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By Mark Brooks, Brian Freeland, Eric Eisinger

Colorado Parks and Wildlife (CPW) operates more dam and reservoir facilities in the State of Colorado than any other single entity and is in the process of completing rehabilitation upgrades to improve operational safety and modernization as required by the Office of the State Engineer. Many of these dam facilities have been operating 50 to 100 years in remote locations presenting challenges related to routine operations, maintenance, and completion of dam safety related repairs. Historically CPW has utilized design-bid-build procurement of construction services to complete these projects which presented additional challenges related to local contracting capabilities and undesirable contract disputes and claims. CPW recently pivoted to a CM/GC procurement process to contract for construction services for dam rehabilitation improvements where the remoteness, seasonal high-altitude limitations, complex topography, availability of local construction resources, stakeholder involvement, and other factors having an increased potential to negatively impact construction success and completion timeliness. The CM/GC delivery method has proven to be beneficial to the success of these projects by early construction contractor involvement as part of a collaborative Integrated Project Team to promote enhanced constructability solutions, innovations to improve quality and cost efficiency, risk identification and mitigation, and schedule and budget adherence. CPW has completed three projects using the CM/GC project delivery approach and select case histories (particularly the most recently completed Alberta Park Dam Rehabilitation in partnership with SEMA Construction) will be presented to demonstrate how this project delivery method overcame project site challenges and constraints to complete complex safety critical rehabilitation projects. The project scope on these projects included improving the capacity and function of service spillways, outletworks, and embankment stability and seepage features, and other dam safety infrastructure.

Oct 7, 2024

By Katie Kontos

The changing landscape of the deep foundations industry is being propelled by many things-most prominently, digitalization, automation and increased emphasis for sustainability. But this is only the tip of the iceberg when it comes to innovation. With all this change ahead, DFI turns to several of its forward thinkers to ask what the future looks like. What challenges will arise? How can the industry prepare to head into the new world of artificial intelligence (Al)? And what can be done NOW to support and empower the future generation of geoprofessionals?

Nov 1, 2024

By Alex Sherman, Gretchen Connelly

The Soo Locks, which date back to the 1800s, are the major cargo connection point between Lakes Superior and Huron. The locks allow the movement of approximately 86 million tons of freight annually through the Great Lakes, the St. Lawrence Seaway and the international markets. Considered a feat of engineering, the locks raise and lower boats to allow passage between the two Great Lakes using a series of gates and chambers that are powered solely by gravity, without the use of any pumps. The Soo Locks are currently made up of the Poe, MacArthur, Davis and Sabin locks. The Poe Lock, the most utilized of the four, was rebuilt in the 1960s to accommodate larger ships. Currently, approximately 90% of the commodities transported through the Soo Locks are restricted to the size of the Poe Lock. The MacArthur Lock, the southernmost lock, is also currently operational. Neither the Davis nor the Sabin Lock are currently in operation.

Jan 1, 2024

By Ryan Nagle, Eric Lindquist, Rob Jameson, Kelli Mccurdy, Serena T. Jang, John Gouchon

This paper discusses the challenges associated with the design and construction of a deep basement in downtown San Jose, California for the 200 Park Avenue project. The aim is to present the critical design issues, how the challenges were addressed, details of the selected shoring design, and the performance of the shoring system during construction. The critical design issues that will be discussed related to retaining the deep excavation and constructing the basement include shallow groundwater, artesian pressures, settlement outside of the excavation, shoring earth pressures, and shoring wall movement. Subsequently, the paper will describe why a tied-back cutter soil mix (CSM) cutoff wall was selected as the shoring system and how it addressed the critical design issues. The design of the tied-back cutoff wall will also be discussed. Lastly, the performance of the tied-back cutoff wall during dewatering and excavation and how it compared to the design will be included. The performance was measured through a monitoring program, including survey data points and piezometers.

