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Mr Bellis
Proposed Development on Switzerland Road
Feasibility Study
RECEIVED ON
24 JAN 2006
DEPT OF LOCAL GOVERNMENTS AND THE ENVIRONMENT
| Received 24-1-06 DOLG06 00122 Town and Country Planning Act 1999 PLANNING and BUILDING CONTROL DIRECTORATE | ||
| Decision | Date and Office Initials | |
| Initial | ||
| Appeal | ||
| Amended | ||
| Returned |
Mr Bellis
Proposed Development on Switzerland Road
Feasibility Study
November 2002
Ove Arup & Partners International Ltd Cameo House, 19 Duke Street, Douglas, Isle of Man, IM1 2BB Tel +44 (0)1624 673445 Fax +44 (0)1624 681559 www.arup.com
Job number 68990/14/DAS/PE
| Page | |
|---|---|
| 1. INTRODUCTION | 1 |
| 2. THE SITE | 2 |
| 2.1 Site Description and Location | 2 |
| 2.2 Geology | 2 |
| 2.3 History | 2 |
| 2.4 Site Walkover and Previous Site Investigations | 2 |
| 3. OPTIONS FOR THE PREPARATION OF A DEVELOPMENT PLATFORM | 4 |
| 3.1 General | 4 |
| 3.2 Overall Stability | 4 |
| 3.3 Level Development Platform – Anchored Retaining Walls | 4 |
| 3.4 Terraced Platforms | 6 |
| 3.5 Summary and Recommendations | 7 |
| 4. FURTHER SITE INVESTIGATION | 8 |
Mr Bellis proposes to construct a private dwelling on the hillside behind Erin Brae in Douglas on the Isle of Man. Arup (Isle of Man) have been appointed to prepare a Feasibility Study for the development.
This report presents the findings of a brief desk study and site walkover, and outlines possible options for the preparation of a development platform. Further site investigation will be required for all of the proposals, which is also discussed.
The site is situated at the northern end of Douglas at OS Grid Reference SC 390 772; see Figure 1. The site is bounded by a lane to the northwest, which runs between Switzerland Road and Little Switzerland, the house and garden of Erin Brae to the south-east, newly constructed apartment blocks called Heritage Homes to the north-east and Piccadilly Court to the south-east.
Until a couple of years ago the site was well kept gardens with a path meandering up the slope from the rear of Erin Brae to the access road. However more recently it has been allowed to become overgrown and is now covered by dense, rough vegetation. In order to form the path the site was once cut into terraces, each around 2 to 3m high. Whilst many of these are overgrown, rock and glacial deposits were exposed at a few locations. On either side of the site rock faces are exposed which have been excavated in order to extend the building platforms for recent developments. Some rock retention works have been carried out on both of the rock faces in the form of rock anchors and buttresses. Anecdotal evidence has revealed that a rockfall (or landslip) occurred during the construction of the apartments to the north east of the site.
The geology of the site was studied on the published 1:50,000 geology map and the associated memoirs. The map revealed that the site was underlain by boulder drift, which in turn was underlain by the Lonan and Niarbyl Flags of the Manx Slate Series of the Cambrian Era.
The memoirs report that the prevalent dips of the Lonan Flags is high, usually ranging from 45° to vertical, generally in a SE or SSE direction, except in a narrow belt running from Clay Head to Douglas Head where they are in the opposite direction. An inspection of the adjacent rockfaces confirmed the direction of the dip was SSE, whereas on the site itself the dip direction appears to be WSW.
The history of the site was studied on maps dated 1869 and 1966, see Figure 3a and 3b.
In 1869 the site was occupied by gardens associated with Erin Brae. The site was terraced and the path was present following a similar alignment to today. There were no significant changes to the site by 1966 with the exception that the slope shown as being slightly wooded at this time. In 1966, Rose Villa and a garage were present to the west of the site, however these have since been demolished and the adjacent site is now occupied by apartments.
