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Gordon Bruce

Military Road Isle of Wight Presentation

Introduction

Good morning /afternoon.

My name is Gordon Bruce and today I am going to talk about the Military Rd on the Isle of Wight.

The A3055 (known as the Military Road) runs along the south-west coast of the island. It passes

over cliffs, which are up to 90 m high and forms an important infrastructure link between the

south and the west of the island, it is also the high point of the Round the Island tour on

account of the spectacular coastal scenery.

However, these cliffs are retreating as a result of coastal erosion at the toe, although the rates of

recession and the modes of cliff failure vary because of the different geological and topographical

conditions along the coastline. Despite having been constructed only 150 years ago there are a

number of locations along the coast where the Military Road has been severed from its original

alignment by coastal recession.

In the 1970s it was recognised that continuing coastal erosion would pose a threat to the Military

Road at a number of locations, and at Afton Down in particular. In addition to observations offailures of the chalk cliffs, instability of the cliff top had also led to the formation of fissures in

the down land slope below the road. These extended into the seaward edge of the road, and

repairs to the carriageway surfacing were regularly required.

As the integrity of the road could be compromised should further recession of the cliff top occur,

an early warning system (EWS) was installed in 1981 to safeguard users of the road following

discovery of the surface fissures. This consisted of a series of tiltmeters connected to automatic

traffic signals, which closed the road in the event of trigger levels being breached. These were

supplemented in 1997 by the addition of a series of extensometers and a settlement cell

installed seaward of the road to identify movement of the upper layers of weathered chalk

material. The instruments were connected to the emergency services and to a data logger, from

which the information could be reviewed remotely by the Isle of Wight Council.

In 1985 the former Isle of Wight County Council submitted a proposal for a diversion of this

section of road, which involved a new cutting further up-slope through the adjacent downland.

This area was recognised as being of international environmental significance, and a public

inquiry recommended that planning permission for the realignment should not be granted.

Therefore consideration has subsequently been given to extending the life of the road by

reducing the risk of cliff instability affecting the road and endangering its users.

Cost-effective measures to stabilise the existing cliff profile were unlikely to be practical given the

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height of the cliffs (.70 m) and were not developed further. Consideration was given to

providing a form of coastal defence to prevent continuing erosion at the toe. However, the

present cliff top position would continue to recede without additional stabilising measures, as

the upper sections of the cliffs would naturally regrade through weathering processes towards a

more stable angle. Furthermore, owing to the high-level environmental designations, it would not

be acceptable to build defences on the foreshore at the foot of the cliffs, and this option was

discounted.

Hence any measures to support the road needed to be based on stabilising the highway itself

along its present alignment. Highway stabilisation at Afton Down was not considered at the time

of the 1988 public inquiry. However, during the review of the failure mechanisms of the cliff in

1997,and considering advances in engineering technology, this option was considered feasible.

PROPOSED STRUCTURE

The objective of the proposed stabilising structure was to support the existing 6 m wide road

while minimising environmental im pa ct and allowing the natural processes of coastal erosion to

continue to remove material from in front of the structure. Prior to considering the range of

engineering options available and selecting the preferred scheme, the constraints needed to be

defined. These were primarily environmental, safety and financial, and included:

(a) minimising visual, ecological and other potential adverse environmental impacts

(b) minimising damage and restrictions on access to the coastal amenities during construction

(c) managing health and safety issues relating to constructing and decommissioning the works

adjacent to unstable cliffs as well as public safety during service

(d) working within financial limits for the investigations and works to be undertaken.

A number of alternative forms of retaining structure were considered during initial concept

design. These included a pile- supported slab using two rows of bored concrete piles and anin-

situ reinforced concrete deck, a reticulated mini-pile solution supporting the road on a series of

small-diameter piles, and a tied retaining wall. Preliminary design proposals for all three forms of

structure were costed, and the tied retaining wall was indicated to be the most economic

solution. Furthermore, it was felt to be the most appropriate in view of the limited access

available to the site, the need to limit disturbance to the cliff, and the requirement that the

structure be decommissioned at the end of its service life. Further development of this form of

retaining wall led to the choice of a wall comprising closely spaced bored piles.

The bored piles would initially be buried and would then be progressively exposed as cliff

recession continues. Based on the rates of recession assumed and practical and cost constraints,

the structure was designed fora

maximum retained height ofup to 10 m, corresponding to the

assumed worst case for the geometry of the cliff, and a nominal 50-year life. The structure

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extends for a total length of 285 m in two sections at Areas A1 and A2. This aimed to provide a

suitable buffer distancebeyond the extent of the structure to ensure that the road is adequately

protected against clifftop recession.

