St Louis, Missouri, like all cities, is faced with infrastructure concerns as it ages and expands. Its sewer systems are no exception. The Metropolitan St Louis Sewer District’s Grand & Bates Storm Sewer Relief Tunnel will extend the city’s system by approximately 2000m and provide the much needed additional capacity in the Grand Glaise Sewer District. Approximately 1,500m of storm sewer laterals will also be constructed to connect adjacent neighbourhoods to the new storm sewer system. Tunnelling contractor, Affholder, has scheduled tunnel completion for late 2003.

Value engineering

The 3.3m diameter TBM driven tunnel is being constructed at a depth of 22.8m to 44.1m. Three large access shafts have been constructed along the alignment (T1D, T2D and T11D), as well as seven smaller diameter drop shafts (T3D, T4D, T5D, T6D, T7D, T8D, T9D and T10D) that will allow the discharge of new or existing local storm sewers into the tunnel.

With a project as large and challenging as this one, strong efforts have been made to reduce costs and production time, and to streamline and enhance the overall design and construction process. To achieve this, consultant, Brierley Associates, in a cooperative effort with contractor Affholder, considered aspects of the project to which value engineering could be applied. These, influenced heavily by the ground conditions, included ground stabilisation and support, anchorage of the shaft wall system and pressure grouting, shaft lining, and ground movement detection.

Stabilisation and support

The ground at the Grand & Bates site is divided into three distinct layers. The first layer of soil consists of primarily loessial and alluvial deposits with some fill. Known for being highly susceptible to destabilisation during shaft construction, this soil requires a positive means of support and control throughout the project duration. The second layer consists of fractured material near the top of the bedrock, including seams of shale, soils and voids, and is capable of delivering high water inflows. The third layer is limestone bedrock, which, while generally competent, contains zones of lower-quality material associated with seams of shale, sinkhole features or faulting.

Due to the diverse geology, ground stabilisation and support was a costly design element in the construction of the access shafts. An approach was developed to minimise the amount of initial support needed, reduce construction time, and decrease the construction cost for each access shaft. After careful consideration, jet grouting was chosen as the means to provide ground pre-stabilisation, groundwater control, and to serve as the primary construction method for the temporary shaft excavation wall.

Affholder selected Hayward Baker, part of the Keller Group of companies, for the installation of two of the access shafts, T2D and T11D, using jet grouting.

The triple-fluid system was selected due to its efficiency in creating a higher-quality grouted soil mass, referred to as soilcrete, than the single-fluid or double-fluid systems in the cohesive soils as encountered elsewhere at the Grand & Bates site. The triple-fluid system uses a high-velocity, rotating water jet, sheathed in a cone of air to erode a column of overburden soils and expel them to the surface. Simultaneously, cement grout slurry is injected below the eroding jets at a lower velocity to tremie place the grout as the rod is withdrawn from the borehole.

Two full soilcrete test columns and one half test column were installed in production column locations before starting the production work. By installing the test columns in design locations, the jet grouting parameters necessary to achieve the required geometry were confirmed. Fast setting cement was used in the production of the test columns so that verification coring could take place the days after installation. All pertinent installation parameters for each test column were documented daily. The first core hole was drilled at the interstice area of the three test columns.

During soilcrete column production, samples of the neat cement grout and in situ soilcrete were taken once daily and cast into 3 x 6″ cylinders for unconfined compressive strength tests. The average 28-day soilcrete unconfined compressive strength test result for both shafts was 6.3MPa exceeding the design criteria of 3.44MPa.

To provide control over construction related ground movements and groundwater inflow, two of the project’s access shaft walls were designed as compression rings, consisting of a series of two rows of interlocking soilcrete columns. Full and half-columns were installed through the overburden soils to create a continuous compression ring that extended to the top of the irregular bedrock surface. The soilcrete supported portion of Shaft T2D is 10.9m i.d. x 16.5m deep. Similarly, soilcrete support at Shaft T11D is 9.7m i.d. x 20.8m deep. The interior shaft column rows were made of full columns with 1.06m diameters, on a 0.9m centre spacing. The outer rows were composed of half-columns with 1.06m to 1.2m nominal diameters placed behind the interstice area of the full columns, producing a nominal wall thickness of at least 1.2m and providing a groundwater barrier preventing potential running ground conditions during subsequent shaft excavation.

