Brighton and Hove Wastewater Treatment Project Infrastructure WSI APPENDIX 1: 2006 GEOTECHNICAL INVESTIGATION RESULTS (ARCHAEOSCAPE) An Evaluation of Geotechnical Borehole and Test Pit Data from Locations between Brighton and Peacehaven, East Sussex C.P. Green, ArchaeoScape, Department of Geography, Royal Holloway University of London, Egham Hill, Egham, Surrey, TW20 OEX, UK Introduction A1.1 This report summarises the findings arising from an evaluation of forty-seven geotechnical borehole and test pit logs taken at the site of a proposed waste water treatment works (WTW/SRC) at Peacehaven and along the route of the associated infrastructure between Brighton and Peacehaven. This infrastructure comprises a tunnelled pipeline from Black Rock (Brighton), following the coast to the existing Headworks at Portobello, then across to the WTW/SRC site at Peacehaven. Along the length of the pipeline are a number of components, comprising shafts and two pumping stations. The geotechnical work was conducted by Soil Mechanics Ltd on behalf of Mott Macdonald. The geotechnical data were subsequently collated and evaluated by ArchaeoScape (on behalf of RPS) with the aim of providing an indication of the range of sub-surface conditions likely to be encountered along the pipeline route and at the WTW/SRC site. The Report is divided into two Parts. Part I provides a description and interpretation of the sediments and sub-surface conditions recorded in the boreholes and test pits. Part II lists the components of the scheme and evaluates the ground conditions and geo-archaeological potential at each component location. Part I Methods A1.2 Forty-seven geotechnical sediment logs located between Brighton and Peacehaven, and at the site of the waste water treatment works at Peacehaven, form the basis of this evaluation. The geotechnical logs consist of 29 rotary and 6 cable percussion boreholes, and 12 test pits. Figure 1 displays a summary of the rotary boreholes Page 16 Brighton and Hove Wastewater Treatment Project Infrastructure WSI (scale, m OD); to emphasise the locations of the rotary boreholes, vertical lines have been used to differentiate three spatial groups (Brighton, Roedean-Telscombe and Peacehaven). Figure 2 displays a summary of the cable percussion boreholes (scale, m OD), while Figure 3 displays a summary of the test pits (scale, m OD). Along the top of each figure are the borehole or test pit reference numbers and in the case of the rotary boreholes (Figure 1) a code indicating the broad topographic character of the borehole site. A key to this coding system is shown at the bottom of Figure 1. In some cases a second code indicating the driller's log or truncation of the sequence is shown, this is also indicated in the key. The location of the dry valleys as recognised by Mortimore (1997) and components of the scheme as described in Part II are illustrated in Figure 4. Figure 5 shows the locations of the boreholes and test pits along the route of the pipeline. Rotary Boreholes A1.3 Twelve rotary boreholes were situated in the eastern suburbs of Brighton (MTR 101A, 101B, 101AA, 101BA, 103A, 103B, 103AA, MBR106, MBR107, MBR108, MTRXC1, MTRXC2) and a further eight along the proposed route between Roedean and Telscombe (MTR 106, MTR 112, MTR118, MTR128, MTR132, MTR134, MTR135, MTR135A). All the other rotary boreholes were within or very close to the site of the WTW/SRC works. The rotary boreholes extend to depths between 25 and 60 metres and in almost all cases terminate in undisturbed chalk of Grade B2 or B1. They provide a good record of weathering and disturbance in the upper part of the chalk and of sediments overlying the chalk. However, sediment recovery was not continuous and gaps in the cores of up to 2m are recorded and may affect the detailed interpretation of contacts between sediment units. The upper 25m of five of the rotary boreholes were not recorded in detail (driller's descriptions only). Cable Percussion Boreholes A1.4 The cable percussion boreholes were all put down in the Peacehaven area. The cable percussion boreholes are shallower (15m and in one case 20m) and provide a less refined record of the sediments recovered than the rotary boreholes. Page 17 Brighton and Hove Wastewater Treatment Project Infrastructure WSI Test Pits A1.5 All the test pits were put down in the Peacehaven area within or very close to the site of the treatment works. The test pits were all put down to a depth of 5m. Results of the Geotechnical Boreholes and Test Pit Logs A1.6 From an evaluation of the borehole and test pit logs, thirteen sediment types can be identified and possible depositional origins can be suggested as shown in section A1.7 below. A1.7 The following is a list of sediment types identified from the borehole and test pit logs (Unit Number; Unit Name; Unit Description): 0; Made Ground 