Owing to the variability in the localised rate and form of the recession of the cliffs, the whole

length of the structure was constructed to a design based on the worst assumed case for the

geometry of the cliff. The location of the structure was determined by identification of the area

most at risk, and the length constructed was governed by financial limits. When continuing cliff

recession results in the structure at any location becoming unserviceable with consequent

closure of the road, the structure will be decommissioned. However, additional sections of the

structure could be added to the lengths of the present structure to prevent the structure being

outflanked by erosion of the cliff adjacent to the structure

Geotechnical Design/Ground Engineering Issues

Rock Slope Stability

Coastal Processes

Fissures

Diverse Geology/ Possibility for inaccurate data collection

Soil stability under high loadings (plant etc.)

Chalk Rock mass different than expected

The geotechnical risk register

The Geotechnical Risk Register is the key in the geotechnical risk management process. It

systematically records and considers all of the risk identified in a structured fashion and ensures that

they are dealt with. Each hazard is considered as 4 steps:

The identification of the hazard

Assessing the probability of it occurring and its impact if it did

Managing the risk identified

Allocating responsibility and action

Risk = Probability x Impact

Good communication between client, designer and contractor is essential for this process to work.

When all parties are working together openly on the project there is a better chance of any risks

being identified and considered early enough to offer solutions, or put contingency plans in place.

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Degree of risk (Probability

* Severity)

Effect Action required

1-4 Trivial None

3-8 Significant Consider more cost effective solutions oralternatives at no additional cost.

9-12 Substantial Work must not start until risk has been

reduced. More resources are required.

13-16 Intolerable Work must not start until risk has been

reduced. Project should not proceed if risk

cannot be reduced.

Interaction Matrix

Sustainability

Sustainability was defined by the 1987 Brundtland Report as meeting the needs of the

present without compromising the ability of future generations to meet their own needs

A twofold strategy could be implemented in the Highway project in order to promote

sustainability. These include:

Mitigation to the environment

Sustainable Construction Methods and Building Materials

Mitgation To Environment

Environmental mitigation is a term used to describe projects or programs intended to offset

known impacts to an existing natural resource such as a stream, wetland, or endangered

species. To mitigate means to make less harsh or hostile. This means that in order to

prevent negative impact upon the Environment during the construction of the retaining wall

and also during the structures lifetime, a number of factors must be considered and a

number of measures implemented.

The first impact I wanted to consider was one that each and every person on the planet has

a part to play in mitigating: -

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Climate Change

There is a general consensus within the scientific community that Climate Change is

occurring due to carbon emissions. What cannot be agreed, however, if this increase is due

to human activity or is due to a natural cycle? While there is uncertainty regarding this issue

it had been deemed prudent to everything possible to reduce carbon emissions through

policy on transport and energy, and legislative controls such as planning and building

regulations. This is collectively known as Sustainability.

As the built environment is a huge consumer of resources and source of pollution, the

construction industry has a significant role to play. It is estimated that building constructions

use and demolition account for nearly half of the UKs carbon emissions. Carbon emissions

are generally used as a simplified or key indicator, of environment sustainability. More

sophisticated systems include Ecopoint system or Eco-footprinting.

Eco-footprinting states that:

The earths renewable resources are currently being consumed faster than

they can be regenerated

Three planets worth of resources would be required if all of the worlds

population had a westernized lifestyle.

The onset of climate change has serious implications for the UK building design. Below is a

list of these implications and the measures I have undertaken taken to mitigate against this:

. Minimizing soil movements around and off site.

Avoidance of synthetics chemicals, PVC, etc.

Designing material dimensions in order to limit off cuts and waste.

Specifying to reduce construction and package waste.

Using higher design loadings in order to maximize the life of the office

building and car park.

Recycling of construction waste

Other issues which I want to mitigate against during the Construction phase include:

Noise Pollution

Noise pollution during construction phase derives primarily from the mechanical plant

employed in construction and from the vehicles that transport materials to, from and within

the site.

Obviously, this would be difficult to mitigate against, as noise during construction is

inevitable. However, construction can be terminated at specified time in order to be less

antisocial in evening hours.

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Water Pollution

Construction site activity produces considerable potential for water pollution.

Construction involves the use of chemical agents, the storage of vehicles fuels, generation of

solid waste and major disturbance to soil.

The avoidance of chemical pollution can be achieved through procedure good practice in

the storage, transport and use of materials, and also proper risk assessments been carried

out and plan emergency immediate actions in event of spillage. Equally important is the

proper management of portable toilets on site.

Often, the most noticeable effect of construction is a potential large quantity of silt to enter

nearby water courses. Such potential must be taken into consideration in the excavation

and filling operations of the project, and also the management of water used on site. For

example, in the washing of vehicles and the cleaning of mud from the road surfaces, some

form of settlement trapping or filter arrangement would need to be put in place.

It is common for road surfaces to accumulate quantities of grit and chemical deposit from

vehicles during drive spills, which would be washed off in heavy rain. A Porus Asphalt

material could be used for the road in order to mitigate against these potential effects.