ALERT-ME | East Leake embankment research site

ALERT-ME logo Site location: East Leake, South Nottinghamshire

The Great Central Railway was originally constructed in the 1890s as a link from the Manchester, Sheffield and Lincolnshire railway to London (Bidder, 1900; Fox, 1900). The section of line used in this study serves as a goods link from the mainline at Loughborough to the gypsum works at Rushcliffe Halt, East Leake.

Daily traffic includes two freight locomotives operated by DB Schenker and GB Rail Freight Ltd. The line is operated by the Great Central Railway (Nottingham) Ltd. from the Nottingham Transport Heritage Centre and is owned by the East Midlands Railway Trust.

Current study

The current investigation focuses on a section of earthworks south-west of East Leake, Nottinghamshire (Figure 1), which were constructed using local materials excavated from adjacent cuttings to the south-west and north-east.

The material was tipped and then compacted by subsequent movement of shunting locomotives and tipping wagons across the tipped material. The tipping method used along this section of the line was not stated explicitly by Bidder (1900), but has been deduced to have been end tipped from current observations and the information recorded by the engineers practicing at the time.

This section of embankment from the East Leake Tunnel (bridge 314) in the south-west to the overbridge (bridge 313) in the north-east forms the East Leake embankment research site (ELERS). The site has been the subject of field investigations since September 2005.

Victorian earthworks are generally very heterogeneous because of the techniques used in their construction, such as end tipping. Several of the data gathered from the field investigations can be integrated to provide an assessment of embankment condition in terms of the variability of fill materials.

Conceptual model for groundwater flow in the aged embankment

Conceptually, the movement of water, whether it be drainage downwards or lateral flow is likely to be controlled by the permeability of the fill network. Observations on core from the embankment indicate a large degree of inter-particle porosity between lithoclasts, and intra-clast porosity, through which water has flowed resulting in particle coatings of gypsum caused by secondary mineralisation (Gunn et al., 2009).

The groundwater drainage pathways within these earthworks appear to include complex vertical and lateral drainage at different depths that result in delays of up to several days in the delivery of water deeper into the embankment core.

Guelph Peameameter tests on pit floors in the granular materials yielded hydraulic conductivity values between 0.1 and 1 cms-1, indicating pervious fill materials 1 m below the surface.

Borehole core, invasive probing and pit observations indicate that the embankment comprises a heterogeneous mass of open, highly porous granular materials including siltstone, sand, gravel and mudstone lithoclasts with low permeability lenses associated with Westbury Mudstone, where it may have degraded to clay and silty clay (Gunn et al., 2007; Gunn et al., 2008). These finer, degraded materials produce low penetration resistance and high frictions on CPT logs, and thus, can be mapped using friction ratio data.

Figure. 2a shows a 2D section of the friction ratio within the embankment, where zones of high fiction ratios indicate lower permeability, and thus, a greater likelihood of lateral flow.

Interpretation

It is possible to infer some potentially interesting features from this interpretation. Water could be perching on two narrow bands of friction ratios up to 2.5 % at the 10 m station (Borehole B) within 1.5 m of the surface, resulting in very low moisture content in materials immediately below.

Water could be flowing laterally at 2.5 m depth along the top an extensive low permeability zone, marked by friction ratios of 4–6 %, that slopes south-westerly (to the left in Figure. 2a), resulting in a greater and earlier recharge below (at 3 m) than above (i.e. about the two narrow bands above 2 m).

The top of this low permeability zone continues at 2.5 m deep until it grades into a high permeability zone (friction ratios 1–1.5 %) just north-east of the 50 m station Borehole F), where it is also overlain by high permeability material.

Water drains readily through the high permeability material but perches on the top of the low permeability zone at 2.5 m depth before flowing laterally and draining to the north-east, resulting in a faster re-establishment of reduced moisture content conditions at 2 m than at 3 m following heavy rainfall events.

Conceptual model

Pore pressure measurements

CPT-based pore pressure measurements in Sept 2005 (after a dry summer) all indicated near zero pressures in the upper parts of the embankment and varying negative pressures through the embankment core and into the bedrock mudstone of the Cropwell Bishop Formation (Figure 2b).

Interestingly, within the top 1.5 m about the Westbury probe the two narrow bands of high friction ratio correspond in depth to two local maxima in suction (or minima in pore pressure); also, the top of the extensive high friction zone at 2.5 m corresponds to a consistent high suction maximum at this depth.

Figure 2b presents a 2D conceptual drainage model through the embankment transect, where low permeability zones coincide with greater pore suctions, over which water will perch and flow laterally. At times when the embankment is completely charged, water will drain and perch on the low permeability Cropwell Bishop mudstone bedrock.

A zone of full saturation will develop above the top of the bedrock, water levels will rise and extend laterally to the lower flanks of the embankment leading to seepage near the embankment toes.

High moisture content and seepage

Flooding at the base of the embankment during the heavy rains of March 2007 and January 2008 confirm seepage from the east toe. Also, during these flood events, the 3 m deep moisture sensors indicated very high moisture contents and the surface moisture sensor numbers 1 and 5 in the lower embankment about the toes, which are under tree canopy in summer, indicated near full saturation.

Thus, in addition to a seepage horizon above the Cropwell Bishop (Mercia Mudstone) bedrock, there may also be another, higher seepage horizon around 4 m or even 3 m below the embankment crest. This may be the case, for example, where the first lift comprising Westbury Mudstone fill resulted in a low permeability zone extending to outcrop at the east flank at this level. It is also possible that any water seeping from this horizon returns to recharge the base of the embankment.

Contacts

Dr David Gunn
Team leader — Geotechnical & Geophysical
Properties & Processes
Email: Dr David Gunn

Dr Jonathan Chambers
Team leader — Geophysical Tomography
Email: Dr Jonathan Chambers