Recent Developments in Landslide Mitigation Techniques

This chapter begins with a brief synopsis of landslide repair methodologies developed over the past 70 years. Early attempts at stabilizing slopes focused on emplacement of toe buttresses and inclusion of subdrainage. As earthwork equipment became larger and more capable, the removal and recompaction of entire slide masses became commonplace. Over the past decade, geotextile and geomembrane products have become available that can significantly alter the options for repair, expecially under conditions of restricted access or poor weather. In the balance of the chapter, I seek to introduce the reader to some of these products and to case histories of ways in which they have been combined to effect novel solutions to slope stability problems.

INTRODUCTION

It has been almost 40 years since Karl Terzaghi's classic paper entitled Mechanism of Landslides appeared in the Geological Society of America's Applications of Geology to Engineering Practice, edited by Charles Berkey. In that interim, little has changed with respect to understanding the theorems of effective stress and progressive failure which promote slope instability.

Since that time, increased development in hillside areas has underlined the importance of understanding the geologic factors promoting instability before beginning engineering analysis or repair. All too often, sites prone to landsliding have been the scene of repeated repair attempts within a few years of each other. Experience over the past half-century tends to suggest that many landslide repair attempts are made without benefit or full understanding of the geometry and hydrologic regimen of the affected sites. In addition, the blind implementation of a traditional engineered repair scheme such as recompaction, may not serve to adequately mitigate all manner and form of future slope instability.

This article will seek to explore some of the more innovative recent techniques available for landslide repair that have come into practice in the past decade or so.

The rational design of a landslide repair cannot begin until the "sensitive factors" of site geology are properly evaluated. In most engineering analyses, the most fundamental factors are: (1) the relative position(s) of the ground water table(s), (2) the fluctuation of ground water levels and the flow volumes ascribable to infiltration or subaqueous flow (aquifers), (3) confirming the presence, character, and geometric extent of both ancient and active landslide slip surfaces.

Mitigation Via Excavation and Recompaction

The earliest engineering attempts at landslide correction likely occurred along railroad and canal embankments in England and France, beginning in the 1830's. As the industrial revolution took root in the late 19th Century, powered excavation machinery, such as track-mounted steam-powered shovels, spearheaded a revolution in earthwork construction. From 1850-1950, most cut slopes were excavated at slopes of 1:1 (45 degrees) or steeper, while fill was placed on embankments of around 1.5:1 (horizontal to vertical). Steeper embankments were accommodated by stacking rock or masonry blocks to create gravity retaining walls, thence filling at 1.5:1 above such structures.

When disaster struck in the form of a slope failure, the style or method of repair depended on cost and the available right-of-way. In rural areas, such as cut slopes on the Panama Canal (MacDonald, 1913, 1947), failed excavations were simply laid back to a more-stable inclination (from 1:1 to 10:1 in some cases). In more urbanized or mountainous areas, where there was little available right-of-way, concrete and masonry gravity retaining walls were most often employed (Ladd, 1935).

Self-propelled earth-moving equipment began to show up on the civil engineering scene in the 1920's as part of the ambitious road building programs being employed throughout the United States. With self-propelled equipment, landslides could be excavated and replaced with some more suitable material, such as drain rock or rip rap. By the 1930's, most large landslide repairs consisted of either partial excavation of the headscarp area and/or the placement of toe buttresses, most commonly over existing creeks or gullies. Such repairs were usually effected in combination with some sort of subdrainage, either withdrawal wells or trench subdrains (Larkey, 1936; Root, 1938; Greeley, 1940). A scheme typical of this early era is shown diagrammatically in the upper half of Figure 1.

By the mid-1940's, sheepsfoot compactors began to be employed for "dry" compaction of large earth embankments (Hansen Flood Control Basin) and rock-fill dams (San Gabriel Dam). Up to this time (1942), only smooth tire compactors with contact pressures of around 40 psi had been available. The sheepsfoot roller allowed contact pressures of around 250 psi, a 6-fold increase in compactive effort (Baumann, 1936, 1937; Proctor, 1933). In the years following World War II, large earthwork projects became commonplace with the introduction of larger, self-propelled hydraulic-powered equipment and the infusion of large projects spawned by the Interstate Highways Act of 1955 and water retention, reclamation, and flood control projects, sponsored by the U.S. Bureau of Reclamation and the U.S. Corps of Engineers.

