Generally yes, a big machine such as a CAT can be driven on the beach to build a seawall on private property. Disturbance to the beach surface such as ruts etc. may require remediation to the conditions existing prior to the CAT being there.
Generally access to Crown foreshore (public beach) would require some form of Crown land tenure such as a temporary permit to provide the opportunity for staff to identify other existing structures such as sewer lines, water pipes etc. requiring consideration by the CAT operator.
Most likely authorizations such as building permits from the local government or temporary works permits for work done from Crown foreshore i.e. public beach, will be required depending on the specific location, the time of year when construction is expected, and the degree of expected disturbance.
More information is available through the local FrontCounter BC office, local government office or the Department of Fisheries and Oceans Fishery Protection Program Toll Free at 1-866-845-6776 or email to: ReferralsPacific@dfo-mpo.gc.ca as construction on private property that may impact fisheries or spawning areas falls within their mandate.
Many kinds of materials may be used to construct a sea wall. For a seawall on private property the local government will most likely have guidelines and requirements for construction based on DFO and MOE standards with respect to environmental concerns. A publication from Stewardship Canada provides an excellent overview of shoreline construction issues. The publication - Coastal Shore Stewardship: A Guide for Planners, Builders and Developers is available here. Page 62 (located in section 3) in particular addresses seawalls.
Note: Some Provincial agency names in this document have changed. Current general responsibilities are available here.
Many kinds of materials may be used to construct a sea wall, although old car bodies are unacceptable.
The responsible party for observing that a seawall is built properly is the agency who issued the building permit or other authorizing document. Other agencies may be involved if conditions are found to be in non-compliance with other related legislation and regulations. As above, the same document from Stewardship Canada has a chart (listing appropriate legislation) and diagrams on pages 34 through 37. See above note about provincial ministry name changes.
The owner of the seawall is responsible for the construction of the seawall. If other land parcels incur storm damage related to the seawall construction, the seawall owner is generally responsible. There are a number of publications that speak to downstream effects of shore barriers such as docks, seawalls, dykes etc. As above the Stewardship Canada publication references this as well starting on page 50.
Yes. Private property generally extends to the high water mark; therefore improvements including seawall construction can be built to the high water mark which may block access along the beach below the high water mark.