GEOSTRATA Magazine

Tunnelling in urban areas is always a challenge, and it’s especially so when construction is proximate to densely populated districts. Such was the case on January 25, 2005, when a 10-m length of a Barcelona Metro tunnel under construction (6.5 m wide × 6.85 m high) collapsed. The Carmel Tunnel is 32 m deep and was excavated using the New Austrian Tunnelling Method (NATM), whereby the strength of the surrounding ground is engaged to strengthen the tunnel structure. The rock into which the tunnel was cut is a series of alternating layers of sandstone and shales. Despite the high cover ratio, (32 m/6.5 m ≈ 5), the failure progressed upward, and two days later a large sinkhole, 30 m in diameter, developed. The sinkhole engulfed an entire building, damaged dozens more, and required evacuation of 1,000 people. Economic losses amounted to 100 million euros, with 30 percent of the payout devoted to building rehabilitation and 70 percent to monetary compensation to the affected people. Fortunately, there were no casualties or injuries.

Some unstable slopes fail dramatically, but most can be characterized by gradual, if not monotonously slow, movements. Yet these latter situations can pose a dilemma when a temporary repair is much simpler and far less costly than finding the root cause of the movements and fixing it. Such was the situation for this case history of slope movements at a sidehill embankment constructed for Route 191 to connect Interstate 91 and Newport, VT. The slope has been the subject of investigations, monitoring, and mitigation construction efforts undertaken by the Vermont Agency of Transportation (VTrans) for more than 50 years.

Aggregate pier ground improvement is commonly used in North America to support buildings. The technology has expanded in the last decade thanks to refinement and introduction of new techniques, more rigorous engineering, improved quality control, and acceptance by the design and construction community. In-place improvement of unsuitable soils offers many advantages, not the least of which is cost savings compared to traditional deep foundations or over-excavation and replacement. Today, thousands of projects in the U.S. are supported on aggregate pier ground improvement.

When first opened in 1940, and as segments were later added to it, the Pennsylvania Turnpike (Turnpike) was principally a four-lane highway with two eastbound and two westbound lanes. As traffic volume increased over time, the Pennsylvania Turnpike Commission (PTC) increased capacity and improved safety by widening the roadway to add lanes and widen medians and shoulders.