This is the clear-up operation after the earlier major landslip in 1877, with Sir Edward Watkin, Chairman of the South Eastern Railway, paying a visit. A storm had washed away part of the 400 feet high cliff near the Martello Tunnel and brought down 60,000 cubic yards of earth, burying and killing three men. The summits of some of the other cliffs were subsequently blown up to lessen the risk of collapse.
There is another photo of the 1915 landslip in the article below and this is accredited to British Railways (Southern Region) to whom copyright would have passed from the SER appearing to confirm my supposition that this series of photos were taken by a photographer working for the railway.
http://www.bgs.ac.uk/landslides/folkestoneWarren.html