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EARS5011 Engineering Geology and Site Investigation
EARS5191 Ground Investigation Methods
Broadway Area Site Investigation
This report outlines the geology, geomorphology and the active processes that
take place in a study area that is located nearby the village Broadway. Abstracts of the
existing literature, aerial photography, and field mapping have been used in order to plot
morphological maps and to identify any potential geotechnical problems. This study
highlights the significant role that the stratigraphic succession plays in the creation of
a variety of landslide types that affect the slopes, above the village Broadway.
Broadway study area
The study area is located on the escarpment slopes to the east of the village of
Broadway (80 m-100 asl), in the vale of Evesham. It extends from the edge of the
village to the top of the escarpment (~260 m asl), encompassing the A44 Broadway
road in the south and Colliers Knapp (~180 m asl), to the north, (Fig. 1).
Fig. 1 Satellite image of the study area.
Topography of the study area
The nature of the underlying geology is reflected in the topography of the site,
(Fig. 2). The soft clays of the Lower Lias form an area of low ground within the base
of the valley, while the more resistant bands of the Middle Lias silts, form the slopes
of the escarpment above this Lower Lias valley. The change from Lower Lias clays to
Middle Lias silts and sands gives rise to a change in slope which is often associated
with the emergence of springs. Towards the top of the Middle Lias the presence of the
Marlstone Rock Formation produces a prominent escarpment. This is evident at
Colliers Knapp where topography changes, forming a distinct flat lithological bench.
Colliers Knapp
A44
Broadway
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Middle Jurassic
Upper Lias
Middle Lias
Lower Lias
Fig. 2, (above) The nature
of the underlying geology is
reflected in the topography
of the site (Modified by
Malcolm Whitworth, 2006).
Fig. 3, (left)
A distinct lithological
bench.
Above this bench the clays of the Upper Lias occupy a narrow zone of ground
rising gradually, small benches are present, as the result of lithological variability
within the Upper Lias (Fig. 3). This formation is overlain by limestones of the Middle
Jurassic Inferior Oolite Group.
Geology – Geomorphology of the study area
The geology of the study area can’t be observed on the surface due to the
presence of Quaternary deposits and vegetation. Existing literature was used in order
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to interpret the underlying geology. During the field mapping artificial cross sections
were used in order to help with the confirmation of the geology.
The geology of the study area is consisting of Lower – Middle Jurassic strata
(Table 1). These strata have a gentle easterly dip, although cambering and faulting
have produced local variations (Fig 4).
Unit Age (Ma) Thickness Lithological description
Solifluction deposits 0.1 ~2 metres Limestone rock gravel/cobbles within a silty
clay matrix
Lower Inferior Oolite 183 25 metres Oolitic and sandy limestones (Outcrops
outside the study area)
Whitby Mudstone Form. 187 6 metres + Lower part outcrops at Broadway. Consists
of dark grey, silty clay and strong oolitic
limestone
Marlstone Rock
Form. 193 7 metres Grey and brown ferruginus, fossiliferous and
sandy limestones
Dryham Form. 193 55 metres Sequence of moderately weak orange-brown
sandstone and subordinate bands of hard
laminated silty clay and clayey silt. Capped
by strong brown closely-jointed fossiliferous
limestone (Marlstone rockbed)
Charmouth Mudstone
Formation 200 40 metres + Hard, dark grey silty clay
Table 1, The Jurassic stratigraphy of the Broadway field area (after Sumbler et al.,
2000)
The geomorphology of the Broadway study area indicates that the slopes of
the escarpment above Broadway are affected by a range of landslide mechanisms;
these slope failures can be described as rotational landslides and mudslides.
The presence of springs is noticeable in the study area. In the centre of the
valley it can be observed that springs have a linear provision, this is best explained by
the presence of a fault (E-W), which cuts through, all of the stratigraphy and displaces
the Marlstone bench. That faulted zone brings into direct contact impermeable with
permeable formations and as a result springs emanate from the ground.
This study has also identified the important role that stratigraphic succession
plays. The lithological variability within the formations anticipates distinct geological
permeability boundaries, between more permeable and less permeable formations.
This mechanism creates spring lines and subsequently due to the nature of the ground
mudslides.
Furthermore the presence of the Marlstone Rock formation acts as a
lithological bench due to being much stronger than the strata above and below it and it
creates lithological benches and rotational landslides.
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Fig 4 Geological map of the area.
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In addition the presence of weak surface materials created and modified under
a periglacial climatic regime, under certain climatic conditions is anticipated to create
solifluction features.
Taking into consideration all the mentioned parameters slope instability
problems are expected for the study area.
Active Processes – Evidence
Landslides
This study has identified a variety of active processes that take place in the
area. Those processes are landslide types that affect the slopes of the Cotswolds
escarpment. According to the landslides the study site was divided into five areas (A,
B, C, D, E), (Fig. 6). It is noticeable that landslides in areas B, C, D, E are associated
with seepage and spring lines resulting from the underlying hydrogeological
variability.
