Archaeological deposits and tunnel constructions: Simulations of groundwater head at the Old Wharf of Bergen, Norway

Abstract

The purpose of this study was to examine the impact of tunnel constructions on the steady-state groundwater head in the archaeological deposits at the Old Wharf (Bryggen) of Bergen. Water infiltration, permeabilities, and boundary conditions were deduced from available data and supplemented by leakage measurements into the existing railway tunnel located upstream of the Bryggen area. The pore water in the deposits at Bryggen has different origins viz. precipitation; leakage from drainage systems; infiltration of seawater; infiltration via the bedrock, and based on these observations, a 2D horizontal – and a 2D vertical numerical model were constructed where the seawater was the boundary at one side of the model and the groundwater divide at the other side. Given estimates of bedrock permeability and steady state infiltration rate, it was possible to calculate groundwater heads in the sediments at Bryggen that were consistent with observations, assuming that all the water percolating into the sediments flow through fractures in the bedrock. By this extreme assumption, a maximum sensitivity of the relation between groundwater flux in the bedrock and the groundwater level in the archaeological deposits was achieved. After simulation of the natural groundwater level (i.e. without any artificial extraction of water in the catchment), the impact of the water leakage into the existing tunnels was calculated. Given the relatively rough estimates of water leakage into the existing tunnels (4 L/day/meter of tunnel), the calculated decline of the groundwater head in the archaeological deposits was less than 1% of the natural groundwater level (<1 cm). If leakage rates were increased to one and two orders of magnitude, the steady state groundwater decline would be from 5% to more than 35% of the reference groundwater level (4–15 cm). The reason for these relatively robust groundwater conditions in the sediments are caused by the boundary conditions: Without any local water extraction in the deposits, the steady-state groundwater head at Bryggen will always be higher than the seawater level. A crucial point for these results was the leakage rates into the existing tunnels. Water discharge into tunnels and other subsurface constructions is the principal risk factor in this study and should be monitored before any new tunnels are constructed.