1.
The General Situation
The church has evidently been suffering from movement
over a long period. About twenty years ago extensive work was carried out at
the east end, where movement had caused vertical cracking in the east window and the wall below it. The remedial work apparently involved a concrete slab beneath the Chancel floor and steel tie rods within
it spanning from north to south. The ends of these ties can be seen at the foot
of the Chancel walls.
The church itself has cracks in the south and
west walls, and the porch has pulled away from the south wall.
The other main concern has been the west Vestry,
which is a single storey flat roofed structure which was built up against the west end of the church in the early twentieth
century. The building is cracked and distorted as a result of foundation failure.
The cracking and movement throughout the building
has been associated with the effects of tree roots from the mature trees that are growing in the south and west of the church,
and at a distance that is normally regarded as being within the line of influence of tree roots.
A site investigation has been carried out, including
two trial pits and a borehole. The trial pits confirmed that the main church
walls are corbelled out below ground and extend to a depth of 700mm below ground level.
The subsoil includes moderately shrinkable clay.
The cracks in the church walls have been monitored
from time to time, and, particularly in the west wall of the church, cyclical movement has continued over the last three years.
2.
Discussion of Movement
2.1
The Church
A check on the distortion in the main church building
reveals that the west end has tipped to the south by about 40mm.
This movement decreases towards the Chancel,
and the Chancel itself is reasonably plumb. There are signs that the tower has
settled slightly compared to the main building, which would not be surprising considering that it is much taller than the
church and so may be exerting a higher pressure on the foundations, but it remains vertical.
The south-west corner, between the south door
and the west door into the Vestry, has settled and the cracking on each side of the affected area reflects this. Elsewhere the cracking on the south wall is associated with twisting in the wall rather than foundation
failure.
A crack over the door from the chancel has been monitored for some time and the results are as follows:
28/3/2006 104 mm
12/9/2006 106 mm
30/6/2008 102 mm
18/9/2008 104 mm
2.2
The Vestry
The Vestry is a separate structure to the church
and it was built 40-50 years later. It would be expected that any initial foundation
settlement in the church would have stopped long before the Vestry was built. Although
the church and the Vestry may share causes of distress the problems are not necessarily the same, so the two structures need
to be considered separately.
The south-west corner of the Vestry has settled
by 120mm, and the west wall leans out correspondingly. There is extensive cracking
above the internal and external doors, and the roof is pulling away from the west wall of the church.
The west wall appears to have been rebuilt in
some areas because the dressed stone window surrounds are in some places not aligned with the adjacent rubble stonework in
the wall. The offset is up to 25mm and this is not an effect that could have
come about due to foundation movement.
By any normal standards the Vestry would be regarded
as no longer fit for its purpose. Repairs are of course possible but these would
need to address the foundation problem by deep underpinning, levelling of the floor, reconstruction of the roof, levelling
of the doors and doorframes etc., the cost of which would clearly be disproportionate to the benefit.
3.
Drainage
There is a system of surface water drainage around
the church taking water from the downpipes to the mains drainage or possibly to soakaways.
The drainage runs to the south of the church were
inspected in 1993 and found blocked with tree roots. No repairs have been carried
out since.
The drains were inspected again in November 2005,
and they were generally found to be clay drains with failed joints, which have allowed tree roots to enter the pipes and block
them. The drains on the north side are only 300mm below ground level.
It is proposed to re-lay all the surface water
drainage because although it is possible to remove roots from drains their removal does not repair the drain itself, and root
infestation will therefore recur. The new drains will be uPVC. It will be ensured that if possible all the drains are connected to the mains and not to soakaways.
As an additional measure a filter drain is proposed
along the north boundary of the churchyard, connecting into the surface water system at inspection chambers via silt traps.
4.
Boiler House Drainage
There is a low level drain from the Boiler House
running beneath the church to an unknown outlet. The drain is blocked and the
Boiler House has been kept dry since 1993 by a system involving a pump and float switches.
The Boiler House is no longer used as such, but it does house some heating pipes and a pump.
It is hoped that the proposed new cut-off drain
will reduce the amount of water entering the Boiler House. Repairs or replacement
of the low level drain is impractical because of its depth and because it passes underneath the church. Clearly the overall ground water and drainage regime on the site is the same whether the Boiler House is
drained via a drain or by pumping.
Any differential foundation movement due to the
relative foundation depths of the Boiler House and the Church would have stopped a long time ago.
5.
Possible Courses of Action
The trees growing to the south and west of the
church are near enough to the building for their roots to affect the moisture content of the clay beneath the foundations. As this varies seasonally, the clay will shrink or expand as the moisture content
changes, and the foundations experience settlement or heave. Provided the whole
building is affected uniformly there need not be any particular concern because the whole building moves together. However if only a part of the building, in this case the south-west corner, is affected, cracking ensues
between the part that moves and the part that does not.
Underpinning is a process by which foundation
loads are taken down to a firm and stable stratum. In the case of this church
underpinning would involve pilling through the shrinkable clay into the more stable sand below it. The piles would need to be sleeved to separate them from the clay.
It is generally considered unwise to underpin
only part of a building, because it can result in the underpinned part ending up being more rigidly founded than the rest
of the building, which can result in further cracking. In this case as the building
is founded on shrinkable clay it is likely to move up and down according to variations in the moisture content of the clay. This can occur due to seasonal moisture level variations in the ground or to variable
water demands from adjacent trees, and is exacerbated by leaking drains.
The cost of underpinning the church would be prohibitive.
A further possibility is to install a root barrier
between the trees and the church. This is to provide a physical barrier to prevent
the tree roots reaching as far as the church. It would need to be of impermeable
material, and to extend to a depth of about 5 metres below ground level. A risk
with root barriers such as this is that they can affect the natural drainage patterns in the ground, and they can attract
roots into any disturbed ground affected by the installation. Any drainage runs
passing through the barrier have to be carefully sealed. This solution does not
therefore offer sufficient guarantees for solving the problem.
6.
The Proposed Solution
6.1
West Wall
For the reasons stated above underpinning of the
church, and a root barrier have been discounted. It is therefore proposed that
the south-west corner of the church should be mechanically stitched back into the main part of the building. The cracks have taken paths of least restraint and so join-up existing openings, but drilled-in steel anchors
can be inserted into the cracked masonry to hold it together.
As long as the foundations continue to move due
to the varying foundation conditions there will be stresses on the masonry. Stitching
existing cracks will help hold the building together but cannot guarantee that new cracks will not open up elsewhere.
6.2
The Vestry
The Vestry is so distorted and cracked that it
is beyond economic repair. The only course of action is to remove it.
7.
Conclusion and Recommendations
The church structural movement is to be controlled
by installing Cintec ties into the walls at the southwest corner and in the south wall.
This will only be a long-term solution if the trees affecting the foundations are also removed. The two sycamore trees to the west of the church are those most likely to be responsible, but this does
not preclude some contribution from the other trees which stand along the south side of the site.
After removal of the trees a period of time will
be needed for the ground conditions to adjust themselves, so the stitching should not be carried out immediately. It should
be noted that the trees will eventually die naturally, with the same affect at that time as will occur when they are cut down,
when short-term re-wetting of the ground will occur.
If the trees to the west are not removed the ground
conditions will continue to affect the foundations of the church. The stitching
work proposed is a pragmatic attempt to reduce the adverse effects of gound movement, but cannot be a long term final solution.
