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北京地区常遇到松散的干砂、砂卵石等不稳定土壤。这种土壤的坚固系数小(<0.6),在顶管掏土时,不能形成“卸荷拱”,因而在顶进过程中,土壤坍塌,造成顶管质量低劣,严重者还会造成地面沉陷,破坏房屋建筑结构实例不少。一九七八年,××胡同污水工程,在松散炉渣回填土中顶管,由于现场狭窄,又没有加固土壤,造成局部房屋产生裂缝,工程质量低劣。
Beijing often encounter loose dry sand, pebbles and other unstable soil. This soil has a low coefficient of solidity (<0.6) and can not form a “unloading arch” during the top tube excavation. As a result, the soil collapses during the top-in process, resulting in poor quality of the top tube and serious ground subsidence , Many cases of destruction of housing construction structure. In 1978, × × Hutong Sewage Project, the top tube in loose slag backfill, due to the narrow site, there is no reinforcement of the soil, resulting in local housing cracks, poor quality of the project.