Hi I am building a single story extension on to a house the extension is concrete beam & block floor with brick /blockwork walls and timber roofing structure with redland tiles. I have dug a pilot hole to see what the ground is like after 400mm I hit chalk, I dug out another 300mm and the chalk is solid. how deep should I dig for the foundations and Is chalk good ground to build on?
"FIRM CHALK" IS CLASSIFIED AS A ROCK FOR THE PURPOSES OF FOUNDATIONS (SO IS GOOD).
LOADS ARE NOT EXCESSIVE IN A SMALL SINGLE STORY DOMESTIC EXTENSION AND IF YOU HAD PLANS PREPAIRED THE DEPTH OF THE CONCRETE FOUNDATION WILL NORMALLY BE INDICATED ON THEM AND THE TRENCH AND CONDITION OF THE SUBSOIL IS CHECKED BY THE BUILDING CONTROL OFFICER WHEN THE TRENCHES HAVE BEEN DUG.
IF YOU REMOVE MATERIAL DOWN TO "FIRM CHALK" THROUGHOUT ALL THE TRENCHES AND THEN REQUEST AN INSPECTION, THE B.C.O. WILL THEN PASS THEM AS READY FOR CONCRETE POUR OR FAIL THEM AND TELL YOU TO DIG TO A FURTHER DEPTH AND ON COMPLETION WILL THEN RE-INSPECT AND PASS (SO EITHER WAY THE JOB GETS DONE).
SO I WOULD SUGGEST YOU DIG OUT THE CONCRETE DEPTH COMPLETELY INTO "FIRM CHALK" WHATEVER THAT DEPTH MAY BE IN AN EFFORT TO HAVE ONLY ONE INSPECTION AND FIRST TIME PASS.
FROM YOUR DESCRIPTION IT SEAMS LIKLEY THAT THE "FIRM CHALK" LEVEL IS AT OR AROUND 600mm BELOW SURFACE SO ADDING THE CONCRETE FOUNDATION THICKNESS FROM PLAN TO THIS DEPTH WILL GIVE YOU THE FOUNDATION TRENCH DEPTH TO AIM FOR WHEN DIGGING OUT.
IN THIS COUNTRY AT 600mm OR MORE DEPTH IN CHALK THE FOUNDATIONS WILL BE ADIQUATELY PROTECTED FROM SURFACE WATER PENERTRATION, DROUGHT SHRINKAGE, GROUND HEAVE, AND THE FREEZE/THAW ACTION.
YOU COULD OF COURSE ASK THE B.C.O. TO GIVE YOU A DEPTH TO WORK TO AS THEY WILL HAVE INSPECTED MANY TRENCHES IN YOUR LOCALITY AND KNOW THE LIMITS/TOLERANCES THEY CAN PASS/FAIL.
MAKE FRIENDS WITH YOUR B.C.O. AS THEY WILL HELP YOU TO BUILD A SAFE COMFORTABLE HOME THAT WILL BE STILL STANDING LONG AFTER WE'RE ALL GONE AND YOUR CHILDRENS CHILDREN MIGHT WELL BE IN RESIDENCE.
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