Principal Characteristics of Limestone Saprolite and Its Forming Mechanisms

The white layers in the vadose zones of carbonate rock weathering sections are carbonate rock saprolites and there is a clay cover in the upper of sections is a necessary condition for the saprolites developing.The white limestone saprolite of a limestone weathering section in Kunming has a porous macro structure and its density,porosity and intrinsic permeability are 1.8081 g/cm3,32.65% and 774.8139×10-3 μm2 respectively.The corresponding parameters of the fresh limestone are 2.6966 g/cm3,0.05% and 0.0084×10-3 μm2 respectively.Limestone saprolite has a strong water-absorbing capacity and a very low water-releasing capacity.Its specific water storativity and specific yield are 30% and 5% respectively.Under a polarized microscope a large number of pores between grains and dissolution pores can be seen in limestone saprolite thin sections poured with cast resin and under a SEM cellular dissolution pores between calcite aggregates and tiny pores between calcite grains can be seen in the saprolite.The transition from limestone to saprolite is completed through weathering fronts of limestone blocks expanding.At the micro scale,a limestone block is a discontinuous grain aggregate and has a huge specific surface area and a strong potential water-absorbing capacity.The vadose zones with clay covers are usually damp and have high relative humidity.When the vapor which invades into a limestone block condenses into liquid water,the water can exert a wedging force on calcite grains and weaken the structural connections between grains.The wedging action makes the calcite aggregate expand to some extent and transform into saprolite crust.After that,the weathering front spread gradually and the rock block is turned into a saprolite block.To sum up,limestone saprolite forms in damp vadose zones through hygro-expansivity and is the product of physical weathering.The leaching of seepage water takes place in the evolution stages of the saprolite.