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Landslide Cases in the Great Lakes: Issues and Approaches

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Landslide Cases in the Great Lakes: Issues and Approaches TRANSPORTATION RESEARCH RECORD 1343 87 Landslide Cases in the Great Lakes: Issues and Approaches TUNCER B. EDIL A review of the experience gained over a decade of studying the arated into two broad groups, mass and particle movements. mechanics of coastal bluff erosion and stability along the Great In the mass group, debris begins to move as a coherent unit Lakes shorelines and the approaches to dealing with this problem (rigid body movement or viscous flow). Movements in which are presented. Shore recession affects planning, design, and particles move individually, with little or no relation to their maintenance of transportation facilities in coastal areas in a significant way. The erosional processes resulting in significant neighbors, are particle movements. The erosional processes mass wasting include wave erosion, solifluction, rain impact and caused by waves, currents, rain, groundwater, and winds seem rill-sheet erosion, wind erosion, sapping, and ice erosion. Another to be mainly particle movements. These concepts are pres­ important process is mass sliding and slumping of bluff materials ented in Figure 1. in response to and in conjunction with the erosional processes. The methods of approach to this problem are grouped as structural (stabilization) and nonstructural (planning and management) solutions. These approaches and the required information to Wave Erosion implement either of them are discussed. Because of the length of the slopes and the variable nature of geology and soil properties, Probably the most significant geomorphic process along the a probabilistic approach to stability analysis has been adopted for Great Lakes shoreline is the erosion and removal of shoreline planning and managing the stabilization efforts. Two specific cases, materials by waves. Wave action is important, both in itself one in an undeveloped segment and another in an urban segment and in initiating and perpetuating other geomorphic processes of the shoreline, are presented to demonstrate the stabilization approach. in segments of the shoreline where bluffs are encountered. The most notable factor that affects the wave erosion in the This paper presents a review of the experience the author has Great Lakes is water-level fluctuation. For the long-term water gained over a decade of studying the mechanics of coastal level changes, the intervals vary from 10 to 30 years and the bluff erosion and stability along the Great Lakes shorelines. magnitudes are up to 2 m. Nearly 65 percent (10 444 km) of the 16 047-km-long Great Lakes shoreline is designated as having significant erosion; about 5.4 percent (860 km) of it is critical. The total damage Sliding and Slumping to the U.S. shoreline of the Great Lakes due to wave action during the high-lake-level period, May 1951 through April Slides (both rotational and translational) and flows (including 1952, is placed at about $50,000,000 (1952 price level). Nearly solifluction) are the two types of movements most commonly 32 percent of the U.S. shoreline of the Great Lakes, not encountered in the Great Lakes region. Coastal bluffs are in including the islands, consists of erodible bluffs. Extensive­ constant evolution because of the combined effects of toe ness of the shoreline formed in erodible bluffs and dunes (an erosion, slides, and face degradation. often complex response of this type of shoreline) to wave Edi! and Vallejo (1) described bluff stability at two sites on erosion makes slope processes an important part of the shore the shore of Lake Michigan. Where unexpected stability recession problem. The shore recession, in turn, affects the occurred, it could be explained in a rational manner by the planning, design, and maintenance of transportation facilities process of delayed failure that results from the unloading of in coastal areas in a significant way. The coastal bluff proc­ clays by erosion. Barring the presence of gross inhomogene­ esses are briefly described, and the methods of approach to ities, rotational slides involving approximately circular rup­ this problem are presented along with two specific cases. ture surfaces have been observed and analyzed in the Great Lakes bluffs formed in cohesive soils (1-3). Deep-seated rota­ tional slips occur in clay soils but are not observed in sands. SLOPE PROCESSES One method of analysis of rotational slides that is accurate for most purposes is that advanced by Bishop ( 4). The failure The interaction of driving forces (gravity and climate) and arc predicted by the Bishop method has been found to com­ soil shear resistance results in a number of processes that lead pare very well with the actual failure surfaces in bluffs in the to debris production and removal. The commonly encoun­ Great Lakes and other places. Using the effective stress tered processes in the Great Lakes coastal bluffs can be sep- approach and the Bishop method, Vallejo and Edi! (5) devel­ oped stability charts for rapid