Water and Effluent Treatment Magazine
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T he original Talla Aqueduct tunnel, built in the 1890s, supplies Edinburgh and Scottish Water's new Glencorse Water Treatment Works with raw water. It runs in a single tunnel for most of its length, split by seven siphons and a number of bridges. The construction is a mix of cut and cover and excavated tunnel and, typically, the tunnel sections are constructed or lined with mass concrete around 380mm thick. There is 19.8km of cut and cover construction, 14.4km of tunnels and 11.2km of siphons. The tunnel sections are 1.8m wide and 2.2m in height and include an arched roof. Access to sections of tunnel are at regular intervals in the cut and cover sections. Siphon chambers are provided at the inlet and outlet to the siphon pipes and overflow chambers are also located up the line of the aqueduct. Plateau One such overflow chamber structure is buried completely on three sides and partially on the fourth as it sits in a saddle at the foot of a hill. Ground level in front of the door is mid-depth of the structure. From this level it slopes up behind the masonry faced retaining walls to a plateau over the structure and aqueduct before sloping up again at an angle of 35-40 ̊ behind the chamber. Tests and surveys revealed some evidence of ground movement and subsidence in the area immediately around the structure was observed. Jacobs was employed by Scottish Water's Capital Investment Delivery (CID) team to provide an outline design and specification for work to ensure the chamber could be returned to the best possible condition. A tender of the works was issued to Scottish Water Construction Delivery partners, with George Leslie selected as the preferred bidder. George Leslie was responsible for the design and construction of the refurbishment and stabilisation works to the chamber. The work was planned to be designed and installed in such a way as not to stress any part of the existing chamber and aqueduct structures. The project design brief included a waterproof structure surrounding the existing chamber structure to exclude all water and thus remove all hydrostatic pressure from the existing chamber. Stress George Leslie, along with its consultant partner Atkins, designed and installed a waterproof bored pile retaining wall and capping beam tied back with rock anchors along the uphill of the overflow chamber to remove the loading on the structure from the hillside under which it is buried. The anchor wall system was designed to carry all of the geotechnical and hydrostatic loads from the uphill slope , as well as remove all such loading from the existing chamber structure. New drainage works between the new bored pile retaining wall and the existing chamber structure in order to relieve all hydrostatic loading from the uphill side of the existing chamber structure was installed. Concrete wall The replacement mass concrete gravity walls on the north, west and south sides of the chamber to remove the soil loadings on these sides of the structure was designed to carry all of the geotechnical and hydrostatic loads. Renovating without impact • Ground movement and subsidence were among the reasons for Scottish Water refurbishing the Talla Aqueduct. The challenge was not to impact the landowner. ONSITE OverflOW STAbiliSATiOn APril 2015 WET NEWS 15 projects specs • Design and construction of refurbishment and stabilisation works to siphon chamber • Construct a waterproof structure surround the existing chamber • install new drainage to move water away from the existing chamber roof • reinstate all ground to the existing levels pre-construction techknow • The work included a waterproof bored pile retaining wall and capping beam tied back with rock anchors • The anchor wall was designed to carry all geotechnical and hyrostatic loads • The mass concrete walls were cast directly on rock head • The roof structure was constructed from precast concrete neeD to know 1 The Talla Aqueduct Tunnel supplies edinburgh and the new Glencorse WTW 2 visual inspections of the tunnel and associated chambers had identified the need to carry out some work to refurbish parts of the tunnel 3 further tests and surveys revealed evidence of ground movement and subsidence in the area immediately around the structure was observed 4 The aqueduct dates back to the 1890s the VerDIct • The aqueduct and its function was maintained during the works to ensure continuous uninterrupted raw water supply to Glencorse WTW. The work was constructed to the design proposed with minimal impact on the landowner in challenging circumstances. Work was designed in such a way as not to stress any part of the existing chamber and aqueduct structures The aqueduct and its function was maintained during the works to ensure continuous uninterrupted raw water supply Also, the mass concrete wall system was to carry all loads from other parts of the works and existing structures as required. The mass concrete walls were cast directly onto rock head. The walls were to incorporate reinforced concrete members to span over the aqueduct so that it is protected and not loaded by any part of the works. This was achieved using compressible fillers. A new precast concrete roof structure with a waterproofed screed spanning from the bored piled wall onto a steel beam spanning across the overflow chamber structure above the existing roof level was installed. Waterproof The soil over the existing structure was removed and replaced on top of the new roof over the chamber. In addition, new drainage was installed to convey water away from the existing chamber roof to adjacent surface water drains. New waterproof mass concrete wing walls were installed with the same geometry as the existing wing walls that shall be demolished and again were cast directly onto rock head. Cracks in the internal structure surface of the chamber were repaired and all internal wall and roof surfaces were covered with a protective coating in order to stabilise the surface. The aqueduct and its function was maintained during the works to ensure continuous uninterrupted raw water supply to the water treatment works. The work was constructed to the design proposed with minimal impact on the landowner in challenging circumstances due to the project location.