Respecto al objetivo específico N º 01:

Patrick J.Doig A.C.S.M., C.Eng., M.I.M.M.

S.A. Healy Co., McCook, Illinois, U.S.A. (formerly Cementation Co. of America, Inc.)

SYNOPSIS

The Inline Pump Station contract was a $10M project that formed an integral part of the Milwaukee Waste Pollution Abatement Program.

The contract involved the sinking of three shafts through saturated overburden and dolomite limestone, and the construction of a large chamber and connecting tunnels.

This paper describes in detail the variety of underground construction techniques required, including shaft freezing, drill and blast excavation, grouting, shotcreting, concrete lining and large diameter pipe installation.

DESRIPTION OF CONTRACT

The Inline Pump Station Contract was bid in August, 1984 and won by the Cementation Company of America Inc., with a bid of $10,012,133. The Engineer’s Estimate was 13% below the low bid. The time allowed for completion was two years. The works had been designed by Howard, Needles, Tammen & Bergendoff and contract administration was by CH2M Hill. The owner was the Milwaukee Metropolitan Sewerage District.

The contract was located adjacent to the existing sewage treatment plant on Jones Island, about one mile south of downtown Milwaukee on the shores of Lake Michigan. The worksite was in the midst of an area of intensive activity related to the Waste Pollution Abatement Program and was somewhat limited in size,

The various elements of the work were as follows:

1. Screening Shaft: 107m (350') deep with 6m(20') diameter in-situ concrete lining and internal divider wall; stub connection to 9m(30') diameter Crosstown Interceptor tunnel and connection to 3m(10') diameter Suction Header tunnel.

2. Ingress/Egress Shaft: 107 m(350') deep, 6 m(20') finished diameter with connection to Pump Chamber.

3. Equipment Shaft: 94 m(310') deep, 6 m(20') finished diameter terminating at roof of Pump Chamber.

4. Pump Chamber: 18 m(60') wide by 12 m(40') high by 34 m(110') long with reinforced concrete walls and shotcrete roof.

5. Suction Header: 82 m(270') long tunnel, incorporating 3 m(10') diameter prestressed concrete pipe liner feeding into three 14 m(45') long tunnels with 1.4 m (4.5') diameter stainless steel liners.

The Inline Pump Station is at the terminus of the Inline Storage System. This is a system of tunnels designed to provide emergency storage of combined sewage and storm water during periods of excessive rainfall. When construction is complete, material will flow into the Screening Shaft where it will pass through coarse screens into the Suction Header Tunnel. From here it will be fed to pumps located in the Pump Chamber, which will pump it to the surface treatment plant.

Notice to Proceed was issued on January 3rd, 1985 and mobilisation commenced immediately. The program envisaged commencing with the Screening Shaft, then the Equipment Shaft and finally the Ingress/Egress Shaft. The Suction Header and Pump Chamber were to be done once access was established

GEOLOGY The stratigraphy at the Inline Pump Station site was as follows:

0–5 m

Silty to gravelly sands, medium dense to very dense.

8 m–25 m (26'–8')

Estuarine Deposits

Layers of medium stiff to stiff silt interspersed with layers of medium stiff clay.

25 m–26 m (82'–85')

Alluvial Deposits Dense sandy gravel 26 m–39 m (85'–128') Lacustrine Deposits

Medium dense to very dense silty sands with stiff clay interbeddings, 39 m–44 m

Silty sands, medium dense to very dense with thin clay layers.

47 m–51 m (155'–168')

Lacustrine Deposits

Medium dense to dense glacial till with small to medium boulders.

Below–51 m (168')

Dolomite

Dense, close to medium bedded with zones of numerous very thin shaley partings, vuggy at depth.

Unconfined compressive strenths 140 Mpa (20,000 psi) average, 300 Mpa (44,000 psi) maximum.

The water table fluctuated seasonally but was generally about 4 m(13') below surface. Permeabilities in the overburden were around 10–6 m/sec and ranged from 10–9 to 10–5 m/sec in the rock. Individual flows in the rock measured upto 13 l/sec (200 gpm) and were usually associated with the vertical or near-vertical joint planes.

