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By Peshawa Mahmod

University of Brighton

Measurement of Chalk discontinuities in the Peacehaven cliff

Introduction The discontinuities characterises in the Chalk is very important in cliff and foreshore stability in Peacehaven. In fact, data collected in Friars Bay Peacehaven by vertical scan line survey using normal Clinometric compass from the base of the cliff to the top. Different type of discontinuities was measured and plotted in Stereonet paper or by use DIPs software for ease interpretation. This report is a part of a cliff slop stability analysis and it describes and discourse possibility of failure in this cliff.

Geology of the Peacehaven cliff (Friars Bay) The majority of the down land of Sothern part of England composed of Chalk (Lord et al, 2002). The Chalk includes of a sequence of cocolith (planktonic algae) and biomicrities micro-porous Limestone to more than 425 m thickness (Young and Lake, 1988). The cliff of Friars Bay in Peacehaven represent outcrop of this Chalk of a period history dating about 80 million years, late cretaceous epoch of campanian stage (Brenchley and Rawson, 2006).

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By Peshawa Mahmod

University of Brighton

Type of discontinuities in Chalk The most important characteristic in the Chalk to engineering properties are style of discontinuities, frequency, aperture and weathering (Lord et al, 2002). In the Friars Bay cliff this type of discontinuities are seen. 1- Fracture caused by weathering: In the top of the section below the top soil about 10m of massively intense fracturing Chalk which is decrease and reduce the quality of the Chalk in this section. This weathering might be due to the mechanical weathering by of plant roots and frost when this Chalk was exposed to the surface (Lord et al, 2002).

2- Bedding plan and marl seam: Generally, bedding plan in this area is horizontal or inclined to about 5 degree in the direction of North. Nodular flint concentrated in the boundary of the bedding plan of pebble to cobble size. In addition with the sequence of the Chalk, Marl seam seen also with different thickness reach to 20 cm in some place of very tight aperture; however, other marl seam is open called griotte. This Marl seams are important to both engineering and hydro geological characteristic of Chalk.

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By Peshawa Mahmod

University of Brighton

3- Inclined joint: These types of joints are located between the bedding plans and it is in general, about 40-50 cm of tight no filling aperture, the inclination of about 40-65 degree. 4- Conjugate joint: These types of joints are the abundance type that seen in this cliff and have a small and large distance. The joints are of more than 65 degree and happen due interaction of the plans in opposite direction. The joints are opening with rough surface and some time filled with clay and Chalk grain Brescia.

Data interpretation In this particular area the direction of the cliff face is about 188 degree to the south and the dip is about 5 degree. The dip of bedding plans is about 5 to 4 degree to the direction of north and to the opposite direction to the cliff face. In fact, there is no possibility to failure caused by bedding plane. There are some joints that have a dip direction to the same dip direction of the cliff face but with less degree; this joints might cause sliding of the Chalk in the direction of the cliff face. However this could effected is small scale because of small distance of this joints that reach to about 50cm. The great probability of failure could cause due to the intersection of joint plan forming conjugate wedge failure to the direction of cliff face. This joints are intersected in the direction of the cliff face caused several failure in this area. 3


By Peshawa Mahmod

University of Brighton

Discussion Different types of discontinuities appear in the Chalk that controls the cliff stability in this area. The rock sliding has a possibility to occur but is small scale, however, the conjugate wedge failure is the highest possibility to occur in different scale also the slickenside show the same probability in the cliff face. Moving to the top of the cliff the highly weathered Chalk could cause problem in any construction and this weather part could be taking in account in any major constructions.

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By Peshawa Mahmod

University of Brighton

Bibliography

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Brenchley, P. J. and P. F. Rawson 2006, The Geology of England and Wales, (2nd ed.), London, the Geology Socity.

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Lord A. J. and Mortimore R. N. (2002), Engineering in Chalk, London, CIRA.

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Young, B. and Lake, R. D. 1988, Geology of the Country around Brighton and Worthing, (1st. Ed.) British Geological Survey London, Crown.

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Measurment of the Chalk Discontinuities in Peaceheaven