You replaced (not repaired) the retaining wall. So, it is an improvement and must be depreciated. The depreciation schedule for yard improvements is 15 years.
You may be able to write off the old wall* (disposition of a capital asset), based on some relatively new rules and write off the demolition portion of the new construction project. See references below for details. You may want to consider having your taxes professionally done to take advantage of these rules.
https://www.thebalance.com/partial-dispositions-3192873
http://evergreensmallbusiness.com/partial-dispositions-and-the-new-tangible-property-regulations/
(link included in above linked article)
https://www.irs.gov/businesses/small-businesses-self-employed/tangible-property-final-regulations
A complementary
provision in the tangible property regulations gives a taxpayer who has made
this election an option to also deduct the removal costs associated with the
partial disposition instead of capitalizing the costs as unitary with the
improvement (Regs. Sec. 1.263(a)-3(g)(2)).
*If you have owned the property 15 years or more (the life of the wall), the write off is not available to you. Even if you owned it less than 15 years, the write off is based on how long you owned it (the longer you owned it, the smaller the write off)
