I'm going to put a regular brick wall in front of the old stone wall.
They will be seperated by about 4 inches and the gap will be filled with
concrete.

The old stone wall is 3.5 feet high and has no real foundation. The base
of the bottom stones are about 2inches below ground level and rest on
crushed stones. The wall is stable and been up about 20 years.

Behind it is soil that goes to the top of the wall. On my side I see the
full length of the wall.

I'm putting in front of the old wall flags that will go about 18 inches
below ground level and stick proud of the soil level by 6 inches. So,
along the length of the old wall at the front will be flags rising 6
inch from ground level.

The new front brick wall will be laid adjacent to these flags. The
inside edge of the new wall and the front edge of the old wall will be
about 4 inches and it will be filled with concrete.

Okay, that's the general setup. How deep should I make the foundation
trench for the front brick wall? And how deep should the cement be in
that trench? Do I fill the trench with cement first to ground level and
let it set and then build the wall on that? Thanks.

"Rich" wrote in message
...
| I'm going to put a regular brick wall in front of the old stone wall.
| They will be seperated by about 4 inches and the gap will be filled with
| concrete.
|
| The old stone wall is 3.5 feet high and has no real foundation. The base
| of the bottom stones are about 2inches below ground level and rest on
| crushed stones. The wall is stable and been up about 20 years.
|
| Behind it is soil that goes to the top of the wall. On my side I see the
| full length of the wall.
|
| I'm putting in front of the old wall flags that will go about 18 inches
| below ground level and stick proud of the soil level by 6 inches. So,
| along the length of the old wall at the front will be flags rising 6
| inch from ground level.
|
| The new front brick wall will be laid adjacent to these flags. The
| inside edge of the new wall and the front edge of the old wall will be
| about 4 inches and it will be filled with concrete.
|
| Okay, that's the general setup. How deep should I make the foundation
| trench for the front brick wall? And how deep should the cement be in
| that trench? Do I fill the trench with cement first to ground level and
| let it set and then build the wall on that? Thanks.
|

I'm puzzled as to why you are putting the flag stones in if you are putting
a new wall up in front of the old one.

You say the old stone wall has no foundation, and is stable. OK. I assume it
is a dry stone wall, so no foundation is perfectly normal. The foundation is
actually the soil it is sitting on, and if you disturb this, you will be
making the wall unstable. The soil foundation can be imagined as a pyramid,
the top of which it the bottom of the wall, and extends downwards with side
slopes of about 35 degrees to the horizontal away from the base of the wall.
If you disturb any soil within this pyramid, you could be asking for
trouble. Just be very careful how you proceed, and without knowing the
length of the wall, I would suggest you open up no more than about six feet
of trench at any one time, put in the flags, form the foundation of the new
wall (see next paragraph), and let it set. Then fill behind the flags with
concrete, and go on to the next section. Any sign of movement of the old
wall, brace the flags with timbers to prevent further movement until the
concrete has set.

Since the flags are going into the ground 18", make that the depth of the
foundation. For a new wall 3' 6"" tall, a 9" depth of 'good' concrete
(1:2:4) should suffice, but better 12" if you hand mix the concrete. That'll
leave the top of the foundation about 6" below the surface. Fine. Doing it
in sections, as I suggest, get hold of some 3 foot lengths of 12mm steel
bar, and stick a couple of them in the ends of each section to prevent any
lateral movement between sections. Then build up with bricks as you
intended. I suggest building the wall in stages (2 feet high at a time) and
backfill with concrete as you go after the mortar has gone off. The backfill
concrete should be a semi-dry mix (1:12) , and not sopping wet!!!

[Credentials at this end:- 25 years as a Civil Engineer specialising in
ground work and foundations.]

Hope that help, and good luck.

jim, Northampton