11.3.4 Heave of excavation bottoms
Excavation will cause the excavation bottom to heave.Too much heaving is usually dangerous for the strutting system and causes settlement of the soils in the vicinity.Under worse conditions，it may lead to the failure of the excavation.Thus，the magnitude of the heave of the soils at the excavation bottom should be monitored constantly.
The heave of an excavation bottom is usually measured with heave gauges，which consist of a cross shaped iron plate，an aluminum rod，and a galvanized iron casing，as shown in Figure 11.11.The cross shaped iron plate is placed below the excavation bottom.It is shaped in a cross to enlarge the contact area with the surrounding soils，so that when the soils below the excavation bottom heave，they will cause the heave gauges to move upward accordingly.The top of the cross shaped iron plate is connected to an aluminum rod extending to the ground surface to measure the heave of the gauge.The aluminum rod is protected by a galvanized iron pipe to seclude the aluminum rod from the soils.
The installation of the heave gauge is as follows：
①Connect the cross iron plate with the aluminum rod.
②Drive the galvanized iron casing into the soil 50 cm below the excavation surface using a wash boring machine and hammer it deeper by another 50 cm.It should be 100 cm below the excavation surface in total.
③Keep boring to a depth of about 150 cm below the excavation surface.Pour 100 cm thick bentonite into the bore.
④Put the heave gauge that is already connected to the aluminum rod into the casing and then push into the soil.
⑤Take some protective measures with the top of the heave gauge（i.e.the top of the aluminum rod）to finish the installation of the heave gauge.
The measurement of heave is to measure the elevation differences of the aluminum rod before and after excavation（of a certain stage）using a level against a fixed point or permanent benchmark.Thus，we obtain the movement of the excavation bottom during a certain stage of excavation.
Figure 11.11 Schematic diagram of the installation of a heave gauge