The Devon Karst Research Society.
EDUCATION PROGRAMME.

Hidden or Buried Karst Features within the Limestones of the City of Plymouth.
1.    THE PROSPECT PLACE KARST PONOR,
The West Hoe Cave System, West Hoe, Plymouth, Devon.

Updated 24 April 2005.


1.0.    INTRODUCTION :
The following edited highlights of text and illustrations are taken from a Site Investigation Report donated to the Society's Archives in January 1975. The Society's Accession No. for this Report is 1551-A.
Although, in many technical aspects, this can be regarded as a fairly "primitive" Report, it is nonetheless particularly interesting, in that it reveals the presence of a substantial buried karst feature in an area where other buried karst features have since been proven to exist over a number years, either during pre-development site investigations or during the progress of the actual construction or development projects. The West Hoe area of Plymouth may be regarded as being a "karstification hotspot" within the east-west belt of the Plymouth Limestones. It contains what we call "The West Hoe Cave System", with component parts of the system dotted around the area, some of which are still accessible and others which are currently hidden and therefore inaccessible. This Webpage deals with one example which exemplifies the latter case.

2.0.    THE REPORT :

"Report on the Difficulties Arising in the Foundations for Flats at Citadel Road - Plymouth.
dated 11th January 1966.

INTRODUCTION.
It is proposed to construct a 19-storey block of flats at the junction of Citadel Road and Prospect Place, Plymouth.
Investigations of the sub-soil commenced with a search of geological records at the Geological Museum, South Kensington. No records of borings were found in the immediate vicinity, but from the 1" Geological Survey Map it was apparent that the site lay within the Middle Devonian Limestone.
It was anticipated that rock would be met reasonably close to the surface and as a preliminary measure five trial pits were excavated by means of a tractor mounted back-acter. Bedrock was not encountered in every pit and where encountered, showed some evidence of weathering. The overburden consisted of made ground and firm increasing to stiff clay. The trial pit logs are attached as Appendix 'A'.
It was decided that the tower should be founded on a raft producing a gross pressure of 2 tons/sq.ft. at a depth of 7-ft., ie. in the natural soil overlying the rock and not on the rock itself.

SITE WORK.
Subsequently, as a result of re-planning, the position of the tower was moved approximately 50-ft. to the southwest. The main contractors, Dudley Coles Ltd., commenced operations in November 1965, by excavating the raft area to a depth of approximately 7-ft.
In this position bedrock was exposed in the raft excavation in the northeast third of the raft area. Furthermore, the material in the southwest corner was inadequate for the support of a pressure of 2 tons/sq.ft. In order to avoid any tendency for the building to tilt, it was decided that the whole raft would have to bear on the rock and local pits were dug within the reduced area in order to ascertain its depth. Within the most southwesterly third of the area, the limit of reach of the machine (12-ft.), did not reveal bedrock. In the remaining third, bedrock was found and later exposed.

The beds in the southwest corner were examined and a number of hand samples removed for examination. The material in these beds was predominantly either firm becoming stiff red clay containing small quartz pebbles, or a soft white or yellow clay. Patches of speckled black and white sand and large quartz pebbles were also encountered. It was thought likely, since the material bore all the appearance of an alluvial deposit, that this was fill. The hand samples were taken to the Plymouth City Museum and examined by their Geologist and Archaeologist, who later also examined the material in situ.

BORINGS.
It was decided that the rock profile should be thoroughly investigated and that this could probably most easily be achieved by rock boring. Geotechnical Engineering Ltd. of Gloucester, were appointed to carry out this work and began Borehole 1. on the 4th December 1965. This boring was open-holed to a depth of 16-ft. and cored beneath, limestone being encountered at a depth of 56 ft 9 ins below the reduced level, ie. 63 ft 9 ins below existing ground level. The rig was then moved successively to positions 2 and 3 where rock was encountered at depths of 4 ft 6 ins and 13 ft 0 ins below reduced level, ie 11 ft 6 ins and 20 ft 0 ins below ground level. In both boreholes bedrock was proved to a depth of 10-ft. Boring work was completed by 10th December, 1965. The cores are available for inspection on the site.

Discussions held with members of the staff at the City Museum, together with a number of references that subsequently came to light from local sources, suggest that the material revealed in Borehole 1., although alluvial in character, is not fill. Worth, in a paper "Geology of Plymouth" - Transactions of the Plymouth Institution, Vol. 5. (1875), has described similar deposits filling pockets in the Limestone of the Hoe. He records instances where deposits 60 feet or more in depth have been located and has advanced a theory to explain the method of deposition. Deep, steep-sided submerged valleys in the Plymouth area have also been described by Codrington in the Quarterly Journal of the Geological Society, 1898. From examination of the core revealed in Borehole 1. and the description in these references, it seems reasonable to conclude that any quarrying activities in the area of the site are limited to surface diggings and the irregularities in the rock profile and the material filling them are primarily natural in origin.

