The pipeline is 1,120 ft long comprising of a 48 inch diameter fiberglass reinforced carrier pipe placed inside a 72 inch diameter steel casing pipe. The microtunnels were constructed in clean, poorly graded sands below the groundwater table with approximately three diameters of ground cover beneath a State Highway and its associated off-ramp structure foundations, multiple sets of railroad tracks, and a variety of utilities. Contact grouting and annular grouting were performed as part of the work.

Background

The City of East Chicago (City) currently operates the existing East Chicago Water Treatment Plant (WTP), near Lake Michigan. The existing WTP has a rated treatment capacity of 24 million gallons per day (mgd). The existing WTP utilises 6.5 million gallons (mg) of treated water storage and a series of high service pumps to supply the City’s water distribution system.

A new WTP is currently under construction to replace the existing WTP, and ultimately increase capacity. The new WTP will be located southwest of the existing WTP, south of Cline Avenue and will have a rated treatment capacity of 17.3 mgd with provisions for expansion to 30 mgd. The facility will utilise a series of high service pumps to pump finished water to the distribution system including the existing 1.5 mg steel elevated tank located on Chicago Avenue.

Raw water from Lake Michigan will be conveyed by a new gravity pipeline installed microtunelling from an intake structure to the new WTP. The new gravity pipeline is 900 ft in length, comprising of a 48 inch internal diameter carrier pipe at a slope of 0.6 per cent installed within a 72 inch internal diameter Permalok steel casing pipe.

The pipeline was constructed in two separate trenchless drives. Drive 1 was approximately 140 ft in length, and was located entirely within city property and passed beneath many critical utilities, including a 48 inch water main.

Drive 2 was approximately 760 ft in length, and passed beneath seven sets of railroad tracks spanning approximately 320 ft and owned by three separate entities, multiple utility lines that included water, electric, gas, and telecommunications up to 48 inch in diameter, and approximately 300 ft of Indiana Department of Transportation right-of-way including a highway off ramp with associated shallow foundations and both the north and south bound lanes of Cline Avenue.

Microtunnelling was chosen as the trenchless method of choice based on the presence of these critical structures, the onsite geology, and the requirement to minimise the risk of ground movement. The intent was for Drive 1 to be used to troubleshoot and eliminate any problems associated with the microtunnelling operation means and methods before performing Drive 2 beneath settlement-sensitive critical structures. Michels Tunnelling was awarded the contract to perform the microtunnelling.

Ground conditions

The project site is located in the Calumet Lacustrine Plain physiographic region of the Northern Lake and Moraine Region in Northwestern Indiana. Bedrock in this physiographic region is generally at depths ranging from 100–150 ft and is overlain by lacustrine deposits.

In general, the onsite soils are derived from a lacustrine environment of beach dunes and regression sequences. The general stratigraphic profile of the site consists of approximately 1 ft of fill material underlain by approximately 30 ft of very loose to dense, poorly graded, fine to coarse grained sand, with less than 10 per cent silt and gravel content, and traces of marine features such as sea shells. This top stratum is underlain by approximately 15 ft of silty sand. Material deeper than 45 ft generally consisted of lean clay with varying traces of silt, sand gravel and shells. Groundwater was found to be approximately 5 ft below ground surface due to the site’s close proximity to Lake Michigan.

Due to limitations stemming from the in-place raw water pipeline system, the new gravity pipeline connecting the existing intake structure and the new WTP had to be installed at depths ranging from 20–25 ft below ground surface (top of pipe) which equates to three to four diameters of ground cover. The material at this depth resembles that of beach sand, clean poorly graded sand, and was classified as ‘fast raveling to running’ above the water table and ‘fast raveling to flowing’ below the water table according to the Tunnelman’s Ground Classification System.

Geotechnical baselines, which the contractor could rely on for bidding and design purposes, were provided on geotechnical baseline sheets incorporated into the contract drawing set.

Geotechnical instrumentation and monitoring

Due to the proximity of critical structures to the trenchless installations (Drive 1 and 2) and running/flowing soil conditions highly susceptible to ground settlement, extra care was taken to develop an extensive geotechnical instrumentation monitoring plan.

In total, seven track monitoring points, eleven monitoring cross sections, six building monitoring points (BMP), and ten surface monitoring points were installed along both drives. All readings were taken by conventional optical survey instruments. Rail track, building, and surface monitoring points consisted of PK nails on ties, columns, and pavement, respectively. Each MCS consisted of four cased stainless steel rods drilled to specified depths below ground. Each MCS monitoring station consisted of one shallow point (5 ft below ground surface) and one deep point (1 ft above crown of tunnel) at the centerline tunnel, and two shallow points offset 5 ft each side from the tunnel centerline.

A threshold value of 0.25 inches, and a maximum allowable value of 0.75 inches of ground settlement, or heave, was written into the contract documents. The threshold value was utilised as the trigger point to start contingency plan implementation; while the maximum allowable value called for an immediate stoppage of mining operation and remediation. The upper limit of ground movement during mining was 0.50 ft in the deep points and 0.36 ft in the shallow points. It was concluded that the majority of the recorded ground movements were caused by vibration from train traffic, complications due to inclement weather and human error in the optical survey.

As part of the contract, railroad flaggers were required to ensure safe working conditions on the railroad tracks and monitoring data was required every two hours during active mining activities and daily if mining activity was stopped.

