What is the purpose of providing shoes in prefabricated drains?

Shoes are normally installed in prefabricated drains for the following reasons:

(i) It avoids the entry of soils into the mandrel by sealing it during the installation of drains.

(ii) It performs like an anchor to retain the drains at the designed depth and to stop the drains from being pulled out during the withdrawn of mandrels after driving the mandrels into ground.

However, the inclusion of shoes in prefabricated drains tends to aggravate the problem of smear effect because the shoes are usually larger in size than mandrels.