GROUTING

Pre-grouting of the shafts and Pump Chamber from surface was specified and was carried out at the direction of the Engineer.

Pre-grouting of the Suction Header from surface was not specified. Additional grouting was directed from underground during excavation.

Surface Pre-Grouting

Pre-grouting of the shafts, which were to be excavated to a minimum 6.6 m(22') diameter, was carried out through holes on a 9 m(30') diameter circle. All holes were drilled to the rock and then advanced in 6 m(20') stages to a final depth of 111 m (365').

Initially, three primary holes were drilled at 120 degrees, followed by three secondary holes between the primaries. In order to check the efficacy of the cover, a centre tertiary hole was then drilled. At the Ingress/ Egress and Equipment Shafts, no further holes were deemed necessary. At the Screening Shaft, two further holes were installed and grouted, both within the limits of the excavation.

Pre-grouting of the Pump Chamber was accomplished with a grid-like arrangement of holes along lines 3.4 m(11') apart with holes spaced 8.2 m(27') apart along the lines within the Chamber. Half this spacing was used for the lines along the north and south walls of the Chamber.

The procedure for each hole was as follows. An NQ size casing was drilled through the overburden and about 600mm (2') into the rock. The casing advancer was then withdrawn and a BQ size drill string entered into the casing. The hole was then advanced 6m(20') in the rock and the drill rods withdrawn.

A packer was then set at the top of the stage and a water test conducted. The Lugeon value was calculated and used to determine if grouting would be necessary. If grouting was required, injection would start with a 5:1 cement mix, followed by successive thickening, until practical refusal had been reached. When the grout had set, the packer was removed and the hole advanced another 6 m(20'). Ultimately the hole was abandoned by filling to the surface with grout and removing the casing.

Drilling of the holes was carried out with either a truck-mounted Longyear HC44 or a track-mounted Acker MPV. Injection was with a diesel-hydraulic mixing and pumping unit incorporating a Moyno 6P6 pump. In general one hole could be drilled and grouted in around 60 hours. During the surface grouting program some 5,000 bags of cement were used. No chemical grouting was done.

Underground Grouting

Virtually no advance grouting was done in the shafts during the excavation, despite grounwater inflows increasing with depth.

Some spot grouting was carried out in arrears.

The Suction Header tunnel was advance-grouted with a series of 15 m(50') long covers. These involved the drilling of four holes at the corners, which were then grouted. A centre test hole was then drilled and a further four holes drilled if required.

The holes were grouted in one stage and all grouting was with cement.

FIGURE I

LAYOUT OF INLINE PUMP STATION 134

A full grout curtain for the Pump Chamber roof was installed from a pilot drift. This was ordered due to the generally wet nature of the rock encountered in other parts of the excavation and the fact that the roof was to have a shotcrete lining. Fans of 9 m(30') long holes, 3 m(10') apart and with a 3 m(10') toe spacing were drilled. These holes were injected in one stage initially with cement. In an effort to dry up the roof as much as possible, chemical grouting was also carried out. Some 2,700 m (9,000') of drilling was required and about 1,000 bags of cement and 7,500 1(2,000 gals) of chemicals were consumed.

Drilling was with track-mounted Gardener Denver PR125 pneumatic drills, consistently achieving 90 m(300') per shift.

Injection was with Peroni piston pumps.

FREEZING

Because of the extent of saturated overburden and a prohibition on the use of dewatering, it was decided to freeze the shafts.

Foraky (UK) were employed as freezing consultants.

Freeze holes were drilled around each shaft on a 10 m(34') diameter circle. There were 34 holes in all, of which half were drilled to depth 55 m(180') and half to depth 27 m(90'). The alternating of holes was to allow a rapid build-up of ice in the top part of the shaft, without a subsequent encroachment at depth. In addition, two observation holes were installed 1.2 m(4') and 2.4 m (8') outside the circle of freeze holes.

