Contents {continued) 3 Detailed Geological Structure Underlying Tip No. 7 1 Superficial Cover of Boulder Clay 2 Mineralogical Composition of Boulder Clay 3 Possible chemical effects on clay from tip water 4 The Underlying Solid Rocks 4 Geological Interpretation of Geophysical Survey of Tip Complex 1 Geological Foundations of other Tips 2 Distribution of Superficial Deposits (i) Borehole Provings (ii) Geophysical Surveys V Hydrological Conditions under Tip No. 7 (i) Water Flow Details 1 Additional Evidence Concerning Hydrological Conditions under Tip No. 7 A Evidence for Fissures in Brithdir Sandstone (i) Borehole P/lV-6 (ii) Inclined Borehole B Effect of Mining Subsidence C Boulder Clay Seal VI Geological Evidence Available after Failure of Tip No. 7 1 Slipped Material 2 The Lower Slip Face 3 The Arched Culvert 4 Details of Tipped Material and Standing Faces A Top Feature B Middle Feature C Basal Feature 5 Water Feeders from Tip Material VII Conclusions Appendices* la Detail Flows of Water lb Water Analyses Ha Permeability Tests lib Geophysical Data 111 Geological Report on the Slip-Scar at Craig Abercwmboi *The main conclusions from the Appendices are contained in the Report, and the five Appendices have not been reproduced here. They can be seen, with other documents presented to the Tribunal, at the Treasury Solicitors Office, 3 Central buildings, Matthew Parker Street, London SW1 by prior appointment. ' ^ 148 Printed image digitised by the University of Southampton Library Digitisation Unit Introduction and synopsis The Report describes the geology and hydrology of the site of the catastrophy at Aberfan and is based primarily on the published Geo- logical Survey maps of the area. The geological maps and sections accompanying the Report include a great deal of additional actual evidence which has been obtained by field work, borehole and geo- physical investigations. The Report demonstrates the coincidence of a set of geological factors, each of which, in itself, is not exceptional, but which collec- tively create a particularly critical geological environment. These are: (i) The presence beneath the tipping area of an embayment or side valley in the steeply sloping side of the main Taff Valley. (ii) This embayment infilled with superficial deposits and parti- cularly with impermeable boulder clay, the upper limit of which rises to an exceptionally high level on the hillside. (iii) The underlying solid rocks dip down the valley side towards Aberfan and are highly fissured. The sandstones contain water in the fissures under a hydrostatic head and are underlain by an impermeable bed (Brithdir Seam and Seatearth). The latter pre- vents downward percolation of the water to the deeper rocks. (iv) (a) The boulder clay cover prevents direct flow of water from these sandstones to the surface. (b) The deep sections of boulder clay, infilling the old valley on the south side of the tip complex, restrict or prevent the normal 'down dip' flow of the water from the sandstones under the tip complex. (v) Mining subsidence has resulted in : (a) A higher proportion of rain water entering the regional rocks through fissure systems—as groundwater. (b) The presence of a narrow zone of extensional strain under- lying the region of Tip No. 7, and resulting in modification of the already established joint system into open fissures nearer the surface, thus providing ready access for under- ground water flow in the sandstones. (c) The presence of a major compression zone due to the effects of mining subsidence on the south side of the tipping com- plex provides a basin structure within which the joints would tend to be closed nearer the surface. This is a contri- butory factor in restricting the 'down-dip' flow of water from the Tip Complex sandstones. The effect of this combination of circumstances is to cause a local rise in the water level in the sandstones under the tipping area until it reaches a height where it flows over the edge of the boulder clay whence it would normally escape by surface drainage channels. In the present circumstances this water has been introduced into the tipped material and ultimately in conjunction with the slope of the ground, and in the presence of plastic boulder clay has led to a con- dition resulting in the failure of the base of the tip. This was followed by the release of the main body of water. The prime cause of the catastrophe was therefore geological. I TOPOGRAPHIC DETAILS The Taff Valley at Merthyr Vale and Aberfan is asymmetric in cross- section with an eastern side to the valley which is steep and upon which surface slopes range between 1 in 16 and 1 in 2 5. The western side of the valley which forms the substance of this report is typified by 149 gentler slopes. Printed image digitised by the University of Southampton Library Digitisation Unit