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1、Caissons are divided into three major types: ① open caissons，② box caissons ( or closed caissons) ，and ③ pneumatic caissons.Open caissons ( Figure 9. 10) are concrete shafts that remain open at the top and bottom during construction. The bottom of the caisson has a cutting edge. The caisson is sunk into place。

2、and soil from the inside of the shaft is removed by grab buckets until the bearing stratum is reached. The shafts may be circular，square，rectangular。

3、or oval. once the bearing stratum is reached，concrete is poured into the shaft ( under water) to form a seal at its bottom. When the concrete seal hardens，the water inside the caisson shaft is pumped out. Concrete is then poured into the shaft to fill it. Open caissons can be extended to great depths。

4、and the cost of construction is relatively low. However，one of their major disadvantages is the lack of quality control over the concrete poured into the shaft for the seal. Also，the bottom of the caisson cannot be thoroughly cleaned out. An alternative method of open-caisson construction is to drive some sheet piles to form an enclosed area。

5、which is filled with sand and is generally referred to as a sand island. The caisson is then sunk through the sand to the desired bearing stratum. This procedure is somewhat analogous to sinking a caisson when the ground surface is above the water table.Box caissons ( Figure 9. 11 ) are caissons with closed bottoms. They are constructed on land and then transported to the construction site. They are gradually sunk at the site by filling the inside with sand，ballast，water。

6、or concrete. The cost for this type of construction is low. The bearing surface must be level，and if it is not，it must be leveled by excavation.Figure 9. 10 Open caissonFigure 9. 11 Box caissonFigure 9. 12 Pneumatic caissonPneumatic caissons ( Figure 9. 12 ) are generally used for depths of about 50 － 130 ft( 15 － 40 m) . This type of caisson is required when an excavation cannot be kept open because the soil flows into the excavated area faster than it can be removed. A pneumatic caisson has a work chamber at the bottom that is at least 10 ft ( 3 m ) high. In this chamber。

7、the workers excavate the soil and place the concrete. The air pressure in the chamber is kept high enough to prevent water and soil from entering. Workers usually do not encounter severe discomfort when the chamber pressure is raised to about 15 lbf / in2( 103 kN /m2) above atmospheric pressure. Beyond this pressure，decompression periods are required when the workers leave the chamber. when chamber pressures of about 44 lbf / in2( 303 kN /m2) above atmospheric pressure are required，workers should not be kept inside the chamber for more than l. 5 － 2 hours at a time. workers enter and leave the chamber through a steel shaft by means of a ladder. This shaft is also used for the removal of excavated soil and the placement of concrete. For large caisson construction。

8、more than one shaft may be necessary ; an airlock is provided for each one. Pneumatic caissons gradually sink as excavation proceeds. When the bearing stratum is reached，the work chamber is filled with concrete. Calculation of the load- bearing capacity of caissons is similar to that for drilled shafts. Therefore，it will not be further discussed in this section.深基础部分词汇扩展( 1) 桩基础的形式 ( Types of pile foundation)勘查工程专业英语( 2) 桩基础术语 ( Basic terminology about pile foundation)勘查工程专业英语。