Anchorage and pressure grouting

Shaft rock grouting and shear reinforcement were performed at 25 locations for Shaft T11D and 28 locations for Shaft T2D. The shear key holes were drilled in a single ring configuration from ground surface through the length of the soilcrete columns and continued another 6m into underlying bedrock.

Due to the karstic nature of the limestone bedrock and the potential for groundwater inflow during shaft excavation, water pressure testing of the upper portions of the bedrock was incorporated into the design. In the event that a karstic, water-introducing feature was encountered in the shear key holes, the design called for the pressure injection of cement grout to seal off joints and fractures in the bedrock, to limit groundwater flow as the shaft was excavated.

After water pressure testing at each shear key hole, a 9.1m long, no. 8 rebar was tremie grouted in place to anchor the soilcrete wall to the bedrock. The reinforcement bar extended 6m into the bedrock, and 3m up into the soilcrete wall. This grouted rebar also acted as vertical spiling, to prevent overbreak beneath the shaft wall during the subsequent blasting operation to the shaft’s design depths for T11D and T2D of approximately 34.4m and 45.7m respectively. Shaft integrity was not impacted by the blasting operation.

Shaft and tunnel excavation

Shaft T11D soil overburden excavation started on July 15 2002 and finished on July 26 2002. Soil overburden excavation averaged approximately 2.6m per shift including hanging wiremesh and shotcrete. Rock excavation averaged approximately 0.9m per shift including rock bolt installation.

Shaft T2D soil overburden excavation started on August 12 2002 and finished on August 23 2002. Soil overburden excavation averaged approximately 2.3m per shift including hanging wire mesh and shotcrete. Rock excavation averaged approximately 0.8m per shift including rock bolt installation.

Drill and blast starter and tail tunnel excavation started on September 17 2002.

The TBM being used for tunnel construction is a Robbins Model 1412-300 specifically designed for hard rock formations. The cutterhead is equipped with 31, 0.43m wedge lock disc cutters. Rotary cutterhead power is supplied by four 315kw water cooled electric motors developing 1,800HP. Boring started on January 14 2003 and, at the time of going to press, has advanced 987m. The finished lining of the tunnel will be a 3.35m diameter reinforced concrete pipe.

Shaft lining

Initially, ring beams and lagging was proposed to supplement the jet grouting pre-stabilisation. However, as part of the value engineering process, a more cost effective and less labour-intensive method of protecting the shaft structure and providing supplemental wall strength was used. A shotcrete option was chosen, reducing costs and accelerating the shaft construction and excavation process.

A nominal 50mm of shotcrete facing was applied to welded wire mesh, stud fastened to the inside wall as excavation of the shafts progressed. This minimal thickness primarily protected the soilcrete shaft wall from freeze/thaw and slab fallout during excavation.

In developing the design of the 1.2m thick shaft compression ring, a maximum compressive hoop stress of 1.24MPa was calculated when the shaft excavation reached the top of bedrock at depths ranging from approximately 16.4m to 22.3m. Based on these anticipated stresses, a minimum soilcrete strength of 3.44MPa was specified in the design to provide a safety factor of 2.8.

However, in the event that compressive stresses in the shaft lining exceeded anticipated values, additional shotcrete thickness could be placed on the shaft wall to increase the wall strength and decrease compressive stresses in the composite wall section. An observational approach was used to verify that the shaft lining was performing as anticipated and to provide a means of determining if additional shotcrete was required. Using a systematic approach, strain was monitored in the shaft wall using a series of vibrating wire strain gages.

