PERCY SQUARE, SOUTH SHIELDS
The study which the following information was taken from is "A Report on Toe Wall and Cliffs below Percy Square", by Maurice Nachshen, November 1948. Attached to it is a geological and soil mechanics report by Skempton, who was brought in to carry out investigations on the site.
THE SITUATION AND HISTORY:-
South Shields is an area on the North East coast of England, near a town called Whitley Bay.
Percy Square is the name of the area in a bay between North Groyne and Swaddle's Hole on the north bank of the Tyne. Here, there are sloping cliffs in boulder clay reaching 100 ft above the beach level. The cliffs are around 200 ft wide in plan.
At the foot of the cliffs is a "Toe" retaining wall, which was built in 1937, to stabilize the cliffs. It performed this job slightly, as the factor of safety in 1936 (before construction of the wall) was 1.10, compared to 1.21 in 1937.
The wall is 1264 ft in length, with a 30 ft return at the east end
On top of the cliffs , within 50 ft of the crest, is situated a very large, 5 storey block of flats. The "James Knott Flats" stretch a distance of 750 ft along the cliff.
OUTCOME OF INVESTIGATIONS:-
The problems noted above were significant, and the time had come to investigate the cause of the deformations of the wall, and make a decision about the way forward for the area. As the flats obviously housed a large number of people, any risk of slope failure needed to be reduced as much as possible. Remedial measures undertaken at this stage would obviously be much less costly and more effective than if collapse was allowed to occur!
Movement at the East end of the Wall: It was discovered from borehole investigations, that in this section there is a thin layer of soft "fireclay" between the stronger boulder clay and the rock.This fireclay layer was level with the base of the wall.
Over time a deep-seated rotational earth slide had developed under the cliff, at the east end of the wall, and because the toe wall here was not grounded in rock, it had moved forward horizontally with the earth mass.
Movement at the Centre Section of the Wall: It was discovered that the lower part of the slope here had bulged forwards, with water running from the foot of the slope, and earth flowing slowly towards the river like a thick fluid.
The causes of this was water logging of the fill material, which interrupts natural drainage of the cliff. The toe wall blocked drainage by banking up the groundwater behind it.
Factor of Safety of Cliffs
|1936 (Before construction of wall)||
|1937/38 (after construction of wall)||
|1948 (when investigations were carried out)||
Each of the two areas described above were to be dealt with separately, but the final remedial work must form a unified whole.
Diagram showing the Remedial Measures carried out at the eastern end of the toe wall.