On the 2 September 2002 an engineer from Arup Isle of Man carried out a site walkover and inspected from the surface a trial pit which had been excavated near to the crest of the slope. The trial pit had been dug a couple of weeks previous to the walkover, therefore the sides had collapsed in and it was too deep to enter safely. The client had provided photographs of the trial pits which were inspected. From the photographs it appeared that rock had been encountered at the base of the pit at a depth of about 1.5m.
During the site walkover a number of exposures were logged along the sides of the terraces. These are shown on Figure 2. Generally the material exposed was highly weathered Manx Slate comprising dense, fine to medium gravel in a slightly sandy, silt matrix. The layering of the bedrock structure was evident. At location 4, at the base of the lowest terrace, moderately weak to moderately strong, thinly bedded, grey siltstone was visible. In places, this had weathered to a hard silt. The bedrock was generally dipping at about 10°/250°, i.e. in a WSW direction.
To the north of the site beyond the access road the ground surface was very variable. A knoll, possibly of rock, was evident on the western edge which became a bowl shaped feature towards the middle of the site. The surface then steeply dipped into a steeper bowl on the eastern side. These could possibly be old quarries, however it was not possible to gain access into this area to inspect any exposures to confirm this theory.
A few years ago, apartment blocks were constructed on the northern side of the site by Heritage Homes. Arup searched the Building Control records for any site investigation reports. The Building Control Officer believed that a site investigation was only carried out following the landslip. However they did inform Arup of the sequence of events. Heritage Homes apparently removed the toe of the slope to extend the platform. Following a weekend of wet weather, a landslip occurred. Heritage Homes called the fire brigade to wash down the slope which caused further landslips. These began to threaten the stability of the lane at the top of the slope and the underground services within it. At this stage, it was thought that a site investigation was carried out and a solution designed which involved concrete buttresses, rock anchoring and meshing.
An inspection of the exposed rock face behind Heritage Homes Apartments revealed moderately weak, generally medium to thickly bedded Manx slate, however occasionally very thickly bedded, with bedding dipping at 65°/165, i.e. SSE.
Due to the steepness of the site, the options available for the preparation of the platform are limited. Access for machinery and temporary works will need serious consideration when deciding upon the final solution. Also it is important that the cost of any temporary works is included in the overall build cost, because for some of the options they could be substantial. All the solutions will require further site investigation and this is discussed in more detail in Section 4.
The overall stability of the slope would need to be considered for all of the options. There are two types of failure of the slope which would need to be addressed during the design. These are:
Based on the information gathered from the site walkover, it appears that the upper part of the slope is covered by a thin layer, approximately 1.5m thick, of superficial deposits overlying the bedrock. There is a possibility that these deposits could slide over the bedrock surface and place an additional force on the back of the lower retaining wall. Therefore it would need to be founded on or anchored into the rock to prevent them also sliding over the surface of the rock. Therefore the anchors may need to be designed to not only stabilise the wall but also retain the wedge of superficial deposits lying over the rock.
As discussed in Section 2.4, the exposed rockface on the adjacent site revealed steeply dipping, clay filled joints which daylight in the face of the slope. The clay creates a surface with a reduced frictional resistance compared to the intact rock mass, therefore a slip surface can form. If any slabs of rock daylight in the face and a release surface is also present then blocks of rock can slide off. Therefore it may be possible that a deep seated failure could occur which could jeopardise the stability of the platform, however this issue would be addressed during the site investigation.
Following the investigation, if the stability analysis revealed that either type of failure could occur, the anchors shown on the attached sketches for all the options would need to be increased in capacity to provide restraint not only to the walls, but also the slope, and the length of the anchors may also need to be increased to extend beyond any potential slip surface within the rock mass.
A level development platform could be formed by constructing anchored retaining walls. Obviously the wider the platform, the greater the height of the retaining walls and so more expensive to construct.
In order to construct a 14m wide development platform, anchored retaining walls up to 6m high would be required; see Figure 4a. Whilst this option would utilise the whole of the potential area, both significant temporary works and earthworks would be required.
In order to provide support to the access road the upper retaining wall would need to be constructed using a top-down construction. It is likely that the face would need to be anchored and meshed in the temporary condition, then the anchors extended to pass through the wall in the permanent case. The anchors would pass beneath the access road and possibly into the land beyond. If this land is not owned by Mr Bellis then permission would be required from the landowner. The anchors would also need to be installed to pass beneath any services in the road.
