I imagine that the photo would have been taken at Harty on the mainland side of The Swale. Interesting to see the large gun also in the photo because at one time, I believe in the late 1800's early 1900's, there used to be an exercise known as "Firing the Shot" which took place periodically at Harty Ferry. Apparently a posse of soldiers marched smartly out to the Harty Ferry, manned guns and fired them, watched carefully by officers. Possibly this was the reason for the soldiers in the photo.
Also, in July 1910, 300 Kent Territorial Royal Engineers from Ashford, Chatham and Tonbridge were encamped in bell tents near the road to Harty Ferry, together with 120 Regulars from Chatham. They had the job of making a bridge over The Swale from the Faversham side to Sheppey to facilitate the passage of troops.
I found both of these snippets from a Bygone Kent article that had been photo-copied and put into a Harty file in Sheerness library, it included a photograph of the bridge building in progress.