8.1 Introduction

According to investigations，most problems encountered in deep excavation have direct or indirect relations with groundwater.Therefore，whether groundwater has been properly dealt with is a crucial point for the success of an excavation.Groundwater⁃induced problems in an excavation may arise from insufficient investigation of groundwater or geological conditions that lead to inability to fully control the groundwater.It may also arise from misunderstanding of the influence of groundwater，so that a wrong excavation method is adopted.Thus，it is necessary to perform detailed investigations of groundwater and its influences on soils or structures during excavation.

The permeability of clay is lower than 10－6 cm／s，from which it follows that the flow velocity of groundwater in clay is rather slow.As a result，to lower the groundwater level or decrease the water content of clay below the groundwater level in a short period requires large amounts of energy or chemical methods to force the movement of groundwater in clay.When the groundwater level is lowered or the water content is decreased，the properties of clay will change significantly.The shear strength will increase and the compressibility will decline.In geotechnical engineering，the methods are often categorized as soil improvement methods.However，soil improvement methods are not the subject of this chapter.This chapter is confined to the introduction of dewatering methods，which are employed to lower groundwater levels.Besides，with the low permeability of clay，that is，the flow velocity of groundwater in clay is small，the possibility of groundwater leaking into the excavation zone，which will cause much inconvenience during construction，need not be considered.There is no occurrence of boiling in clay，either.Therefore，when clay is encountered in an excavation，the groundwater level can be ignored in practical engineering applications，except in cases of the strengthening of soil.

The permeability of sand or gravel is usually greater than 10－3 cm／s，which follows that the flow velocity of groundwater in sand or gravel is rather high and groundwater probably will leak into the excavation zone during excavation and cause much trouble.In the worst cases，it may bring about the loosening of soils，sand boiling，or upheaval failure.To avoid such conditions，it is necessary to design comprehensive dewatering schemes before or during excavation.On the other hand，with the higher flow velocity of sand or gravel，simple methods such as pumping are usually enough to lower the groundwater levels.

Goals of dewatering can be summarized as follows：

①To keep the excavation bottom dry.With the higher flow velocity of groundwater in sand or gravel，groundwater may well flow into the excavation zone，which will cause inconvenience for construction.To keep the excavation bottom dry，the groundwater level is generally lowered to 0.5⁃1 m below the excavation surface.Groundwater flows so slowly in clayey soils that flowing of groundwater into the excavation zone will not occur.There is no need to lower the groundwater level in clay.

②To prevent leakage of groundwater or soils.To excavate in sandy or gravelly soil，with a high groundwater level，using either soldier piles or sheet piles，which are not satisfactorily watertight，or diaphragm walls or bored piles with joints that may have defects will risk the possibility of the leaking of groundwater into the excavation zone through the retaining wall.The leakage of groundwater and soils may lead to disastrous results and bring about collapses and failures of excavations in the worst cases，when the leaking is great enough to enlarge the holes in the retaining wall.

③To avoid sand boiling.To keep the excavation bottom dry when excavating in sandy or gravelly soils requires lowering the groundwater level within the excavation zone to 0.5⁃1 m below the excavation bottom at least.While excavation proceeds，the difference between the groundwater levels within and outside the excavation zone grows larger.When the hydraulic gradient around the excavation bottom grows larger or equals the critical hydraulic gradient of soils，sand boiling will occur.Many methods are available to avoid sand boiling.One of them is to lower the groundwater level outside the excavation zone.However，the possibility of ground settlement outside the excavation zone has to be considered.

④To forestall the upheaval failure.There exists a permeable layer（such as sand or gravel）underlying the clayey layer.The water pressure from the permeable layer will generate an upheaving force against the clayey layer.When the water pressure acting on the bottom of the clayey layer is larger than the total weights of the clayey layers，upheaval failure will occur.One of the methods to prevent the occurrence of upheaval failure is to lower the piezometric pressure of the permeable layer by pumping.

⑤To keep the basement from floating.With the completion of excavation，one starts the construction of basements.In sandy soils，with the light weight of structures during the stage of basement construction（the weight of the basement only），the phenomenon of the floating of the basement is likely to happen if the weight of structures is smaller than the water pressure acting on the foundation base.Once the floating phenomenon has happened，with the differential heaves of the foundation，the floated basement will not necessarily sink back to the original elevation while building construction proceeds，which may lead to damage of the structures.In the worst condition，the basement may need to be demolished or reconstructed.Therefore，dewatering is usually required at the stage of basement construction to keep the upheaving force on foundation bottoms smaller than the weight of structures during construction.Dewatering is to be continued till the upheaving force is smaller than the weight of structures during construction.