11.5.2 Groundwater level

The instrument for groundwater level is the observation well where a vertical pipe with many holes，enveloped with two layers of nylon net，is placed.The installation procedure of the vertical pipe is as elucidated in the following（see Figure 11.23）：

①Bore to the designed depth using a drill machine.

②Fill the bottom of the bored hole with sand.

③Place the water observation pipe into the hole and fill the space with sand.

④Take some protective measures on the top of the vertical pipe.Thus is finished the installation of the observation pipe.

After groundwater flows into the pipe and the water level inside reaches the stable state，the water level in the pipe is then the groundwater level.As with the open standpipe piezometer，the water level in the pipe can be measured by using a water level indicator.

The aim of the observation well is to monitor the change of the groundwater before or during excavation and to supply information for the determination of the excavation method，the design of water pumping，and the management of construction period.