By Steven Pryce, Cowi (Cooling Prize Finalist 2023)
1. Introduction
As the industry drives towards a more sustainable future, there has rightly been significant emphasis on innovative thinking in the design of new infrastructure. However, of equal importance is how we maintain and manage existing infrastructure to make it fit for the future. This is particularly relevant to infrastructure associated with the water and sewerage industries. As changes in the natural environment alter the ground in which decades-old infrastructure has been built, the maintenance of such assets is likely to become increasingly challenging. This will necessitate innovative solutions to facilitate the maintenance of these assets.
The focus of this paper is to explore forensic ground investigation and geotechnical analysis methods to enable the repair of a broken section of deep sewer in the Glasgow area. Complex geological and hydrogeological conditions, as well as the proximity to and condition of adjacent assets and property, created a uniquely challenging environment for this repair. While this paper aims to explore the geotechnical analysis and design methods associated with the repair, further consideration should be given to the relative cost and effort associated with this. That is to say that if greater focus was given to the long term maintenance of the assets at planning stage, including utilising the benefit of expertise on local geological conditions, then such complex and costly geotechnical interventions may have been avoided completely.
2. The problem
The project pertains to a broken section of foul sewer, identified in 2020, which resulted from pipe defects that were a legacy of previous ground improvement works undertaken at the site. The chronology of events leading up to the pipe break is discussed in subsequent sections. Given the sewer sits below the resting groundwater level, significant temporary works were required to facilitate the repair. This is further complicated by the close proximity of adjacent residential properties. The geometry of the sewerage network and surrounding environs is presented in Figure 1.
3. Challenging ground conditions
Complex geological and hydrogeological conditions exist at the site, which create difficulty for the maintenance of the existing assets. The site is initially underlain by 2m of made ground deposits, which comprise reworked natural soils mixed with concrete blocks associated with the historical use of the site. This is underlain by Clyde Alluvium to approximately 16m below ground level (bgl). These deposits comprise fine and uniformly graded silty sands and sandy silts. The deposits are of low strength, high compressibility, highly susceptible to changes in moisture content and liquefiable. The deposits are underlain by glacial deposits comprising low strength, low permeability clay.
A hydrostatic groundwater level of 2m bgl (rising seasonally to 1.2m bgl) is present. Considering the sewer invert depth of 5.5m bgl, this creates challenging temporary works requirements to facilitate maintenance or repair works. The ground model and indicative groundwater drawdown profile (based on permeabilities of 1 x 10-5m/s and 1 x 10-6m/s) is presented in Figure 2.
4. A brief site history
Cowi’s involvement at the site stretches back to an initial ground collapse event in 2012. Anecdotally, it is understood that at this time a manhole was replaced in the playing field utilising sheet piling with local dewatering. On completion of the works, the sheet piles were extracted, resulting in disturbance/quasi-liquefaction of the soils and subsequently a significant settlement event, which caused the loss of serviceability of the original pumping station infrastructure. During this event, settlements of up to 500mm were observed, as indicated in Figure 3.
This event caused severance of pipe connections into chambers, as well as deformation of the access road and significant differential settlement of a kiosk structure. Therefore, it was concluded that the pumping station required complete replacement.
Following extensive ground investigations, a ground improvement solution was designed by Cowi. This was implemented in 2016 and was followed by the reconstruction of the pumping station. The ground improvement solution comprised the innovative use of dewatering (via deep wells) to lower the hydrostatic groundwater level, therefore increasing effective pressures within the near surface Clyde Alluvium and effectively compacting the soils under their self-weight.
These works were implemented successfully and facilitated the reconstruction of the pumping station. However, settlements were incurred in the ground supporting adjacent infrastructure and residential property, leaving them with a low residual tolerance to additional settlements. Following the completion of the works, cleaning of the existing pipe network was scheduled; however, it is unclear if this was undertaken. It is hypothesised that existing defects in the pipework were exacerbated over time by fine soils inflowing through open pipe joints, which led to the eventual pipe break. A timeline of events is presented in Figure 4.
