CHAPTER 7

DEFORMATION AND CONTROL SURVEYS, SLUMGULLION LANDSLIDE

by David J. Varnes, William K. Smith, William Z. Savage, and Philip S. Powers

Introduction

In 1985, the Colorado Geological Survey planned aerial photography of the Slumgullion landslide at a scale of 1:12,000 as part of its growing program on landslide hazards and offered the USGS an opportunity to share in the results. Prior to the photography, R.L. Schuster and others set out plastic strips as targets on a number of photo-control points on stable ground. Visibility between points was not a consideration and their horizontal and vertical positions were not determined at that time. In 1990, the USGS contracted for new aerial photography at scales of 1:14,000 and 1:6,000, so the 1985 points were retargeted and other control points were incorporated into a 5-km-long triangulation-trilateration control net that initially connected 17 points on stable ground. This control net, which will be briefly described below, allowed for the photogrammetric compilation of large-scale (1:1,000) topographic maps of the Slumgullion landslide from the aerial photographs taken in 1985 and 1990.

The first detailed measurements of movements of the active part of the Slumgullion landslide were begun in 1958 by D.R. Crandell and D.J. Varnes (Crandell and Varnes, 1961), who established lines of markers on the upper, active part. Movements on the body of the active slide and advance of the toe were measured occasionally for more than a decade. At the narrowest part, where the landslide is only about 160 m wide, the velocity was found to be about 6 m per year near the center and slightly less at strike-slip faults along the margins, as shown in figure 1. Movements during the period 1958-68 of a station near the center of the slide (and, for a shorter period, of a point at the base of the southern part of the toe) are shown in figure 2. Measurements were continued at a few places and at irregular intervals through 1973. In 1991, we began to investigate possible movements on the inactive part of the landslide below the active toe to determine the effects of loading by the active toe. These investigations are described in this chapter.

Control Surveys

At the time that the triangulation net for aerial photographic control was targeted, the direction and length of the future flight line were known, but the aereal coverage of individual photograph frames was not known. Because of this, the triangulation stations and targeted photo-control points turned out to be not very well distributed for photogrammetric compilation, particularly for control of the more detailed 1:6,000 photography. The distribution of the 22 triangulation points, which were in place by 1993, and some of their names are shown in figure 3. Their positions are referenced to a coordinate grid based at station WINDY (Coordinates 10,000 m north, 10,000 m east) at the Windy Point Overlook on a spur off State Highway 149. Local north was estimated from the available 1:24,000-scale topographic map. Elevations are referenced to U.S. Coast and Geodetic Survey bench mark H 169 on the old part of the landslide, near its southern border, a small distance west of the curve of State Highway 149 (fig. 4).

Although the distribution of triangulation points was sparse owing to difficulty in selecting mutually visible points among obscuring ridges and within the heavy forest around the landslide, the photogrammetrists were able to construct topographic maps at 1:1,000 and other scales. These became bases for a variety of subsequent geologic and deformation studies by geologists working on the active slide, including several from the Italian National Research Council.

Deformation Surveys Below the Active Toe

As work progressed on the active slide, we turned our attention toward the apparently inactive part to determine whether it was responding to loading from the 1 m per year advancement of the 40-m-high active toe. The quickest and most accurate means to detect possible movement, although only in a vertical direction, was by leveling. The initial step was to lay out, in 1991, loops of level circuits from bench mark H 169 northeastward in front of the active toe. As bench mark H 169 is itself on the old slide, about 230 m from the active front, and may be in an unstable area next to a ravine, we also ran levels from the bench mark southwestward along State Highway 149 beyond the old slide into a supposedly stable area. The principal circuits (labeled green, blue, red, and black, respectively, from southwest to northeast) are shown in figure 4.

The first remeasurement of level lines, in 1992, showed that, during 1 year, some points close to the active toe had moved down as much as 20 mm, while points 50 to 100 m from the front had subsided smaller but still observeable amounts. These findings made it necessary to determine whether horizontal displacements and deformations also were occurring. In 1992, to detect these horizontal movements, we laid out quadrilaterals and triangles with sides of 20 to 90 m using electronic distance measurement (EDM) in three areas in front of the active toe and located stations in a closed traverse about 1 km long around the principal (blue) level circuit. A dense triangulation-trilateration net perhaps would yield more closely spaced data concerning deformation, but that was impractical in this forested area. The quadrilaterals and triangles were remeasured 2 months later. All results of these surveys as of the end of the 1992 field season, were summarized and subsequently published in U. S. Geological Survey Open File Report 93-577 (Varnes and others, 1993).

The principal level lines, quadrilaterals, and triangles were again remeasured in the spring of 1993. Total changes, generally for about a 2-year period during 1991 to 1993, are shown in figures 5, 6, 7, and 8. Continued vertical movements near the active front resulted in the accumulated changes shown in figure 5. All movements were down except at three places very close to the active toe--two on prominent rising rolls at the northwest border of the toe near station T13 and at TP7 near the southeast border on wet unstable ground--and at TP77 along the highway (see figs 5 and 6). The use of points in the broad hillside shoulder and ditch along the highway has become questionable. Point TP79, showing the large 29.2 mm depression during a period of 1 year, may have been run over by road equipment; TP77, showing a 4.5-mm rise in 1991-92 was later torn out of the ground. Otherwise, the subsidence along the active front noted in 1992 continued into 1993, with some indications that depression is more rapid near the northern part of the front than at the southern part.

The braced quadrilateral shown in figure 6 near the curve in sTATE Highway 149 was laid out in 1992. Unrecognized problems during the resurveying in 1993 apparently involved the westernmost station, TP 200. All three triangles that include that point do not close within acceptable errors, even though the horizontal angles observed at TP 200 added up satisfactorily to 360E. Farther north, in the quadrilateral and triangle at POND (figs. 6 and 7), four of the eight lines had changes in horizontal distance of more than 4 mm in 1 year. All of these lines involve station TP 204, which is on marshy ground and may be unstable.

The four triangles in the red level circuit on the bench northeast of POND (see fig. 8) show many changes in distances and angles during 1 year that are larger than the probable errors of adjusted observations. The probable errors are not yet established, but, in small triangles such as these, they are believed to be about 2-4 mm in distance and about 5 seconds in angle. The measurements made so far suggest that stations TP 30 and TP 202 are both moving away from station TP 31A, which is close to the active front. Triangle TP 32-TP 202-TP 32B appears to be contracting slightly and uniformly.

Conclusions

Surveys to date indicate that the inactive part of the Slumgullion landslide is responding to changing loads caused by the advancing toe and is not as stable as previously assumed. In general, ground is depressed in front of the advancing toe, the amount decreasing with distance from the toe. However, three points very close to the toe have risen significant amounts (20-80 mm) in 2 years. We are continuing to monitor these movements and expect a clearer deformation pattern to emerge.

Acknowledgments

We thank our colleagues who assisted at various times in the leveling and deformation surveys, including M. Arattano, S. Cannon, M. Chiarle, S. Diehl, D. Eversoll, R. Fleming, M. Giardino, R. Guzzi, M. Paresi, J. Savage, and K. Varnes.

References Cited

Crandell, D.R., and Varnes, D.J., 1961, Movement of the Slumgullion earthflow near Lake City, Colorado, in Geological Survey Research 1961: U.S. Geological Survey Professional Paper 424-B, p. 136-139.

 Varnes, D.J., Smith, W.K., Savage, W.Z., and Varnes, K.L., 1993, Control and deformation surveys at the Slumgullion Slide, Hinsdale County, Colorado--A progress report: U.S. Geological Survey Open-File Report 93-577, 15 p., 1 pl.