evaluation of the state of sta­ CERMES, Ecole Nationale des Ponts et Chausees, 93167 Noisy-le­ bility of actively evolving Great Lakes coastal slopes. These Grand Cedex, France. Current affiliation: Department of Civil Engi­ charts indicate the stability status as well as the type of poten­ neering, University of Wisconsin, Madison, Wis. 53708. tial failure, whether deep or shallow, to which the bluffs may 88 TRANSPORTATION RESEARCH RECORD 1343 MASS MOVEMENT GRAVITY Sliding VIBRATIONS FORCES : Rotational: SUIJ1lS Translational: CLIMATE Block Slide Slab Slide Flow Solltluction PROCESSES : Debris Flow SHEAR STRENGTH !!ABIICLli MQVliMlilH RESISTANCES : VEGETATION Wave Erosion Wind Erosion STRUCTURAL SYSTEMS Ice Erosion Rill Erosion Sapping FIGURE 1 Forces, resistances, and slope processes in Great Lakes. be subjected. The geometric changes can be discerned from thaw normal to the slop face at which failure occurred was the stability charts. measured to be 0.25 m. A translational slide in which the moving mass consists of a single unit that is not greatly deformed, or a few closely related units, may be called a block slide. An example of such Rain Impact, Rill and Sheet Erosion, Sapping, a failure involving a block of fractured till in the upper part Wind, and Ice Erosion of a coastal bluff in Milwaukee County, Wisconsin, was reported by Sterrett and Edi! (6). Translational slips can also occur in These processes are also important in general mass wasting a homogeneous soil mass. In particular, granular materials that occurs in the exposed coastal slopes of the Great Lakes. such as sand and gravel fail in surface raveling and shallow In slopes formed in granular material, these processes are slides with the failure surface parallel to the slope surface. dominant. A description of these processes given by Sterrett Similar failures occur in a mantle of weathered or colluvial (7) determined on the basis of field observations that most of (granulated) material from clay slopes and are referred to as the material removed from the slopes during summer is by slab slides. An infinite slope analysis is often representative way of sheet-wash and rill erosion. Sterrett (7) found that the of such failures. Sterrett (7) reported slab slides with a depth universal soil-loss equation, in its modified form as suggested of about 6.0 m from Milwaukee County. This depth coincided by Foster and Wischmeier (10), is useful in predicting soil closely with the depth of desiccation cracking and soil struc­ loss from steep slopes. ture change from fine to prismatic beds to massive intact blocks. Sterrett (7) also observed that frozen slabs of soil measuring 0.6 m x 10 m x 13 m failed in early spring, and STRATEGIES FOR DEALING WITH attributed this failure to differential melting of the bluff face. ACTIVELY EVOLVING SLOPES The most significant characteristic of the coastal slopes in Flows and Solifluction many areas of the Great Lakes shoreline is the fact that they are actively evolving natural slopes-the slope geometry con­ Flows commonly result from unusually heavy precipitation, tinually changes. This characteristic sets these slopes apart thaw of snow, or frozen soil. The flows observed in the Great from other natural slopes in terms of stabilization approaches. Lakes bluffs take place mostly in spring and result primarily There are basically two approaches to the problem of actively from ground thawing and snow and ice melting. Therefore, evolving coastal slopes. The first approach involves structural they can be classified largely as solifluction. Solifluction is the or stabilization solutions on a site-specific basis. The structural downslope movement of water-saturated materials that fol­ approach, with some additional considerations, is similar to lows thawing in previously frozen slopes. Vegetation appears other natural slope stabilization efforts. A proper stabilization to be the most restraining factor for solifluction. The size of program should include (a) protection against wave action in the flows along the western Lake Michigan shoreline varies all cases, (b) slope stabilization against deep slips if needed from 0.3 to 0.6 m wide up to 15 to 20 m wide and 21 m long. (important in the delayed instability often observed in bluffs A number of approaches for the analysis of solifluction fail­ formed in stiff clay soils), and (c) stabilization against face ures have been suggested. Vallejo (8) introduced a new degradation and shallow slips. Shore protection is a major approach to the analysis of solifluction that reflects the par­ component and may be more costly than the slope stabili­ ticulate structure of the flowing mass. Vallejo and Edi! (9) zation. The problems associated with the execution of this applied this analysis, with successful field verification, to a category of solutions are of two types: (a) many attempts are coastal bluff in Kewaukee, Wisconsin. The critical depth of not engineered and fail to cope with the problems, and (b) Edi/ 89 those engineered solutions often neglect to consider all aspects and by the regrading of the bluff to a stable slope angle.
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