Muddymike wrote:

On further inspection of the images I think you are right, The top/left
chamber may connect to the centre one further down, if so its certainly a
three stage septic tank. Ten rows of bricks down sounds high for the water
table. Is it that high outside the brickwork? If not it may be that the
chambers are still retaining water. If the rubble inside is loose it could
be the remains of the old lid structure. These were sometimes concrete slab
and sometimes corbeled brickwork the beginnings of which can be seen.


Thanks all, some useful pointers. To answer a few questions:

I had rejected the idea of a row of outside toilets. They are not the
same size, they are probably a bit small, the outlet pipes are in the
wrong place – I would expect them to be at the rear, they all flow into
different places and it was probably built the same time as the house so
one toilet I could possibly accept but not three. The house was built
with two inside toilets as it was. One each upstairs and downstairs.

A septic tank seems more likely. I hadn’t realised that they came in
three parts. It would also be consistent with the left hand pipe coming
from the direction of the house and the right hand pipe flowing towards
a ditch at the bottom of the garden although I haven’t dug to find where
either actually goes. The left hand pipe is also in line with where the
soil stack used to be.

It is possible that the construction was mirrored under the lawn and
that the front edge was some form of centre walls. That would explain
why it is a straight run of bricks doesn’t match the other side with the
corbelling. Most of what you see in the pictures was under a paving slab
path - an extension of the original concrete path so the slabs could
have been laid when the septic tank was abandoned.

What is not so clear is why there would be a septic tank in the first
place. The whole area as laid out in the 1930s and is only a couple of
miles from the centre of Cambridge (and just of the main road to boot)
so I would have thought it would have had mains sewerage from day one.
However, although the first eight houses (4 pairs of semis) in the road
were built just before the war all the others were built after. Mine was
one of the first (maybe even the first) to be built post-war. Is it
possible that this was a semi-temporary structure put in so that the
house could be built before the sewers were extended to that end of the
road?

And yes, the water table is normally at about that level. When I
replaced the lead water main a couple of years ago the new pipe had to
be laid under water to get it to the regulation 600mm depth. Mind you
that was in a particularly wet February. I am told that the geology is a
gravel filled gault clay basin. There is also a ditch at the bottom of
the garden which is reportedly supplied by a spring further upstream.
Certainly that area of garden can get pretty waterlogged in the winter
and stay moist even in a dry summer. This may be why, if it is a septic
tank, it was built so high.

If I get a chance I will dig some more over the weekend. If it is a
septic tank then it should have a solid base at some point and I would
like to find it.

Thanks again,

Andrew