Oct 31, 2023

By Jason Hertz, Tony Amis

Low to zero carbon ground sourced heating and cooling solutions face limitations in densely populated urban areas, especially when building footprints span entire sites. Despite these challenges, alternative approaches exist. This paper explores one such alternative—embedding geothermal loops into a building's foundations, connected to a ground-sourced heat pump. This innovative method efficiently caters for a significant portion of the base heating and cooling load, with an air-sourced heat pump incorporated for any remaining heating and cooling requirements. The focus of this paper is on the successful deployment of energy foundations in a 250,000 ft² commercial building in lower Manhattan, New York. The building, now fully operational as an all-electric structure, serves as a case study. Insights gained from this project provide valuable lessons for future endeavors. The paper not only discusses the technical aspects of the installation but also offers operational performance data gathered over the past six months. By presenting real-world applications and lessons learned, this paper aims to contribute to the knowledge base for similar projects in urban environments. The insights shared here can guide future initiatives seeking sustainable and efficient heating and cooling solutions, particularly in densely populated areas.

Oct 7, 2024

By S. Harteis, D. Dolinar

Many mining operations rely on bulkheads to provide a barrier between impounded water or slurry and active mine workings. However, bulkhead failures could cause and have caused catastrophic flooding that puts the underground workforce at risk. The National Institute for Occupational Safety and Health1 (NIOSH) in collaboration with the Mine Safety and Health Administration (MSHA) is conducting research to evaluate the adequacy of existing design practices for water and slurry bulkheads. A key component of this research effort is documentation of the performance history and design parameters for bulkheads installed in underground coal mines during the last 20+ years. This research is part of a larger effort to develop general design guidelines and procedures for constructing and maintaining bulkheads that will help to ensure their long-term structural integrity, while significantly reducing the inundation risk for miners. This paper presents the results of NIOSH’s research to identify state-of-the-art bulkhead designs, including design criteria, leakage monitoring systems, and emergency warning systems. Underground observations and evaluations of existing bulkheads suggest that the most important design factors influencing their performance are the interface of the bulkhead with the surrounding strata and the potential magnitude of the hydraulic pressure to which they may be subjected. When a bulkhead has failed, leakage has generally been through the surrounding strata or along the bulkhead/strata interface, with the failure potential along the interface increasing with hydraulic head. It was also determined that development of a monitoring program to ensure the long term structural integrity of bulkheads is an important safety consideration. Current monitoring procedures range from weekly visual inspections to constant monitoring via pressure transducers and fluid level indicators. Where possible, these devices are used in conjunction with a computer based mine monitoring system to alert mine personnel when an emergency condition exists at the bulkhead installation.

Jan 1, 2006

By Lauren Speakman, Mark Reinders, Thomas Fiala

In the heart of downtown Toronto, a full city block at 481 University Avenue and 210 Dundas Street West has become the site of one of the most complex heritage preservation efforts ever attempted in North America. The development of the United Bldg., a 50-storey mixed-use tower, required the complete in-place retention of two culturally designated façades: a 9-storey Beaux-Arts structure built in 1928, and a 10-storey Modern Classical building completed in 1961. The City of Toronto By-law 1393- 2017 designated the façades of both 210 Dundas Street West (Maclean Pub- lishing Company Building, 1928) and 481 University Avenue (Maclean- Hunter Building, 1961) as heritage sites due to their architectural, historical and contextual significance. Regulations imposed by this bylaw prohibited dismantling or relocating any part of the heritage façades. All original finishes brick, stone, concrete and steel - had to remain undisturbed and fully supported throughout construction. At 10 storeys tall, the façade retention system (FRS) designed for this project was a monu- mental engineering and construction marvel that was unprecedented in the industry. The complexity of this project was only amplified by the spatial lim- itations imposed. Dense urban infra- structure - including streetcars, sub- way tunnels and congested utilities not only restricted the available options for the FRS design but also imposed stringent monitoring criteria. The solution selected needed to restrict ground settlement to fewer than 3 mm (0.1 in) and façade movement of 15 mm (0.6 in). This led to the development of a highly integrated hybrid support system, merging elements of the existing structure with the FRS and the support of excavation (SOE) design to allow four levels of underground construction beneath the retained façades and structure. On the United Bldg. project, Canadian specialty contractor Green Infrastructure Partners was responsible for delivering the demolition, shoring scopes and façade retention. This included the full demolition of two brick buildings and the partial demolition of 210 Dundas Street West and 481 University Avenue. The shoring scopes executed the full deep foundation system including secant piles, micropiles, caissons, tiebacks, walers and shotcrete. The façade retention entailed the fabrication, supply and installation of all FRS steel and connections, as well as the jacking of the retained structures onto the new foundations.

