Secondary lining backfilling method for small-clear-distance triple-arch tunnel

The invention relates to a small-clear-distance triple-arch tunnel secondary lining backfilling method which comprises the following steps: S1, cleaning an inverted arch cavity: removing a secondary lining template, and then cleaning sundries between a primary lining and the secondary lining; s2, cushion layer construction is conducted, specifically, a cushion layer is laid on the top of the second lining, the cushion layer is a sand material mixed with water swelling rubber and cellosilk, a sand material mixed with asphalt or a sand material mixed with bentonite, and the thickness of the cushion layer is 5-15 cm; s3, filling the inverted arch cavity: filling rubble concrete above the cushion layer to form a filling layer, wherein the gap between the filling layer and the waterproof plate is greater than 15cm; s4, backfilling: filling a gap between the filling layer and the waterproof plate with a filling material to form a backfilling layer, wherein the filling material is expansive concrete or expansive mor