NANT-Y-MAEN 133 Acres Twvn S^tsJf aoXQOQ 55 Acres CENTRAL COMPLEX 81 Acres Ceuhfen • ^ if^ngej SOUTH STREAM 12 9 Acres lUHmtnt OfrdcHf 5.1 Location plan showing catchment areas 150 Printed image digitised by the University of Southampton Library Digitisation Unit 1 Topographic features Western side of Taff Valley Inspection of the Ordnance Survey contoured map (Fig. 5.1) indi- cates the presence of three topographic units as follows: (i) Northern Area This unit lies east of Cnwc, extends southwards through Nant-y- Maen and is characterised by surface contours which have a predomi- nantly north trend. The average slopes of the valley sides range between 1 in 2-88 and 1 in 3 25 with the steeper slopes tending to occur above the 800 ft contour where by reason of control by the geological succession there is less uniformity of slope. (ii) Southern Area From the neighbourhood of Hafod-tanglwys Isaf southwards the sur- face contours show a predominant north-east or north-north-easterly trend. The average slope of the valley side ranges between 1 in 2 88 and 1 in 4 with the steeper gradients occurring on the higher parts of the hillside, particularly above the 900 ft contour. (iii) Central Area The region lies between the limits of (i) and (ii) above and exhibits swinging curved contours which in parts of their course follow a north and north-north-westerly direction. The area is characterised by overall gentler slopes which show a progressive range of gradients from 1 in 3 above the 900 ft contour through 1 in 4,1 in 5,1 in 5 6 descending the valley side to a position adjacent to the abandoned railway lines. It is on a part of this central area that the Tip Complex is situated, extending from about 1175 A.O.D. to circa 625 ft A.O.D. This area of gentler sur- face gradients lies immediately west of Merthyr Vale Colliery and from a purely topographic point of view offered an advantageous site for colliery waste disposal. 2 Topographic control In the areas (i) and (ii) above there is a general topographic simi- larity, and this is directly related to the geological sequence and struc- ture of the solid or consolidated rocks which form the valley sides. The detailed topographic features are principally decided as a result of differential erosion of the relatively hard and softer rocks in that succes- sion. The precise position and direction of these features is dependent upon the disposition or structure of these rocks on the valley side. In the Central Area (iii) only the higher part of that region above approxi- mately 900 ft is subject to this type of control. The lower and gentler gradients are related to a changed geological circumstance which involves the presence of unconsolidated superficial deposits concealing the solid rocks. These deposits have a structure and a distribution pattern which is markedly different from that of the consolidated strata. There is therefore a relationship between topographic detail and geological sequence and structure along this portion of the western slopes of the Taff Valley. 151 Printed image digitised by the University of Southampton Library Digitisation Unit H SURFACE DRAINAGE PARTICULARS 1 General introduction The principal source of water on hillsides is derived from rainfall, other sources, e.g. snowfall, may be locally or temporarily important, and atmospheric precipitation in the form of dew though realistic in quantity is of subsidiary importance. The disposal of rainwater on a hillside is effected in the following three ways: (i) A proportion of the total rainfall enters the surface rocks to pro- vide an underground storage, some part of which may later be recirculated as seepages or springs. (a) Water will enter the rocks if the latter are porous, i.e. voids are present between the grains when the water fills the voids and saturates the rock to a level referred to as the water table. (b) Water will also enter the rock if the latter is permeable, i.e. the water is able to traverse the rock through joints, fissures or fractures, and under certain conditions water will be stored in the joint system under a hydrostatic head. (c) Ready intake of water will occur if the rock is both porous and permeable. (ii) A proportion of the total rainfall collects on the surface into drainage channels as 'run off water. This proportion is higher on impervious rocks such as clays and on non-porous or imper- meable rocks. The proportion also depends on the surface gradient. (iii) A proportion is directly returned to the atmosphere by evapora- tion and is climatically dependent in its proportion. On the hillside such as the western slopes of the Taff Valley near Merthyr Vale and Aberfan these proportions are normally set and stable since they depend upon the physical characters of the rocks and their geological structure, and upon climatic factors.