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St Mary's Church, Garthorpe, Leicestershire

03 April 2009

STRUCTURAL PROBLEMS WITH LEANING AND FRACTURED WALLS.

Ian Hume (Consultant Conservation Structural Engineer to The Churches Conservation Trust)

Introduction.

GarthorpeThe village of Garthorpe is about 5 miles east of Melton Mowbray in Leicestershire and St Mary’s church sits on rising ground overlooking a shallow valley next to the Old Rectory but rather apart from the rest of the village.  Like most ancient churches in England, St Mary’s was constructed over a long period with different elements being constructed at varying times.  

Both arcades date from the early 13th century whilst the clerestory which they support is 15th century.  Parts of the chancel and aisles are of 14th century construction whilst the tower is 15th century.

A Victorian restoration was carried out by J Day of Leicester in 1895-96 and at that time “an alarming crack in the west face” of the tower was noted.  

Relatively recent inspections showed that there were many cracks about the church but none of significance in the tower so apparently this problem was addressed at that time.

Structural problems 1998 to 2007.
The main, and most obvious problem, noted when Ian Hume visited Garthorpe in 1998 was the outward lean of the north wall of the north aisle and numerous cracks associated with this movement were apparent in the north aisle. However there were also a number of significant fractures at the west end of both arcades and in the south wall of the south arcade.

Monitoring  
Measuring gaugeIt was apparent that whilst the movement had been going on over a very long period (as shown by an assortment of repairs of varying ages) it was clear that some movement was continuing.  Quite what was moving and in which direction was not entirely clear so it was therefore decided to set up a rigorous monitoring system.  This was done using measurements taken with a measuring gauge with brass screws set in plastic plugs drilled into the wall as targets.  This method gives and accuracy of better than 0.1mm which is significantly better than necessary for this type of movement.  

Also plumb line measurements were taken at three locations inside the church along the north wall.  In view of the amount of movement and bearing in mind cost restrictions, it was decided to do this using a simple plumb line but, by using properly fixed top and bottom targets, good repeatability was obtained.  It is felt that accuracy of about 1mm is achievable with this technique.  As the wall leaned outwards in excess of 220mm, there seemed little point in spending large sums of money to achieve a higher degree of accuracy.

Monitoring commenced in 1999 and is still continuing.  Several of the fractures have showed little or no significant movement whilst others have opened up significantly.  Unsurprisingly the cracks which have opened significant amounts (in one case nearly 5mm but more typically in excess of 1mm) are those in close proximity to the leaning wall.  As regards the plumbing records, the plumbing point at the east end of the north wall of the aisle shows an outward movement of about 13mm, that at the centre of this wall about 6mm and that at the west end only 4mm.  It was concluded from this movement, which monitoring showed to be progressive rather than cyclical, was that radical action should be taken.

Trial pits.
Three trial pits were dug around the outside of the north aisle walls.  All showed footings of rough-hewn stone founded on clay subsoil at depths of about 300mm below existing ground.  Previous experience of mediaeval churches and other buildings of that age suggests that this is normal – perhaps even quite deep by comparison with other cases.  There were tree roots showing in the trial pits.

Possible causes of movement
MonitoringThe best first step is to install ground drainage close to the walls or to ensure that any existing ground drainage is functioning properly.  It is often found that this results in movement ceasing or at least, slowing so dramatically that no further action is needed.   At Garthorpe this work was done some years ago so clearly excessive ground water or excessive changes in ground moisture content was not felt to be a basic cause of the movement.  A very large tree in the garden of the Old Rectory is most likely to be a contender for the main source of the movement.  Taking the tree down was not really an option, partly because it is not within the boundary of the churchyard.  Major trimming would be useful.  However removal of the tree might merely exchange one problem for another as once the tree stops drawing water from the ground, the clay on which the church is founded, will almost certainly swell and ground heave will result in different but probably just as serious cracking.

Remedial measures.
It has been decided to underpin the most seriously affected end of the north aisle wall.  This is to be done in a traditional way by excavating to a depth of about 1.8m below the existing ground level in short sections not exceeding 1m in length and backfilling these with lightweight concrete.  The excavation will be made in the usual “hit and miss” pattern ensuring that the concrete has been allowed to set for at least seven days before the adjacent section is dug out.  In this case the underpinning will extend about 600mm in front of the outside face of the wall but will only penetrate as far as the inside face of the wall.  This is to avoid any work within the church and is deemed to be adequate due to the lean of the wall being so severe as to preclude there being any significant load under the back face of the wall.

Lightweight concrete has been suggested to reduce the amount of residual settlement due to installing the underpinning.  As the weight of the lightweight concrete being used is about the same as the weight of the soil being removed, the load on the soil at 1.8m below ground will be about the same as it is now; thus no further settlement should take place. As the wall is so far out of plumb it has been recommended that temporary support should be given to the wall whilst the underpinning is being installed. Monitoring continues.