It shall be unlawful to make any such excavation or tunnel in any way contrary to or at variance with the terms of the permit therefor. Proper bracing shall be maintained to prevent the collapse of adjoining ground; and in excavations, the excavation shall not have anywhere below the surface any portion which extends beyond the opening at the surface. No injury shall be done to any pipes, cables or conduits of the city department. No unnecessary damage or injury shall be done to any tree or shrub or the roots thereof.
('72 Code, § 92.049) Penalty, see § 96.999