Case Study: Horizontal Infrastructure 01 · PDF filecreate a slurry that can be displaced to...

Case Study:Horizontal Infrastructure 01

Parklands West Infrastructure Repair. September–December 2014

New Horizontal Civil Infrastructure: Gravity Wastewater Installation, Parklands West, Christchurch.

Project tags: Horizontal Infrastructure, Horizontal Directional Drilling (HDD), Trenchless technology

Truline Civil Limited 174 Marsden Road West Coast, Greymouth 7805 T 037686862 [email protected] http://www.trulinecivil.com

Truline Civil Limited

Gravity Wastewater Instal lation, Parklands West.

Project overviewThis project was necessitated by the urban development, the urban extension particularly in the Prestons area. Major

developments of Ngai Tahu’s on both sides of Prestons Road. Prestons is the city's biggest subdivision, designed for 2,600

homes and 8,000 residents. It is being developed by CDL Land NZ, Ngai Tahu and Foodstuffs South Island. The area was

well under-capacity for the discharge of sewage to the city's treatment facility at Bromley. Which required an upgrading of

the sewer main to a 350 mm diameter pipeline. It is quite a significant sized pipe, and it also is in parallel with the existing

wastewater pipeline carrying effluent from the Belfast freezing works. The only realistic corridor was through the new

subdivision at Tumara Park. Which is Putake Street, Rothesay Place. That was a high quality recent development which was

not realistic to open trench at the required depths of around 3–4m. Hence the requirement to Directional Drill. This was

undertaken in very soft, highly saturated silts, with a high water table requiring de-watering of entry and receiving pits and

special treatment through the drilling process due to the in-situ groundwater pressures both due to the depth of the pipeline

lay and the groundwater table. Free-flowing saturated sand.

Client: City Care

Principal Stakeholder: Ngai Tahu Property

Duration: Sept–Dec 2014

Project Engineer: Aurecon

Site Supervisor: Grant Senior

Client Project Manager: Fathi Hassneiah

Consultant Engineer: Aurecon

HDD Operator: Liam Richardson

Drill shot overview

PIlot drill-shot diameter: 125 mm

The gravity wastewater main pipe diameter: 350 mm

Back reamer: reamed a 380 mm hole on the way back

For Christchurch's growth it is a required additional discharge, high-capacity pipeline to cope with urban expansion. Fitting a

steel 90° “elbow” bend in the main sewage line at over 4m depth in very challenging, saturated sandy ground, and within 2m

of an existing operating wastewater pipeline. This ingenious back-to-front drilling method both keeps the entry pit dry, and

yet also allows pulling both ends of the 350 mm Wastewater line through the steel walls of the pit for subsequent welding to

the right-angle bend—all without damaging the new pipe, or disturbing the existing infrastructure. The backreamer displaces

the additional 225 mm diameter of the borehole as cuttings. The ability to suspend the cuttings, and the ability to flow

cuttings out of the hole as a sediment laden slurry are important. The mixing ability of the back reamer is equally important.

During the back-reaming process, the main objective is to mix the cuttings from the back reamer with the drilling fluid to


Gravity Wastewater Installation, Parklands West. Sept–Dec 2014 1

create a slurry that can be displaced to the side of, or discharged out of, the bore hole to make room in the hole for the

product being pulled back.

Interview with Grant Senior“This is a main sewer line that is being laid on a grade. It is all on pretty flat grades, 1:500 roughly, and because we have only

got a tolerance of +/-5 mm, we have got to be pretty accurate. First we have to “map”, or GPS survey down the path of

each shot on the ground, map it out by spraying down GPS waypoints every metre all the way along every pilot drillshot

path, this provides a highly accurate map on the ground which defines the path we must align the pilot drill to perfectly in

order to meet the very tight specifications.

So we mark down a GPS reading every meter and drill as close as we can to it.”

Very challenging near-surface conditionsBut the near surface conditions that this pilot shot is going into is running sand due to the very high water table. The water

table is only about 1m down, and we are drilling down about 4 m, so at all depths 1m and below the sand is pressurised

and completely saturated with water, and therefore, because it is running sand it is very easy for your pilot drillshot to go off-

line, and likewise your pull-back of the new pipeline. Yes, so there is that and there’s also the fact that getting your “bite”

back in that ground is very difficult.