The holes were drilled by Test Drilling of St Louis using an Ingersoll Rand rig. Holes were drilled 180 mm(7") diameter with a tri-cone bit under mud containing 1.2 kg of bentonite per litre (10 lb/gal). A 6 m(20') conductor casing was used.

Progress during the course of the work averaged around 6 m(20') per hour.

As each hole was completed, a 100 mm(4") diameter casing with a closed end was placed in the hole. When all the holes at the shaft had been drilled and cased, the deeper holes were surveyed using oil well techniques. This service was provided by both Eastman Whipstock and Sperry Sun. Surveying of the holes was required to establish if tolerances had been met. If two holes had diverged an unacceptable amount, the casing was removed from the intervening shallow hole and it was deepened to 55 m(180'). In rare cases, an additional 55 m(180') hole was drilled. The casings were also pressure tested for leaks.

Thereafter, a 50 mm(2") diameter polythene hose was placed inside the casing to act as a delivery tube with return up the annulus. The holes were connected up to 200 mm(8") diameter delivery and return headers and thence to the brine circulating unit. This had “warm” and “cold” compartments which actually differed in temperature by about 0.5C(1F). Three pumps each capable of 40 1/sec (600 gpm) provided circulation. All piping was insulated.

The system was charged with calcium chloride brine which was delivered to site as a 38% solution. This was diluted to give a solution with a specific gravity of 1.29. Cooling of the brine was achieved by passing it through two 64t(70T) refrigeration units, These comprised a heat exchanger using ammonia as the refrigerant, which was cooled by a 150kw (20hp) York compressor.

The two observation holes were equipped with thermocouples at vertical intervals of 6 m(20'), which were connected to a Honeywell recorder.

Once the system had been established, brine circulation began at a rate of 60 1/sec (1,000 gpm). Brine was quickly brought to below −30 C (−20 F). Closure of the ice wall was indicated by water rising in the central pressure relief hole. This took around 25 days. Development of the wall to design strength, as determined from observation hole temperatures, took a further 7 days. At this point excavation commenced.

Freezing continued during excavation and lining, with the flow of brine being regulated in an attempt to maintain the ice wall width without greatly increasing it.

Freezing of the Screening Shaft was discontinued in November, 1985. Readings taken at 55 m(180') depth in the observation holes, indicated that temperatures did not rise above freezing until March, 1986.

SHAFT CONSTRUCTION Freeze Excavation

The basic method of excavation was with a 1.34 cu.m (1.75 cu.yd.) clamshell, suspended from a 45t(50T) crawler crane.

Hand-held pneumatic breakers were used to trim the walls. As the ice encroached further into the excavation, a Kubota KH28 mini-backhoe fitted with a hydraulic breaker was introduced to break up the major amount of the frozen ground. Ice encroached 600 mm (2') down to around 40 m(130') depth, increasing to about 1 m(3') at 43 m(140') and freezing all the way acrooss the bottom of the shaft at 46 m(150'). At these depths drilling and blasting was commenced with the backhoe being used to load into 2.5 cu.m. (3.3 cu.yd.) muck buckets. Drilling of blast holes was done with Victor air augers.

Liner plate and ring beams were used to secure the top 6 m(20') of the shaft and thereafter mesh and vinyl were pinned to the wall to guard against falling material. Insulating blankets were hung for about 12 m(40') below the liner plate to counteract the thawing effect of the sun. The shafts were sunk to 55m(180') before being concrete lined.

Excavation progress per eight-hour shift was around 1.2 m(4') in unfrozen ground, 600–900 mm (2–3') in areas with less than 600 mm (2') of ice encroachment and 300– 600 mm (l–2') with the majority of the shaft floor frozen. A standard crew consisted of a foreman and four miners with a crane operator and toplander on surface.

Rock Excavation

Excavation was by drill and blast using hand-held sinkers. Where the shaft was wet, half-sump rounds were taken. If conditions allowed, a full-sump was blasted. In general, full rounds were 1.2 m(4') deep, requiring 110 holes for a full-sump.