5. Ground investigations for current repair
Extensive ground investigations were undertaken with two primary aims: to forensically analyse discreet changes in ground conditions by depth and by area to determine a hypothesis for the pipe break identified in 2020; and to ascertain geotechnical parameters to be used in the design of temporary works to facilitate the repair. The works comprised the extensive use of cone penetration tests with measurement of porewater pressure (CPTu), which were supplemented by dynamic probing in areas of restricted access, and borehole installations for monitoring groundwater levels. Geotechnical parameters were largely ascertained from CPTu testing; however, some laboratory testing from Mostap samples was undertaken to validate the correlations from CPTu data.
The CPTu results indicated a consistent profile of low cone end resistance (qc ) values immediately above the pipe invert for positions running along the line of the foul sewer, in comparison with positions outside this area. This trend was also present for positions in the pumping station area; however, the qc values were slightly higher by comparison. This assessment is presented graphically in Figure 5.
Based upon the foregoing, it was hypothesised that a disturbed profile existed along the length of the sewer, which was likely associated with difficult and problematic installation techniques. It is considered that disturbed conditions were likely to historically exist in the pumping station area, but these were improved to some extent by the 2016 ground improvement works (see Figure 4). This information was used to determine the extent of the proposed (2023) repairs and form a view on residual risks to the network on completion of the works, as well as additional maintenance and investigation requirements.
6. Conceptual design
Following agreement that the problem was one of a broken/open pipe(s) at the location of the observed ground depression, the client instructed to proceed based on a local repair to the sewer. Cowi developed a conceptual design for the repair. The low residual tolerance of adjacent infrastructure and residential property to additional settlements from the proposed works was the key driver in the decision-making process for the design of the temporary works. Consideration was also given to minimising further disturbance to the residents of these properties.
The design comprised a sheet piled cofferdam to a depth of 16.5m bgl to allow keying-in to the low permeability clay layer. This allowed simplification of the dewatering design and greater control when reducing the groundwater levels, as well as a reduced risk of base instability within the excavation. To mitigate the risk of liquefaction resulting in possible ground collapse, the sheet piles were specified to be left insitu following completion of the repair and were specified to be driven using low energy techniques. Dewatering was designed using an array of vacuum assisted ejector wells.
There was a risk of high levels of settlement from the sheet pile driving impacting the existing sewerage infrastructure at the interface of the piles. As such, compaction grouting beneath these assets was specified to give the pipework resilience against this settlement. The 2016 works identified a high permeability groundwater feed affecting the site in a north west to south east bearing.
This created an increased risk of drawdown settlements at the open ends of the sheet piles (around the pipework) leading to additional settlements at the adjacent houses. To mitigate this, permeation grouting was specified to reduce the net horizontal permeability of the ground and therefore reduce the drawdown radius. A plan showing the conceptual design solution is presented in Figure 6 and a conceptualised 3D view of the sheet piling design is presented in Figure 7.
7. Anisotropic conditions – compressibility
Given the sensitivity of adjacent infrastructure and residential property to additional settlements, it was considered that traditional approaches to settlement calculation were insufficient to provide the level of confidence required.
As such, a back analysis of the site monitoring data from the 2016 project was undertaken to statistically analyse the ground performance and therefore give further confidence of how the strata were likely to react to the proposed dewatering.
A detailed study on predicted settlements associated with sheet pile installation was also undertaken; however this is not discussed in this paper.
The back analysis utilised extensometer data from the 2016 project, which measured settlements in each stratum resulting from dewatering. Alongside this, constrained modulus values (M) derived from CPTu testing before the implementation of the 2016 works were plotted and split into statistical trends representing the 5th, 50th and 95th percentiles of the data set (see Figure 8).
Design values of M were derived for each of these trendlines to calculate settlements at each 0.5m depth increment using traditional calculation methods. The results of this assessment were compared with the extensometer results to draw the following conclusions on the observed behaviour of the ground (see Figure 8):
- The stratum between 2m and 9m bgl closely followed the 5th percentile of the constrained modulus data (meaning, the more onerous end of the data set). This is likely due to the water being drawn down below this stratum and the subsequent effects on the ground, which is similar to an immediate impact type load.
