Wolf Creek Dam Wolf Creek Dam, located on the Cumberland River in south central Kentucky, has experienced below-dam seepage since 1968. The Joint Venture constructed a 980,000 ft (90,000 m ) concrete barrier wall using a secant pile wall (SPW), combined barrier wall (CBW), or secant pile system (SPS) technique. Most of this minimum 2 ft (0.6 m) wide concrete barrier wall is being built to depths up to 277 ft (84 m) using innovative construction techniques in variable subsurface conditions. (For more infor- mation about this project, see “2013 OPA Winner: Wolf Creek Dam Rehabilitation” in the September–October 2013 issue of Deep Foundations magazine.) 2 Comparing Different Excavation Methods Each of the following drilling techniques has advantages and disadvantages. Here are our thoughts: • Bored piles with Kelly bar and rock tools use a conventional drill rig equipped with a rock auger, bucket and core barrel. The only technical limitation of this method is the capability to drill through boulders and into sound rock. A chisel may be necessary to break boulders and reduce rock consistency, which may affect the average production rate. • A full face down-the-hole (DTH) hammer uses a bit with the same diameter as the pile to be excavated. Possible stratigraphic inhomogeneity does not create problems in the advance rate; in addition, cuttings are transported directly on the surface by the same air-flow that operates the bit. The limitation is the extensive use of air, which may need to be avoided because of environmental constraints. • A cluster DTH hammer uses a special bucket equipped with six DTH hammers (5.5 in [140 mm] diameter each) and involves direct or reverse circulation. The position of the hammers is studied to cover the entire pile cross section during bucket rotation. Cuttings are collected into the bucket’s body, which can be cleaned on the surface. Heterogeneous soils, with fractured rock and boulders present, may damage the hammer. To avoid this risk, a “load sensing” system is used. However, with a series of DTH hammers mounted on the same body, this is not possible and if one of the DTHs is not hammering rock, it will break. • Large diameter pilot hole and reaming combines full face DTH and a conventional Kelly bar system. To excavate a pile, first, pre- boring is performed by a DTH hammer, then the pile is reamed up to the desired diameter with a conventional piling rig. • Small diameter pilot hole and reaming is similar to the large diameter method, however, pre-boring is performed by a 16 in (406 mm) diameter DTH hammer to excavate about 15% of the pile cross section. Then, pile reaming is facilitated by the empty coring of the DTH with a rock auger or bucket equipped with a stinger to maintain the tool guided into the already-excavated central part of the pile section. U.S. Army Corps of Engineers and contractor Treviicos-Soletanche 2 The drilling-guide pontoons fixed to the drilling string The drilling device lowered into the excavating holes The table below shows a comparison of these techniques, based on a hypothetical project in which piles, 3.3 ft (1 m) diameter and 82 ft (25 m) deep, are installed into the following stratigraphy: from 0 to 66 ft (20.0 m): sand with cobbles and boulders (TCR = 50); and from 66 to 82 ft (20.0 to 25.0 m): basalt (TCR = 80; RQD = 100; UCS = 10,150 to 14,500 psi [70 to 100 MPa]). The table lists the ratio between the installed total power and productivity, based on a 10-hour working shift. The table shows the best equipment solution (in terms of the ratio between power required and daily production) is a large pilot hole with a reaming operation. This method becomes more advantageous by adopting two reaming rigs in combination, with one rig performing the pre-boring. With this scenario, daily produc- tion increases to 262 ft (80 m), i.e., 3.2 piles per day, with a power requirement that increases from 65 to 99 hph/ft (160 to 242.5 kWh/m), due to the contribution of the second 70 ton (70-tonne) class machine with an installed power of about 330 kW (443 hp). This method has a power requirement similar to a conventional large-size piling rig; however the production rate is four times higher. Power requirements of excavation for several drilling systems Method Power Equipment Type and Class (-) Bored Piles With Kelly Bar Full Face DTH Cluster DTH Large Pilot Hole and Reaminbg Small Pilot Hole and Reaming piling rig - 100 ton piling rig - 100 ton 7 air compressors piling rig - 60 ton 4 air compressors piling rig - 70 ton 2 air compressors piling rig - 20 ton 2 air compressors piling rig - 70 ton piling rig - 70 ton Installed Power (kW) 480 2.048 1.196 778 330 618 330 Estimated Power Rate (m/h) 2.0 5.0 3.0 10.0 4.0 12.0 3.0 Requirement per Linear Meter (Kwh/m) 240 410 399 160 162 10 h Shift Production (m) 22 50 30 40 30 (piles) 0.88 2.00 1.20 1.60 1.20 DEEP FOUNDATIONS • JAN/FEB 2014 • 73
