Damage due to subsidence
The city's surface is comprised of more than rock and soil - it also contains water. Groundwater in urban areas, is often viewed as a problem -- it is pumped and drained to keep buildings dry. However, changes in groundwater levels could affect the stability of ground.
The weight bearing capacity of the ground is dependent upon its composition and the groundwater in the pores of the soil masses. When groundwater pressure drops, the pore-pressure lowers, resulting in compression of the geologic masses. This is called mechanical subsidence. When the groundwater level of the upper ground layers falls, pores that were full of water may fill with air. In this case, in addition to the mechanical subsidence, secondary subsidence may occur in soils with high-organic content. Lowering the groundwater increases the air supply to begin the decay of organic material. However, if the organic material remains under water, the material stays intact.
Such decay can occur in marshes or in waste disposal sites that are rich organic content (for example, sawdust under Oslo Central Train Station.). In the same way, a lowering the groundwater level can also damage building foundations made of timber. Subsidence can take place suddenly, resulting in serious building damage.
Bergen’s pier and wharf houses (The Brygg) and a section of town called Vågsbunnen are designated World Heritage Sites, and are examples of how important groundwater is for the preservation of organic culture deposits and the historical buildings above them. The slow decline of groundwater for several decades has resulted in subsidence, which in turn has resulted in the loss and damage to the protected cultural strata. Many historical wooden houses are tilted, and many sidewalks and streets are damaged due to subsidence. NGU’s researchers have helped find measures to raise the groundwater level and rescue the wharf houses.
Bjørvika, a new area of Oslo, is built on settlement-prone clay deposits, as well as large masses of organic material, found meters under the surface. Massive construction activity in recent years may have caused a decrease in the deep groundwater level and possibly even in the upper groundwater level. This could have contributed the accelerated sinking and movement, a rate faster than the geoengineers expected. Infrastructure has been damaged by this already, and could mean higher maintenance costs in the future. Satellite-based radar interferometry makes it easier to reveal subsidence and movements in the ground.
Although groundwater may be a determining factor in the stability and in conversation of archaeological sites, groundwater's role is often not considered in pre-investigation for construction projects. Note too, that groundwater does not limit itself within political border, hence, a need for systematic monitoring of the groundwater level over a broader area.
