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Hi All
I have just finished my mechanical inspection pit in the garage and managed to get it all fully waterproofed and warm using fibreglass.
I read a lot of information posted by a lot of people all over the web who complained that digging the hole and finishng it nicely was not the problem but keeping it waterproof was. Never one to be beaten (more that I like a challenge!) I decided to give it a go. So this is what we did:
1. Marked out the absolute maximum width of the pit, disc cut the concrete and broke the rest of the concrete floor to give us access to the soil within which we were going to dig the pit.
2. Dug a hole out 6.5ft down by required width and length. The soil was very well compacted and so we did not require any structural support to hold the sides up.
3. lined the floor with damp proof membrane and layed a temporary concrete floor of 3 inches or so, not worrying about it being level or perfect. NOTE * by this time the water in the bottom was reaching 2ft or so over the period of a couple of days! Goes without saying that pumping this water out before laying the floor was a requirement.
4. Once the floor has set, line the entire pit with damp proof membrane pushing it down as much as possible (otherwise the weight of the concrete will pull it down!). Leave about 2 to 3ft of membrane over the sides as this can always be trimmed after the concrete has been poured in.
5. Using shuttering ply, erect a frame in place leaving the correct width of concrete space around the pit. Brace every which way possible internally as the weight of the concrete might blow the shuttering so spending more time here securing the shuttering will work out better in the long run. Weused 8 x 4 sheets and left a lot poking out over the top of the pit so when we wheel barrowed the concrete in we didn't need to worry about spilling any inside the pit.
6. Around the newly erected frame, screw some 4x2 all around the top at floor level. This is to create a ledge all the way around the pit that you can rest timbers on to cover the pit when not in use.
7. Pour the concrete! Recommend a mix-on-site lorry as they were fantastic for what we wanted. You will need a vibrating poker to vibrate any air bubbles out of the mix once poured. Before it goes off go around with a flot and level with the floor.
8. Once the concrete has a good face on it say 3 or 4 days, carefully remove all the shuttering and bracing. You'll no doubt be standing in about 4ft of water by now! Don't worry we'll get around this.
9. Pump all water out, create a sump hole that is about 1 ft lower than the temporary floor, clean the debris and put a small layer of gravel in the bottom, get a bucket that fits the hole and drill lots of little holes in it around the sides and bottom. Fit a water pump in that is capable of pushing the water out over the pit, we used a marine bilge pump that did the job. This sump hole will ensure the water level around the pit is kept as low as possible. If you want to be really snazzy, fit a float switch and an electronic timed relay to automatically keep the water level down.
10. Where the concrete walls meet the temporary floor, using a disc cutter or angle grinder cut a grove in "v" shape all along the edges so that it creates a water channel to the sump. It needs to be below floor level. lay conduit in the water channels and ensure the flow is towards the sump hole.
11. Re-lay your floor so it is the correct level etc.
12. Pump all water out and get some G4 Damp Seal and paint it on the concrete areas that fibreglass will adhere to (do this in sections else you'll ruin it by standing on it) following the instructiions. This will give the fibreglass something to bond to.
13. Fibreglass all over the pit floor and walls just above the water table. If you are confident in your fibreglassing abilities, fibreglass over the sump hole after removing all the electronics. If not, leave it all in place and ensure it is wired safely and correctly then fibreglass over it. NOTE * when fibreglassing ensure you keep the temperature raised slightly in the pit to allow the resin to go off properly. Also, DO NOT let the resin get damp or affected by water during curing as it will ensure it doesn't go off!
14. Use gel coat with a coloured pigment (I chose white) to go over all the fibreglassed areas and allow to go off.
15. Hey presto! One extrememly water tight pit!
Hope this helps others, you can of course fibreglass over an existing pit but the method would be slightly different - if you need to know how to do this let me know.
I have just finished my mechanical inspection pit in the garage and managed to get it all fully waterproofed and warm using fibreglass.
I read a lot of information posted by a lot of people all over the web who complained that digging the hole and finishng it nicely was not the problem but keeping it waterproof was. Never one to be beaten (more that I like a challenge!) I decided to give it a go. So this is what we did:
1. Marked out the absolute maximum width of the pit, disc cut the concrete and broke the rest of the concrete floor to give us access to the soil within which we were going to dig the pit.
2. Dug a hole out 6.5ft down by required width and length. The soil was very well compacted and so we did not require any structural support to hold the sides up.
3. lined the floor with damp proof membrane and layed a temporary concrete floor of 3 inches or so, not worrying about it being level or perfect. NOTE * by this time the water in the bottom was reaching 2ft or so over the period of a couple of days! Goes without saying that pumping this water out before laying the floor was a requirement.
4. Once the floor has set, line the entire pit with damp proof membrane pushing it down as much as possible (otherwise the weight of the concrete will pull it down!). Leave about 2 to 3ft of membrane over the sides as this can always be trimmed after the concrete has been poured in.
5. Using shuttering ply, erect a frame in place leaving the correct width of concrete space around the pit. Brace every which way possible internally as the weight of the concrete might blow the shuttering so spending more time here securing the shuttering will work out better in the long run. Weused 8 x 4 sheets and left a lot poking out over the top of the pit so when we wheel barrowed the concrete in we didn't need to worry about spilling any inside the pit.
6. Around the newly erected frame, screw some 4x2 all around the top at floor level. This is to create a ledge all the way around the pit that you can rest timbers on to cover the pit when not in use.
7. Pour the concrete! Recommend a mix-on-site lorry as they were fantastic for what we wanted. You will need a vibrating poker to vibrate any air bubbles out of the mix once poured. Before it goes off go around with a flot and level with the floor.
8. Once the concrete has a good face on it say 3 or 4 days, carefully remove all the shuttering and bracing. You'll no doubt be standing in about 4ft of water by now! Don't worry we'll get around this.
9. Pump all water out, create a sump hole that is about 1 ft lower than the temporary floor, clean the debris and put a small layer of gravel in the bottom, get a bucket that fits the hole and drill lots of little holes in it around the sides and bottom. Fit a water pump in that is capable of pushing the water out over the pit, we used a marine bilge pump that did the job. This sump hole will ensure the water level around the pit is kept as low as possible. If you want to be really snazzy, fit a float switch and an electronic timed relay to automatically keep the water level down.
10. Where the concrete walls meet the temporary floor, using a disc cutter or angle grinder cut a grove in "v" shape all along the edges so that it creates a water channel to the sump. It needs to be below floor level. lay conduit in the water channels and ensure the flow is towards the sump hole.
11. Re-lay your floor so it is the correct level etc.
12. Pump all water out and get some G4 Damp Seal and paint it on the concrete areas that fibreglass will adhere to (do this in sections else you'll ruin it by standing on it) following the instructiions. This will give the fibreglass something to bond to.
13. Fibreglass all over the pit floor and walls just above the water table. If you are confident in your fibreglassing abilities, fibreglass over the sump hole after removing all the electronics. If not, leave it all in place and ensure it is wired safely and correctly then fibreglass over it. NOTE * when fibreglassing ensure you keep the temperature raised slightly in the pit to allow the resin to go off properly. Also, DO NOT let the resin get damp or affected by water during curing as it will ensure it doesn't go off!
14. Use gel coat with a coloured pigment (I chose white) to go over all the fibreglassed areas and allow to go off.
15. Hey presto! One extrememly water tight pit!
Hope this helps others, you can of course fibreglass over an existing pit but the method would be slightly different - if you need to know how to do this let me know.