Parklands West Infrastructure Repair site map

Keeping the pilot borehole open in running sandThe challenge is trying to keep your borehole open because it just wants to keep on collapsing continually. The reason it

does that is… it’s like, when you stand on the shoreline at the beach and the wet sand washes around you feet, and you

sink down into it, then when you try to get your feet moving again, you find that you’re well and truly stuck because the wet

sand has sucked onto your feet like quicksand. Well, if you get a 100 m length of steel drill rods underground you can

imagine how if you get a couple of little wee collapses in the drilling fluid encasing your drill shot, it only has to be a small

collapse in your Drilling Fluid “jacket” in the middle of a 100 m line of your drilling rods—the running sand, will fall in under

pressure and adhere to the steel pipe, it literally sucks onto the drilling rods, like an adhesive or a vacuum, and there is just

no amount of horsepower that is going to shift it.

For drilling in running sand we use the DrillFlex® systemWe use a DrillFlex® System, which is a very thin fluid as our drilling fluid, it flows easy but when it’s not under pressure it will

spring out and “hold it”, by which I mean the DrillFlex® fluid does the job of holding the borehole open and keeping your drill



L to R: The HDD bit and sonde. The Putake Rd site just prior to the next drillshot entry trench being dug. The pilot Rothesay Road drillshot set 30m back from the destination 4 m deep in the 90° bend sheet piling pit gets underway with a little help.

rods jacketed in fluid. Then it will set-off again really fast, meaning that when your fluid starts pumping again, when it is once

again under pressure and needs to move, it will all fold over and start flowing again.

As an operator If you think “Gee I’m in the sand I need to use heaps of bentonite” your mud slurry gets really thick and what

you can actually end up doing is when you start pumping again you’ll rip a big hunk of that filter cake off the side of your

bore hole and the bore will collapse in.

So there is a lot to consider with these saturated sand near surface conditions;

• you’ve got to try and stop the water coming in, and

• you’ve got to try and hold the sand out, and

• to get your flows right in these sorts of conditions too without washing your borehole clean is a bit tricky too.

The quality of our HDD installation is very tightly monitored by consultant engineersOnce the Gravity Wastewater Line is installed the consultant engineers have been potholing down on it every 9m and GPS-

ing it to check that we are on line with our tracking to the path and the gradients or fall of the pipeline—and if we are out by

more than 5 mm we have to dig it up. So we’ve got to be perfect, we’ve got to be spot-on every single time.

Near surface conditions vary from sandy to layers of peatAnd in some of these near surface conditions, staying true to your tracking and gradients that’s really difficult to do; there’s

layers of peat down there that you cannot anticipate, with roots and tree stumps and all sorts of things which can bind-up in

your back reamer as it rotates, causing it to wander off the true line of the pilot bore.

How do you install a 90° bend in the new Wastewater pipeline right alongside an operational

main sewer at a depth of over 4m in running sand? What we've actually done here is we've set ourselves back, set the HDD back 30m or so, and we've drilled down into this

sheet-piling, into the 4m deep entry pit in preparation for the Pilot Drillshot up Putake Road. Once we have pulled back the

new pipe, the Putake Drive pipe into the entry trench we will be able to fit the 90° bend joining it to the Rothesay Rd pipe.

How did you get the Rothesay Road pipeline into the entry pit without damaging the

existing pipeline? We used the same “back to front” method we are about to use. You see, down at the bottom of that pit at a depth of

roughly 4m, is the 500 mm diameter actively pumping sewer from the Belfast freezing works—it is an older pipeline made of

strong but relatively brittle reinforced spun concrete.

The design of the new Wastewater network

we are installing allowed us a mere 25 mm

of tolerance vertically beneath that pipeline.

To drill in under that we had to drill through

the pit perpendicular to the existing pipeline

and get the pilot bore to pass just

underneath the pumping Belfast freezing

works sewer. It was critical that we

absolutely stayed right on line there—I mean

if your steel drilling rods flexed or whatever,

they would just push straight up through the

sewer—and bear in mind that it’s not just

sewage as such, it is all the chemicals,

effluent and so on. So, Superfine tolerances,

no room for error.



The 90° bend in the pipe to connect the new Rothesay Road and Putake Drive segments of the wastewater line..

