A review of conventional techniques for subsurface characterization of landslides

Abstract

Naturally occurring landslides can be difficult to characterize and often exhibit anomalies in controlling geometry and consistency. The characterization of landslides in engineering geology practice is usually based on program of subsurface exploration; focused on identifying subsurface materials, structure and stratigraphy, hydrologic conditions, and appropriate strength parameters. On native slopes bereft of anthropogenic modification (excavation), geomorphic evidence of previous movement and its relative depth and scale are often discernible to the experienced eye. This paper seeks to describe the various techniques that have been successfully employed in site-specific subsurface investigations of prehistoric landslides. These include considerations of limiting geometry (for planning subsurface exploration and sampling), overcoming problems with site access, exploratory trenches, small diameter borings, downhole logging of large diameter borings, and geophysical techniques. These procedures can be combined to develop more realistic cross sections along the vector of maximum landslide movement, so the displaced mass can be realistically analyzed. The respective advantages and disadvantages of these techniques are discussed, with baseline references on subsurface exploration.

Appendix: Baseline references

The following list presents the most-often-cited references on subsurface exploration. Hvorslev’s report (1949) is a comprehensive reference upon which most subsequent standards are based. It was developed by the ASCE Committee on Subsurface Sampling between 1938–1949 and was adopted as a Corps of Engineers’ standard in 1949. It was reprinted by The Engineering Foundation in 1962 and 1965. Although these methodologies emphasize site exploration and testing for evaluation of foundation conditions rather than evaluating land slippage, this publication has been cited in numerous legal proceedings as standard practice for subsurface site characterization across much of the US, and elsewhere. It is wise to be familiar with the applicable standards for subsurface exploration, sampling and testing in any given area.

Over the past 35 years the NAVFAC DM-7.1 (U.S. Navy 1982) manuals have been the most widely cited publications pertaining to subsurface exploration standard of practice, with respect to number of borings and frequency of sampling (during the 1950s and 1960s the Hvorslev [1949] was the most widely cited reference). Some of the most cited references on subsurface exploration over the past 35 years have been: AASHTO (1982), Acker (1974), ASCE (1976, 1986), ASTM (1971, 1996, 1997), Cedergren (1989), FHWA (1990), Haley and Aldrich, Inc (1984), Head (1989), Hunt (1984, 1986, 2005), Lowe and Zaccheo (1979, 1991), McCalpin (1996, 2009), Scullin (1983, 1994), Stout (1977), Wroth (1984), WP/WLI (1993), U.S. Army Corps of Engineers (1960, 1972) and U.S. Bureau of Reclamation (1989, 1995). Most state transportation departments employ AASHTO’s testing standards but may include additional test standards (e.g., California Department of Transportation 2001).