The West Seattle Transfer Tunnel involved the construction of a 10,223ft long, 13ft diameter tunnel to transfer sewage from the West Seattle drainage basin to a secondary treatment facility south of Alki Point, in the Puget Sound.

The tunnel was driven through overconsolidated glacial and interglacial soil deposits at depths of up to 400ft using a Lovat M-157 Earth Pressure Balance Tunnel Boring Machine (EPBM), with a grouted single pass trapezoidal segmental lining.

Geotechnical Aspects

Ground conditions at the tunnel horizon (black line)  comprised the Pre-Vashon deposits (green), a very dense, wet, fine-coarse, interbedded sand and gravel, overlain by the Lawton Clay (grey), a hard interglacial lacustrine clay. 

The tunnel was driven from east to west with the initial 800ft of the tunnel driven through the Lawton Clay before passing through a transition zone into the into the Pre-Vashon deposits for the remainder of the tunnel. 

Groundwater was present throughout the length of the tunnel with a maximum hydrostatic head of 60ft in the Lawton Clay, decreasing to between 8ft and 14ft above the tunnel in the more permeable Pre-Vashon deposits with the resultant risk of unstable and flowing ground conditions.

Settlement Analysis
Prior to commencement of tunnelling, the Contractor appointed SubTerra Inc to carry out a detailed evaluation of anticipated tunnelling conditions, and a settlement analysis based on the selected tunnelling method and lining system.

Following traditional settlement trough predictions, the estimation of maximum settlement was refined using probabilistic methods to reflect the variability of the glacial deposits, with the minimum, most likely & maximum values for key parameters used, rather than single points.

The results of this analysis provided a range of settlement values for each tunnel section and included a mean estimated value, and 90% confidence interval values of settlement. 

Construction

To minimise the potential for ground loss and possible surface settlement, the TBM was required to have Earth Pressure Balance capabilities.  The TBM was allowed to operate in non EPB mode where ground conditions allowed, and where surface settlement potential was low. 

Lovat EPBMs are equipped with a series of progressive ground control features which depending on the ground conditions range from open mode (belt conveyor and flood doors), to semi-EPB mode (belt conveyor, flood doors and pressure relieving gates), to full EPB mode (screw conveyor).

Assembling the Lovat EPBM in the east portal excavation.

To assist in ground control during semi-EPB mode, additives (bentonite, foam and polymers) were injected into the face.  This minimised ground loss and maintained settlement within the specified limits such that it was not necessary to use the machine in full EPB mode.

Settlement, a major concern at the outset remained within the specified limits.

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