Determining Corrective Action for Highway Landslide Problems

Determining Corrective Action for Highway Landslide Problems R. F. BAKER, Engineer of Soil Mechanics State Road Commission of West Virginia THE PROBLEM of landslides has plagued highway problems dealing with land• highway departments throughout the slides in unconsolidated materials. The country for many years. For some states, primary emphasis was to be towards the and particularly West Virginia, the dam• correction of existing problems. How• age caused by earth movements represents ever, it was felt that the principles a major expenditure, one that involves should be applicable to the problem of hundreds of thousands of dollars annually. design. Over 80 percent of the area of West The basis for the study was the writ• Virginia is located in a landslide-sus• er's experiences in West Virginia, com• ceptible area. The total number of bined with general theories from geology, landslides on the state highways has soil mechanics, and highway engineering. never been established. However, the The analysis as advanced is for consider• writer estimates that this total will ation in the study of ail landslides in approach 1,000 on the 31,000 miles of unconsolidated material, with the ex• primary and secondary roads in the ception of those of the nature of fluvial state. transported material, i. e., the water The complexities of the landslide prob• present is far in excess of normal soil lem have very few parallels in highway moisture, and the debris is a "relatively engineering. The literature on the subject small proportion of the flowing mass" carries numerous references to case (2). histories, but none outlines a systematic, Since one of the primary aims of the complete approach to the solution of a study was to consider the applicability given problem. The recent bibliography of the various corrective measures, the published by the Highway Research Board investigation could have been accomplish• (1) offers a complete summary of the ed by a study of existing landslides that publications relative to mass movements. have been treated. Such an approach The work of geologists on landslides has was used by Price and Lilly (12) in 1942. been and is of considerable value. The However, a direct study was impossible classification systems suggested by since as a routine department function Sharpe (2) and Ladd (3) assist tremen• there were requests to investigate over dously m understanding the complicated 100 landslides during the past three variety of movements that occur. From years. Due to a personnel shortage, the viewpoint of corrective actions, the the demand necessitated superficial report by Ladd is perhaps the most com• analyses but it was decided that the prehensive contained in the landslide program lent itself to the development literature. Numerous engineers (4, 5, of aprocedureto evaluate the movements. 6,7,8,9,10,11,15) have discussed the In addition, it became possible to study application of the theories of Soil Me• the applicability and usefulness of various chanics to the analysis of the stability corrective methods. The theories ad• of a landslide, but there are few details vanced in the following are not complete concerning the determination of the effect for three vital factors remain in the of a corrective action in terms of stability. evaluation: (1) observation of those The study that led to the following landslides that have been corrected by theory was designed to prepare an ap• the methods outlined herein; (2) a more proach to the analysis and correction of comprehensive study of flow movements. the center of gravity of the moving mass advances in a downward and outward direction. " It will be noted that the time -SLIP SURFACE MO SUP SURFACE element is involved in the definition only by the term "rapid displacement. " The terms slip-plane, slip-surface, and surface of failure will be synonymous Figure 1. Differentiation between slide and flow (after Sharpe). A slide is a and will refer to the surface that sepa• movement of a block of material, whereas rates the mass in motion from the under• flow is entirely internal deformation. lying stable material. Permanent solutions will be defined and (3) investigation of less costly meth• as corrections with an anticipated life ods for correcting landslide problems. of at least 50 years. An expedient solu• The writer is aware that the analysis tion will be considered adequate for a is an over-simplification. Extensive period of a few months to 5 to 10 years. study and evaluation is still very nec• All corrective actions will be classed essary, but for the immediate future a as one of two types, elimination or con• working tool is available. trol. The actions involving elimination depend generally upon avoiding or re• DEFINITIONS moving the landslide. Control methods are defined as corrections which produce The following definitions will be used a static condition of the landslide for a throughout. Some of the terms may be finite period of time. argumentative and general, but it is the While there have been many classifica• opinion of the writer that the following tion systems proposed, the bases for the are most applicable to the engineering classifications have most generally been phases of the landslide problem. related to cause and effect of the move• Landslides have been defined by ment rather than the mechanics. One Terzaghi (3) as follows: "The term notable exception is the system proposed landslides refers to a rapid displace• byHennes (7). For a quantitative analysis ment of a mass of rock, residual soil, of a design or correction for a given or settlement adjoining a slope in which landslide, the most satisfactory classifi- Figure 2. A definite slip-plane, identifying the movement as a slide. 3 cation is one which differentiates on a stressed beyond their "fundamental basis of the effect of the forces and strengths," and as a result, slow but resistances at work. Thus, the major constant internal deformations occur. primary classification would appear to be landslides in consolidated materials, BASIC FUNDAMENTALS and those in unconsolidated materials. IN LANDSLIDE ANALYSES A second primary differentiation would divide the movements into those with a From the observation of landslides slip-surface and those without a slip- in West Virginia, and from a review of surface. This latter grouping was out• the literature on landslides and soil lined by Sharpe (2), who termed the mechanics, the following statements former as slides and the latter as flows. have been outlined by the writer (13) as The principle is pictured in Figure 1. being fundamental to the analysis of a The movements in flow conditions are landslide relative to its correction as a the result of internal deformations. highway problem. It should be empha- i Figure 3. Typical flow movement. Note the characteristic roll of the material at the toe. Some movements originate as a flow and develop into a slide. For the purpose of the following sized that the statements apply prima• analyses, the term slide (Fig. 2) will be rily to highway problems and may not be defined as all landslides which involve of value from an academic viewpoint or unconsolidated material in which the for landslide analyses for other purposes. movement is along a slip-surface. The 1. There are numerous instances terms flow and creep will be defined as where the control of the landslide will those movements which do not have a not be the best solution. For instances slip-surface, the movement resulting that involve the use of an elimination from internal deformation. A flow (Fig. corrective action that avoids the land• 3) will be further defined as being caused slide, halting the movement is not gen• primarily by excessive water. The term erally a factor in the solution (Fig. 4). creep will be differentiated in accordance 2. Determination of "the" cause of with Terzaghi's concept (3) that failure a landslide is not always essential to an occurs at a considerable depth due to accurate solution to a highway landslide the load of the overlying material. The problem, and is always secondary in layers at the deeper elevations are importance to an understanding of the / Figure 4. The slide involved in the pictured location is at the right. Tlie problem was solved by shifting the roadway into the stable bedrock at the left of the picture. mechanics of the movement. The surface become of primary interest cause of a landslide is often argumenta• (Fig. 5). tive even after all the available facts 5. For a given landslide problem have been determined. In many cases, there is more than one method of cor• one cause or another may have been the rection that can be successfully applied. straw that broke the camel's back. Of A common misconception that should be more importance than the cause, is the clearly dispelled is that for a given land• realization that increased stability will slide there is one and only one solution. result by eliminating or minimizing the The inference that is undoubtedly intended effect of any contributing factor, par• is that for any given landslide, one meth• ticularly that of the effect of the force od is the most desirable from a considera• of gravity. tion of economics, appearance, construc• 3. The works of man can measurably tion problems, etc. accelerate or decelerate the rate of 6. The decision as to the corrective movement of a landslide toward the action to be used for a given highway topographic bottom of the area. Land• landslide problem is eventually reduced slides are recognized by geomorpholo- to a problem of economics. This is a gists as being a major landforming proc• statement of an obvious fact, but it is too ess.

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