Matin,
this is a good question and one very good book that you could pick up at your local or school library is one entitled "Forgotten Highlander"; Author: Alistair Urquhart; ISBN: 978-1-616-08152-2. In the book Mr. Urquhart shows how not only defenses were inadequate but that the "ruling class" of the British Officers assigned to Singapore never fully prepared the troops for any sort of battle on the Island nation as often during training exercises in the jungle the Officers would cut the training short or extend 'tea breaks". Mr. Urquhart goes on to say that he and many like him never got to fire a shot against the Japanese and that the "bluff" worked against them there Singapore.
Part of the reason in my view is the consideration of how the Imperial Dai Nippon forces would kill wounded soldiers in the hospitals by bayonet along with 200 Doctors and Nurses. This occurred all over Southeast Asia and before capitulating the attrocities in Hong Kong had made their way to Singapore; they only continued in Singapore after what little fighting had began. I also believe that it was a combination effect of the Japanese being able to attack multiple locations at once on 7 December (8 December after one crosses the International Date Line in the Pacific Ocean) and the ill prepared effects the British and Dutch had in the Pacific Rim. One lone attempt of Sir Churchill to send Battle Ships HMS Prince of Wales and HMS Repulse were both sunk on 10 December 1941 in the South China Sea by Japanese (Mitsubishi) Naval Airplanes carrying bombs. The sinking of these two great battleships were mentally demoralizing to all the British troops in the region at the time. Add to the fact that there were diversionary landings of Japanese Forces and at one point the Japanese Forces are moving across land by bicycle to get into position of attack. Enraged by the stubborn stand put up by the Malay Regiment soldiers, the Japanese exacted revenge by storming into the nearby Alexandra Hospital, bayoneting and killing patients and staff on sight (as referenced above).
By this time the Japanese advance had forced British troops to fall back to a perimeter around the Municipality. This was their last defense. Around this the Japanese converged. Not only were water supplies falling to a critical level but supplies including food, fuel and ammunition were also running low. These reasons along with mounting civilian casualties, led Percival to make the momentous decision to surrender.
These are thoughts to consider on the matter - the book I referenced above should be in your library and information can be easily looked up. Percival in my view was not one of the best Field Commanders that the British had.
All the best,
Gerry