Baystate Medical Center - Geotechnical.
This was one of the most informative tours we took from a geotechnical standpoint. It had a wide range of soil stabilization methods in the small footprint due to close proximity of work to the operating hospital and adjacent roadways, none of which could be disturbed.
Along the roadway a system of soldier piles was used, where steel piles are driven and infilled with timber lagging as the site is excavated. This was later left in place as the site was backfilled. In the deeper excavations soil anchors were also used to tie the wall back into the soil under the road. This method requires drilling into the soil at an angle, inserting cables that are grouted into place and then tensioned using hydraulic jacks. The system is shown in the photos. The center photo shows the heads of the soil anchors in the shallower excavation. Where deeper excavation was necessary additional anchors were placed with angled waler blocks spanning between the soldier piles, seen about 1/3 up the pile in the last photo.
The sides of the excavation closest to the existing buildings used secant piles, a series of overlapping augered concrete piers. These are placed and then excavated on one side. The advantage is that these have less impact on the surrounding soil and structures (vibration/noise) than the steel piles. To the right you can see how close the basement walls were placed to the secant piles.
For piping and othe utilities very large trenches were required between existing buildings. The main trench is shown, supported by steel struts across the trench welded to the walers (longitudinal steel members along the trench). This system laterally supports the piles in the walls at a given height by spanning the earth pressures across the trench to the equal opposing pressure. These struts are being placed in the left photo and then shown from below after excavation was complete.
Where neither soil anchors nor struts were able to be placed, excavation walls were supported from within the structure being constructed using a raker system (steel members angled back to a foundation). First you can see these rakers from the inside of the new structure basement. Boxouts were provided in the walls around the raker. The next photos show a raker exiting the boxout and attaching to provide support to a soldier pile.
In many places the soil was backfilled against the basement wall. However in the left photo a large cavity will permanently exist below the ground to relieve pressure against the basement. In the right photo the space between the soil and structure will be filled with a geofoam (white blocks shown) to minimize the soil pressure.
