Report on geotechnical field trip to isle of wight

Report on Geotechnical Field Trip to Isle of Wight

1- Overview: Geology of the Isle of Wight

Fig.1 Geological Map of the Isle of Wight with its legend.

The Isle of Wight is located in the South of England, between 5 and 8 Km from the coast in the English Channel. As the rest of England, the Geology of the Isle is mainly sedimentary. Specifically materials from the early Cretaceous (127million years ago aprox.) and middle Palaeogene (30 million years ago aprox.) are found. The map above shows the distribution of the different rocks types. The north half of the isle is formed by Paleogene, while in the half south there are Cretaceous Rocks.

2- Alum Bay

At Alum Bay, we find groups belonging to the Eocene and Cretaceous, being these two geological ages separated by anunconformity (although we cannot find an angular discordance), which are located in the contact between the Cretaceous Chalk formation and the Eocene Reading Beds.

Fig.2 Picture of Alum Bay from the north side where we can see the chalk formation. In general, without going into detail we can say that these materials are found along from the beach. Period Formation Barton Clay Materials Mainly darkclays Others Presence of lignite bands interbedded and ironstone. Presence of iron sandstone and lignite bands Some clay bands interbedded. The sands colour is more striking Presence of a claystone flint grown in the spot Big translational slide Presence of flints. Presence of pipes due to the seepage filled with grey sand

Eocene

Bracklesham Mainly light white-yellow sands, Group with darkclay bands interbedded Bangshot Yellow -grey sand Sands London Clay Dense dark brown silty clay Reading Mainly red-brown silty clays Formation White Chalk Chalk

Cretaceous

Concerning the stability of the cliff there is along the bay; we conclude that it is not good. This is due to the presence of several landslides across the section, many of them small, but which still seem active today. Inthe section we have outlined, we have only represented the most notable and potentially dangerous slides, these being:    Translational Slide in Reading Beds formation. Mudslide in London Clay group. Other minor debris slides which could become larger in the future.

With regard to the comparison between our section, and the printed section, we believe that while the second has a more accurateand comprehensive information regarding the geological knowledge, our section is more useful in engineering work, because it reflects the geologic information needed for the job, without getting into useless details, and moreover it locates and describes the several slides in the section. 3- Military Road

The Military Road, located on the west side of the Isle of Wight, runs almost the entirecoast from north to south in this part of the isle. Since its construction has been a path of great importance for the islanders because of its beautiful views and important tourist impact these have.

Fig. 3 – Military Road View at Afton Down The problem of this ancient route is that the erosion caused by wind and sea on the coastline is endangering its course, and as a consequence of theirimportance is not considered the fact of changing its route to the inside of the island. So the decision was taken to protect the road to longer term possible, provided that the measures used have no environmental impact. The road is affected by this problem in several points, which will analyze the following two:

3.1- At Afton Down

In the near Afton point we have to analyze, the road is overa chalk cliff towering 75 to 90 meters above sea level. In the distance that separates the road from the edge of the cliff we could see very clearly the existence of large cracks several meters long, and no observable depth or 1 to 3 meters depth.

Fig.4 View of the cracks As mentioned above, at this point the road runs over the formation belonging to the Cretaceous chalk found in the...