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By Takashi Enomoto

"The cement deep mixing (CDM) method has a wide range of applications, both on land and at sea. This paper describes the features of the equipment on CDM vessels engaged in maritime construction in Japan: the DCM No. 3 barge, the DCM No. 6 barge, and the DCM No. 8 barge. The DCM No. 3 barge is a large vessel with a mixing area of 5.7 m2. The design of the elevator of the deep mixing unit on the DCM No.3 barge enables us to conduct deep underground/ sea operations. This enables us to conduct mixing at depths of up to 52.6 m below the seabed and up to 70.5 m from the water surface. Additionally, since the tower is installed on the center of the barge, it has excellent stability. The DCM No. 6 barge and DCM No. 8 barge are mid-sized vessels with a mixing area of 4.6 m2. The DCM No. 6 barge was fitted in 2013 with energy conservation equipment and upgraded to an environment-friendly CDM barge. The energy saving upgrade used electric power, instead of hydraulic power, by inverter controlled alternating current motor for both mixing unit and main hoisting winch. This enables the using on effective use of regenerative power when the deep mixing unit is lowered. A feature of the DCM No. 8 barge is that it has an inclinable tower located at the bow section for elevating the deep cement mixing unit. Since the tower can be greatly inclined, the hull has a lower center of gravity and greater stability when the vessel turns. This also allows the vessel to pass below bridges and simplifies cruise preparations, outfitting and dismantling work.INTRODUCTIONThe cement deep mixing method (CDM method) involves mixing and stirring a cement-based solidifier into soft ground to rapidly bring the ground to the desired firmness. This method is used to suppress ground shear deformation, increase strength, increase slip resistance, and mitigate liquefaction and subsidence. It has been recently used in Japan in the construction of revetments at controlled waste disposal sites, both to boost the strength of the seabed and to prevent the seepage of waste matter and other pollutants. Undertaking CDM construction work at sea requires specially constructed marine construction CDM vessels. Since CDM construction at sea entails many more restrictions on working conditions than construction on land, the vessels themselves sometimes need to be modified. This paper will describe 1) the remodeling of the turrets and mixing machinery for mixing to great depths in deep water (DCM No. 3 barge with eight axes); 2) the upgraded of equipment on board CDM vessels after accounting for environmental considerations (DCM No. 6 barge with four axes); and 3) the use of a inclinable tower (DCM No. 8 barge with four axes)."

Jan 1, 2015

By Athul Prabhakaran, Ramin Motamed, Ahmed Elgamal, Milad Jahed Orang, Reza Boushehri

Documented case histories from past earthquakes have exhibited the devastating effects of the liquefaction phenomenon on the performance of buildings that are often presented in the form of excessive settlement of shallow foundations. Example earthquakes include the 1964 Niigata earthquake in Japan, and more recently the 2010-2011 Canterbury Earthquake Sequence in New Zealand, and the 2011 Tohoku earthquake in Japan. All of the listed events comprised several examples of severe liquefaction-related damage to the buildings founded on shallow foundations. Although the liquefaction-induced building settlements have been experimentally studied extensively since 1964 using shake table tests and dynamic centrifuge experiments, studies focusing on cost-effective mitigation strategies are still limited. In this study, the performance of helical piles as a cost-effective countermeasure to minimize the damaging effects of liquefaction-induced ground settlements on buildings founded on shallow foundations is studied. Two large-scale shake table experiments were conducted at the University of California, San Diego (UCSD) to investigate the effectiveness of helical piles in liquefiable grounds. Results from these two experiments highlight a highly improved performance of a shallow foundation supported on helical piles. These largescale shaking table experiments demonstrate the efficiency of using helical piles as a cost-effective countermeasure to reduce the liquefaction-induced building settlement.