Fig 5 The typical landslide sequence observed in the Cotswolds. “Inland landslide
hazard assessment using walk over surveys and geomorphological mapping
techniques.” Powerpoint Presentation Whitworth,M 2006
The typical landslide sequence observed in the Cotswolds (Whitworth, M., 2006)
consists of:
• Cambered strata in the Inferior Oolite which caps the upper part of the escarpment.
• Zone of large scale rotational landslides below the Inferior Oolite.
• Zone of successive shallow rotational landslides.
• Extensive shallow mudslides and translational landslides.
Rotational landslides The dominant geomorphological feature of the Broadway study area is the
large rotational landslide at Colliers Knapp (area A), (Fig. 6). The landslide extends
from the Lower Lias into the Middle Lias. There is evidence that a new rotational
block is created (concave break of slope in Colliers Knapp) (Fig. 13). Furthermore
there is a building, of recent age, at the bottom of the escarpment that shows evidence
of structural damage. Taking all this evidence into consideration it can be said that it
is an active landslide.
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Fig 6. Lansliding areas in investigation site (Image from Whitworth, M. 2006)
Mudslides Mudslides are a common landslide type in the study area, often associated
with spring or seepage zones. Poor drainage characterizes such types of landslides. It
noticed that springs that emanate from Marlstone formation can be associated with
the hummocky ground.
On the south facing slope below Farncombe House (area B) (Fig. 6), there is
an extensive mudslide system. The lower part of it may have been reactivated more
recently, by a spring which occurs half way down its length. Evidence for this recent
activity includes the disrupted ridge and furrow lines in the valley bottom.
In the central part of the study site (area C), on the west facing slopes of the
valley, a distinct mudslide lobe can be observed emanating from a spring line that
doesn’t seem to trigger any extensive shallow mudslide activity, since a field of
ancient ‘ridge and furrow’ above this landslide still remains intact. However
knowledge of its presence is important because it is located nearby a faulted zone that
has displaced the strata approximately 70 m laterally.
On the north facing slopes, (area D), there is an extensive active mudslide,
which follow the line of a spring down slope onto the lower slopes, where the existing
stream is deflected by the toe of the mudslides. The hummocky ground creates a
turbulent texture. This hummocky topography indicates a possible recent phase of
shallow mudslide movement.
Solifluction features
The site has been subject to periglacial (cold non-glacial) conditions during
the Quaternary. This cold climate created a characterised mantle of periglacially
derived slope deposits including frost shattered bedrock (Fig. 9). These deposits are
of varying thickness and are prone to slope movement, since they tend to behave in a
plastic way, when the pore pressure is high. In area E solifluction features can be
observed.
A
B C D E
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Fig. 7 Geomorphological map of the area
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Fig. 8 Morphological map of the area
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Fig. 9 Exposure of frost shattered bedrock within the Marlstone Rock can be seen in
the ditch above Colliers Knapp.
Recent Landslide Dating Method
It is clear that slope movements in the study area have disrupted the remains of
ancient ridge and furrow cultivation systems. The age of the landslides can be
estimated by study of the remnants of ridge and furrow farming. According to the
Enclosure award map for Broadway parish (WRO BA 368, r264.72) most of the fields
in the Broadway area were enclosed in 1771 (Whitworth, M. et al 2000). The ground
where the ridge and furrows remain straight and uniform must have remained stable
since 1771 (assuming no more furrows were dug post-1771) and the fields where the
ridge and furrows have been disturbed must have moved since 1771( as noted by
Chandler 1970) (Fig 10).
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Fig 10 Map showing disturbed and undisturbed ridge and furrow cultivation remains.
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Geotechnical problems and parameters
The study area is characterized by poor drainage and mass imbalance.
As mentioned before the lithological variability within the formations
anticipates distinct geological permeability boundaries, between more permeable and
less permeable formations. The permeable formation behaves like aquifer and
discharge water at the contact with the impermeable formation. That mechanism
creates spring lines. The soil in that area becomes more saturated; its strength
becomes diminished, which initiates mass movements. This scenario represents areas
B, C, D, E, (Fig. 5).
In area A there is the dominant geomorphological feature of the Broadway
study area a large rotational landslide at Colliers Knapp. As mentioned it is active but
fortunately there no nearby infrastructures, the slope’s angle is low so it isn’t an
urgent geotechnical problem, (Fig. 11).
Fig. 11 Colliers Knapp
Areas D-E; these areas (Fig. 12) show the highest and most intense recent
shallow mudslide activity. There must be a parameter that deteriorates the ground
conditions. This new parameter is the A44 Broadway Bypass. It is very possible that
there was a change (increase) in the slope’s angle during the construction, or that the
slope was loaded by the embankment materials used for the road’s construction,
which led to the slope’s unstable condition. Furthermore the A44 construction must
have deteriorated the drainage of the overlying slope, which anticipates increased
water seepage and increased spring flow in the lower areas.
