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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.

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