Almost 35,000 people were affected by the 2007 flooding in Hull and more than 10,000 homes had to be evacuated. Following these devastating effects, the East Riding of Yorkshire Council set up a partnership with Hull City Council (HCC) with the purpose of investigating solutions to reduce the threat of repeat flooding.

With the assistance of the Environment Agency (EA), the council undertook ground surveys and complex computer modelling of rainfall.

As a result, the proposed Anlaby and East Ella Flood Alleviation Scheme (AEEFAS) was identified as part of a suite of schemes that will reduce the risk of flooding to Hull and Tranby area, from ‘severe’ to ‘moderate or low’.

The AEEFAS scheme includes two attenuation lagoons for rain water collected from the two catchments the project serves. The lagoons are of such a capacity that they are both classed as reservoirs under the Reservoirs Act 1975. The collection structures are connected via a large storm water culvert and a connecting watercourse. The largest of the lagoons will be located on the former Sydney Smith School site and will contain new sports pitches. For the majority of the time the lagoon will remain dry, only to be filled during severe wet weather events.

The AEEFAS project is a GBP 22M (USD 28.4bn) scheme in three phases:

  • Phase 1 began in July 2017 and consisted of improvements to the existing Western Drain and a new culvert under Beverley Road.
  • Phase 2 works began in February 2018 and involved the construction of a new flood water storage lagoon, expansion of an existing lagoon, construction of 800m of flood water channel and an Energy Dissipation Unit (EDU) outfall to a storm water tunnel. A short section of 1.8m internal diameter cut and cover storm water culvert is also included to connect the tunnelled section of the culvert to the EDU.
  • Phase 3 consists of the 1.3km of 1.8m inner diameter storm water tunnel between a flood flow inlet structure at Kerry Pit at the north of the scheme and Tranby Ride. NMCN is the main contractor for Phase Three works.

Pell Frischmann is providing geotechnical and geoenvironmental support to the East Riding of Yorkshire Council’s Infrastructure and Facilities team for Phase 3 with Cowi providing specialist tunnelling support.

The tunnel is approximately 1.3km long and the specialist sub-contractor delivering this is Active Tunnelling Limited. The tunnel is required to run beneath the hills present between two valleys and the Humber flood plain. An Energy Dissipation Unit (EDU) is located at the end of the tunnel, which consists of a large concrete basin structure with 14 no. concrete diffusers. This is to remove energy from the flood water prior to being discharged into an open water course.

Two shafts have been constructed to facilitate the construction of the stormwater tunnel. A 6m diameter and 9m deep permanent access shaft at Tranby Lane (TS1) splits the 1.3km of tunnel into two drives of 723m and 597m. To split the longer of these drives into two separate drives, an intermediate shaft was proposed by the contractor during the tender period to allow early release of the Kerry Pit site at the northern limit of the scheme. The 4m diameter and almost 15m deep intermediate shaft will also remain as a permanent shaft to provide further access to the tunnel.

Tunnelling through the Chalk

The main geological formation is the Burnham Chalk formation. This is described by the British Geological Survey as white thinly-bedded chalk with common tabular and discontinuous flint bands; sporadic marl seams. As a northern province chalk it is typically of higher density than southern chalks. This is primarily due to mechanisms prior to and post diagenesis. As dry density is related to the Unconfined Compressive Strength (UCS), this makes Northern Province chalks harder to tunnel through. The flint bands also pose a challenge to tunnelling as flint is a very high strength material and is also very abrasive.

Chris Hitch, principal engineer at Cowi, explains: “There is the potential for very large tabular flints up to several metres in size (Paramoudra flints) within the Burnham Chalk. These would cause significant problems to a tunnel boring machine although have not been encountered to date at the AEEFAS site.”

The tunnel is driven through the Burnham Chalk formation for almost its full length. There is only a short section either side of the main shaft (TS1), which is Glacial Till – a stiff over-consolidated clay.

Laboratory test results gave a wide range of UCS values for the Burnham Chalk. The maximum value recorded was approximately 16MPa.

Spoil produced by the tunnelling activities is transferred to several local chalk pits for supply to local industries for use alongside freshly quarried chalk..

