OR/13/006 Data acquisition, quality and presentation

From Earthwise
Jump to navigation Jump to search
Entwisle, D C, Hobbs, P R N, Northmore, K J, Skipper*, J, Raines, M R, Self, S J, Ellison, R A, and Jones, L D. 2013. Engineering geology of British rocks and soils - Lambeth Group). British Geological Survey. (OR/13/006).

* Geotechnical Consulting Group (GCG)

The data presented show that the geotechnical properties of the Lambeth Group and its units are extremely variable, even within an area and within a unit. The data presented here shows the changes with depth and primarily the median values for comparison between different units. They should not be used for design purposes. The box and whisker plots in Appendix 5 represents the variation in values of some parameters. It is, therefore, important to carry out site and lithology specific investigations for use in design.

Data sources and coverage[edit]

The majority of the data presented and assessed in this report were extracted from site investigation records for the motorway and trunk road network, underground railway lines and large water pipelines. In addition, a small number of tests have been carried out at BGS’s laboratories and are referred to. The site investigation records used were those held at the British Geological Survey National Geoscience Data Centre (NGDC), with additional data requested from a number of sources in Association of Geotechnical and Geoenvironmental Specialists (AGS) digital data transfer format. These site investigation records provided good quality data. Most of the selected reports were for investigations produced after 1985, although data from earlier site investigations were added where it was considered of high quality or in areas where there was little or no recent data. AGS digital data transfer format data were chosen wherever possible.

The data records selected were either from near surface site investigations along the outcrop or, in the case of central London, from investigations for tunnels. Data for the Lambeth Group have been split into four areas as shown in Figure 5.1, reflecting the lithostratigraphic variation, present outcrop, the extent of current and potential development, and the availability of data sources.

Figure 5.1    Areas 1 to 4 used in the analysis of the geotechnical data, with the outcrop of the Lambeth Group and borehole sites shown.

Most of the data are from Areas 1 and 2 with fewer boreholes and data points occurring in Areas 3 and 4. The depth of investigation of the Lambeth Group varies between the areas. Many of the site investigations in Area 1 and 2 are for tunnels whereas those for Areas 3 and 4 are mostly for roads. This is reflected in the data profile plots presented in Section 6. However, there are a few deep investigations in Area 3 and 4, most notably for bridge construction along motorway or main road routes. The total data set for the Lambeth Group comprised values for approximately 4,900 test samples and 4,000 in situ Standard Penetration Tests (SPT) from almost 1,500 boreholes and test pits, abstracted from 104 site investigations.

Data quality[edit]

The data sources primarily comprised recent contracts carried out by ground investigation contractors for the Highways Agency or its predecessors, and investigations for major railway lines such as the Jubilee Line Extension (JLE), Crossrail and the Channel Tunnel Rail Link (CTRL). The extreme values in each data field were examined, and those that appeared to be gross errors were deleted. Nevertheless, it has to be recognised that much of this original data, as in other geoscience fields, was potentially 'dirty' in the statistical sense. The values in the database often reflect the final result of a succession of field, laboratory and transcription procedures, during which one must expect errors to occur, however small or infrequent. Some data, such as stratigraphy, are interpretative and hence subjective. The information that could be extracted from borehole descriptions varied to some degree with the age of the site investigation but also the drilling method. In some areas a full assessment of the variability of the Lambeth Group would require closely-spaced boreholes with, preferably, continuous, undisturbed sampling. In such instances, lithostratigraphic details are likely to be missed when using standard cable percussion drilling with selected undisturbed sampling intervals often used in site investigation practise, or as required by BS5930 (BSI, 1981[1], 1999a[2], 1999b[3], 1999c[4]). In some site investigations rotary drilling was employed to retrieve continuous core in plastic liners. This method provided generally good quality core, and the related borehole records were particularly useful for providing good lithological descriptions and lithostratigraphical interpretation.

Classification of lithostratigraphy[edit]

Wherever possible the lithostratigraphic classification in this report follows Ellison (1994)[5]. The various lithostratigraphic divisions are listed in Table 5.1. In many instances detailed, or correct, geology (stratigraphy) was not indicated in the site investigation reports or borehole logs. This necessitated revised lithostratigraphical classifications being assigned to the borehole logs (and associated measured data for particular borehole intervals) based on known lithostratigraphy of particular areas and from borehole descriptions. This proved difficult to do in some cases and required very good lithological logs such as those available from more recent site investigations for tunnels under London.

With regards recognition of the Lower and Upper Mottled Clays of the Reading Formation, their separation is relatively straightforward in those areas where the lower part of the Woolwich Formation is present between them, but elsewhere this is more difficult. However, based on the observations of Skipper (1999)[6], this has been attempted where the borehole descriptions are of high quality. Skipper (1999)[6] observed a dark organic horizon above a bleached bioturbated horizon within the Mottled Clay and designated this as representing the boundary between the upper and lower Mottled Clays of the Reading Formation. The boundary indicates a depositional hiatus followed by faunal activity, weathering and then inundation by organic-rich waters in a low energy environment. However, this organic horizon may be thin and difficult to observe during normal cable percussion drilling and sampling operations.