Keeping the entry pit dry over 4m down in near-surface conditions of running sand The preparation for this next shot is another example of the same method, the aim is to prevent all the sand just flowing in a

400 mm entry hole large enough for the new pipeline and filling up the entry pit. To prevent that, what we have done instead

for this drillshot, is to cut a 150 mm diameter entry hole into the sheet-pile wall, a 15 cm target hole is only just big enough

for our 12.5 cm pilot-bit to enter the pit.

Forming and excavating the sheet-pile entry pit Sheet-piles, as you can see, are a system of big steel pile sheets that all interlock (to form a box) and we drive them in with

the digger 8m down to make the walls of a trench pit that is 5m deep into water-saturated sand. Once the “box” is formed

below ground we dig the pit out inside the sheet pile box. Then there is the matter of keeping the pit dry, which I’ll get to in a

minute.

Sub-surface steel walls and aluminium shields reflect the HDD locator signal transmitted

from the sondeWhen you are coming through in proximity to the sheet-pile trench wall with the sonde it’s really difficult because you’ve got

your sonde, which sends out a combined GPS locator, depth-finding signal—a signal which is transmitted upwards and out

in a “V” shape—and if the signal hits any steel, especially a wall of steel, it reflects back, interferes and distorts giving you all

sorts of confused readings. This means that when you get within a few metres of your sheet piling box trench, or even an

aluminium trench shield, the reflected signal starts throwing all the sonde readings out, making them unreliable, and it is only

the experience of dealing with this difficulty as an HDD operator that means you are able to get the pilot bit to go precisely

where you want it to.



The 5m deep destination sheet-piling pit containing the operational main sewer.

Overview of the critical first 30 m of the Putake Drive pilot boreSo, in overview, we are still managing to “pile it in”… in other words, we sheet-pile a pit over 4m down to the specified

position of the new Wastewater line 90 degree bend that we are installing and we must be precise and work very, very,

carefully around the existing pumping sewer from the Belfast Works.

Next step is that we drive the pilot bit to enter the tiny entry hole cut out of the pit wall. We are doing this by positioning the

HDD set well back from the pit. The entry point for this shot is a 30 m distance from the pit. Over that 30 linear metres of

approach to the pit the pilot bit descends to a depth of over 4 m and by being precisely on track and on-gradient we will get

the pilot bit to pass through that critical 15 cm diameter target entry hole I mentioned—bear in mind that hole is 4m down

and no bigger than your head will fit through, so we have no room for error whatsoever. You can imagine if the pilot shot

goes off-line at all to the left, the right, up or down, bang! You’ll hit the sheet-pile and you’re stuffed! You see the thing is, in

sand you’re not going to get a second chance, you can’t just pull back and have another go at it “it’s too late mate!” So

yeah, on this drillshot our aim is to drill in, through this hole in the sheet-pile, parallel to this big Belfast sewer, out through the

other sidewall of the pit to an end point receiving pit 90 m further down Putake Drive.

Pulling back the two sections of new Wastewater line to weld the 90° bend in the pitFrom that point up there in the receiving pit at the end of the pilot bore, 90 m up Putake Drive, there we’ll attach the

backreamer and pull the sewage pipe back into this pit. Note, we did have trouble with pulling the section into the pit on this

side of Rothesay Rd, where we’ve got the reamer all tangled up in roots in a horrible peat layer, we had to cut the pipe off

and pull a short piece back across the road. But that is exceptional, so far we have installed this new sewer line from right

down near Burwood Road down there, come all the way to here so we’ve completed a few hundred metres of installation.

Then from here out to Mairehau Rd we’ve got another 6–700m to do.



As the sonde comes into within a few metres of the sheet-pile trench wall it becomes increasingly difficult to pick up the GPS and depth-finding signal, this is when the skills of well-trained, experienced HDD operators come into play.

Keeping the 5m deep entry pit dry by means of a high capacity “well-point dewatering

system”The 5m deep sheet-pile retained pit in water

saturated sand is kept dry by a “well-point

dewatering system”. You cannot just stick a

big pump down into the ground 5m and

“suck away” to keep your entry pit dry,

because of course that’s not going to work,

you’ll just end up sucking sand, not water,

and then the whole area around the pump

will just collapse in on you. So instead, what

you have to do is put down all these well-

point lances against the sheet piles. and you

can see all the water pumping out through

the system of well-point lances, they are all

pumping flat-out and the water table is so

low here that the system can’t keep up. In

these sub-surface conditions, if you wanted

a completely dry hole you’d need—see how

we’ve got two pumps going, big pumps—to

keep the entry pit completely dry you’d have to set up another system of well-points around, further out from the sheet-pile

walls of the trench, and put in an additional pump system. These two big pumps are currently tapped out pulling this amount

of water out of the ground to give us a dry hole.

