Eddy, hi.
How did it go with the Guy from Severn & Trent? Any conclusive conclusions?
As for some other proposals you mention in the last post?
1/. Pointing the internal brick and adding a DPM of some sort.
A/. The water if as we seem to suspect is indeed ground Water will simply circumvent the bricks and come up directly from the "Solumn" [the ground under the house] in effect a down and up move by the water, where if it can not get in via the bricks on the external wall it will come in by percolating under the founds and gain access that way. it is a perennial problem attempting to retro fit a DPM / DPC.
2/ Filling a part of the underfloor void with concrete?
A/. to attempt to use "conventional;l concrete simply will not work, there is not enough room to pump the concrete in and at the same time level it off.
B/. Fear naught, there are available "Specialist" Foamed Concrete suppliers. This material is a [best described as ] lightweight "foamed mix" that is self leveling, that is you do not have to get in there with it to get it to a level, it will do that itself, goggle Foamed Concrete Suppliers in your area. Problem is that you will have to do this on a room by room, or in this case [underfloor] on a compartment by compartment basis so that you are filling a defined area of the underbuilding, containing the concrete within that area with a shutter at the entrance to that area. What i am trying to elaborate on is that your underbuilding will be a series of roughly room shaped areas, you need to fill them up one by one, starting at the furthest point away from where you introduce the pumps hose.
C./ The suppliers truck comes with a small but effective pump attached, the hose length is limited so Pre-Planning is absolutely VITAL in determining how and where you place the discharge hose in the foundations.
D/. I have used this Idea successfully to stop a very serious ingress of contaminated ground water from entering an underfloor area, the contaminant was Central Heating Fuel oil that effectively and massively destroyed the paint applied DPM on the walls and Solumn of the property, in effect the Kerosine ate the DPM.
E/. How deep to take the foamed concrete. i would suggest that you infill to a depth of a few hundreds MMs above the flood level that you have now, what will then happen is that water, being as lazy as humans will opt to infiltrate the next nearest easiest area, your neighbours underfloor void.
F/. What then happens if your neighbour does what you have done, but takes his foamed concrete higher than yours, you will again get water in up to the level of the Concrete in His foundation / underbuild area.
3/. Fitting a sump and a sump pump?
A/. many "purists can say that the underfloor area will be "damp" but to have running water in there is not a good idea, because, running water will introduce m,ore free moisture in the air within the underbuilding. Will tend to cool down the house. Costs some money to run, but not cost nearly as much as installing Foamed concrete.
B/. Provided that you have a really good ventilation under the floors then this option is worth looking at.
C/. problem with this system is that if you attract the water from your neighbours property as well, then it could be argued that He is benefitting from your endeavors, at no cost to him, remember water is lazy, it will naturally percolate / run to the lowest point available, don't you just love Gravity.
D/. Next problem is where do you pump the water to? nearest rain water down pipe, job done.
3/. Back to the french drain.
A/. Your drains Invert levels are above the foundations, you could excavate the external french Drain to the level of the founds, run the drain to a "Catch pit" then pump from there? all work external? no ness in the house. From the catch pit the outfall from the catch pit pumps into the rain water system or if you have a combined system into that.
B/. if there is enough room in the Garden a "Soakaway" may be an answer, this is dependant on the nature of the soil surrounding the property, if it is Boulder clay forger it, Sand is a good option. it could be anything between.
B/. Advantages of this system are, the actual french drain track could be very narrow indeed, there are some small excavators that will happily dig a 300.mm wide track to the depth you need, followed by "Geotextile Material" and then infilled with suitable stone, plus a perforated "Wavin Coil" pipe at the base.
See what you think.
Hope this assists.
Cheers.
Ken.