水工隧洞的衬砌有什么作用？

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One day, when Bill was    小题1:  home from school, he heard a noise coming from the bushes（灌木）.He went over to see   小题2:   it was. Behind a bush, he found a black dog. Bill went back home as soon as he    小题3:   with the dog and showed her to his parents. Bill's dad found that the dog had a broken leg,    小题4:   they brought the dog to a doctor for animals by car. On the    小题5:   Bill's parents decided that Bill could keep the dog if they couldn't find the owner.
When they arrived,the doctor checked the dog. He told  Bill that she  小题6: going to have babies! The next day, Bill   小题7:   signs（牌子）like “FOUND DOG BLACK” around his neighborhood. He   小题8:   wrote his family's phone number on the signs.
Two days later, Bill got a call   小题9:   the owner of the dog. The owner thanked Bill. When he came to pick up his dog, he said Bill could have    小题10: of the babies. Bill was very happy! Finally Bill got a baby dog from the owner. He named him Rosco and liked him very much.