The conventional quasi-three-dimensional technique has a high efficiency in the simulation of regional groundwater flow in multilayered subsoils

with the assumption that groundwater flow is essentially horizontal in the aquifers and vertical in the aquitards. This assumption requires a two orders of magnitude permeability contrast between the adjacent soils

which is not conducive to a wide application of the conventional technique. In this study

calculation theory is extended to be applied to regional groundwater flow in general multi-layered subsoils. Firstly

the partial differential equation governing two-dimensional horizontal flow in each soil is derived with the average hydraulic head as the main variable and considering the infiltration at the upper and lower interfaces. The velocity continuity conditions at the soil interfaces provide the coupling condition among the soils. Then

the vertical variation of the hydraulic head in individual soil is obtained by quadratic interpolation between the average head and the head at the upper and lower interfaces. A quasi-three-dimensional finite element model is presented based on the extended calculation theory. Each soil is discretized with the same two-dimensional horizontal grids

and the fluid mass balance is established at each node. The extended quasi-three-dimensional technique has a higher accuracy and greater applicability

as it can overcome the limitations of the conventional technique

and simultaneously get a reasonable simulation of the horizontal and vertical flow in each layer without sacrificing the calculation efficiency. The extended technique provides an efficient and competitive scheme to simulate regional groundwater flow in multilayered subsoils

because it is convenient to solve free surface problems and deal with singular problems of underground structures

and compute a high-precision three-dimensional representation of the hydraulic head distribution.