Hitch says geological challenges, posed by chalk, have been faced working with a full face TBM, which is not pressure balanced. It is a close face TBM mixed shield provided by Active Tunnelling. The TBM is approximately 2.18m external diameter to install 2.14m outer diameter FP McCann pipe jacking pipes. The inner diameter of the pipes is 1.83m.

There are no additional waterproofing requirements to the standard seals. The FP McCann pipes are designed to take a single seal between each pipe.

The TBM was launched twice from TS1, once for the pipe jack drive to Tranby Lane and once for the drive to the intermediate shaft. The TBM was also launched from Kerry Pit at the very north of the scheme to the intermediate shaft.

Tunnelling works began in August 2018 with Drive 1 (597m), between the main shaft (TS1) at Tranby Lane and the start of the cut-and-cover tunnel at Tranby Ride was successfully was completed in February 2019. Drive 2 between the main shaft (TS1) and the intermediate shaft was completed in September 2019. The final drive between Kerry Pit and the intermediate shaft is due to be completed in November 2019.

Hitch explains that there have been a few delays due to the stress that the ground conditions were placing on the TBM, in particular the gearbox and motors. The risk of further breakdowns was mitigated with uprated components and shift working being carried out to compensate for time lost. The contractor undertook noise monitoring before commencing shift working to assess the impact of this on the local community and to ensure compliance with the Public Health Act. It was fortunate that residential receptors were reasonably remote from the majority of the works owing to the works being carried out in primarily rural areas.

Tunnelling in the source protection zone

The tunnel passes through Burnham Chalk formation, which is designated as a main aquifer due the high fracture permeability of the Chalk that provides a high level or water storage. Parts of the site are placed within an outer (Zone 2) groundwater Source Protection Zone (SPZ), which relates to a number of abstraction boreholes, the closest is a potable water borehole (Yorkshire Water’s Springhead Pumping Station) located approximately 1.2km to the north-east of the site and supplies water to most of Hull.

A Hydrological Risk Assessment (HRA) and a Groundwater Monitoring Baseline Report for the scheme have been completed by Pell Frischmann on behalf of East Riding of Yorkshire Council. The HRA set out Control Levels and Trigger Levels for turbidity, pH and electrical conductivity (EC) along with baseline groundwater monitoring results. The Groundwater Monitoring Baseline Report presented information from data loggers left in monitoring wells for a year prior to construction.

The contractor NMCN commissioned WYG to review and update the Pell Frischmann HRA, taking into account the results of the Groundwater Monitoring Baseline Report and NMCN’s method statement for tunnelling works. This presented an updated conceptual site model, monitoring scope, control levels and compliance limits which groundwater monitoring during site works were controlled against.

The updated HRA was submitted to the Local Authority and Environment Agency for comment and confirmation that the proposed approach was acceptable.

At the end of the tunnel the Energy Dissipation Unit (EDU) is located, including a large concrete structure with dissipation baffles.

Several groundwater monitoring points were installed during the ground investigation for the works and were monitored in advance of the works to obtain baseline readings and during the works to ensure there was no impact on the aquifer due to the tunnelling works.

The ground water is below the tunnel drive. The invert level of the tunnel is approximately 27.5m AOD at the inlet at Kerry Pit at the north of the site. At the most southern point of the bored tunnel the invert level is approximately 16.5m AOD. The groundwater level in the area of the tunnel was found to be around 1m AOD so there is no potential risk for any fluid to migrate to the aquifer.

Maintaining Alignment

Day to day control of tunnel alignment was through the use of tunnel lasers set on independent frames at the base of the drive shaft or pit. Chris says

“Because of the use of skips to remove spoil along the tunnel to the drive shaft and the fact that pipe jacking involves the full pipe string moving tunnel lasers were the only practical method of daily tracking of alignment,” Chris adds. “Due to the phenomenon of laser drift, which can be experienced in longer drives with temperature variation along the tunnel as found on the AEEFAS scheme, frequent survey checks were completed to verify the laser alignment.

This combination of methodologies to check the alignment has proved successful for all the drives.”

Standard monitoring surveys of the Tranby Lane were completed by the contractors to confirm that no significant settlement had taken place. Settlement calculations were completed in advance of construction to assess the level of settlement expected.

In terms of health and safety, rescue plans are in place for work in a confined space. The tunnel and shafts are classified as “Medium Risk” confined spaces. As such suitable methods of confined space working were required which included detailed rescue plans.