transfer values and have the ability to stiffen soft soils and improve the settlement behavior of associated foundations. High productivity rates and the ability to easily install hollow bar elements in low headroom or tight access conditions provide economical foundation solutions. Hollow Bar Technology A hollow bar is a continuously-threaded steel element that is drilled into the ground whi le cement grout is injected simultaneously through a sacrificial bit at the bottom of the pile. The hollow bar acts not only as the reinforcing element, but also as the drill steel and grout conduit. A variety of bit types are available depending on the soil type, and bits can be modified to adjust the drill hole diameter. Using self- drilling injection elements without casing allows grout to freely penetrate into the soil, forming a “ragged” profile and enhancing bond values. Steel grades vary from 68 to 87 ksi (469 to 600 MPa). Hollow bars have many advantages: • The installation method eliminates the need for temporary casing and for top- up grouting after casing removal. • Production rates are typically increased by 2 to 3 times. • Increased production rates result in an overall decrease in project cost. • Bar sections are typically fully threaded and can be cut and coupled at any length, allowing them to be used in restricted access or limited headroom areas. • The one-step installation is ideal for work- ing in loose or submerged soil conditions. • The bars accept high loads in complex configurations, where vibration is restric- ted, and in difficult ground conditions. Innovative Applications In addition to providing conventional structural support and earth retention applications, hollow bars are used for a variety of innovative solutions: • Foundations for tall and slender wind turbine structures subjected to dynamic load changes are well suited to hollow bar micropiles. Light, mobile drilling equipment used to install hollow bars can access remote locations or areas of sloping ground that larger drilling equip- ment used for conventional foundation solutions cannot easily access. • The one-step, low headroom process allows installation of hollow bars in basements for seismic retrofit of existing buildings to accommodate earthquake shaking and liquefaction. • They can be used for improved railway construction including: upgrading of embankments by soil nailing, widening of railway embankments using a com- bination of micropiles and soil nails, a new track base slab system using micro- piles, and foundations for overhead electrical masts. • They can be used in combination with strand anchors to provide seismic retrofit solutions. The horizontal load induced by seismic forces generates eccentric bending moments in the foundations, resulting in twisting of the structure and opening of the joint between footing and ground. Hollow bars resist compression loads, and post-tensioned strand tendons installed within the hollow bar element resist the dynamic horizontal seismic shocks without uplift of the footing. Europe’s Entry to Micropile World Cup Exhumed hollow bar element showing grout body (photo credit Con-Tech Systems) 68 • DEEP FOUNDATIONS • NOV/DEC 2014 A recent hollow bar project, a new building for the Silesian Museum, was selected as Europe’s entry to the First World Cup of Micropiles competition at ISM’s 12th Inter- national Workshop on Micropiles in Krakow, Poland, June 2014. The competition was established to select the “best” micropile project in the world since the previous workshop. The European entry competed against projects from North America, South and Central America, and Oceania/Asia. Installation of hollow bar micropiles at Silesian Museum (photo credit TITAN Polska) The project was completed by TITAN Polska under the direction of project managers Natalia Maca and Jakub Sierant. It involved construction of a new building for the Silesian Museum in Katowice, Poland, situated on a former coal mine site. The new building had three levels and was constructed almost entirely below ground, requiring a 16 m (52.5 ft) deep excavation adjacent to post-industrial 19th and early 20th century buildings. Upgrading the existing coal mine structures to modern- day museum facilities required substantial renovation of the existing buildings, including lowering the existing-grade basement up to 4 m (13 ft), creating a tunnel under one of the buildings and significantly increasing structure loads. A micropile underpinning solution provided minimum interference with the existing terrain and close integrat ion of surrounding facilities and buildings of the old mine with the new structure. The construction schedule was very tight, and difficult geological conditions were encountered including complex soil layering, mine cavities, faults, loosened zones and rock mass movement. A total of 1,400 m (4,593 ft) of hollow bar micropiles (TITAN 52/26, TITAN 73/56 and TITAN 73/53) were used to underpin the renovated historical buildings both as a temporary support for the structures located in the excavation affected zone, and as permanent