The difficulty in satisfactorily founding the block had, by these discoveries, become severe. A specialist firm of piling contractors able to provide suitable equipment for the installation of piles in such strata were approached for a budget price for piling the southwest corner of the raft. This price was necessarily very flexible since the extent of the work was not at all clearly defined. However, it was clear that the cost would not be less than £10,000.

GEOPHYSICAL SURVEY :
Since even within a small area the depth to rock varied very significantly, further borings would be costly and haphazard in providing the information we required. It was therefore decided that the rock drill should be dispensed with and that a geophysical survey using resistivity methods should be substituted. Mr W.R. McKnight, a Geophysical Consultant, was appointed to carry out the resistivity survey and site work was carried out on the 19th December, 1965 and 3rd January, 1966. It was decided to investigate the rock profile not only to the southwest, but also to the northeast with the object of exploiting the possible re-location of the block. Eight trial pits were excavated in the alternative new area some 55-ft. northeast, in order to investigate the viability of this scheme prior to application for Planning Approval. Rock in all cases was encountered within 10-ft. to 12-ft. of ground level.

CONCLUSIONS :
The sections obtained by resistivity methods are shown as Figs. 1. and 2. [not given in this Webpage].
Fig. 3. shows a plan of the site on which are superimposed all of the information obtained. Also shown is the revised position of the tower block and the approximate contours of the rock surface.

It will be seen that there is a narrow fissure of some depth running approximately east-west through Borehole 1. Although the fissure is quite narrow, by coincidence the siting of this borehole fell within its boundary. The fissure is probably along the line of a minor fault associated with a more pronounced fault well to the west of our site, which has been subsequently eroded by water action during the recession of the Ice Cap. To the south and west of this fissure the rock is markedly more shaley in character but approximately the same depth below ground level. There is no indication of any major swallow hole, clay sink or possible slip-plane within the area of the site. The rock configuration south of the fissure makes a significant contribution to the stability of the site.
No information has been found to suggest that a submerged cliff face or abandoned and re-filled quarry exists within the site boundary. All evidence points to disturbances below about 15-ft. being entirely natural in origin and in no way impairing the stability of the proposed building either in its earlier position or in the latest revised position.
The revised position is recommended in view of :-
(i)  its being more remote from the disturbed area and
(ii) major economies in the foundation can be achieved, since bedrock is within about 10-ft. of existing ground level."

APPENDIX 'A'  : TRIAL PIT E. (North Side).
Fill.................................................................2ft 6ins.
Firm silty clay.................................................2ft 6ins.
Limestone cobbles in firm silty clay matrix........5ft 0ins.
TRIAL PIT B.
Soft fill, clay, ashes, cobbles............................4ft 0ins.
Soft clay and rock fragments............................4ft 0ins.
Light brown shale with clay bands.....................2ft 0ins.
Partially weathered shale (inclined at 80º).
TRIAL PIT E. (South Side).
Fill.................................................................5ft 0ins.
Limestone cobbles in sandy silty clay matrix.....3ft 0ins.
Bedrock or large boulder.
TRIAL PIT C.
As above, but weathered shale not properly exposed.
TRIAL PIT F. (North Side).
Topsoil...........................................................1ft 6ins.
Fill.................................................................1ft 0ins.
Limestone cobbles in firm clay matrix...............1ft 0ins.
Shaley rock (slate?) dipping at 80º....................6ft 0ins.
TRIAL PIT D.
Topsoil and fill.................................................2ft 0ins.
Limestone cobbles & fragments in clay matrix...5ft 0ins.
Limestone bedrock or extensive outcrop.
TRIAL PIT F. (South Side).
Topsoil...........................................................1ft 6ins.
Fill.................................................................4ft 6ins.
Limestone cobbles in clay matrix.
Also large pockets (2ft 6ins.) of sandy
decomposed rock............................................3ft 6ins.
(The above pits were in the area of the garages.) (The above pits were in the area of the tower block.)


FIGURE 3.

..........
Figure 3.  Showing the position of Boreholes, Trial Pits, Resistivity Survey (Transect) Lines, and the karstic limestone rock-head contours below the Site, to an original Scale of 1 : 240.
Note the position, shape and depth of the Fissure, within the context of adjacent rock-head contours indicated on the survey.
[Main Image of Fig. 3. is 773 kB]


..........
Borehole Log No. 1. (sheets Nos. 1. and 2.)
(co-incidentally located within the Fissure).
[483kB and 470 kB]

..........
Borehole Log No. 2. [200 kB].................................................................................................Borehole Log No. 3. [416 kB]

[end of Report]


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