Significant earthworks would also be required. Much of the excavation works is likely to be in rock which would require breaking out. Due to the narrowness of the site, it would be very difficult to create an access ramp down onto the lower terrace, and the access from the bottom of the slope would be via a narrow passage through the garage of Erin Brae and up a series of steps.
Consideration would also have to be given to the foundations of the proposed dwelling as half of the building would be on bedrock whilst the other half would be on backfill to the retaining wall, or glacial deposits. It is likely that the foundation construction should comprise ground beams spanning from the rock at the back onto the retaining wall with a suspended floor construction. However, this would need to be confirmed during the detailed design.
Therefore whilst this option would provide the largest development platform it is not the recommended solution as it is likely that the cost of the temporary works would be far more expensive than the gain of a few metres of building platform. In addition, health and safety would have to be given serious consideration during the construction of this works.
The advantages of this option are :-
The disadvantages of this option are :-
In order to construct an 11m wide development platform the anchored retaining walls would need to be approximately 4m high; see Figure 4b.
Whilst the development platform is reduced by about 3m the extent of the works is still quite significant. The retaining walls would need to be anchored into the bedrock and substantial amount of material would need to be removed from site. Significant temporary works would be required in order to construct the lower retaining wall as this is required half way up the slope. It is likely that a bench would have to be constructed below the lower retaining wall on which to sit plant. The retaining wall could then be constructed in lifts being backfilled and anchored as it increases in height. The anchors would need to be at a depth which would pass below the foundations of the proposed building.
The advantages of this option are that both the temporary and permanent works are not as large as for Option A. Therefore the cost of constructing this option would be less. However
the same health and safety and construction implications as discussed for Option A would still be relevant.
In order to create a 9m wide platform the height of the retaining walls reduce to 3m. At this stage it is assumed that these would require anchoring especially the wall at the lower level. However unlike options A and B where it is likely that two rows of anchors would be required for each retaining wall only one anchor is likely for this option. However this would need to be confirmed during the detailed design, to ensure that sufficient toe restraint can be achieved by embedment of the wall. If this is not achievable then anchors would be required, see Figure 4c.
Since the wall is only 3m high it may be possible to cast the upper wall in bays approximately 2-3m wide. However this would need to be confirmed following a site investigation to determine the rockhead profile. If the rockhead is found to be close to the surface then it may be possible to excavate the face and use only localised bolting in order to stabilise the face prior to the wall being cast. An alternative method of construction would be to remove existing stone wall and dig a trench 3 to 4m deep, place a preformed reinforcement cage and concrete the wall. Then the material could be excavated down in lifts and anchors installed as required. In order to construct the lower wall a toe could be constructed which would improve the stability of the wall hence possibly negating the need for an anchor. However this would need to be confirmed during the detailed design.
The quantity of excavated material to be removed from site for this option is quite significant, as there is very little fill required to backfill behind the lower wall. However, if the platform is raised by about 1.5m, it is possible that a cut/fill balance may be achievable; see Figure 4d. The upper wall would reduce in height to 1.5m and therefore it is likely that no anchors would be required if sufficient embedment could be achieved. The disadvantage is that the lower wall increased in height to around 4.5m, and therefore an additional row of anchors would be required to ensure stability. The lower wall is the more difficult to construct due to the steepness of the slope, and therefore the cost of any temporary works would also increase.
Whilst the building platform is much smaller for this option, the cost of the temporary works would be much less. Therefore it is likely that the cost per square metre of building platform is going to be much lower. This option would also be much safer to build. Whilst any working practices on slopes can be dangerous, this solution would allow ramps to be constructed between levels for plant and machinery, and the retained height of the wall is much lower therefore it will be safer to construct. However, the health and safety and construction implications discussed for Option A should still be considered.
Figures 4d and 4e show possible options for a terraced solution. These options have the advantage that the extent of the temporary works is reduced considerably and less earthworks and disturbance to the slope would be required.