Fig. 12 Areas D-E
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Site investigation plan – investigation of the geotechnical impacts of the active
processes
Having located the active processes in the study area and the mechanism that
creates them a ground investigation plan can be proposed (Table 2), taking
consideration two factors;
In the study area the majority of site investigation can be carried out without
significant use of rotary drilling techniques
The majority of classification and index testing will be carried out on samples
taken from boreholes, trial pits and shafts.
Technique Area A Area B Area C Area D Area E
Geophysics √ √ √ √ √
In situ examinations √ √ √ √ √
Probing √ √ √ √ √
Light percussion
drilling √ √ √ √ √
rotary drilling √ √ √ √ √
Table 2 Proposed site investigation techniques
Geophysics can be carried out as a first attempt to acquire a basic idea of the ground
conditions in the study area. Geophysics can contribute to assess the ground condition
beneath the site with a low cost, (“For the same amount of dollars as one drill hole,
the equivalent cost geophysical survey can collect data over 1 to 160 sq. km.,
depending on the method,” GeoExplo Ltd).
Different Geophysics techniques can be used to assess ground condition data;
Electrical resistivity
Assessment of the lateral variability, of the near surface. Useful for the upper
Lias formation, which consists of many layers with different stiffness.
Classification of the subsoils, into groups with similar geotechnical
characteristics, which will lead in the distinction between cohesive and non
cohesive.
So this technique can be applied in the South-West-North facing slopes that
consist of cohesive – no cohesive clays
Seismic Refraction techniques
This technique can be applied in the South-West-North facing slopes, so as to
determine the position of the rock head which will indicate the appropriate areas to
start examination in situ.
Examination in situ can lead all investigations, or subsequent geophysics. Trial
pits and shafts provide by far the best method of recording both the vertical and the
lateral ground conditions. The study area makes trial piting, difficult in certain areas
(B, C, D, E),since there is a great hazard of collapsing of unsupported trench due to
the nature of the ground. Nevertheless with a low cost, shallow trial pits and shafts
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(with a parallel direction to the slope) can be constructed in those areas, in places that
are expected to meet rockhead.
Probing can be used as a qualitive guide to the variations of ground conditions,
providing valuable profiles and assessing the variability of the site. It is a cheap, rapid
test and it can be deployed in the whole study area.
Light percussion drilling (Shell and Auger) can be performed in the majority of
site investigation (soft clays), while rotary drilling is easier to use in area A (strong
intact rock).
All the above techniques must aim in
Mapping of the impermeable Marlstone Rock formation which as mentioned
plays a significant role in the study area.
Assessing ground moisture content
Piezometers must be deployed in the areas with existing springs. Those
piezometers should be monitored periodically in order to observe the ground water
table and to estimate the critical level that triggers the mass movements (shallow
mudslides and/or solifluction). The A44 Broadway Bypass and the nearby areas must
be monitored, since they are expected to show intense shallow mudslide activity;
inclinometers, extensiometers, crack meters and other monitoring instruments must be
deployed along A44 in the problematic areas, (areas D, E)
Potential changes in the site over 120 year time scale
In the future, over 120 years time scale it is highly possible that the north and
the south area of our study area will have been affected by the active processes that in
the present take place. This will be area A and areas D-E. In area A (Colliers Knapp)
a new rotational Block will have been created, where is now the concave break of
slope. This means that the back scarp is going to extend upwards to the Middle Lias.
(Fig. 13). As a geotechnical problem, it is of little significance, since the hazards are
already spotted and it is highly unlikely to take place any further infrastructure
development in that area.
Fig A concave break of slope in Colliers Knapp, which is highly possible to evolve
into a rotational block.
The areas, bordering the A44 Broadway Bypass, (areas D-E), are expected to
show intense shallow mudslide activity. This scenario is based on observation of
deformation which has already occurred in the area since the bypass was constructed
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in the mid 1990’s and the assumption that the climate conditions and the deformation
rate will be the same. As a geotechnical consideration this area is of high risk since it
will affect A44 Broadway Bypass.
To conclude within the next couple of decades subsidence may start to occur
underneath the carriage way and cracks appear in the road surface. That is why proper
investigation of ways to improve the area’s drainage must be carried out.
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References
Clayton, C. et al. 1995. Site investigation. 3
rd Edition. Blackwells.
Cooke, R.U. @ Doornkamp, J.C 1990. Geomorphology in Environmental
Management 2nd
. Clarendon Press, Oxford.
Whitworth, M. C. Z., Murphy, W., Giles, D. P. and Petley, D. N. 2000. Historical
Constraints on Slope Movement Age: A Case Study at Broadway, United
Kingdom. Geographical Journal. 166 (2), 139 -155
Whitworth, M. C. Z., Giles, D.P. and Murphy, W. 2002. Landslides of the
Cotswolds Escarpment, Broadway, Worcestershire UK. Journal Proceedings of
the Cotteswold Naturalists Fieldclub, XLII Part 2, 118 – 127.
www.geoexplo.com
www.googleearth.com