By Ramin Motamed, Anna Panorska``, Joseph A. W. Toth

In this study, a dataset of 145 bi-directional and static load tests was used to evaluate the load-displacement response with respect to unit side resistance for drilled shafts comprising fully and partially mobilized data. A total of 612 unit side resistance curves were calculated which included 52 fully mobilized curves showing a maximum response. A new approach using the double tangent method, is proposed in calculating the maximum unit side resistance from partially mobilized load-displacement data. The double tangent method was applied to each load-displacement response to estimate the ratio of mobilized resistance between the intersecting tangents and peak measured resistance. It is observed that as the mobilization of side shear approaches a maximum, the ratio approaches unity. Using the fully mobilized curves, regression models were developed using correlations between ratios of estimated resistance at the double tangent, peak, and maximum measured responses. Regression models are evaluated for the following factors: predominant soil type and drilled shaft construction methods. Comparisons of calculated and measured maximum values of unit side resistance are compared for each model. Although these factors each influence the load-transfer behavior, a single global regression model including all the data showed promise in yielding accurate results.

By Sherif Elfass, Gary Norris

Bi-directional load testing of drilled shafts has been widely adopted as a means to evaluate their load carrying capacity. However, in order to reach failure, most of these tests are carried out on sacrificial shafts using expensive sacrificial elements such as hydraulic jacks. A proposed low-cost bi-directional testing technique is discussed. It mechanically loads the end bearing soil below a drilled shaft, against its side shear resistance. The technique was originally established as an alternative to post-grouting to reliably account for the forfeited end bearing resistance. However, since it loads the shaft in a manner similar to conventional bi-directional tests, it can be used to not only load test sacrificial shafts, but production shafts as well. Furthermore, unlike conventional bi-directional load tests, the load is applied through surface jacks. These jacks, along with other load test components, are movable and can be reused to load test all the shafts of the project thus presenting an economic benefit. The concept of this technique has received favorable feedback from a few consulted departments of transportation, and while promising, has not been field tested to date. A U.S. patent was issued for the tip loading module and the loading technique described.

Sep 8, 2021

By Richard Schmidt, Lijun Deng, Mohamed Abdelaziz, Weidong Li

Helical piles are widely used as a foundation supporting a variety of different structures. Understanding the axial behaviour of helical piles is required to confirm the engineering design. Helical pile research is often conducted in the field, but this method can be prohibitively expensive in some cases. Geotechnical centrifuge modeling offers an alternative method for the helical pile research and practice. A series of axial load tests of helical piles installed into cohesive soils were conducted on the geotechnical centrifuge at the University of Alberta. The objective was to investigate the effects of inter-helix spacing and soil strength on the axial behaviour of helical piles. Four types of aluminum model helical piles including one single-helix pile and three double-helix piles with various inter-helix spacing ratios were tested. Both compressive and tensile loading tests were conducted. All model piles were instrumented with multiple strain gauge stations. The clay had an undrained shear strength of approximately 50 kPa and 120 kPa. The test results indicate that there is a transitional failure mode between the two widely recognized failure modes, i.e., individual plate bearing and cylindrical shearing. The shaft adhesion was significantly reduced by the helix. Axial capacities were comparable to the field test results.

By Bernard H. Hertlein

The introduction of LRFD methods for structural design has been a gradual process by AASHTO, ACI, AISC, ASCE, and the FHWA, which has largely been ignored by the deep foundations industry. In 2007, however, the industry has been obliged to embrace LRFD for any deep foundations on projects involving federal funds. The authors are concerned that the risks caused by the need for, and the difficulty of providing, reliable quality assurance, will result in unnecessarily high safety factors that will defeat part of the purpose of LRFD. A primary attraction of LRFD for owners and engineers alike is that it promises more efficient design, and therefore cost reductions. Unfortunately, a key element of LRFD philosophy is often overlooked - that the applied safety factors can be rationally reduced, often allowing less material to be used, thus saving costs, but only if all the risk factors involved are identified and closely controlled. This means that both the quality of the materials and the quality of the construction work must be adequately monitored, and held to an acceptable standard. In the case of deep foundations, the soil in which the foundation is constructed is also a critical component that must be taken into account in the assessment of risk factors.

