Characterization of Shallow Soil Slopes in Greenville County, South Carolina

Kimberly Henning; Senior Geology Major; kjhenni@clemson.edu

Advisor: Scott Brame
brames@clemson.edu

Introduction

Landslide hazards maps have been created for several counties in North Carolina in response to several high profile slope failures in that state, some of those slope failures resulted in fatalities and extensive damage to property. Using their model and working closely with the NC Geological Survey, we are developing a landslide hazard for the most northerly section of Greenville County where steep slopes associated with the Blue Ridge escarpment exist.

In order to create this hazard map, it was necessary to acquire several requisite skill sets.  A good part of the summer was spent learning how to input and display digital data into a Geographic Information System (GIS). The software used to create the hazard rankings and map, called SINMAP, is a complicated program that requires both GIS skills and knowledge of the mechanisms and inputs that control slope failures. Those requisite skills were acquired during this past summer.

Abstract

The goal of this research is to create a Geographic Information System (GIS) based landslide hazard map that can be made available to land managers in the upstate of SC.  The study area is the mountainous region in northern Greenville County, SC that includes Jones Gap State Park and Caesars Head State Park. This area currently lacks any hazard mapping excluding FEMA Flood Zones. The landslide hazard map will be constructed using the GIS based Stability Index Map (SINMAP) program.  Four counties in North Carolina have previously been mapped for slope failure potential by the North Carolina Geological Survey. These hazard maps are designed to prevent and minimize damage to local communities. Soil cores collected at Jones Gap and Caesars Head will be used to determine soil properties and shear strength.  Combined with high accuracy digital elevation derived from lidar data, slope failure potential maps will be constructed.

Stability Index Map (SINMAP)

Landslide hazard maps are created using digital elevation maps (DEMs), slope movement deposits, and a Geographic Information System (GIS) based Stability Index Map (SINMAP). Digital elevation model (DEM) methods are used to delineate slope characteristics and specific catchment areas that are required inputs into SINMAP. DEMs represent the topographic surface of the Earth. Two types of DEMS are available: digital surface modeling (DSM) and digital terrain modeling (DTM) are needed for specifications in elevation and drainage modeling.

In addition to the DEM, the accuracy of the rankings produced by SINMAP are greatly enhanced by site specific data such as soil cores collected in the study area that have been analyzed for shear strength, soil composition, parent material, organic matter, and permeability. As with any model, the more accurate the inputs are, the more accurate is the output. Another critical aspect in landslide assessment is the nature of the groundwater system and how it responds to large rainfall events.  We want to use SINMAP to predict shallow soil slips that produce landslides when groundwater levels are raised such that elevated pore pressures exist in the subsurface.

The mountainous regions of upstate South Carolina typically have thin soils and receive in excess of 50 inches of rainfall each year. Steep slopes, excessive rainfall and specific vegetation types are the main drivers of slope failures. Jones’ Gap and Caesars Head State Parks are both covered in dense forests and have the steepest slopes in South Carolina. The likelihood of landslides increase with each factor (high elevation, dense vegetation, excess moisture) and are more likely to reoccur in the same areas. Accordingly, previous slope movement deposits will be mapped and entered into SINMAP .

SINMAP is a raster-vector data model that runs under the infinite plane slope model with pore pressures interpreted using steady state models of hydrology (1). The infinite plane slope model runs under the assumption that the slope extends for a long distance (i.e., infinitely) and has a subsurface profile that is consistent along the entire slope. In this idealized model, the failure plane is parallel to the slope. Properly calibrated, SINMAP can predict slope failures that result from major rainfall events.

Materials and Methods

Having mastered the the requisite skills to operate the software, the site specific data collection described below will be conducted this fall and entered into SINMAP. Six cores will be collected from Caesar’s Head and Jones’ Gap State Parks. The Caesar’s Head cores will be referred to as cores CA(1-3) and CB(1-3) and Jones’ Gap cores will be referred to as cores JA(1-3) and  JB(1-3). The core samples will be tested for shear strength, soil composition, parent material, and permeability.

After obtaining a research permit to collect samples in Jones’ Gap and Caesar’s Head, the following steps will be taken.

  • A one-to-two foot opening is dug onto the side of an idenified shallow soil slip to provide a work base.
  • One foot long, three inch diameter cores (A) and six inch long, two inch diameter cores (B) are hammered into each working base site and removed once the soil is compacted.
  • These are taken to the lab where they are cut, opened, and photographed.

Each core will undergo direct shear analysis.

  • The CA(1-3) and JA(1-3) cores are all trimmed to a cylinder of 2.5” diameters and 1” length. They must fit into the direct shear machine in order to be tested. 
  • Each core is placed into a direct shear machine and sheared at normal stresses of 500 psi, 1000 psi, and 2000 psi to get the effective shear strength at failure. 

Each area that a core is taken from will also undergo soil tests through the agriculture services lab in Clemson,  SC.

  • A standard soil test including tests for pH, buffer pH, extractable phosphorus, potassium, calcium, magnesium, zinc, manganese, copper, boron, sodium, and lime requirements and recommendations. Calculations for cation exchange capacity, acidity, and percent base saturation are also included per the soil testing website.
  • An organic matter test will also be conducted.
  • The soil testing lab recommends these steps to collect a sample (2):
    • Use a soil auger, spade, or shovel.  Samples from cultivated areas should be taken from the surface to a depth of 6 to 8 inches.  Samples from pasture or turf should be taken only 2 to 4 inches deep.
    • Soil cores should be approximately the same size throughout their depth.  No adjustments are needed if using a soil probe. To do this when using a spade or shovel, take a thin slice from the side of a V-shaped hole.  Take sub-samples from at least 12 locations within the sample area.
    • Place the sub-samples in a clean plastic bucket and mix thoroughly.
    • Do not heat a moist soil sample to dry.  Spread it out on newspaper and let air dry overnight.  Crumble and mix before it hardens.
    • Label or number the soil bag.  Fill bag to fill line (the equivalent of 1 – 2 cups).  Do not use the same number for more than one sample.
  • Use digital elevation model (DEM) of South Carolina to begin creating SINMAP.
  • Use information from core samples and soil samples to validate or update input information from DEM.
  • Run SINMAP program.