OR/14/051 Borehole records of the ground surface
|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.|
Before assessing ground surface change related to road construction using borehole records it must be established that the ground levels recorded in pre-construction borehole and trial pit logs provide reliable estimates of the actual ground surface. In lieu of information on the accuracy or precision of the methods used to derive the ground levels during the site investigations, tests were conducted for sample boreholes datasets for which no ground surface change is expected to assess potential differences between the recorded value and the DTM.
The first test, in the northern area, was conducted using a set of boreholes drilled in 1972 for a possible bridge crossing over the River Nairn, east of Daviot, which was not subsequently chosen for the road route (Figure 2).
A second test was conducted utilising boreholes drilled after the main road development at Ballinluig (in the south; Figure A1-2 and Moy (in the north; Figure A1-1) to examine how well their start heights corresponded to the DTM surface level.
The two test areas are located in rural upland settings with little urbanisation in the vicinity of the road and surface change due to non-road related construction or excavation activities is not likely to have resulted in measurable change over the period since the boreholes were drilled (up to approximately 40 years). Similarly, agricultural activity is limited and is unlikely to have been associated with ground surface change in the vicinity of the road since its construction. Natural processes of erosion and deposition may influence ground surface levels, particularly in the vicinity of rivers, or on unstable slopes prone to landslides. The A9 route within the test areas generally avoids river floodplain areas and inspection of aerial photographs and geological maps indicates that there are no significant recent or past landslips evident in the vicinity of the road.
The River Nairn test — boreholes drilled but no road constructed
Thirty boreholes were drilled in 1972 as part of a site investigation for an alternative location of the A9 viaduct over the River Nairn near Daviot. The boreholes lie approximately 1.2 km to the north-east of the site where the actual viaduct was constructed and lies 300–400 m east of the closest segment of the A9 (Figure 2).
The difference between the borehole start height (HB) and the DEM surface level (HPT) was investigated for 28 of the boreholes from this site investigation where the start height is known. The difference between the 1972 ground surface level and the c.2010 surface level given by the PGA DTM was derived as
Hdiff = HPT - HB (Equation 1)
The mean of Hdiff for the 28 boreholes is -0.42 m (SE = 0.44, St dev = 2.3) and a 1 sample t-test indicates that it is not significantly different to zero (p = 0.36). This result demonstrates that the ground level as recorded in the boreholes drilled in 1972 corresponds well with the DEM ground surface level. This result suggests that that measurable surface change is likely to be minimal in areas away from the main road construction. Furthermore, by demonstrating that the ground levels recorded in the borehole logs from 1972 accurately reflect the modern ground surface, this test supports the inference that the recorded surface levels in pre-road boreholes reflect the real ground surface prior to road construction.
Recent boreholes and ground surface change
Site investigations were conducted as part of road improvement works between 2005 and 2007 in the vicinity of Moy, in the northern test area, and Ballinluig in the southern test area Figure A1-1 and Figure A1-2. Analysis of Hdiff values for 25 boreholes drilled between 2005 and 2007 in the Ballinluig area, yields a mean difference of 0.08 m (SE = 0.22, St Dev = 1.09) which is not significantly different from zero (p = 0.71). In the Moy area, 60 boreholes were drilled along the road route in 2005. Analysis of Hdiff for all the Moy boreholes gives a mean of -0.01 m (SE = 0.13, St Dev = 1.00), which is also not significantly different from zero (p = 0.916).
These results indicate that the borehole ground surface levels are consistent with the PGA DTM representation of the ground surface (captured between 2005 and 2010), providing further support for the interpretation that ground levels in site investigation boreholes provide a reliable record of the ground surface prior to construction activities.