With the British Expeditionary Force largely under French Command, their defence strategy was decided by the French defence strategy. This, as is well known, relied on a single supposedly impregnable line - The Maginot Line. With Belgium neutrality during the phoney war, the Allies were concerned about the fact that this line stopped at the France / Belgium border. As part of the extension of the Maginot Line along this border, the British built many pillboxes and trench systems covered by huge anti-tank ditches - the so called Gort Line. Although the intention was to develop defences in depth, there simply was not the time to do this.
The Gort Line consisted of a series of mutually supporting strong points at regular intervals. Anti-tank guns, in short supply, were given shell proof concrete cover. Machine guns, sited on fixed lines, were also given concrete protection. Infantry defences were developed in line with Infantry Training 1937, weapons-pits for 2 - 3 men dug first then developed into a trench system. Due to a high water table in places, breast works were sometimes needed. A bulldozer could be used for this - to scrape up the earth to form the breast work. But the trenches were all dug by hand as the many traverses needed could not be dug with machinery. The huge anti-tank ditches were dug both by hand and machinery and were fully revetted.
Above: Artists drawing of anti-tank ditch (left) and trench (right) - BEF France 1939/40
Timber was in short supply meaning the same shuttering had to be reused over and over again in pillbox construction. There was also some confusion in which weapon mounts to construct in the pillboxes as it was not clear if certain parts of the defences were to be manned by British or French troops!
Dannert / double apron wire obstacles were erected in great depth. This was to allow the wire to also act as an anti-tank obstacle (wire would wind around the tanks drive sprockets).
Concrete mixer used in construction of BEF defences BEF pillbox (IWM O 1785)
The Maginot Line also lacked any depth - the outpost line was so weak that the Germans were freely able to patrol it and in his book The Battle of France, Andre Maurois describes as "agonizing" the fact that he did not see any troops upto ten miles behind the line.
The second main part of the Allied defence strategy was to be a rapid advance into Belgium to halt the expected main German attack. But the main German attack developed through the Ardennes with disatsrous results for the Allies. There is a good account in John Hosfall's Say Not The Struggle of the BEF's retreat, fighting from various canal / river 'stop-lines'. He describes the infantry battle with platoons / companies fighting in hastly dug weapons pits or from buildings.
The failure of the Maginot Line strategy was to have a lasting affect on British Home Defence strategy. As for the Maginot / Gort Lines, "was it even occupied, all this concrete" wondered Andre Maurois.
The Battle of France, Andre Maurois, The Bodley Head 1940
Dunkirk, Major General Julian Thompson, Sidgwick & jackson 2008
Say Not The Struggle, John Horsfall, The Roundwood Press 1977
Engines of War, War Office, Adam & Charles Black 1941