The following three case studies illustrate the use of small- diameter directional drilling in rock followed by drilling enlargements at depths exceeding 300 ft (91 m). The fourth case study involves excavation through rock at depths greater than 800 ft (244 m). Copenhagen Metro – Permanent Retaining Walls The transportation system in Copenhagen, Denmark consists of multiple train and bus systems. Two new subway lines are under construction that will form a circular route around the city center with 9.6 mi (15.5 km) of track connecting 17 stations. For station construction, one of the greatest challenges is linked to limitations imposed by the location of jobsites within a densely populated urban area. It was necessary to eliminate vibrations and disturbances to adjacent structures and to minimize waste materials. The other major challenge was the sub- surface conditions. The stratigraphy consists of glacial till from the ground surface to 66 ft (20 m) deep, com- posed of clay, silty sand and sandy gravel with random granite boulders (20-40 in [500-1,000 mm] size range), underlain by limestone (UCS value 435 to 5,800 psi [3 to 40 MPa]). A layer of very hard flint (UCS > 58,000 to 87,000 psi [400 to 600 MPa]), ranging from a few centimeters to several meters in thickness, is embedded within the limestone. Trevi S.p.A., which was awarded the bulk of the foundation work, chose to use cased auger pile (CAP)/cased secant pile (CSP) technology to build the diaphragm walls needed to sustain the shoring for 14 stations. For the remaining three stations, hydromill technology was adopted because the depth to be reached exceeded the maximum depth possible with CSP. Using the CAP/CSP method, it is possible to install piles directly adjacent to existing foundations, avoid disturbance to the surrounding ground, and accurately and precisely position the piles. The use of protective casing enables a high degree of vertical control, typically with less than 0.7% deviation. Also, with the CAP/CSP method piles are augered concurrently with driving the casing into the ground. When necessary, augers can be advanced below the casing depth using a double independent rotary system. Concreting is done through the auger shaft during auger extraction. Reinforcement is then lowered into the fresh concrete, after which the temporary casing is removed. To 72 • DEEP FOUNDATIONS • JAN/FEB 2014 construct a secant pile diaphragm wall, p r i m a r y a n d secondary piles are overlapped by a few inches. A guiding wall ensures the diaphragm wall alignment and guides the casing from the top. For the Copenhagen project, a total of about 313,320 linear ft (95,500 linear m) of 40 in (1,000 mm) diameter piling has been installed using Soilmec SR-100 drill rigs, up to a maximum depth of 92 ft (28 m). In order to drill through the flint layer present at variable depths, Trevi pre-drilled 8 in (200 mm) diameter holes along the entire pile depth, with five holes in the primary piles and three holes in the secondary ones. Arapuni Dam – Structural Rehabilitation Arapuni Dam is located on the Waikato River near Hamilton, New Zealand. The dam, completed in 1928, is about 164 ft (50 m) tall and is founded on ignimbrite, a volcanic rock. From the beginning, the dam has been affected by occasional under-seepages and undergone subsequent grouting campaigns. A soil investigation indicated that there are four main fissures underlying the dam. A joint venture of Trevi, Mighty River Power Diagram of the reaming tool and Brian Perry Civil was awarded a contract to install cut-off walls across the fissures. Trevi proposed using secant piles drilled from the dam’s crest. The holes were drilled about 200 ft (60 m) deep into the dam’s concrete and then 100 to 130 ft (30 to 40 m) into the underlying ignimbrite. The depth of the holes and the existence of a gallery at the dam’s base required the use of directional drilling. To form four separate cut-off walls, 136 piles were drilled with a maximum deviation of 1% over 330 ft (100 m) depth with superb results in reducing leakage. A significant excavation challenge involved drilling an accurate small-diameter pilot hole (smart hole) up to 3.9 in (100 mm) diameter, reaming the hole to 15.7 in (400 mm) diameter, and then drilling adjacent holes using the previous holes as a guide. Each completed panel was approximately 8 ft (2.5 m) long. The JV developed an innovative solution to achieve overlapping holes without one hole deviating into the other. The natural tendency of the auger would be to follow the path of least resistance, causing the hole to deviate from vertical toward the already-drilled hole. Trevi used a guide system with two large “pontoons” attached to the drill string to hold the drill bit in position to drill the second hole so that it properly overlapped the first.