In parts of the Hampshire Basin, the Lambeth Group is often coarse-grained giving rise to difficulty in distinguishing between the Upnor and Reading formations. Where this distinction was not possible to determine from borehole logs, the coarse-grained deposits were simply classified as ‘Lambeth Group’ (LMBE). It should be noted that the ‘basement beds’ of the Reading Formation shown on some geological maps of the Hampshire Basin are here interpreted as the Upnor Formation. Note: ‘Woolwich and Reading Beds’ in the western Hampshire Basin are thought to have been mapped as part of the West Park Farm Member (Bristow et al., 1990), the basal member of the London Clay, but is considered here to be part of the Lambeth Group

Table 5.1    Stratigraphical subdivisions of the Lambeth Group used in the geotechnical database, after Ellison (1986)[7].
Formation Stratigraphical position Units in the database
Woolwich Formation Upper Woolwich Upper Shelly Clay (striped loam)*
Lower Woolwich Laminated Beds Lower Shelly Clay
Reading Formation Upper Reading Upper Mottled Clays
Reading Formation Mottled Clays
Lower Reading Lower Mottled Clays
Upnor Formation Upnor Formation Upnor Formation

[Note: * indicates no geotechnical data available]

Data sub-division and presentation[edit]

Statistical tables and plots used to illustrate the range and distribution of data values according lithostratigraphy (Figure 5.1) and location (Areas 1 to 4; Table 5.2) are presented in see Geotechnical properties and in Appendix 4. The Areas were selected by stratigraphy and geography and do not have any significance outside the database, and are not represented by uniformly distributed data. In addition, the samples from any general location should not necessarily be considered as representative of that area as a whole.

A variety of plots including ‘line’ and ‘scatter’ plots are used to display correlations of various key geotechnical parameters (see Geotechnical properties). For example, selected geotechnical parameters are plotted against depth or other parameters, in order to determine variations caused by depth, and other related factors. Weathering may be related in a general sense to depth below ground level, but this is not a simple relationship of decreasing weathering with increasing depth in the case of the Lambeth Group. It should be noted that the plots of parameters with depth should be treated with some caution as they may contain random errors, and are meant only to give an indication of general trends. As such, depth relationships shown here should not be used in design calculations. Median values, 50th percentile value, are used in Section 6 to illustrate differences between the different units.

In addition, summary statistical analyses of the data are given in the form of tables accompanied by ‘extended box’ plots that give a graphical representation of the range and distribution of the geotechnical data with respect to area and lithostratigraphy. These are presented in Appendix 4.

OR13006textbox.jpg
Table 5.2    Areas used in geotechnical data analysis and the associated
geological units and codes used in the geotechnical database.
Area Location Lambeth Group Formation Units
1 Central London Woolwich Formation (WL) Upper Shelly Clay (UPSCL)
Laminated Beds (LBED)
Lower Shelly Clay (LSCL)
Reading Formation (RB) Upper Mottled Clay (UMCL)
Lower Mottled Clay (LMCL)
Upnor Formation (UPR) Upnor Formation (UPR)
2 East London, Kent and South West Essex Woolwich Formation (WL) Laminated Beds (LBED)
Lower Shelly Clay (+ lignite beds)
(LSCL)
Reading Formation (B Lower Mottled Clay (LMCL)
Upnor Formation (UPR) Upnor Formation (UPR)
3 North Essex, Suffolk, Hertfordshire, Buckinghamshire,
North east Hampshire, Berkshire, West London, Surrey
Reading Formation (RB) Mottled Clay (Upper and Lower Mottled Clays undifferentiated) (LCM)
Upnor Formation (UPR) Upnor Formation (UPR)
4 Hampshire Basin Woolwich Formation (east only)* (WL)
Reading Formation (RB) Mottled Clays (Upper and Lower Mottled Clays undifferentiated) (MCL)
Upnor Formation (UPR) Upnor Formation (UPR)

[Note: * indicates no geotechnical data available]

References[edit]

  1. BSI. 1981. BS5930: Code of practice for site investigation. British Standards Institute, London, UK.
  2. BSI. 1999a: BS5930: Code of practice for site investigation. British Standards Institute, London, UK.
  3. BSI. 1999b: 2007. Code of practice for site investigation: Amendment 1. British Standards Institute, London, UK.
  4. BSI. 1999c: 2010. Code of practice for site investigation: Amendment 2. British Standards Institute, London, UK. 192pp.
  5. ELLISON, R A, KNOX, R W O'B, JOLLEY, D W, and KING, C. 1994. A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geological Association, 105, 187–197.
  6. 6.0 6.1 SKIPPER, J. 1999. The stratigraphy of the Lambeth Group (Palaeocene) of South East England. Ph.D. Thesis. Imperial College, London, and Natural History Museum.
  7. ELLISON, R A, and LAKE, R D. 1986. Geology of the country around Braintree. Memoir of the British Geological Survey, (England and Wales), sheet 223. British Geological Survey, Keyworth, Nottingham, UK.