Tzarchei Rabim:[1]

It is permitted to perform Melacha on Chol Hamoed for the sake of the public.[2] This however only applies to needs of the public that relate to the bodily needs of a person, while other public needs are forbidden to be performed during the Moed.[3] [Public matters that pertain to bodily needs may be performed even if it involves professional work [Maaseh Uman] if it is needed for the sake of the Moed. If, however, it is not needed for the Moed then it is forbidden to perform a professional work, and only amateur work may be performed.[4]]

Examples: For example, one may fix roads, and remove from them any hazards. One may mark graves to warn Kohanim from going there. One may fix Mikvaos.[5] One may fix a bathhouse.[6] It is however forbidden to build a Shul during Chol Hamoed.[7]

_____________________________________

[1] See Shulchan Aruch 544; Shemiras Hamoed Kehilchaso chapter 11

[2] Michaber 544:1; Mishneh Moed Katan 2a

[3] Rama ibid; Beis Yosef in name of Rashba

[4] M”A 544:1; P”M 544 A”A 1

[5] Michaber ibid

[6] M”A 544:1

[7] Rama ibid; Beis Yosef in name of Rashba