By using an innovative installation apparatus developed by Ron Williams of Geotechnology Inc, the gauges were pressed into selected uncured soilcrete columns immediately after completion. With Hayward Baker’s assistance, a total of twelve Geokon Model VCE-4200 vibrating wire strain gauges were installed at each of two access shafts with four gauges installed in quadrants at deep, intermediate and shallow levels.

Using an average soilcrete tangent modulus of 1,341MPa for Shaft T2D, and 1,320.4MPa for Shaft T11D, determined from unconfined compression tests completed on strain gauged core samples and vibrating wire strain gauge data obtained by Geotechnology as the shaft was excavated, corresponding compressive stresses in the shaft lining were calculated. In the event that measured stresses exceeded 1MPa, additional shotcrete would be applied to the shaft wall to provide additional support

The shotcrete lining design proved to be more than adequate as the maximum hoop stresses monitored at Shaft T11D were approximately 0.62MPa, monitored approximately 1.5m above the top of the bedrock.

Ground movement detection

The proximity of the access shafts to houses and roadways was a primary concern. Shafts T2D and T11D were approximately 1.8m and 0.3m from the curb, respectively. This warrants constant monitoring. Fragile brick sewers in proximity to the works were also a concern. The jet grouting did not produce any harmful vibrations affecting the sewers. Constant maintenance of the borehole annulus contributed to the controlled soilcrete column diameters, eliminating the risk of disturbing nearby utilities.

The soilcrete access shaft walls were designed to provide a rigid ground support and effective impermeable barrier to groundwater inflow. The pressure grouting program was effective in controlling ground water during excavation of the fractured rock. In addition, little groundwater inflow or construction related ground movements were noted as the blasting required to advance the shafts to their design depth was completed, demonstrating the effectiveness of the soilcrete/bedrock seal and the grouted vertical rebar spiling. However, the contingency measures that were incorporated into the design remained available during shaft construction in the event additional ground control and support were necessary.

Data collected from the strain gauges indicates that the two soilcrete access shafts are performing as anticipated, with no unacceptable high stresses and no ground heave or settlement. In both shafts, the maximum hoop stress occurred approximately 1.5m above the bottom of the soilcrete wall. As each shaft was excavated to the bedrock, strain gauge data indicated a progressive increase in measured hoop stress. However, once the excavation reached the bedrock surface, measured hoop stress remained relatively constant as drill and blast operations were completed to advance each shaft to its invert depth. Mark Rybak, project manager for Affholder stated, “The strain gauges did, indeed, provide a safety factor for us and a comfort zone for the sewer district. Actually being able to see the strain gauge readings in graph form provided us with an easy-to-read graphic representation of the stresses the soilcrete columns were being subjected to.”

Saving time and money

Cost, time and constructability are the issues that govern the development of the value engineering approach. According to Mark Rybak: “The redesign cut costs and time in that there was less soilcrete, no ribs and boards to install as additional support and the wire mesh and shotcrete turned out to be a very cost effective and adequate alternative.” As a result, excavation of the overburden soils to a depth in excess of 18.2m was completed in only nine days, permitting the rock blasting and mucking operation to get underway much sooner than anticipated.

The value engineering enabled the tunnelling contractor to reduce their original bid price of US$32.9M to US$31.2M, a difference of US$1.7M.

In addition to the substantial cost and time savings, the jet grouting procedure provided an additional level of security, as it sealed off areas where significant groundwater inflow was likely. This benefit is exemplified in the cases where several of the 3m diameter drop shafts were being constructed along the tunnel alignment without grouting or ground pre-stabilisation. Pressure grouting was used at T5D to remediate the inflow to a degree. At drop shaft locations T3D and T6D, labour costs per foot were approximately 50% more than at the others, due to the inflowing groundwater.

Related Files
Section through T11D shaft showing geology and support
Measured compressive hoop stress vs excavation depth at T11D
The triple fluid system of jet grouting
Plan of the T11D shaft’s soilcrete system