SURVEYING IN REDCLIFFE CAVES. BRISTOL.,. 1953 – 4. Description of the cave system.

The system known as REDCLIFFE CAVES is a man made system excavated in the sandstone ridge which is approximately bounded by Redcliffe Way, The Floating Harbour, Bathurst Basin and the New Cut.

The cutting of the caves probably commenced during the Middle Ages.  This report, however, is not concerned with the historical side of the cave system, although it is worth noting that the caves appear to have been connected with the Slave Trade and also with the quartering of prisoners from the Napoleonic Wars, who constructed the diversion of the Avon into its present course through the New Cut.

The caves appear to have been formed by driving a series of entrances from the north side of the ridge and linking these by excavating all the sandstone between them to form a system having an almost constant roof height and level floor supported by a series of irregularly shaped and positioned pillars of the original sandstone.

No attempt was made by the excavators to introduce any form of order or symmetry to the system, the position and size of the pillars bearing little relation to the function which they have to perform, in supporting the roof.

In course of time it was found, not surprisingly, necessary to supplement the original system of pillars by walls arches and fillings.   These were of stone, brick and more recently of concrete.   The process is still being continued as more support becomes necessary.

During the War, bombs penetrated the cave system, the roof of which is only about 15-feet below ground level.   The ash filling which was subsequently introduced has, according to old accounts, cut off portions of the system completely.

In places wells have been sunk from the surface which go right through the cave system and on through the cave floor.

A circular wall of stone has been built for each well from floor to ceiling of the cave, thus giving the effect of a continuous shaft when viewed from the surface.

In many places, where the original excavation has not been modified, the marks of the tools used can still be seen on the walls.

The walls have, in some instances, almost closed off large portions of the caves and thus it is often necessary to make long detours to reach the far side of a particular wall.   However, holes existing in some of the walls proved . useful and sights were often taken through such holes.

Objects of the Survey.

As originally planned, the surveying of the Redcliffe system was undertaken with two objects in mind.   The first to obtain a plan of the cave system and the second to give members of the B.E.C. an opportunity to undertake surveying work in Bristol during the week, under conditions reasonably similar to those obtaining in a natural cave.

The permission of the City Engineer’s Department, under whose management the caves come, was applied for and obtained in November,  1952 and the first trip took place in January, 1953.

Survey work in 1953.

The first few trips were spent in exploring the system.  It was found to be difficult to obtain a mental picture of the shape and direction of the cave owing to the maze like nature of the system.   It was finally decided to run a series of closed polygon traverses through the cave, using modified Astro compasses mounted on tripods, and tapes.   The error involved by ignoring elevations was agreed to be very small owing to the extremely level nature of the floor.   By the end of February, a large portion of the system had been covered in this manner and the results plotted and checked.

It was decided, at this stage, that the system would provide an interesting testing ground for an experiment in the use of a Plane Table for underground work and a suitable table was constructed by one of the surveying party.   This was used to obtain the position of walls and pillars until a halt was called in the work at Easter, when it was felt that the coming of the summer would interfere with the survey.

Survey work in 1954

The survey was restarted in April, 1954.  It was now felt that, by this time, the second of the original objects of the survey had been carried out, and so the work in 1954 was done by a small team of surveyors, the object being solely to complete the survey.    During the intervening time work on the 1953 results had brought slight discrepancies to light and so work started with a check on some of the earlier polygon traverses.   As a result, an earlier misinterpretation of the position of one of the station marks was discovered, and after resurveying this traverse work on the rest of the system was resumed.

The methods described previously, were used up until June, when a new method was adopted.  It had been found that, providing care was taken in setting up the Plane Table, it could be used to plot line traverses to the same order of accuracy as the Astro compasses and hence the line survey and detailing could proceed simultaneously.  Permanent stations were still used, so that the work could be checked if it became necessary.

By October, 1954, the survey covered a large part of the system.  Further Work would entail entry into portions of the caves which had been the scene of much rock fall.  Special permission would have to be obtained before a survey was concluded.   A few small passages were, however, investigated.  A lower grading has been claimed for these portions of the survey.

Surveying Methods.  Polygon Traverses.

The methods adopted for the polygon traverses conformed to the usual practice for cave surveys of this type.  Survey stations were selected at suitable points and their position recorded on the roof by means of carbon markings from acetylene lamps.  The caves, although damp, are not as wet as normally encountered and the markings thus made appear to be permanent.  The roof, being level, afforded an excellent place for marking since accidental obliterations were avoided by this method.

A plumb line was then dropped from the station mark, and the Astro compass on its tripod positioned underneath.  A light was similarly placed under the next station mark, and a bearing and distance taken.  A bearing of the last station was then taken in a similar manner and the Astro compass moved to the next station.

A 100ft. steel tape and a 66ft. cloth tape were used, the cloth tape being checked against the steel tape before use and a correction made.

Surveying Methods.  Plane Table Operation.

As a result of the experience gained in the use of a plane table for such work, the following method was evolved and is recommended to any cave surveyors who may be considering the us: of a plane table.

A team of three surveyors was used although four can be employed.  No.1 operates the plane table and commences the work by setting up the table under a survey station, or at some other convenient point.  At this stage, the others assist by erecting the plumb line and setting up lights from other stations which are visible from the operating point.   No.1 then takes sights onto all these stations draws the appropriate lines on the record card and labels the card with the position and any other details.  The table is now set up and orientated.