Jan 1, 2007

By Michael W. Neill, O&apos

The performance of foundations that combine the high bearing capacity of rafts (mats) with the settlement reduction capability of piles is described through a summary of 24 case histories. Structures included are high-rise commercial office buildings, governmental buildings, industrial facilities and residential buildings, and subsurface conditions range from soft clay to stiff soil overlying rock. Pile types are bored, augered and driven. Since piled rafts are not commonly used in the United States, most of the case histories are from overseas. In all cases the performance of the foundations was monitored during and after construction. Particular emphasis is placed on measured settlements of the piled-raft foundations, distribution of resistance between piles and raft, and general analysis methods that were used by the designers for various geotechnical circumstances. Finally, patterns of behavior are summarized. In most cases, the piles were not designed to mobilize their full capacities under sustained loading of the foundation; therefore, under long-term loading conditions, the piles typically carried more load than the raft. This paper is the result of a study conducted by ITC-18 (Deep Foundations, Willem Van Impe, Chair) of the International Society for Soil Mechanics and Geotechnical Engineering by a subcommittee chaired by the author.

Jan 1, 2003

By Gokhan Inci

Stability problems with dam spillways on existing landslides and appropriate mitigation measures to remedy them are ongoing concerns for geotechnical engineers. The first successful application of drilled piers as stabilizing structures under the spillway was at the Trinity Dam in Sicily, Italy. ?Underpinning? was considered as an alternative to spillway relocation during the Alpaslan 2 embankment dam design in eastern Turkey. Based on the available literature the first application of drilled piers to stabilize a spillway chute in the U.S. was at the Ralston Dam in Colorado. Ralston Dam, which is a 175-foot-high zoned fill dam located 15 miles northwest of downtown Denver, was built in the late 1930s. The concrete spillway for the dam was built on the Pierre Shale Formation in the left abutment. Since reservoir operations began, mass movements have occurred downstream of the chute area. These movements have required periodic spillway maintenance. After the occurrence of recent movements and because of high piezometeric levels, URS was contracted by Denver Water to develop a more permanent solution by the installation and monitoring of additional instrumentation, analysis of slope stabilizing measures, design, repair, and reconstruction of the distressed section of the spillway chute. With state of the art modifications that include: 1) a stabilizing berm with a toe trench that improves the global stability; 2) seepage prevention and control measures that prevent the infiltration of water; and 3) the structural support of the replaced chute section with drilled piers and reinforced concrete the probability of future displacements or other problems have been reduced for the Ralston Dam Spillway.

Jan 1, 2007

By Richard Guenther

As available space in urban centers becomes more scarce, builders and developers are increasingly turning to vacant, existing structures to meet cities' growing needs. The developers of City Ridge Roadside Development and North America Sekisui House (NASH) - - found a suitable site for their "urban village" at the former Fannie Mae headquarters complex on Wisconsin Ave. NW in Washington, D.C., which had been vacated in early 2018. The urban village will consist of eight new structures centered around the building that currently sits on the site. All told, the project will include 687 residential units, a 150-room hotel, over 150,000 sq ft (13,935 sq m) of retail space, and 62,000 sq ft (5,760 sq m) of office space. A below-grade parking structure will extend across most of the footprint. The Fannie Mae building, which was designed in the Colonial Revival style to mimic the Governor's Palace in Williamsburg, Virginia, was named a historical landmark following a request from the development team. The central portion of its existing masonry structure was built in 1956 on pile caps with small-diameter cast-in-place piles. Two wings, which were added in 1962, were built on belled caissons. There are three levels above grade and one below.

