Common ITC is allowed and NOT full.
Rain harvesting pit is covered under 'plant' as per Explanation given below sub-section 6 of Section 17 of CGST Act. As per legal dictionary meaning, 'for a manufacturer 'plant' means 'apparatus' also which is used for manufacturing the product. Thus it is covered under 'Exclusion clause' and not hit by Section 17 (5)(d).
The department may go for 'civil structure' which is out of exclusion clause. Since the burden of proof is cast upon the manufacturer (registered person) under Section 155 of CGST Act, you are to prove that rain harvesting pit is covered under the meaning and scope of 'plant'. The Hon'ble Supreme Court has defined, 'what is plant'. You can take the shelter of that definition to counteract the department's contention, if such situation arises.
You will have to be pro-active.