No.2 now proceeds to the first portion of wall to be detailed and places a light level with the sighting arm of the table.   No.1 then takes a bearing onto this light No.2,who has also the free end of the tape, then places this at the same point and No.3 pulls the tape taut (standing over the plane table) and calls out the distance to No.1, who then records it.  When this has been done, No.1 l gives a signal and the process is repeated.

From the description given, the method may appear slow, but in practice, with an experienced team, detail work may be done rapidly.  In the case of this survey, the team became so used to working together, that each member was able to anticipate the requirements of the others so that very few requests were necessary and work proceeded almost in silence.

The remainder of No.3’s job consists of illuminating the plane table, so that No.1 can concentrate on recording. Whenever a No.4 is available, he takes over this part of No.3’s work.  This results in a crowd round the plane table however, and is best avoided by built in illumination.  In any case, No.3 has to take great care to avoid jogging the table or obstructing No.l1who has to move right round the table as the work progresses.   On the other hand, No.3 must keep close to the table as the tape must be read from its centre.

No.2, in addition to providing the stations along the walls, must decide on the best places from which readings should be taken and illuminate the stretch of wall so that No.1 can see to draw the connecting lines between the points.

Nearly a hundred plane table records were produced by this method during the course of the survey.

Surveying Methods        ….Plotting.

The polygon traverses were first laid out to a scale of 1:100 on a drawing board.   In the case of those portions of the survey where the entire work was carried out by plane tabling, the required lines were taken off the records and tabulated with those from the Astro compasses.   Errors were then distributed and the final line survey produced.

When all the stations had been thus positioned, the station positions were transferred onto tracing paper and each plane table record traced through.  Since the records were produced to the same scale, direct tracing was possible.  After correction of the plane table records, the entire survey was reduced to a scale of 1.200 and finally drawn on a stable paper in drawing ink.  The plans were duplicated from this master copy and hand coloured in “Pelican” waterproof drawing inks.

Suitable location marks were incorporated on the master copy so that the overlying streets could be added to the copies in a contrasting colour.

Accuracy.

An accuracy equivalent to the Cave Research Group No.6 grading has been claimed for the main portion of the survey.   The degree of closure of the polygon traverses was consistent, the angular readings closing better than the distances on both methods.   No closed traverse was accepted with a discrepancy of more than a foot in station position.

The plane table results were found to vary by an amount depending on the radial distance from the recording point.  The maximum distance used in the records was 30 feet, and the variation on pillar or wall position at this distance amounted to nearly one foot.  However, since features at this distance from the recording point were usually common to at least two and often three adjacent tablings, a mean position could be established.   This variation of  a 3% in radial distance was felt to be mainly due to the visual “filling in” between recorded points on the plane table records.

Another advantage which was used to reduce some cumulative inaccuracies consisted of making use of the straight sections of wall.  An open traverse, including both sides of such a wall, should not show any tapering and over a long stretch of wall, a slight angular discrepancy will produce such a taper which may then be distributed over the traverse.

The Survey.

Owing to difficulties mentioned on page 3, it was found not possible to survey the entire system, hence the dotted line on sketch does not represent the complete cave system.

Sketches of the system are appended to this report. The first shows the division of the system, for the purposes of the survey, into series having a relatively closed periphery in accordance with normal cave practice, the second shows the system complete with all pillars and walls.   The maze like nature of the system will be appreciated from the second sketch.  The shapes shown for pillars are those approximately midway between roof and floor, i.e. the “waists” of the pillars.   Since most of the natural pillars are vaulted into the roof and floor to a considerable extent, the effect is to constrict the cave from the point of view of visibility and in some cases ease of movement.

Possible Extent of the Gave System.

In common with nearly all underground places, exaggerated accounts of the extent of the system are to be found.    In the case of Redcliffe, however, we have a good guide to the probable extent, since this must be in any case limited by the shape of the sandstone ridge under which it lies. The western end of the present system consists of natural rock over its entire length so that extensions to the west are ruled out.   A few cuttings exist in the cliff face to the west, but these are separate from the main system.

To the South West, a tunnel definitely joints the railway tunnel and possibly crossed it.  This appears to be an isolated tunnel, however, and on the most optimistic estimate, may have once been a “back door” coming out by the New Cut.

The area between the South Eastern Series and the Shaky Series is undoubtedly full of workings, which extend right round the entire eastern face of the surveyed system.

It is very probably that the workings go under Redcliffe Hill and that an entrance exists or existed in the church. This would probably represent the most Easterly point, as the gradient begins to fall from here onward.

Thus, it would appear likely that the area surveyed is at least half the total system at its greatest extent and may well amount to two thirds.

Conclusions.

A survey of the accessible portions of Redcliffe Caves, has contributed to the surveying experience of those taking part and afforded an excellent opportunity for the use of the Plane Table as an aid in underground work to be studied.

As a result of the experience gained, the Author has since used a Plane Table under natural Cave conditions.  In conclusion, the author would like to acknowledge the help given by many Club Members on this project and in particular the regular plane table team without whose continued support, the survey would not have been possible.

S.J. Collins,

January 1956

The first image shows the plan to scale 1:200

The second image shows an outline plan showing the surface features and the names of the series/extensions.