Britain experiences a large number of landslips annually and slope movement within the Ironbridge Gorge greatly contribute to most occurrences. This is due to the cutting of the gorge by Severn River into a geological structure of weak rock materials. In addition, the continued riverbanks erosion has weakened the gorge's sides making it prone to sinking. Jackfield is a rural area located on an unstable part of the Ironbridge Gorge, where several landslips have occurred. Moreover, Jackfield is prone to flooding by the river from time to time but, it is the 1952 landslip that devastated Jackfield. The situation at Jackfield is compromised by mining, which has created voids at depth causing ground subsidence, plus the loading of the slopes with mine waste Therefore, this paper highlights geological information of Jackfield which favors the high number of slides occurring.
Geological Model for Jackfield
Location of Jackfield
The Jackfield site is bounded by the River Severn to the north, the Jackfield Tile Museum to the west and the footbridge across the Severn near The Boat Inn to the east; the southern boundary runs approximately NW-SE through Woodhouse Farm. The slopes on this side of the river are used for pasture and have little or no trees except for either side of Salthouse Road and the lower valley sides which are situated towards the westerly and easterly boundaries.
Jackfield lies on the southern bank of river Severn within the Ironbridge gorge. The village is located in an unstable part of the Ironbridge Gorge, where several landslips have been recorded (Scatena & Lugo, 1995). In line with this, in April 1952, a mega landslip occurred in the Ironbridge Gorge at Jackfield leading to the destruction of more than 20 households with other houses descripted on the paper. Outlined below are the locations of selected landslides in the UK.
Figure 1 -- Locations of selected landslides in the UK.
The Geology of the Gorge
The geology of the gorge is very complex with alternating beds of permeable rocks such as limestone and sandstones, and more impermeable layers of grey shale, clays, and thin beds of coal seams as well as ironstones. The frequent layers of clay, often 5 metres thick produce slip zones where one layer can slide upon another with through flow water acting as a lubricant.
The slope in Jackfield has been subjected to rapid degradation by melt waters during the last glacial period and the high groundwater levels associated with post glacial conditions (Bentleya & Siddle, 1990). This makes the riverbanks to be weak and can easily slide.
Figure 3 -- Main features of a landslip.
In addition, river erosion undercut the valley sides and removes support at the slopes along the Salthouse Road. Besides, valley development has initiated a cycle of land sliding activity as slope processes strive to reach equilibrium (deBoer, 1992). As opposed to river and rain, the accelerated degradation of the slopes is aided by the relatively incompetent geology comprising embedded clays, mudstones, sandstones and coals, and the geological dip of the beds.
In Jackfield, the dip of the strata is shallow towards the east which could facilitate ground movement towards the river. Furthermore, the geological structure in the form of faulting may be providing conduits for groundwater and back scraps for slope movement, for example The Doughty and Tuckies Faults.
The ground movement observed from the geomorphological mapping identified land units of multiple rotational back tilted blocks of various stages of degradation and translational failures with secondary rotational failures.
The presently active translational failure in the Salthouse Road sub-unit is a reactivation of the 1952 slip. Ground movement of this area is currently occurring, evidenced during a recent walkover. The failure in 1952 was triggered by heavy rainfall and subsequent high groundwater levels; this is likely to have been the trigger for the most recent ground movement, although no data is available for actual rainfall in this period.
The failed area of the lower slope contains substantial made ground deposits where the slip plane lies at the interface with the underlying clay and further upslope the slip plane can be observed at ~5m depth within the weathered clay.
The areas of instability coincide with areas that have been mined at shallower depths than Area B. For Red Tile Clay and Sulfur Coal (Meyer, Schuster, & Sabol, 2002). The combined effect of more than one seam being mined may have involved greater bed separation of the overlying strata and subsidence, which has reduced the integrity and strength of the material resulting in the localized instability along the Salthouse Road subunit.
The area of the 1952 slip is located on the outside bend of the river where more turbulent water erodes the toe and removes support of the ground upslope. Borehole instrumentation has availed that the Salthouse Road sub-unit is currently active and areas outside this sub-unit are relatively stable.
In this region, the groundwater levels are 5m bgl on the lower slope of the Salthouse Road sub-unit and 2m to 3m bgl outside this unit. This suggests that given the slopes are currently active; slope stability is very sensitive to increases in the groundwater level. The sensitivity of groundwater was analyzed using slope stability software and found that areas of the slope are prone to failure with a rising water table.
Furthermore, perched water tables may occur above the in situ clay material within the relatively more permeable made ground which will further reduce the strength of the material.
According to survey monitoring has identified up to 5m NNW movement since 1994 and almost 850mm of settlement in the slipped area. The average rate of movement is ~0.5m / year. The principle cause of ground instability is considered the erosion of the slopes by river action and the high groundwater levels following periods of heavy or prolonged rainfall. The consequences of mining in the area may also be affecting the site.
The movement of the debris slide in the area of the Salthouse Road sub-unit is slow with accelerated periods of movement following high water infiltration. Survey monitoring, during 2003, has identified lateral ground movement to the NE (towards the river) of up to 0.2m and settlement of up to 15mm. Inclinometer monitoring (CP 14) identifies made ground to be slipping on in situ material, which may be subject to sub-artesian water pressures. Ground displacement is ~20mm downslope; all other boreholes show negligible displacements.
A further area of concern is in the Jackfield site, where a section of slope previously failed in 1952-53. The major failure zone lies within 5m of the surface where the slope materials comprise weathered clay and mine spoil (Wolman & Gerson, 1978). Although, the slope is relatively gentle in inclination, erosion at the toe of the river bank by river flow is removing downslope support.
You’re 81% through this paper. Sign up to read the full paper.
Sign Up Now — Instant Access Already a member? Log inAlways verify citation format against your institution’s current style guide requirements.