Feb 1, 2022

By Kirty Dave

Foundations in any contract are the most uncertain parts of the contract and most of the "Construction-Contract-Disputes" are either directly related to foundations or arise as 'Side-effects of changes in promises related to foundations. Foundation Engineering is defined as ' The art of selection, designing and constructing the elements that transfer the weight of structure to the underlying Soil or Rock." (Ralph B. Perk). This art started becoming a science since 1920, when scientific efforts to determine the behaviour of sub-surface materials were made. The art and science merged to give new techniques and the science started dominating the field as more and more refinement came in techniques to predict more accurately the characteristics of the sub-surface elements. Inspite of good thorough investigations and design, the contractual promises drafted could leave some room for differences between the parties and above all the prudency expected of both the parties may fall short of the expectations raised in the contract. This may give rise to disputes which may need to be resolved as per principles of law. This paper attempts to initiate a dialogue amongst Foundations-Engineers and Contractors as to how do we minimise the differences arising out of or related to contracts concerning "Foundations".

Jan 1, 1996

By Masaaki Terashi

The machine for the hybrid method called JACSMAN is ordinarily a dual shaft machine. Each shaft has augers at three different levels and nozzles for jetting at two different levels. The distance between two shafts may be changed according to the particular requirements. Figure 4 shows single shaft and a treated soil column created by the method. The diameter of the auger is 1.0 m. As the jetting nozzles locate at the periphery of the auger, jetting can achieve the additional treated soil column beyond the diameter of the auger. The diameter of the jetting improvement is fixed at 2.3 m.

Feb 10, 2003

This paper summarizes seven years of Italian ex­perience with the use of large calibration chambers for the calibration, under closely controlled stress and strain conditions, of some in situ testing devices which are used in sand deposits. At the present time two calibration chambers are operating in Italy, one at E.N.E.L.-C.R.I.S. (Milan) (*) and the other at I.S.M.E.S. (Berga­mo) (**). Both chambers can accomodate cylindrical samples 1.2 m in diameter and 1.5 m in height. A detailed description of these chambers can be found in Bellotti, et. al (1982).

Jan 1, 1987

By B. Philip Shull, Allen W. Cadden, Johanna M. Simon, Michael S. Senior

A 670 linear meter (2,200 linear foot) large-diameter sanitary sewer interceptor installation at depths ranging from 3 to 10 m (10 to 30 ft) below existing grade in an area with a high water table requires careful attention to design and performance of the system during construction. Highly variable soil and rock conditions at the site necessitated the design and construction of various excavation support systems along the alignment. In addition to the subsurface condition challenges faced at this site, the proposed sewer pipe was to be installed between an existing active combined sewer main and a high pressure gas main feeding the nearby city. What better way is there to monitor performance of the system than to require a detailed instrumentation system? Instrumentation system specifications and designs are becoming more commonplace in the industry, but it is always important to sit back and take a look at the whole picture. What are you looking to gain from your monitoring? What pieces of information are useful? What interpretations can you derive from the instrumentation you have selected? This paper will address the instrumentation system used to monitor the 670 m (2,200 ft) excavation from the specification requirements to the implementation and interpretation. This case history will delve into the challenges faced and methods used to overcome those challenges on this particular support of excavation (SOE) project. A combination of soldier pile, lagging and sheeting, and shoring systems was used to achieve the necessary excavation support. In designing the systems, attention was given to the configuration of the bracing system to allow the sections of largediameter pipe to feasibly be installed in the excavation while maintaining the SOE system.

