Gov. Jay Inslee’s order to suspend tunneling at the Highway 99 project is unjustified because Seattle Tunnel Partners was already solving a sinkhole problem, project manager Chris Dixon says.

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The state’s order last week to suspend digging at the Highway 99 tunnel “is wrongful and unjustified,” Seattle Tunnel Partners (STP) says.

The contractors insist they already responded appropriately to a sinkhole that suddenly formed Jan. 12, in the wake of tunnel-boring machine Bertha’s cutter head. STP immediately filled the divot with a mixture of 250 cubic yards of concrete and sand.

The letter, signed by STP manager Chris Dixon, accompanies a root-cause analysis that Gov. Jay Inslee and the Washington State Department of Transportation (WSDOT) demanded last week. That sinkhole came hours after a barge tipped and spilled muck into Elliott Bay.

“Stopping the [tunnel-boring machine] at its current location is not recommended and increases the risk of creating additional sinkholes,” says the contractors’ study, obtained Monday by The Seattle Times.

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Dixon’s arguments represent only one side of the dispute.

The analysis is to be reviewed by a panel of independent, state-chosen construction experts. WSDOT must issue written permission for STP to continue the deep dig.

Bertha has advanced 190 feet since undergoing repairs and reassembly late last year in a giant vertical launch pit, and has moved 1,280 feet since tunneling started in July 2013.

WSDOT spokeswoman Laura Newborn, who did not release the report, said documents from STP are still under review and “the lines of communication are open.”

No date has been announced for tunneling to restart.

In its root-cause analysis, STP says that it’s not possible to determine unequivocally what caused the sinkhole.

“The potential reasons are a pre-existing void, a ground loss in the face of the [tunnel-boring machine] or a combination of both,” it says.

A clause in the $1.35 billion contract allows the state to suspend tunneling to correct unsafe conditions for workers or the public.

On the other hand, the nearby Alaskan Way Viaduct showed no damage. It is shielded by a row of buried concrete pillars, which STP installed to protect it from this kind of predicament.

Dixon argues that Bertha should immediately restart and regain its momentum for the next 250 feet, until reaching Safe Haven 3, a concrete-lined rest stop.

After inspections there, the plan is to burrow under the viaduct and past a few brick Pioneer Square buildings — perhaps the riskiest spot in the 1.7-mile route from Sodo to South Lake Union.

The governor’s transportation adviser, Charles Knutson, said last week, “the remaining space is the only opportunity to test the machine and the operation of it, and make sure it is ready to go for the longer drive. When an incident happens like this for the sinkhole, this is the time to correct the mistake that happened and make sure it doesn’t happen again.”

Other highlights from STP’s analysis:

• Tunnel workers recently measured the excavated soil by weight, but not also by volume, as they are supposed to do and did earlier in the project.

• Buried sensors showed some ground motion in the area a day and a half before the sinkhole formed — which prompted a meeting among top STP engineers, plus a WSDOT consultant. But the surface looked normal. Contractors increased their injections of grout and bentonite clay around Bertha to reduce potential voids.

• Loose sand and gravel there are susceptible to sinking, they were disturbed by last year’s repairs, and an even weaker layer of fill soil sits only 30 feet above.

• Bertha is descending at 2.4 degrees, into deeper and what is supposed to be more consistent dirt.

• Since the winter restart, Bertha has averaged less than one-third of its specified thrust and torque, as the team moves slowly and cautiously.

In related news, STP turned off its deep groundwater-removal wells around the repair vault last weekend, Newborn said.