The wall shown in the photos below will eventually become the east portal wall. According to Caldecott-tunnel.org, most walls are built from the base upwards.
First, an auger drills large holes, ninety feet deep and several feet apart. Concrete is poured into the circular holes and allowed to harden. Next, a series of holes are drilled between the original holes, overlapping the edges. Steel cages are dropped into the holes for extra support and then concrete is poured into the hole. When the concrete hardens, it forms a series of underground interlinking piles.
Starting at the top of the wall, the dirt in slowly excavated revealing the front of the wall. Supports are added to the wall as the excavation goes deeper. According to Caldecott-tunnel.org, the machine in the lower photo is drilling holes to insert tie-back cables, which as the name indicates, will tie back the wall against the hillside. The semi-circular pattern on the wall in the top photo is the outline of the tunnel. The tunnel will be excavated straight through the portal wall. This could begin in as little as three weeks.