Unigel explosive was used for the bulk of the holes and Hercosplit for the trimmers. Nonel half-second delay detonators were used for initiation A powder ratio of around 1.6 kg/cu.m. (2.75 lb/cu.yd.) was achieved. Holes were drilled 40 mm(1−5/8") diameter with cruciform bits and hexagonal steels averaging around 90 m(300') of drilling per unit.

Mucking was generally with the Kubota, although an Eimco 630 was also used. The Kubota could load a 2.5 cu.m. (3.3 cu.yd.) bucket in four minutes compared to seven minutes for the Eimco 630.

Support was by the use of 1.5 m(5') long split-set rock bolts arranged on a 1.2 m (4') pattern. Mesh was added by the use of 450 mm(18") split-set inserts where required.

Crew make-up was the same as for frozen excavation with three eight-hour shifts being worked each day. A typical cycle for a 1.2 m (4') advance was as follows:

Drilling 7.0 hrs

Lining of the shafts was generally done in two lifts. Initially the shaft was lined from 55 m(180') depth to surface prior to turning off the freeze. Upon reaching final depth, the remainder of the shaft was lined.

Lining was carried out with a 6 m(20') high steel form, working from a three-deck stage. The form was hung from the stage with four chainfalls. The stage was suspended from two 40 kw(50hp) New Era winches, each with a 23,000 kg (50,000 lb) line pull. Concrete was lowered into the shaft in a 1.2 cu.m. (1.5 cu.yd.) bucket and placed in the form through elephant trunking.

Design thickness for the concrete was 450 mm(18") in frozen ground and 300 mm (12") in rock. The shaft was excavated to 7.2 m (23.5') in frozen ground and 7m (23') in rock. In each case, the average thickness of concrete was 600 mm (24").

Around 80 cu.m. (100 cu.yd.) of 28 Mpa (4,000psi) concrete and 640kg (1,400 lb) of rebar were needed for each pour. Joints were built with 150 mm (6") deep waterstop.

Using a standard mining crew working three shifts, it was possible to achieve a pour per day. Additional time was needed to set up the operation and further installation of panning in wet areas. Patching and contact grouting was carried out as a separate exercise towards the end of the contract.

Screening Shaft Internal Divider Wall

The divider wall was required to allow screening for the material entering from the 9 m(30') diameter tunnel, to provide guideways for a clean-out clam to be lowered from surface and to provide a housing for a surface overflow pipe. As such, it was very intricate.

It was formed by using a 4.6 m(15') high steel form built by Economy Forms, using a combination of standard and special panels. The form had to be broken down into four sections for each move. The work platforms were integral with the form.

The wall included 12 steel guides, a 900 mm (36") diameter steel pipe and very dense rebar, inhibiting the ability to vibrate internally. Superplasticised concrete was therefore used to ensure a satisfactory finish. The design quantity of concrete for each pour was 28 cu.m. (36 cu.yd.) with actual usage indicating a 7% wastage factor.

136

The standard crew was used with the addition of two carpenters. After a very long learning period, production reached one pour every 40 hrs.

SUCTION HEADER CONSTRUCTION

The Suction Header was designed as a length of 3 m(10') diameter conduit, feeding into three 1.4 m (4.5') diameter conduits.

The 3 m(10') section was to be lined with pre-stressed concrete pipe and the smaller sections with stainless steel pipe.

Suction Header Tunnel Excavation 3m (10')

As the concrete pipe was 3.6 m(11'8") external diameter, it was decided to mine the tunnel as a 4 m(13') modified horseshoe.

Drilling was with a Joy, crawler-mounted two-boom pneumatic jumbo. Sixty holes, 3 m(10') long, were drilled per round and these were loaded with a combination of Unigel and Hercosplit and initiated with Nonel detonators. Explosives consumption was 3.9 kg/cu.m. (6.8 lb/cu.yd.) and the average advance was 2.7 m (9'). Drilling was with 45 mm (1–3/4") diameter button bits averaging 90 m(300') per unit and 25 mm(1") round steels averaging 170 m (550') per unit.