In constructing larger cuts and fills, some were invariable placed across ancient landslide deposits without benefit of engineering geologic input. By the late 1950's, a new style of repair came onto the scene known by most practitioners as the "recompacted buttress fill", shown diagrammatically herein as the lower half of Figure 1.

Figure 1: A: The original approach to landslide repair was to buttress toe areas in combination of limited removal of the upslope area, trimming back the headscarp and installing wells to draw down the water table. B: As earth-moving equipment became more capable, the entire mass of a landslide could be excavated and recompacted in a buttress fill.Often such buttresses are constructed with subdrains.  

Buttress fills remain the most commonly employed method of landslide repair in the United States today. They are identical to new-construction embankments in that they employ shear key benches which are excavated beneath zones of disturbance (landslide slip surfaces) or potential distress, such as soil and organic horizons. In landslide repairs, subdrains are almost always included at the heels of the key benches in order to alleviate pore pressures which promote landslippage (Forbes, 1947).

Removal and replacement techniques carry with them their own inherent liabilities. For starters, the landslide material must necessarily be excavated, carried, and stockpiled at an adjacent location. In instances of large slides, it is possible to excavate in one area while simultaneously placing this same muck in another area already excavated and prepared for fill placement. However, in steep terrain, available stockpile area may be scant, therein requiring the construction of temporary stockpile fences, against which fill may be stored.

A second liability is the normally high moisture content of the slide material, which is usually excessive in the season following earth movement. Quite often, the slide material requires scarification, drying, and/or mixing in order to bring moisture levels down close to optimum for placement at 90% to 95% relative compaction. This requires additional handling, warm sunny days (or appreciable wind), and a larger working area.

Lastly, simple recompaction of low-strength materials can be dangerous in that compactive effort does not change the mineralogical make-up of the parent material. Soft-expansive clay will still be expansive clay, if only not as soft. Even though compacted to a high degree of achievable density, the fill is still able to absorb additional water through swelling or mineralogical absorption by cation exchange with percolating groundwater. It is for these reasons (and subdrain clogging) that many recompacted buttress fills have failed over the past 35 years.

Conventional Retention Structures

A wide variety of retention structures have been successfully employed to repair landslippage where high value structures are inextricably involved with the repair. The types of structures are basically divisible into four main categories:

a.gravity structures,

b.cantilever structures,

c.flexible/bulkhead walls,

d.retained structures, and

e.combination structures; incorporating one or more of the above methods

Examples of the traditionally employed wall structures and engineered retention systems are presented in Figures 2 through 6.

Figure 2 - Various type of gravity retention structures. Such structures depend upon their sheer mass as a resisting force to the load imposed by a hillside. This is the earliest type of retention structure, having been used by Assyrians and Egyptians beginning around 2900 B.C.

Figure 3- Various types of cantilever retention structures. Such structures came into use with the advent of pile driving, which dates to Roman times. The use of large-diameter augers allows such structures to be constructed in stiff soils and soft rock.

Figure 4 - Various types of retained structures - those employing tension elements. The cost and feasibility of such structures is almost wholly dependent on drill rig access and drillability of the ground.

Figure 5 - Various types of flexible retention structures, or those that deflect in order to shed their imposed loads. Such deflection lessens wall loads by allowing the ground mass to mobilize its shear strength (Rankine Active Pressure Theory) 

 
Figure 6 - The Loffelstein®, or Loffelblock®; retaining wall, shown here, is a design concept emanating from Austria which is now produced in the United States. Extremely economic, its primary application is for slopes under 22' high with an angle of internal friction greater than 30 degrees. In the case shown, the wall was constructed on a 20% longitudinal gradient to support a highway cutslope. Such walls can be built for $12 to $15 per square foot (in 1988 U.S. dollars).