The Rondout Bypass Tunnel Shafts – Shotcrete Lining

The Rondout-West Branch Tunnel (RWBT), a segment of the Delaware Aqueduct (the Aqueduct), was built from 1937 to 1944 and provides about 50% of New York City’s total water supply. The tunnel is concrete lined and has a finished inside diameter of 13.5 ft (4.1 m). It is about 45 mi (72 km) in length and runs in the southeasterly direction from the Rondout to the West Branch Reservoirs.

O’Hare Plaza West Drive and Executive Garage Restoration

While infrastructure projects such as parking garages are often the last projects to be funded due to their utilitarian nature, they are important to the health of commercial properties since they are often the first impression for tenants and visitors. At O’Hare Plaza, the parking garage serves the 700,000 ft2 (65,000 m2) office complex.

The Neville Island Bridge Project

The Neville Island Bridge Rehabilitation project originally came out for bid in 2018. The bridge is located in Allegheny County, PA and within PennDOT’s District 11. In the bid package, it was noted that construction could not begin until a nesting pair of Peregrine Falcons had fledged their young, which translated to a late summer start.

Moyie Dam Repair: Back to the Future

Moyie Dam at Bonner’s Ferry in northern Idaho started initial construction just before WWII. They shut down construction during the war. After the war was over, the dam went into service in the late 1950s.


The Tuscarora Mountain Tunnels in Pennsylvania have a fascinating history. The older of the two tunnels, built between 1938 and 1940, was designed to carry a railroad that never came into service. Industrialists of the day, William Henry Vanderbilt and Andrew Carnegie, were backers of the scheme which was halted when they struck a deal with rival railroad owners.


On August 14, 2021, the Caldor Fire started in Northern California near Pollock Pines, CA. The fire ended up consuming almost 222,000 acres (90,000 hectares) and seriously threatened South Lake Tahoe. Besides the over 1000 structures that were lost, the fire also burned along a major portion of the El Dorado Irrigation District’s Canal (EID) which provides drinking and irrigation water to numerous communities throughout the area

I have a project with a segmental retaining wall. The project is located in Maryland. The CMUs of the retaining wall have significantly deteriorated in several areas. The geo-grid fabric appears to be in good condition in the areas where it could be observed. I suspect the deterioration is caused by saltwater runoff during snow events combined with the freeze/thaw cycles. There are some localized areas where the masonry units have completely disintegrated. The largest area being approximately 4 ft2 (0.37 m2). Is it feasible to remove the deteriorated masonry material, down to a sound surface, and shotcrete the face to restore (or exceed) the structural integrity of masonry units? If so, will this restore the structural integrity of the segmental retaining wall?

Shotcrete is routinely used for the repair of deteriorated concrete masonry and brick. Shotcrete should easily have a minimum compressive strength of 4000 psi (28 MPa) when properly shot and cured. According to the National Concrete Masonry Association, current CMU units have a minimum 2000 psi (14 MPa) compressive strength. Older CMU had a lower 1500 psi (10 MPa) minimum. Thus, the shotcrete placement will be significantly stronger and less permeable than the in-place CMU. This should give the wall better resistance to saltwater and freeze-thaw exposures thus extending useful life.

Depending on the depth of the new shotcrete placement, you may consider mechanically tying the shotcreted layer back to the sound CMU with epoxy or mechanically embedded anchors or j-bolts. Thicker sections may also benefit from the use of a steel wire mesh or fibers.

When shooting onto existing CMU sections, the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. Shotcrete placed onto an existing CMU surface will provide an excellent bond IF the following conditions are met:

  • Make sure the surface is roughened and clean.
  • The amplitude of roughness should be +/- 1/8th in. (3 mm) or more.
  • A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
  • Bring the CMU surface to a saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
  • Make sure the shotcrete placement is properly executed with high-velocity placement and quality materials.
  • The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
  • Be sure the shotcrete contractor is using an air compressor able to produce at least 185 CFM (5.2m3/min) for wet-mix and 375 CFM (10.6 m3/min) for dry-mix (gunite) of air flow at 100 to 120 psi (0.7 to 0.8 MPa).
  • Use of an ACI-certified shotcrete nozzleman is recommended.
  • No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.

Finally, without details on the loading conditions, and structural details of the original construction, we cannot comment on the structural integrity of the repaired wall. You should contact a professional engineer experienced in concrete repair to evaluate the structural integrity of the wall when repaired.