GEOLOGICAL ASPECTS OF LANDSLIDES
CONSTRUCTION DELAYS CAUSED BY LANDSLIDE CORRECTION CAN BE A SOURCE OF GREAT COST. THEREFORE, IT BEHOOVES THE HIGHWAY ENGINEER TO BE FAMILIAR WITH THE CHARACTERISTICS OF SLIDES THAT HE MAY ENCOUNTER. THE FIRST STEP IN ANY LANDSLIDE ANALYSIS IS COLLECTION OF INFORMATION TO ANSWER SUCH BASIC QUESTIONS AS (1) WHAT IS THE GEOLOGY OF THE LANDSLIDE AREA, (2) WHAT SUBSURFACE CONDITIONS DEFINE THE SOIL OR ROCK MATERIALS AND GROUNDWATER CONDITIONS IN THE SLOPE, AND (3) WHAT IS THE EXTENT OF THE MOVING MASS, BOTH LATERALLY AND WITH DEPTH. GEOLOGIC CHARACTERISTICS AND PROBLEMS VARY FROM ONE MATERIAL TO ANOTHER. FOR EXAMPLE, TALUS OR COLLUVIUM DEPOSITS ARE IN REALITY TEACHEROUS AND DANGEROUS PILES RATHER THAN THE GOOD MATERIALS THEY APPEAR TO BE. THE STABILITY OF TALUS SLOPES IS GREATLY AFFECTED BY FLUCTUATIONS IN GROUNDWATER LEVEL AND BY THE CONDITION AND EXTENT OF WEATHERING OF THE BEDROCK. SMALL CUTS MAY CREATE MASSIVE SHEAR STRAINS THAT RESULT IN FAILURE 200 OR MORE FEET INTO THE BEDROCK. CORRECTION FOR TALUS SLOPE MOVEMENTS USUALLY REQUIRES EXTENSIVE ALTERATION OF SUBSURFACE DRAINAGE AND SELECTED UNLOADING OF THE MOVING MASS. IN PRECONSOLIDATED CLAY OR CLAY SHALE A RELATIVELY SHALLOW CUT MAY INDUCE SLOPE FAILURE BECAUSE WHEN UNLOADED THE CLAY CAN EXPAND ONLY IN THE VERTICAL DIRECTION, CAUSING AN IMBALANCE THAT MAY EXCEED THE SHEAR STRENGTH OF THE CLAY. FAILURE IN THESE CASES IS PROGRESSIVE; IN ONE CASE A SLIP-OUT INCREASED BY A FACTOR OF 75. EVEN THE SAME MATERIAL MAY BEHAVE DIFFERENTLY IN SOME CASES, AND FOR THIS REASON IT IS IMPORTANT TO BE FAMILIAR WITH THE GEOLOGIC HISTORY OF THE MATERIALS. AN EXAMPLE IS GIVEN IN WHICH, BECAUSE OF CERTAIN VOLCANIC ACTION, A GEOLOGIC ANOMALY AT THE BASALT--SHALE INTERFACE CREATED A MYLONITIC SEAM IN WHICH THE AVERAGE STRENGTH WAS FAR LESS THAN THAT OF MOST NATURAL CLAY SOILS.
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Authors:
- Gedney, D S
- Conference:
- Publication Date: 1970-4
Subject/Index Terms
- TRT Terms: Bedrock; Clay; Engineering geology; Excavations; Geology; Groundwater; Landslides; Preconsolidation pressure; Road construction; Shale; Shear strength; Slope stability; Slopes; Talus
- Old TRIS Terms: Preconsolidated clay
- Subject Areas: Geotechnology; Highways;
Filing Info
- Accession Number: 00230085
- Record Type: Publication
- Report/Paper Numbers: pp 31-35
- Files: TRIS
- Created Date: May 1 1972 12:00AM