Mucking was with a 1.2 cu.m. (1.5 cu.yd.) International crawler loader dumping directly into muck buckets at the Screening Shaft. Support was provided by split-set roof bolts as required.

The excavation was continually interrupted by having to stop to grout and by the presence of large flows of water. Towards the end of the drive, water became less of a factor and a typical cycle for a 2.7 m (9') advance was as follows:

Drilling 7.9 hrs

The concrete pipes were supplied by Lock-Joint Co of New Jersey. They were 3 m(10') long and weighed over 18t(20T).

They incorporated a double gasket, bell and spigot joint.

A purpose-built, hydraulically-operated pipe carrier was obtained for the transportation and placing of the pipes in the tunnel. This consisted of a long needle beam with wheels and outriggers at either end and a set of bull wheels at the front. As the carrier ran on track, a sub-invert was poured, incorporating 100 mm(4") angle-iron as track sections. These were set to line and grade so that the pipe could be laid directly on them.

The first pipe to go into the tunnel had an integral bulkhead which prohibited the use of the carrier. For this pipe, a steel cradle was fabricated which ran on Hillman rollers. The pipe was lowered down the shaft with a 145t(160T) American crawler crane and pushed into the tunnel with a Caterpillar 910 loader. Once in position, jacks were used to lower the pipe and cradle onto the rail. The cradle was left in place and the rollers withdrawn.

The procedure for the remaining pipes was to lower them to the bottom of the shaft and thread the carrier through. The pipe was then pushed to within 1.5 m (5') of the previous pipe and the front bull wheels lowered into the previous pipe. The front carrier wheels were raised and the pipe pushed to the point where the joint was about to close. The front and rear outriggers were then lowered, the front ones inside the previous pipe and the rear ones onto the tracks. The pipe was then lowered onto the track and pushed home with jacks.

This operation originally took a number of days due to the the gaskets on the pipe being oversize, making joint closure difficult. Once this had been corrected it was possible to install a pipe in eight hours.

Backfilling of the pipes was done in 25 m(80') lengths. Slick lines were placed to the exterior of the pipes and the end pipe bulkheaded. A 100 mm(4") diameter delivery line was installed from surface down the shaft and into the tunnel. A specially designed 14 Mpa (2,000psi) backfill concrete incorporating a high percentage of fly-ash, was used. This was discharged from ready-mix trucks on surface into the delivery line and flowed under its own head through the delivery and slick lines and around the pipe. No problems were experienced with blockages.

Tunnel Excavation and Pipe Installation 1.4 m(4.5')

These tunnels were driven as 2.4 m(8') square headings from the Pump Chamber using jacklegs. Mucking was with an Eimco 630 dumping directly onto the Chamber floor. These three headings were a total of 40 m(135') in length and were excavated in 17 shifts.

The 1.4 m(4.5') pipe liners were supplied by Progressive Fabricators of St Louis as a straight and elbow section, which bolted together and onto the concrete pipe. Installation required no special equipment. Timbers were laid on the floor of the heading and the elbows were pulled in with a chainpull. The straight sections were pushed in with a loader. Each heading required 7 shifts for pipe installation. Backfilling of the pipes was done with a Conspray concrete pump, using exterior slick lines.

PUMP CHAMBER CONSTRUCTION

The Pump Chamber was designed with an elliptical roof, curved sidewalls and flat end walls. Resin dowels, 5 m(16') long and 25 mm(1") in diameter, were required at 2.4 m (8') centres in the roof and walls.

Excavation

The Chamber was opened up with a 5.5m (18') wide by 3.7 m(12') high pilot heading driven along the roof from the Equipment Shaft to the Ingress/Egress Shaft. This was excavated with the Joy jumbo and the Caterpillar 910 loader. The pilot heading was used for the grout cover and the installation of extensometers and dowels. Thereafter, the roof of the Chamber was opened out to full width and the remaining dowels and the shotcrete lining installed.

The bulk of the Chamber was excavated in three lifts from a sinking operation at either end. Blast holes were generally

The bulk of the Chamber was excavated in three lifts from a sinking operation at either end. Blast holes were generally