The following section gives a summary of the findings of two previous studies conducted into the problem of windthrow by researchers at UCG. Both studies were concerned with the stability of sitka spruce trees planted on wet mineral soils.

Castledaly Study:

The first study was conducted by Conor McMenamin in 1992 at a Sitka spruce plantation in Co. Galway, Ireland. The site preparation technique used was double mould board ploughing.

The study consisted of a series of destructive tree pulling tests, dynamic rocking tests, the storm monitoring of a single tree and a series of laboratory tests.

Summary of results:

1. Destructive tree pulling tests:

Table 1 summarises the results of the tests. The trees tested in this manner had been cut off at a 6m height and pruned. The direction of pulling was at right angles to the direction of the plough furrow.

1. Dynamic tests:

• Tree cutoff and pruned
• Mechanical rocking device
• Instrumentation
• Rocked until looping action induced
• The dynamic tests trees were flooded to saturation

The results of the rocking tests may be summarised as follows:

• Pore water pressure accumulated after overturning moment reached a level between 5kNm and 7kNm.
• Hydraulic fracture, pumping and liquefaction were observed in both trees.
• Subsequent to fracturing smaller forces caused larger displacements than was the case previously.

1. Storm Monitoring

A single tree in a forest was fully instrumented and its response to natural wind loading was recorded. The results are summarised below:

• An average wind speed greater than 5m/s caused significant dynamic loading.
• The recorded overturning moments reached as high as 7KNm
• The pore water pressure increased on both the windward and leeward sides of the tree

1. Laboratory testing:

The most important element of the laboratory testing was the use of a cyclic triaxial test to investigate the strength and deformation characteristics of a soil when subjected to repeated loading. The results are summarised in table 2.

Ballyfarnon study (Carey, P. 1993)

• Conducted at Ballyfarnon forest, Co. Sligo, Ireland.

Two site preparations
1) Double mouldboard ploughing and
2) Mole Drainage

Summary of Results:

1) Dynamic rocking tests
1 Four trees tested
2 Two from each plot
3 Same methods as previously descrived
4 Test trees were flooded prior to rocking

The main results are summarised below:

• In all cases pumping, hydraulic fracture and liquefaction occurred
• Pore water pressure accumulated when the induced moments reached between 5kNm and 7kNm
• No obvious superiority of the trees on the mole plot was evident.

1. Laboratory Testing:

• Cyclic triaxial: the main results from cyclic triaxial tests are summarised in table 3.

Simple shear testing: the main results are given in table 4.

• The ratio of cyclic shear strength to the monotonic shear strength varied between 37.8% and 48.9%
• A comparison of the relative performances of a peat with the gley found that the peat could take greater strains than the gley
• Also the build-up in pore water pressure was not as substantial in the peat as that in the gley.