 for h2 = 13 m at this site.

Assuming an Odenstad type of failure mech- anism, developing immediately following initial bank failure, a failure angle af = 45" would appear to be a reasonable assumption. Equating E,, = Ei with ap = 45",

when (c,,), = 0, as assumed by the author, yhl/c,, = 2, as obtained by the author. It is reason- able to assume that the basal plane has a lower undisturbed strength due to anisotropy (say (c,,) = (c,,) ( r , /2) and that this strength is further reduced by the extensive flow distortion (Ladanyi et al. ( 1968) indicate (c,,), = c,,/4 as an average value). For an anisotropic soil, the re- moulded shearing resistance could be as low as (c,,), = c,,/8. For the South Nation River site, as well as many others, hl= h2 + H/2. Then

From experimental correlations for earthflows Mitchell and Markell ( 1974) proposed the general use of an earthflow chart in which

Equation [3] predicts the lower range of the field observations summarized by [4]. More generally, [3] can be expressed as

where S = c,,/(c,,), and D = h2/H. The value of D is site specific and the limits of can be established theoretically. Further research

on the rate of decrease of shearing resistance, anisotropy in undrained strength, and other factors is needed before realistic values of S can be established for use with this analytical model. Indeed, this mechanism would appear to depend on weak layers or strength anisotropy in order to be critical.

The above analysis, like the author's, is essen- tially a static upper bound limit analysis. A velocity V o was assigned on Fig. 2 to avoid defining a failure displacement. It is apparent, however, that neither subsidence nor flow can fully develop with- out internal remoulding of the soft sensitive layer. The next advance in a theoretical analysis would be to consider the acceleration and deceleration of the flowing soil mass using energy functions. The acceleration at any time t could be given in the form

where N, is a static stability number and f (d ) is the, as yet unknown, deceleration function due to energy dissipation in the spoil. A plastic flow model (Mitchell 1978) is more amenable to this type of analysis than the Odenstad model and some suc- cess has been achieved using a critical state energy dissipation function. Preliminary results indicate that retrogression R is not directly proportional to slope height H but these results are currently con- sidered valuable only as a framework in which to interpret field observations. For the typical pro- files found in the Ottawa - St. Lawrence lowland areas, Mitchell ( 1978) has suggested, from field correlations, the following empirical relations for estimating the maximum potential for retrogres- sion:

R < 100 m forN,< 5 P I

R < 100(Ns - 4) m for N, >, 5

where N , = yH/c,, is obtained as the minimum value in the profile, down to a 'kinematically admissible' depth below the toe of the slope.

It should be pointed out, however, that the rela- tive independence of retrogression R from the slope height or profile characteristics is not altogether inconsistent with the Odenstad model. Equation [5] can be written as

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Page 5: On the retrogression of landslides in sensitive muddy sediments: Discussion

450 CAN. GEOTECH. J. VOL. 15, 1978

for an average value of a = 58'. When D ap- proaches unity (homogeneous clay), R approaches zero and the potential for earthflow is minimal. For a profile consisting of a stiff layer overlying an equally thick layer of softer sediments (D = 0.5), R = O.SSH(N, - 4.5). Stiff layered profiles that include relatively thin softer layers have lower values of D. Using D = 0.25, for example, R = 0.38SH(Ns - 3) . As D approaches zero, R approaches zero, indicating the requirement of a soft layer to initiate failure. The value of S would depend on anisotropy as well as strain softening and would generally be higher in the stiffer layered profiles. In addition, the value of H required to initiate failure would be higher in stiffer sediments. Thus the value of SH would be expected to increase as D decreases and, as a result, the value of R could be relatively independent of the slope height H or the profile characteristics. For typical profiles and assumed values of S (varying from S = 4 for homogeneous clay to S = 16 for stiffer anisotropic profiles) [8] would predict

[9] 50(Ns- N,) m < R < 100(Ns- Nf) m

where 2.5 < Nf < 5 represents the theoretical value of yH/c,, required to initiate failure. It should also be noted that values of Nf less than 4.5 require tension cracks extending to depths greater than H/2. The author suggests (p. 597) that this is a highly improbable condition and this writer would agree except under specific geological conditions where internal water pressures in intercalated silts or sands could provide the condition of zero effec- tive horizontal stress. Mitchell and Klugman (1978) discuss the mechanisms by which internal groundwater pressures in layered, intercalated pro- files can create extremely rapid sapping and con- tribute to earthflows.

The relationships developed by the author and in this discussion should not be applied directly to the very stiff banded sediments found in the

Saguenay - Lac St. Jean region and further north- east to the Outardes and Toulnustouc rivers, in Quebec. Seepage pressures, which are of no con- sequence in the + = 0 analyses, may be extremely important in earthflows in banded sediments that include intercalated sand and silts.

Field examination of the detail shown on Fig. 1 led this writer to conclude that there is a gradual transition from initial rotational retrogression to a plastic flow phenomenon in the South Nation River landslide. The Odenstad wedge failure mechanism, as discussed by the author, would likely develop in the transitional stage. Since overall retrogression appears to depend mainly on the value of yH/c,, landslides (at different locations) may terminate at any stage. Minor retrogressions terminate in the rotational stage, major retrogressions may ter- minate in the transitional stage, and earthflows must involve a plastic flow phenomenon. An evaluation of the retrogression potential is of major signi- ficance with regard to route selection and regional development planning. Evaluation of both retro- gression and subsidence allows a closer estimate of potential damage due to flow of spoil. The author's analytical treatment of the Odenstad model has obviously contributed to terrain evalua- tion in sensitive soils.

EDEN, W. J. 1956. The Hawkesbury landslide. Proceedings of the 10th Canadian Soil Mechanics Conference, Ottawa, Ont. pp. 14-22.

LADANYI. B.. MORIN, J . P., and PELCHAT, C. 1968. Post-peak behavidur of sensitive clays in undrained shear. ~ a n a d i a n Geotechnical Journal, 5, pp. 59-68.

MITCHELL, R. J. 1978. Earthflow terrain evaluation in Ontario. Ontario Joint Transportation and Communications Research Programme, Project Q-53, Research Report 213.30 p.

MITCHELL, R. J., and MARKELL, A. R. 1974. Flowsliding in sensitive soils. Canadian Geotechnical Journal, 11, pp. 11-3 1.

MITCHELL, R. J. , and KLUGMAN, M. A. 1978. Mass instabilities in sensitive Canadian soils. Engineering Geology. In press.

ODENSTAD, S. 1951. The landslide at Skottorp on the Lidan River. Proceedings, Royal Swedish Geotechnical Institute, 4, pp. 1-38.

TAYLOR, D. W. 1937. Stability of earth slopes. Journal of the Boston Society of Civil Engineers, 24, pp. 197-246.

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