So you can’t actually see the exit hole into the sheet-pile trench where the pilot drill will exit the trench headed for the

terminal point of the drillshot, 90m up Putake Drive, you can’t see the hole because the sand has all bled in.

So how do you pull a new 350 mm Wastewater line through a steel wall at a depth of 4m in

running sand without your entry pit filling in on you?What we do is pull the 350 mm sewage line back with the HDD towards the wall of the pit until the collar is just sticking

through the 15 cm diameter entry hole we’ve cut in the sheet-pile (the collar hooks onto the back of the reamer, and then,

attached to the reamer is the new sewage line). This means that when the reamer is right there, pressed right up against the

back of the sheet piles, we get the gas in, cut a hole, then immediately pull the line the rest of the way through the sheet-pile

into the trench. This keeps to a minimum the amount of liquid sand that bleeds in through the larger diameter (400 mm) hole

the sewage line has just pulled through into the trench.



One of the two pumps dewatering the sandy soil adjacent to the 5m sheet-pile pit.

The pilot drillshot that started 30m back up Putake Drive, enters the sheet-pile pit wall on the left precisely through the centre of the waiting 30 cm diameter hole cut 4m down in the sheet-pile trench wall. The Rothesay Rd striped wastewater

So we pull the new Wastewater line back to the GPS point where the 90° elbow bend is fitted to the pipe. As you can see

this trench is pretty deep down into wet sand, over 4m deep here, Unfortunately you can’t really see what we’ve done but

the surveyor was here this morning and measured all of this out and he found that we are within 2 mm of specification.

With a pilot bore through running sand, how do you pull back the Wastewater pipeline precisely on the 1:500 gradient?To recap quickly, this first pilot drillshot is starting from just up Putake Drive there, just beside the container 30m away, We’ll

drill down through this point here in the trench wall 4m down, where we’ve already cut the entry hole in the sheet-pile—you

can see sand bleeding in there which has covered the hole so it is obscured. We’ll be drilling through there, out the other

side of the trench and off up Putake Drive and then we’re going to carry on from there, drilling all the way down to Mairehau

Road.

So this first shot we’ll pull back down from the pothole at the endpoint 90m up Putake Drive there. We’ll pull this first shot

back into the trench here, and there you can see blue steel “elbow bend” unit over there that is going to be welded into the

sewage line at that precise GPS point 4m down in the trench.

From the end point of this first shot we will go further up Putake Drive and drill back in this direction to the end point

receiving trench of this first shot, towards this trench. So this shot we will be drilling our way towards the entry point 90m up

Putake Drive and then laying, or installing the new sewer line back to the 90° bend here by pulling it back towards this

trench. So our method, doing the drill-shot and the back-reaming this way, back towards the trench. This method seems

back-to-front, but, as you will understand in a moment, the reason that we are pushing out a pilot from this trench and

pulling the pipe back towards it is because, what is not obvious to the casual observer, is that when we must pull the pipe

back into the trench here, on a highly accurate grade with a 5 mm tolerance. In order to be able to achieve that we

need a long, flat tail on the pipe at the point where it enters the pilot bore, pulled back by the HDD. Putting it another way,

the new Wastewater pipe must be introduced into the pilot hole, perfectly in line with the pilot shot in running sand. If it is not



Detail of a de-watering lance.

perfectly aligned, the pipe will go “off grade” as it is pulled back. As you can see here, if we drilled the pilot bore in the other

direction and pulled back from the other direction up Putake Drive there, well… who wants to excavate a trench to enable a

long tail here in these ground conditions, a trench 4m deep extending back from to the sheet-pile pit and risk having it all

collapse on you? There is that to consider and the fact that pulling a sewage pipe down through a small hole into the pit and

out the other side without damaging it as it passes through the steel walls would be nigh-on impossible. But because we

use this back-to-front method, then as you can see we are pulling the new Wastewater line in nice and flat, and perfectly on-

grade with no risk of damage to the pipe.



The site of the next destination pit along Putake Drive. The 4m deep sheet pile pit is on the horizon.

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