Brief notes at 6-week stage (16th November 2019)

After 6 weeks and despite many signs stating the road is closed, we still get up to 1000’s of pedestrians and drivers who refuse to believe the road is actually closed!

Initial survey

Before commencing the Harbour Wall repairs, it was necessary to establish the existing condition – of the wall and nearbyconstructions and busines premises. There was a preliminary survey in several phases. This was carried out prior to work commencing (30th September) with a main aim of checking external condition of nearby buildings. This would enable any damage caused by pile-driving and related operations to be determined

There was then an internal survey using old-fashioned methods (camera and clipboard). (peter.christie@dorsetcouncil.gov.uk).

Demolishimg the old kiosk building @ Ferry Steps

The next phase of Harbour Wall repairs was to demolish the old storage and sales kiosk, built above the waling beam next to the Ferry Steps. Initially, the building was shrouded in temporary sheeting, checking for and removing any potentially hazardous materials. Then the “JCB” moved in a flattened the site in short order.

Further work includes replacing parts of the existing waling beam (anchoring structure for the tie-rods holding existing and new sheet piles in place – the tie rods are alleged to extend some 23 metres underneath the adjacent buildings).

Pile Driving operations

The main work involved pile-driving new sheet piles about 0.5m from the existing wall surface profile, about 18m into the subsoil. Originally, it was suggested a 4.5 Tonne hammer would be used to start the piles into position, followed by a more substantial (45 tonne?) ship-mounted hammer to drive the piles fully home. We have no specific information (as public) but we think the current hammer is a half-tonne hydraulic type – the noise is quite substantial and vibrations presently lie around 20 – 50 milliGals.

The piles will be cut as needed for drainage and tie-rod fixings. The gap will be back-filled with reinforced concrete, with ancillary works to fix in new sections of waling beam. The whole project is being (part?) funded from a £1.9 million Coastal Regeneration Scheme opening the way for the Harbourside Redevelopment Scheme(ca £3.3million?). When the repairs are completed, there are plans for new Harbourmaster and fishing buildings plus a scenic walkway around the Peninsula perimeter. Any flood defences would be funded from a different scheme.

Vibrations

Discussions of vibrations arising from pile driving and consequent effects on nearby structures tend to get a bit complex. In very general terms, ground displacements of 2mm/sec or more poses some risk of minor damage to older buildings (such as Listed Grade 2 Aaran House), while displacements above about 5mm/sec may cause significant damage. Hammer weights about 0.5 Tonne and frequencies about 1/sec tend to fall into the range of MMI (Modified Mercalli Index) 3 (noticeable but very little damage) while “heavier” impacts and higher frequencies can push perceived effects up to MMI 4 (minor damage) or even MMI 5 (significant damage, difficulty maintaining balance). The Council advised us to close for the duration and we took their advice.

Above, we see a few graphs of accelerometer readings vs time, during the early weeks of pile driving. Note the acceleraometer was not too accurate or sensitive and the sampling rate was low – which may cause some peaks to be “missed”. Using the archaic non-SI unit, the Gal, we can see background levels around 2 milliGal, rising to around 50 milliGal, as the operations got nearer. There is no easy relationship between Gals and mm/sec ground displacements but the MMI scale would suggest 30 to 50 milliGal = MMI 3, roughly. We have observed only slight plaster cracking and a little dust fall.

Closure of Esplanade to Custom House Quay road and pavement junction

Meantime, up to 2,000 pedestrians per day and 100’s of motorists are “visiting” the site, at least as far as the road closure next to The Roundhouse b&b. It is quite amusing to see all those stern faces crumple into astonishment when they find out a half-dozen “road closed” and “diversion” signs were not just dumped for fun and the road into Custom House Quay really is closed (6 weeks till now: another 6 weeks to go approximately).

The Council (DCC, formerly WPBC) have advised the nearest properties to close during the period of works, for safety, noise and disruption reasons. Hence, we are closed (since 1st October) till the Christmas period (scheduled completion 23rd December). We are hoping for compensation due to business interruption.

Reopening in 2020 – early in January

We apologise for any inconvenience to guests who would have wished to book our guesthouse this autumn, but the works were essential and closure was unavoidable. It was only a few years ago a section next to Condor Ferry Berth collapsed. Allegedly that was in relatively sound condition compared to the current repair section. Of course, public safety is paramount and works have continued as rapidly as feasible. We thank all our guests for visiting us in the past and we hope to see you all again next year. Happy Holidays!

(See mid-stage update)