Large TBMs like this generally allow access to what is called the face (the area being excavated) via a double bulkhead system. The ground conditions in Seattle require that the machine be able to control the rate of excavation at all times such that a large sink hole does not form due to uncontrolled loss of ground. This is done by keeping pressure on the face.

The pressures required to keep the face stable are a function of the soil properties, lithology, and groundwater. The typical way for someone to access the face is to pressurize ahead of the machine head with gas and have workers enter under pressure (known as a dive or an intervention). Workers generally are limited in how long they can be under pressure and must go through decompression.

Edit: real quick calculation on my phone puts them at about 70 psi or 4.5 atm of pressure to do a manned intervention, possibly higher for safety

Disclosure: underground tunneling is my profession.