By James P. Hambleton, Anastasia Nally

This work focuses on the numerical simulation of multi-plate anchor systems (e.g., helical anchors) in sand subjected to vertical loading. In assessing the stiffness and capacity of these multi-plate anchor systems, full awareness of the abilities and limitations of the various analysis methods must be understood. This work first summarizes studies completed by others and then goes on to assess the failure mechanisms of multi-plate anchors in sand and the influence of (1) plate width-to-depth ratio, (2) number of plates, and (3) relative positioning of plates. The analysis makes use of (1) conventional limit analysis, (2) so-called modified limit analysis that employs reduced strength parameters to account for the influence of soil dilatancy, and (3) the displacement-based finite element method, which considers elastic as well as plastic deformation leading to failure. The work critically reflects on limitations in the current analysis methods for helical ground anchors. INTRODUCTION Helical anchors are a relatively low cost foundation type, typically consisting of a steel rod and single or multiple plate attachments. In recent decades, they have seen rapid growth, and as outlined by Merifield and Sloan (2006) and Hambleton et al. (2014). Design techniques for helical anchors have previously been predominately empirical, as much of the past research in this field is experimentally based. It is common practice for helical anchors to be modeled as horizontally oriented plate anchors in both numerical and experimental analysis. This work focuses on analysis based on the finite element method, including so-called finite element limit analysis and the conventional displacement-based finite element method, with some comparisons made to experimental studies. Numerical simulation of anchor pull-out capacity in cohesionless soils is stunted by the many difficulties faced when using numerical analysis with cohesionless soils. Additional variations can be seen when comparing the serviceability based load capacities to the ultimate load. In this work, both the force-displacement response (stiffness) and the ultimate anchor capacity are assessed, with more focus on the latter. This work focuses only on analysis techniques for multi-plate anchors with vertical loading in cohesionless soils. Many studies have been completed in this area, some of which assume a rectangular plate in a plain strain model (Rowe & Davis, 1982; Merifield & Sloan, 2006; Cerfontaine et al., 2019). Whereas others use axisymmetric conditions (Baker and Kondner, 1966; Saeedy, 1987; Murray & Geddes, 1987; Ghaly & Hanna, 1994; Sakai & Tanaka, 1998; Merifield et al., 2006;) and minimal studies considered both axisymmetric and plane strain in the one study (Vesic, 1971; Tagaya et al., 1988; Murray & Geddes, 1987; Sarac, 1989).

Jan 1, 2019

By Jonathan Ernst

The Bruce Museum is an art, science and natural history museum in Green- wich, Connecticut. The museum was originally built as a private residence in 1853, and has undergone several reno- vations, the most recent dating to 1992. The current renovation is part of a $60 million capital expansion plan for the "New Bruce." The renovation includes a 43,000 sf (4,000 m2), three-story addition that will more than double the size of the museum, adding space for new ex- hibitions, education and community spaces. The proposed bottom of the foundations for the renovation sit up to 16 ft (5 m) below the Bruce Museum's existing shallow foundations.

Aug 1, 2023

By Alison Savage, Michael Terry

"For decades, the International Market Place (IMP) has been an icon in the heart of Waikiki in Honolulu, Hawaii. It was famous for Duke Kahanamoku’s restaurant where people flocked to hear Don Ho sing in the ’60s and ’70s. Later, it was a bustling, open-air hub of small vendors who peddled everything from Hawaiian souvenirs and jewelry to food court meals and farmer’s market produce. Small buildings and shacks that housed the businesses were located on a curved tract of land — just one block off the beach — that stretches from Kalakaua Avenue to Kuhio Avenue. The entrance to the complex is dominated by a huge, old (circa 1850) Banyan tree. This tract is owned by the Queen Emma Land Company. Queen Emma was a Hawaiian Queen from the 1800s. Revenue from the land has supported the Queen’s Medical Center in Honolulu for decades.The reimagined IMP is owned by Taubman Centers, of Bloomfield Hills, Mich., and CoastWood Capital Group of San Francisco, Calif. The IMP will offer a distinctive collection of upscale fashion and lifestyle retailers, including Hawaii’s only Saks Fifth Avenue store, as well as ten sit-down restaurants, including concepts by award-winning chefs Michael Mina and Roy Yamaguchi. The mall features two large open-air courts, the first of which has the huge Banyan tree as its entryway centerpiece. The second open-air court, called Queen’s Court, includes a stage and mature monkeypod trees and a few palm trees. Foundation SelectionAfter a geotechnical exploration that was directed by SME, Plymouth, Mich., and performed by Geolabs, Honolulu, it was discovered that the historical site had hidden and challenging foundation conditions. The subsurface conditions included a very deep but now in-filled ancient river valley containing more than 180 ft (54.9 m) of lagoonal deposits consisting of normally consolidated plastic clayey silts and silty clays with blow counts mainly varying in N-value from 1 to 6. The silt was underlain by several soft coralline layers, some overconsolidated clays and various basalt layers. The static groundwater level on the site was at a depth of about 5 ft (1.5 m), closely matching sea level.Although driven piles had previously been used as the preferred deep foundation support for adjacent structures, this area of town is now fully developed and local noise ordinances preclude their use. Augercast piles were initially considered, but when the general contractor dck/FWF, Pittsburgh, Pa., asked for bids for the augercast piles, several foundation contractors offered alternate systems. The practical installation depth of augercast piles was limited and due to the depth of the in-filled alluvial valley, this system could likely have left soft silt below the tips of the piles over much of the site."

