Følgevirkninger af råstofgravning under grundvandspejlet

Summary and conclusions

Mining alluvial deposits below the groundwater table causes drawdowns to occur in the pit and around the quarry. When calculating drawdowns in the aquifer resulting from the mining activities it has been customary to use the theories from water well testing by considering the mining process as one of groundwater pumping. However, this analogy is not physically correct since pumping removes water from the pit, whereas quarrying moves groundwater from the aquifer to the pit as a result of changing the porosity from the actual value to exactly 1. It should be expected, that actual drawdowns in the aquifer will be smaller than the ones calculated in the way mentioned. This is also the experience gained in actual cases of mining below the water table.

In order to develop a method which more accurately describes the actual process, the mathematical equations governing the groundwater flow have been written down and solved analytically. The solution has been tested against actual data from a gravel pit in Skåne and shows very good agreement. The formulas derived involve mathematical functions well known from well testing theories and should therefore be relatively simple to use in practical applications. Det solution presented has been developed in cylindrical coordinates, since it proved to be unpractical to apply any other geometry. Cylindrical coordinates are, however, the most practical geometry, since the circular pit is an acceptable approximation to most polygon shaped quarries with edges having approximately the same lengths.

The mathematical solution shows the surprising result, that the drawdown in the pit is time independent. During the first stage of quarrying an abrupt water table drawdown is established and this drawdown does not increase with time. As the circumference of the pit expands, the volume of groundwater flowing into the pit balances the volume of aquifer material excavated in such a way, that the drawdown is constant. In the aquifer proper drawdowns will increase with time, but these drawdowns are limited since at the pit-aquifer interface drawdowns are equal at all times.

The formulas derived for calculation of drawdowns assume a constant rate of excavation. In reality there will interruptions and during such periods the water level will recover partly or in full. The pit, therefore, will act as a buffer and reduce drawdowns. This is believed to be the main reason, that no substantial drawdowns seem to have been observed in and around quarries after longer periods of excavation. The formulas, therefore, can be used to calculate the maximum drawdowns as a result of quarrying below the groundwater table, and such drawdowns must be expected to exist during the early phases of quarrying only.

As a consequence of this, it is recommended that quarrying is initiated by establishing as large a pit area as possible, and that the excavation rate is slow at the beginning. Further, it is recommended, that the consequences of excavation are documented through continuous monitoring of groundwater levels in the pit and in the aquifer, and keeping records of aquifer material excavated on a daily or weekly basis. Finally, rainfall should be recorded on a daily basis.