Co-ordinating with the railroad flaggers, combined with below freezing temperatures, proved to be major obstacles in obtaining the required monitoring readings until tunnelling was completed. Freezing temperatures and windy conditions also affected the visual and physical ability of the surveyors.

Tunnel construction

Drive 1 (140 ft) between Jacking Pit 1 (JP1) and Receiving Pit 1 (RP1) was completed at a rate of 35 ft per day between 12–17 November 2010, and Drive 2 (760 ft) between JP1 and Receiving Pit 2 (RP2) was completed at a rate of 76 ft per day between 30 November–10 December 2010.

JP1 was circular in shape, 30 ft in diameter, and constructed to a depth of 30 ft below existing ground surface using conventional excavation methods and installation of steel sheet piles as the temporary support system. Actual length of sheet piles was approximately 20 ft as a result of sloping down of the top 10 ft around the pit. RP1 and RP2, constructed in similar fashion, were approximately rectangular in shape to accommodate connections to the existing raw water pump station pipework (RP1) and the new raw water pump station pipework. A series of dewatering wells were installed around the perimeter of each pit to facilitate pit excavation and control water flow into the pits during construction.

An Akkerman slurry MTBM, with a 77 inch excavated diameter, was used for the mining. Slurry microtunnelling has several advantages over other types of mining methods and was the preferred method of trenchless installation for two main reasons: 1) the ability to accurately install the pipeline at design grade due to laser guidance and enhanced steering capabilities; and, 2) the ability to regulate the slurry line pressure to provide an active counter resistance to the excavation face. Throughout the mining process, no significant overmining or loss of slurry water to the ground was observed. An intermediate jacking station was installed immediately behind the MTBM in the pipe string but was never utilised.

Poorly graded fine to medium grained sand below groundwater table is highly susceptible to producing flowing ground conditions, much like liquefied sand, if left exposed and unsupported. During Drive 1 mining, the over-cut void (approximately 1.5 inches) between the casing and the ground was immediately filled with flowing sand, and resulted in a rapid increase of jacking load (20 per cent over 20 ft). After completing approximately 70 ft of mining, bentonite was injected through a single grout port installed immediately behind the MTBM trailing gear which reduced the jacking force build up. In order to further reduce and control the skin friction on the pipe string in Drive 2, additional bentonite ports were installed in the MTBM trailing gear and bentonite was pumped at a rate of four bags per casing (20 ft long section). In addition, polymer was added to the bentonite to increase viscosity. The introduction of bentonite and polymer around the casing during mining as opposed to bentonite only resulted in an immediate drop of jacking force (from approximately 350–250 tonnes over 60 ft). The formation of a plastic filter cake around the tunnel helped to stabilise the flowing ground as an added benefit of using the bentonite polymer mix.

The impact of improved bentonite injection used for Drive 2 is evident from the lower jacking force per foot of tunnel. The improvements to the bentonite injection system included additional injection ports immediately behind the MTBM, continuous pumping for the entire drive, and the presence of a full-time mud engineer onsite at all times during active mining. With only skin friction and no face pressure being recorded while pushing the MTBM into the receiving pit, the jacking load for the Drive 1 was about 190 tonnes per 130 ft (1.46 tonnes per ft), and for Drive 2 was about 290 tonnes per 740 ft (0.39 tonnes per ft).

Three sinkholes appeared near the jacking pit. The first sinkhole was in Drive 1 immediately behind the sheet piles, followed by the second sinkhole approximately 12 ft from the sheet piling. The third sinkhole was along Drive 2, approximately 10 ft from sheet piling.

A number of factors contributed to the sinkhole occurrence:

• Ground disturbance during installation of sheet piling;
• The dewatered state of the ground around the jacking pit, and
• The soil being carried from the MTBM face along the pipe string into the jacking pit during the initial launch of the MTBM into ground.

In the case of the sinkhole formed during Drive 2 mining, spoil was washed into the jacking pit for approximately 3–5 minutes, and a hole approximately 4 ft x 4 ft wide and 5 ft deep occurred ten minutes later. Chemical grouting was performed around the entrance eyes for both Drive 1 and Drive 2 but provided little ground stabilisation. All three sinkholes formed outside of railroad, utility and roadway zones of influence and were backfilled and stabilised immediately.

Contact grouting was performed through the grout ports fabricated into every casing section. To prevent the likelihood of flowing sand entering the tunnel while uncapping the grout port, each grout port was pre-installed with a ball valve before the casing was jacked in the ground. Grouting cut-off pressure was kept below 10 psi (generally just above the hydrostatic head) to prevent blow-out and buckling of the steel casing. During contact grouting, two grout valves on either side of the active grout port were opened and observed for bentonite venting and grout return. Grouting of the annular space between the casing and carrier pipe was performed via slick lines placed in the annular space. Bulkheads were constructed at the ends of both Drives 1 and 2 to contain the grout and help ensure complete encapsulation of the carrier pipe.

Conclusion

The project’s success can be attributed to the extensive settlement monitoring, low settlement and heave threshold limits, fast contractor remediation, careful attention to machine pressures, and using Drive 1 with the least settlement-sensitive structures as a learning-curve.

Schedule co-ordination and continual discourse with third parties including utility companies and railroads throughout the microtunnel operation led to well-informed, effective relationships that did not impede construction efforts.

The introduction of the bentonite polymer solution as pipe lubricant was a major factor benefitting ground stability and jacking load reduction. The use of remote automated instrumentation for settlement monitoring would increase efficiency, allow for fast data transmission, and eliminate human error and the number of personnel required.