1; Topsoil; Brown to dark brown sandy clay with fine to coarse angular to sub- angular flint clasts, penetrated by many modern roots 2; Flinty Head; Comprising of colluvial deposit of brown to dark brown more or less sandy clay with fine to coarse angular to sub-angular flint clasts 3; Chalky Head; Colluvial deposit of brown more or less sandy clay with fine to coarse angular to sub-angular flint clasts and sub-angular to sub-round clasts of weathered chalk 4; Sandy Head; Colluvial deposit of brown to dark brown sandy clay 5; Flint Gravel; Water-laid deposit of fine to coarse angular to sub-round flint clasts 6; Solution pipe infill; Variable mixture of sandy and stony clay, infilling larger solution features in chalk 7; Woolwich Beds; Palaeocene sand and sandstone overlying Chalk; sometimes iron cemented 8; Disturbed/displaced chalk(Chalk grades Dc and Dm); Structureless mass of abraded chalk gravel, sand and silt, often stained orange or yellow mixed Page 18 Brighton and Hove Wastewater Treatment Project Infrastructure WSI with variable amounts of angular to sub-angular flint (and occasionally sandstone - probably sarsen), light brown silty loam and brown or dark brown sandy clays. This unit probably includes both colluvial Combe Rock deposits and chalk severely disturbed by periglacial processes and mixed with overlying Tertiary and Quaternary sediments 9; Weathered chalk(Chalk grades C5, B5, B4); Chalk, often stained orange or yellow, retaining its original structure, but often with open joints infilled with silty chalk or silt, clay or sand derived from overlying Tertiary or Quaternary sediments; or small solution features with similar infills 10; Sound chalk(Grades B3, B2 and B1); Chalk, slightly weathered and unweathered in situ chalk 11; Chalk with flints; Driller's description 12; Beach shingle; Includes both the Pleistocene Black Rock Raised Beach and Recent beach shingle Interpretation of the Geotechnical Boreholes and Test Pit Logs A1.8 Rotary Boreholes: The sequences recorded from the rotary boreholes provide the most reliable record of sub- surface conditions. These sequences fall into three broad types, as indicated below: A1.9 Characteristics of Type A, B and C: A - Sound chalk (Grades B3, B2 or B1) overlain by less than 5m of disturbed/displaced and weathered chalk and/or Quaternary sediments B - Sound chalk (Grades B3, B2 and B1) overlain by more than 5m of disturbed/displaced chalk and/or Quaternary sediments C - Sound chalk (Grades B3, B2 and B1) underlying Woolwich Beds sands and sandstone A1.10 On this basis, the rotary boreholes have been assigned as either Type A, Type B or Type C as discussed below. Page 19 Brighton and Hove Wastewater Treatment Project Infrastructure WSI Type A A1.11 The Type A group includes MBR106, MBR107, MBR108, MTR101A, MTR101B, MTR103A, MTR103B, MTR128, MTR134, MTR135, MPR100C and MPR102. These boreholes can be divided into three sub-groups. A.i This sub-group includes two low-level boreholes (MBR106 and MBR107) put down on the foreshore in the Black Rock area from ground surfaces below 6m OD. MBR 107 passed through Made Ground down to 3.81m OD and directly into sound chalk (Grade B3). MBR106 passed through Made Ground down to 2.86m OD. Below this level core recovery was incomplete but sediment logged as 'raised beach' was encountered intermittently between 2.26m and 0.76m OD and the borehole reached sound chalk (Grade B3) at 0.26m OD. The interpretation of deposits in borehole MBR 106 as 'raised beach' is accepted in the Interpretative Report prepared by Mott MacDonald (2005). However, the beach deposits recorded in borehole MBR106 almost certainly represent the modern beach, buried during the construction of the marina. At levels between 0.76m and 2.26m OD, they are some 5.6m below the lowest level at which the base of the Black Rock Raised Beach has ever previously been recorded (Hutchinson and Millar 1998 - see discussion of boreholes MTRXC1 and MTRXC2 below). The possibility that these deposits represent the Black Rock Raised Beach can therefore be discounted. A.ii Borehole MTR128 was located near the axis of a dry valley that reaches the coast at Saltdean (DV4 of Mortimore 1997). This is the longest of the dry valleys reaching the coast between Newhaven and Brighton and is cut down closer to the present sea level than the shorter and more severely truncated valleys (Figure 4). The borehole passed through 4.01m of Made Ground to reach sound chalk (Grade B3) at a depth of only 4.45m (10.51m OD). This result is somewhat unexpected as Mortimore (1997) states that 15m of weathered chalk underlies the dry valley here. It seems possible therefore that the zone of deeply weathered chalk beneath this dry valley is relatively narrow, possibly truncated by the downcutting of the valley, and that the borehole lies outside this zone of deep weathering.