Option E, shown on Figure 4e, would involve the construction of a number of smaller retaining walls. In order to limit the height of the lowest retaining wall for buildability it would be necessary for the upper wall to be 1m high. Therefore, a step down from the road into the first floor level of the building would be required. If however it is required that the first floor level is at the same level as the access road then it is possible that the lowest retaining wall is replaced by steel or reinforced concrete columns, Option F. This will be discussed in more detail in the following section.
Health and safety would require careful consideration during the design and determining the method of construction of the lower retaining wall as it needs to be built on a steep part of the slope. As with Options A to D, the anchors would need to be installed at a level which would not interfere with the buildings foundations.
Option F, shown on Figures 4f and 4g replaces the lower retaining wall with steel columns. This solution could also be used to extend either the building area or to provide a patio balcony at the front of the property. If the concrete columns are racked out and also form V shapes then it may be possible that only two concrete bases would be required. However this would need to be confirmed during the detailed design. One serious concern for this solution would be the stability of the concrete bases therefore it may be required that an anchor is needed to restrain the lateral forces in the base. However the lateral forces may be reduced if the columns are raked; see Figure 4g.
As discussed earlier, these options would also reduce the quantity of fill required to be moved off the site. Due to the steepness of the section of the slope behind the columns it may be necessary to anchor and net this area of the soil slope to prevent localised landslips which could undermine the floor slab. Anchoring may also be required on the slope below the foundations, again to prevent failure of the slope which could jeopardise the stability of the foundations. Again this would need to be confirmed following a site investigation and during the detailed design. It should also be noted that any anchoring would be restricted to below the building footprint therefore no permissions from third parties would be required.
When considering the most appropriate solution, two main factors must be given careful consideration, these are:
Since the site is on a steep slope, the costs of the temporary works to maintain a safe working environment could be substantial. Bearing in mind these factors, we would recommend that if a terraced platform is acceptable then Option G is adopted. This solution would provide an aesthetically, pleasing structure whilst maximising the building area and providing a veranda along the front of the proposed property. However, should a level platform be required, then Option D would probably provide the most cost effective solution
Some further site investigation will be required irrespective of the option chosen for preparing the development platform. The main reasons for the investigation would be to determine:
Bearing in mind the steep bedding, and clay filled joints and surfaces visible on the adjacent site, it is important that the structure of the bedrock is understood thoroughly to prevent any landslips occurring as was experienced on the adjacent site. Due to the steep nature of the site further trial pitting would be difficult and health and safety must be given careful consideration. Trial pits would be beneficial to allow the geological structure of the rock to be measured, in order to establish the overall stability of the slope. However if the excavation of trial pits is considered too dangerous the most suitable method for determining the rock head profile would be hand held drilling techniques. A series of probeholes could be drilled down the face of the slope using rope access techniques. A suitable anchor point would be a vehicle parked on the access road at the crest of the slope. Whilst hand held drilling is likely to provide sufficient information to plot a profile of the intact rock it is unlikely to provide much detail on the thickness of the weathered zone and no information on the direction and dip of the bedding of the rock. Therefore it is recommended that 1 or 2 boreholes are drilled on the site. The location of the boreholes should be determined once a preferred option for the development platform has been chosen. However it is likely that access will determine the locations. It would be possible to carry out one borehole by making access through the stone wall as was done to dig the trial pit, since this wall will need to be demolished to carry out the construction works anyway. It may also be possible to lift a small drilling rig onto the path which runs along the slope. The elevation of this path is approximately 23mOD and therefore a borehole approximately 6 or 7m deep would be required. For health and safety reasons it may be necessary to install some anchor points to fix the drilling rig and the operators to, to prevent them falling. These could be installed when the hand held drilling probes are being carried out.
Whilst a brief site walkover was carried out at the beginning of September it would be recommended that a detailed geological mapping of the surrounding area is carried out at the time of the site investigation to try to extend our understanding of the geology and the surrounding area. The mapping should also include gaining access beyond the upslope side of the access road. However this land is not owned by Mr Bellis and therefore permission to enter the land would be required.
FIGRNS
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