Mauka Is the Hawaiian word for inland or towards the mountain, which is exactly where the City of Honolulu plans to tunnel soon. The Hawaiian island of Oahu was formed by two volcanoes more than three million years ago, Waianae, on the western side, and Koolau, to the east. They are now both extinct and home to the state’s capital, Honolulu, and most populous island. ¬
Like many cities in the US, Honolulu is dealing with aging sewer infrastructure and sewer overflows. To meet a consent decree, it is constructing a 3-mile (4.8km) long wastewater tunnel with an inside diameter of 10ft (3m) through the former caldera of Koolau.
The basalt that will be encountered along the alignment is fairly unique, and the wastewater tunnel in general is the first of its kind for the city. While there are highway tunnels, the only other sewer tunnels are limited to about 6ft (1.8m) in diameter, explains Lori Kahikina, director of the department of design and construction for the city. "This is going to be a major undertaking for us," Kahikina says.
The City and County of Honolulu will be advertising their Kaneohe- Kailua gravity sewer tunnel in February/March — a TBM excavation of approximately 3-miles from the Kaneohe Wastewater Pre-Treatment Facility to the Kailua Regional Wastewater Treatment Plant, three shafts and installation of a 10ft (3m), 3in (76.2mm) fiberglass pipe with the annulus backfilled with low density cellular concrete. The construction contract will be required to complete within three years and the estimated project cost is USD 150M.
It should be noted that prospective bidders will need to obtain their Hawaii contractors license at the time the bidder’s pre-bid statement of qualifications is submitted and not at the time of bid opening, as previously stated last year. The bidder’s pre-bid statement of qualifications forms are anticipated to be due in early March.
Kaneohe Bay
Honolulu is technically designated as the urban part of the island, along the southeastern shore. But as a consolidated city-county it encompasses the entire island. According to the 2010 census, some 337,000 people live in Honolulu proper and a total of 953,000 inhabit Oahu.
On the eastern side of the island, the areas of Kanoehe and Kailua both have small commercial areas but are predominantly residential. Between the two is where the alignment spans, and much of the tunnel is beneath the undeveloped Oneawa Hills — the former caldera of the volcano.
The current wastewater system comprising a 42in (1m) force main built in 1976 serves more than 90,000 people, over an area of 57 square miles (147.6sq. km).
Among the options considered for the consent decree was another force main under the environmentally sensitive Kaneohe Bay. Another was the gravity sewer tunnel alternative through Oneawa Hills.
"We actually, from the beginning, wanted to the do the tunnel, but we needed to get EPA approval because our consent decree says ‘back up force main,’ — and so that went through Bay," Kahikina says.
"We finally got EPA approval to go the tunnel route. We’re staying away from bay," she emphasises.
The gravity sewer tunnel will convey wastewater flows from Kaneohe to Kailua and will have three shafts. On the Kaneohe side the shaft will be about 55ft (16.7m) deep, and at the other end, the Kailua shaft will be about 95ft (29m) deep. The third shaft, along the alignment is 200ft (61m) deep, but that access shaft is located on one of the higher points of the Oneawa Hills. Tunnel construction will primarily be staged from the Kailua shaft, from which the TBM will launch.
Boring in Basalt
Kahikina estimates the geology is about 90 per cent basalt and another 10 per cent soils. The city is specifying continuous probing ahead of the TBM and pre-excavation grouting to control groundwater inflow.
Getting an accurate picture of what the geology is like proved challenging during geotechnical boring. "Every time we do a boring there is a slight variation," says Kahikina. "Oahu was formed with volcanoes. So you could have lava tubes underneath there."
At the same time, Oneawa Hills has earned its name, with the topography so hilly and inaccessible, vertical bores needed to be done at a slight angle. In the end, this also allowed the team to get a more diverse cross section of the ground.
Speaking with Tom Pennington, a project engineer with Jacobs Associates, which is working as a subcontractor to designer Wilson Okamoto Corporation, he explains, it’s lava in a form not often encountered in tunnelling.
"Usually when you’re in these caldera environments you can have basalt flows but you tend to get a lot of basalt dykes, which are more laminar, vein-like structures in the volcano — that’s actually how the lava gets to the surface."
Most of the rock along the alignment is dyke basalt versus flow basalt.
"Then you also get these basalt breccia deposits, which is more of formation that forms during the eruptive phase," Pennington adds. "You can get other rock fragments mixed in as this stuff is coming up to the surface.
"It’s not your traditional Pahoehoe and A’a flows that most of the islands are comprised of. Normally when you think about Hawaii basalts you think about those two types of basalt and we really don’t see too much of that in the vicinity of the tunnel," says Pennington.
This will mean more variability during TBM excavation, for example, from one 10ft (3m) section to the next. He explains, "It’s variable in the sense that there is weathering present. Not from atmospheric effects, but more internal weathering, chemical weathering, just due to the formation of volcano, and then variability in different types of dykes and different thickness, orientations and strengths."
Pennington later adds, "we can experience high strengths even though there is this variability, and alteration to the rock due to its geologic history. We still can encounter relatively fresh, unweathered basalt, which can be up to 30,000 psi (207MPa), pressure strength and that’s always a challenge for tunnelling work.
"We’re fortunate in that we don’t think we have too much of that. We have it, but on average I think our strengths are a lot lower than that."
The city expects one of the biggest challenges for tunnel construction will be finding a TBM that can go through the different rock conditions and that contractors will want to source a hybrid tunnelling machine.
Compared to regions such as the Pacific Northwest or Iceland, the variation is smaller in scale and tends to be a little weaker because of the weathering.
Looking at the H3 highway tunnel on Oahu, which is located more centrally in the island, even there the geology is different, Pennington says. "That area has more of the traditional basalt flow deposits and less of the dyke complex, the breccias, the kind of basalt that we are anticipating"