Jan 1, 2016

By Jeff Starchuck, Harald Kullmann

"In 2005, an inspection showed that the Deception Bay Wharf in the Canadian Arctic was at the end of its service life. Although Deception Bay is closed with landfast ice up to 5 ft thick, shipping operations continue year-round, with additional voyages in the summer open-water season to supplement mine supplies. The rehabilitation scheme that allowed minimal interruptions to shipping involved concrete caissons cast on barges and an ingenious placing system.WorleyParsons Westmar of Vancouver, B.C., performed the inspection under contract to the owner, Xstrata Nickel, Montreal, Quebec. The wharf, on the coast of the Nunavik region of Quebec, was originally constructed in 1971 for an asbestos mine, and is now used to support Xstrata’s Raglan nickel mine, importing fuel for p o w e r g e n e r a t i o n a n d general mine supplies, and exporting nickel concentrate to smelters in the south. Emergency repairs were essential to allow operations to continue at the site, while the parties considered permanent rehabilitation options and secured funding. The repairs were done in the openwater seasons, and included reinforcing the failed interlocks above water level with steel patch plates and injection grouting of the cell fill below the port crane in 2005. In 2006, workers installed a circumferential belting system below water level.Westmar held critical planning sessions with the mine operation personnel to find ways to stretch the limits of the mine supplies and nickel storage facilities. In the end, the parties agreed to shut down construction activities four times during the ice-free season to allow access to the dock site.Westmar developed a plan that allowed mine operations to continue during construction, and Xstrata approved the plan and budget in September 2006, for execution in 2007. Rapid construction was essential. To accommodate the short construction season and maintain minimum levels of service for the mine, the work was radically different from conventional Arctic deep-sea port construction. Like the original Deception Bay Wharf, construction practice was to use steel sheet pile circular cells with the consequential heavy support services for the labour force. Construction duration was typically two seasons. The plan for Deception Bay was executable in one season based on a concrete caisson structure. Only a modest amount of schedule float was needed."

Jan 1, 2009

By Charles G. Crans, Naresh Gurpersaud

"Toronto’s Union Station, a National Historic Site occupying an entire city block, is the busiest commuter rail transportation hub in Canada. An extensive project is being undertaken to revitalize Union Station while maintaining the historical significance of the building and minimizing disruption to commuters. A significant stage of the revitalization involves excavation indoors, to a depth of up to 5.5 m below the existing floor level to create space for a new transit concourse and a new lower-level pedestrian retail concourse. A hybrid excavation support system utilizing micropiles as soldier piles was constructed over an 18 month period between 2010 and 2011.Development and testing of connections between the micropiles, shotcrete facing, lagging, walers, and tiebacks presented significant challenges. The existing viaduct structure was constructed in the 1920’s on land reclaimed from Toronto Harbour, so there was significant (and ongoing) uncertainty with respect to buried obstructions including relic timber wharfs and abandoned steel and timber piles. Collectively all these conditions precluded the use of conventional methodology; new techniques, installed using modified equipment, and for the first time in the Greater Toronto Area, were required.Details of the existing site conditions, installation challenges, design, construction and performance of the hybrid excavation support system inside this historic structure are presented.INTRODUCTIONUnion Station in Toronto, Ontario is the busiest passenger rail facility in Canada, with 65 million passengers annually and an average of 200,000 passengers each day. Construction of the station was completed in 1920. It has been designated a National Historic Site of Canada and inducted into the North America Railway Hall of Fame in the Facility and Structure category.The Union Station Revitalization Project (USRP) was tendered in 2010 and awarded as a 6-year, $450M construction management / trade contract package. Included was a large scale grade reduction, referred to as “The Dig Down”, to create two basement levels and this required extensive excavation support constructed entirely indoors within the active station setting."