- The stratum between 9m and 12m bgl closely followed the 50th percentile (or average) of the data set, which was as expected and provides confidence in the statistical and numerical relevance of the data set.
- The stratum between 12m and 16m bgl closely followed the 95th percentile of the data set (meaning, the less onerous end). This is likely due to a pressure distribution effect below the reduced dewatering level, which is otherwise difficult to quantify.
The conclusions of this assessment allowed these statistical trends to be used in the forward prediction of dewatering settlements from the current works and, hence, provide a more forensic assessment of how the ground was likely to behave.
8. Anisotropic conditions – permeability
While the conceptual design included sheet piles being advanced to the depth of the low permeability clay layer at 16m bgl, which would effectively limit the drawdown settlements outside the extent of the sheet piled excavation, it was acknowledged that a perfect seal would not be achieved due to gaps in the sheet piles where bridging over sewer pipes was required. As such, an assessment was required to ascertain the drawdown settlements, which may impact the existing pumping station and residential properties.
The 2016 ground improvement project identified that the net horizontal permeability of the ground varied, and a high permeability feed was noted to impact the site in a north west to south east bearing. It is conjectured that this follows the route of the pipework and therefore may be associated with the granular pipe bedding. To determine the likely maximum drawdown settlements affecting the two closest residential properties (one at a north west bearing from the works and one to the north east), a back analysis of the results from the 2016 project was undertaken.
The back analysis involved plotting surface settlement values from the 2016 works with respect to direction and distance from the ring of dewatering wells, as shown in Figure 9. Drawdown settlement curves were plotted for various values of coefficient of permeability (k), and using the monitoring data points, best-fit drawdown curves were also plotted. These curves confirmed that a higher permeability feed affected the site in a north-west to south-east bearing. The assessment provided characteristic values of k with respect to direction from the dewatered area that were used to calculate the likely maximum drawdown settlements affecting each house.
Calculation of the maximum drawdown settlements allowed determination of a suitable criterion for the permeation grouting, which, if implemented successfully, could provide confidence that additional drawdown settlements were unlikely to affect the houses.
9. Conclusions
This paper explores how site monitoring results can be used in concert with ground investigation techniques to provide a more informed view of ground parameters. Developing this complete picture of the ground performance allows tailoring of mitigations to reduce impacts on adjacent assets and infrastructure, as has been shown with the specification of the compaction and permeation grouting in this study, and the selection of their success criteria. The analysis methods presented in this paper represent modelling predictions only, which shall be reviewed and validated using site monitoring data during the implementation of the works.
While the focus of this paper is on the analytical methods utilised in the design of the sewer repair, consideration must be given as to why the maintenance of these assets has proved so difficult. Further consideration is required at planning stage as to how the assets will be maintained over their lifecycle, with acknowledgement of potential changes in the environment induced by climate change. Such considerations in this case might have eliminated the requirement for the complex geotechnical interventions discussed and the costs associated with implementing them.
10. Acknowledgements
The author is grateful for the contributions of Donald Cook, Dimos Koungelis, Andrew Kennedy and Liam Jardine in the preparation of this paper. Thanks are also extended to George Leslie Ltd for its collaboration on this project and for providing permission to present the work.
11. References
Bracegirdle, A et al. (1996). A methodology for evaluating potential damage to cast iron pipes induced by tunnelling.
Ciria C796. (2021). Assessing the impacts of construction-induced ground movement on framed buildings.
Koungelis, D et al. (2019). Findings from a dewatering trial used to densify loosened natural soils. XVII European Conference on Soil Mechanics and Geotechnical Engineering.
Preene, M (2020). Conceptual modelling for the design of groundwater control systems. QJEGH Quarterly Journal of Engineering Geology and Hydrogeology.
Robertson, PK and Cabal, KL (2014). Guide to Cone Penetration Testing for Geotechnical Engineering, 6th Edition.