OR/14/051 Methodology

Whitbread, K. 2014. The geomorphic impact of road construction: a case study of the A9 in Scotland. British Geological Survey Internal Report, OR/14/051.

Data sets

Site investigations were conducted prior to a major episode of construction forming the modern A9 route in the late 1970’s. Records of the boreholes and trial pits from these investigations are stored in the British Geological Survey (BGS) digital borehole database the Single Onshore Borehole Index (SOBI), and original paper logs are available as scanned images. These records include the borehole start height and drill date where originally recorded in the original borehole log. All borehole data along the A9 route in each of the test areas was extracted from SOBI. Topographic maps of varying detail and quality were provided in the original site investigation reports along with cross-sections along the road route. Digitisation of these records was beyond the scope of this study, but digitisation of maps and sections would to help constrain the pre-road ground surface in future studies.

Two high-resolution DEMs that represent the modern ground surface, the NEXTMap Britain dataset and the more recent ‘PGA’ dataset are available for whole the UK. The NEXTMap Britain dataset comprises surface elevation data derived from flown interferometric synthetic aperture radar (IfSAR) obtained between 2002 and 2003 (IntermapTechnologies, 2004). The PGA dataset was derived from analysis of stereo aerial photographs obtained between approximately 2005 and 2010. Details of the resolution of the two datasets are given in Table 1. The relative accuracy of the two DTM datasets with respect to Ordnance Survey terrain data is assessed in DEM assessment.

Data preparation

Borehole records containing information on the drill date, start height, and X and Y coordinates were extracted from SOBI for boreholes located within 50 m of the centre line of the A9 and plotted as point shapefiles in ARCGIS. Values of the surface elevation at the borehole point were extracted for each record from the DEMs using Spatial Analyst Tools.

The distribution of made ground (MGR) and worked ground (WGR) associated with cuttings and embankments, respectively, along the A9 route was mapped in ARC GIS using a combination of field surveying (in the southern test area only), Ordnance Survey (OS) 1:10 000 scale raster maps, OS VectorMap, aerial photographs, and Google Street View.

Attribution of borehole records

Ground surface levels for each borehole point were extracted from the DTM using ARC Spatial Analyst tools. The borehole records were also attributed according to whether they occur in areas mapped as made or worked ground by spatially joining the borehole and artificial ground Shapefiles. Analyses of the resulting data, including statistical tests, were conducted using Excel and Minitab.

DEM re-interpolation

The PGA DTM for each site was converted to grid of points from which the points occurring in areas of made ground and worked ground were selectively removed by clipping to the artificial ground polygons. The clipped point clouds were then converted back to raster grids using different interpolation functions to estimate the pre-road ground surface in the areas of artificially modified ground. The interpolation functions used to derive the ‘pre-road’ DTM were Inverse Distance Weighting (IDW), spline and triangulation; details of the functions are given in Appendix 2 - Interpolation functions.

Volumes of material input or removed along the road route were derived using the Cut-Fill function in ARC GIS Spatial Analyst tools. This function compares the ‘pre-road’ DTM the modern DTM, to identify regions of net gain or net loss and derive their volumes.