Chapter 9: Road drainage and the water environment

Memorandum
(Continued)
Page 5 of 6
5. Groundwater Assessment
Swinford Road Cutting (area around S9)
Ground investigation data is limited with only six boreholes within the vicinity of the proposed cutting, the
majority of which are over 100 m from the underpass. Three groundwater monitoring installations are
located in BH216, BH212 and BH323 (as shown in Table 4.1). BH216 and BH212 have been monitored
on a monthly basis between April 2004 and August 2006. BH323 has been monitored monthly between
August 2005 and August 2006. Screened response zones are installed at varying depths within the Lower
Lias Clay (BH216 & BH212) and River Terrace Deposits (BH323).
Cross Section A-A’ (attached) indicates the area is underlain by up to 4.5 m of Glacial Till underlain by
Lower Lias Clay with River Terrace Deposits situated to the northeast. The underpass is likely to cut
through the Glacial Till into the Lower Lias Clay during the construction phase. Both strata will also be
exposed in the 1:3.5 cut slopes.
Groundwater strike and monitoring data indicates that groundwater is in hydraulic continuity across the
geological strata with an average head of 111.5 mAOD. Proposed finished road level is around
108.1 mAOD at its deepest point, therefore the risk of encountering groundwater during the construction
of this particular element of the scheme is high.
A14 – M6 Link (area around S1)
Cross Section B-B’ (attached) and the BGS geological map indicate that the proposed A14-M6 Link is
underlain by Lower Lias Clay with an area of Glacial Till up to 4m in thickness situated immediately west
of the underpass.
Nine borehole logs and nine groundwater monitoring installations are available in the assessment of the
groundwater regime (as listed in Table 4.1). Response zones are installed at depths within the Lower Lias
Clay between 85.93 mAOD and 118.32 mAOD.
Groundwater monitoring data indicates that groundwater is in hydraulic continuity across area with an
average head of 109.6 mAOD. Proposed finished road level is approximately 107 mAOD at its deepest
point in the region of the underpass, however this drops to the east to tie in with existing ground levels.
The risk of encountering groundwater during the construction of this particular element of the scheme is
therefore considered high.
Further hydrogeological assessment
Although there is a high risk of encountering groundwater at both locations during construction, the
hydraulic conductivity of the Glacial Till and Lower Lias Clay is thought to be relatively low. The geological
memoir states that the Lower Lias deposits are predominantly argillaceous (very fine-grained) in nature. In
several localities boreholes to depths of over 60 m have not encountered groundwater in these
formations. At best they are reportedly capable of yielding around 1-2 m 3 per hour, generally from thin
bands of fissured Limestone within the shales and clays.
A number of the GI borehole logs indicate that no groundwater was encountered during boring, however
groundwater is reported to have entered the exploratory hole overnight or following installation of the
standpipe. This suggest that even though the intercepted strata is saturated (i.e. lies below the water
table), the hydraulic conductivity of the Glacial Till and Lower Lias Clay is so low that the bores take a
significant period to fill to the standing water level. Based on this and the published data referred to above,
the volume of groundwater seepage into the cuttings is likely to be relatively slow.
6. Conclusions & recommendations
The risk of encountering groundwater during the construction of both the Swinford Road Cutting and the
A14-M6 Link is considered to be high based on the available information. However, the hydraulic
conductivity of the Glacial Till and Lower Lias Clay is relatively low; therefore the volume of groundwater
seepage into the cuttings is likely to be slow (around 1-2m 3 per hour).
Jacobs Engineering U.K. Limited
Jacobs_MemoA4.doc