Jan 1, 2017

By Patrick J. Lydon, Gregory A. Terri

"The recently opened Mansueto Library at the University of Chicago campus was a 2012 OPA runner-up. The dramatic elliptical shaped dome of this structure tells only part of this structure’s story. The underground automated storage and book retrieval system below ground required excavations as deep as 55 ft (17 m). The system that allows the University to store many of its publications well below grade also required an approximately 50 ft (15 m) clear span for the perimeter walls between the lower level and street level slabs. The latter requirement allows the conveyors to operate freely to retrieve and transport publications up to the lobby level. The design-assist-build team worked together to develop the most cost-effective solution for below-grade work in this challenging urban and geologic environment.The perimeter of the new oval-shaped library is over 620 ft (189 m) long and sits in a central location on campus—adjacent to the Regenstein Library and two city streets and the Henry Moore sculpture, which commemorates the location of the first self-sustaining controlled nuclear reaction. The new library construction not only had to meet challenging structural and geotechnical requirements, but also had to be sensitive to its active and historic surroundings. The soils at the site consist of about 25 ft (7.6 m) of sand, followed by lean clay, generally increasing in stiffness to the top of the dolomitic limestone. Five to 10 ft (1.5 to 3 m) pockets of saturated sand also lie on the north side of the site at about 45 ft (13.7 m) below grade. The top of weathered rock varies in elevation from 59 ft (18 m) below grade in the south to 69 ft (21 m) in the north. There is 1 to 2 ft (30 to 60 cm) of weathered rock. Groundwater at the site is about 7 ft (2 m) below grade.The new structure would extend 55 ft (17 m) below grade through saturated sand strata and into weathered rock for part of the structure’s footprint, so it was critical that the new permanent retention system minimize groundwater seepage laterally through the structure’s wall as well as piping through the weathered rock below the structure. The perimeter walls had to be very stiff to minimize ground movement around the structure’s perimeter due to the significant clear span between slabs."

Jan 1, 2013

By James M. Tantalla, Roderic A. Ellman, W. Allen Marr, Scott R. Bamford

"Construction of the powerhouse for the Willow Island Hydroelectric Project required a 100-ft deep excavation inside the limits of a cofferdam and embankment-seepage cut-off wall through soil and rock adjacent to the Ohio River. Portions of the rock had preexisting weak plans and slickensided surfaces with low frictional strength. Stability analyses were performed for critical sections of the excavation to evaluate the stability of the soil slopes and rock cuts using buckling, tension crack, and limit equilibrium analysis techniques. Loading conditions included the 100-yr and intermediate floods, normal pool, sudden drawdown, and seismic design conditions.Stability analyses indicated adequate factors of safety for soil slopes and for rock cuts through the landside embankment sections. However, the results of deep-seated stability analyses through the rock supporting the riverside cellular cofferdam indicated unacceptable factors of safety. Consequently, the riverside portion of the excavation required measures to increase stability. Long, high-capacity rock anchors and deep dewatering wells to lower the site piezometric head were chosen for this purpose.Thirty high-capacity rock anchors were installed on the upstream, riverside, and downstream sides of the cofferdam rock cut. Required rock anchor loads varied from 1,968 to 2,074 kips. The designed anchors consisted of multiple, seven–wire strands with up to 59 strands per anchor. Bond lengths used an allowable bond stress of 100 psi based on the rock bond measured with pull-out tests. Maximum bond and anchor lengths were 50 ft and 140 ft, respectively. Measurements of forces in selected strands and displacements of the excavation showed excellent performance."

Jan 1, 2017