Sep 1, 2025

By Jose Caicedo, Elmar Buhl

Polyurethane based ground injections are most frequently used to stabilize shallow foundations that have under- gone settlement due to soil shrinkage, softening of the ground by water (e.g., heavy rainfall or defective sewer pipes), or planning errors regarding the bearing capacity. The expansive reaction of the injected polyurethane, together with the counter-pressure of the foundation, results in a ground improvement of the soil layers underneath the foundation. When more material is injected, a releveling of settled shallow foundations or concrete slabs is possible. Due to more frequent reuse of existing buildings in urban areas, we have seen an increase in requests regarding the reuse of the existing foundations under increased loads. Sometimes the load increase can be accommodated by the existing foun- dations without any modifications, but it is often the case that the new load exceeds the bearing capacity of the old foundation. In this case, there are several options to strengthen the foundation. Most commonly, sectional underpinning and retrofitting micro- piles along the foundation are used to increase the bearing capacity. An alternative option is the strengthening of the ground under the existing foundations to increase the bearing capacity and to minimize settlement due to the new load. In the last few years Uretek, a provider of polyurethane based ground injections, has frequently applied these injections to successfully perform the necessary ground improvement in the context of building reuse or the addition of new stories to buildings. This article will present a case study of a residential building in Hamburg, Germany, where an additional story was planned and the bearing capacity of the existing shallow foundations was not sufficient for the new load. In this case expanding geopolymer injections were used to gain the necessary bearing capacity and the effect was monitored by an in-situ load test. A second case study will briefly illustrate an experimental project where ground injections were used to increase the skin friction of a micropile foundation at a parking garage in Munich before the addition of three new stories.

Mar 1, 2025

By Andrew Evans

As healthcare facilities evolve to meet the needs of expanding urban popu- lations, the demand for sophisticated excavation support and deep foundation solutions becomes increasingly critical. One such project is the new campus for Children's Medical Center in Dallas - an ambitious expansion set to serve the region for decades to come. At the core of this high-profile project is Oscar Orduno, a Dallas-based specialty contractor. This project, like many at the intersection of healthcare infrastructure and urban development, brings unique geotech- nical challenges. Texas. The expansion includes new clinical buildings, research facilities and a network of support structures - all of which rely on robust and technically sound below-grade construction. The specialty contractor joined the project to deliver an excavation support solution, ensuring that the develop- ment's deep foundations and per- manent basement wall would meet all performance requirements. Given the urban context of the site, as well as being adjacent to ongoing hospital operations, and the substantial depth of excavation, the scope demanded a high level of coordination, technical capability and adaptability. At the center of the site is a perma- nent basement wall, reaching depths of nearly 70 ft (21 m) below street level. The wall's design had to account for immediate earth retention during ex- cavation as well as the long-term structural needs of the new hospital complex. The total project scope con- sists of several new towers, across 33 acres (13 ha) and increasing the total hospital footprint by more than 2 mil- lion sq ft (185,800 sq m). These chal- lenges were met through an integrated system of soldier piles and tieback anchors, engineered and installed to stringent standards.

Sep 1, 2025

By Abdul Amir Alhelu

The Cardinal Hill slope movement repair was an urgent project, and a high priority, time-sensitive project for the Louisville Water Company. Cardinal Hill is a critical water distribution facility that serves a majority of Jefferson and Bullitt Counties, and parts of Hardin and Nelson County, Kentucky. On March 14, 2019, the Louisville Water Company issued a certificate of emergency based on evidence of impending slope failure, which could cause a catastrophic break in a 60 in (1.5 m) diameter transmission line and loss of a 30 million gal (114 million L) reservoir. The reservoir would fail due to the pipe failure. The subsurface conditions generally consisted of a stiff silty clay extending to about 27 ft (8 m) below the shoulder of the road, where a weathered siltstone was encountered. The siltstone was about 3 ft (1 m) thick and underlain by shale. The earth auger sampling tools encountered refusal material (shale) at about 30 ft (9 m) deep. The boring was extended about 5 ft (1.5 m) into competent shale using a roller bit. Two borings were drilled on the slope on the south side of the fence in the neighbor's yard, encountering refusal material at 11 ft (3 m) near the driveway and at 4.5 ft (1.3 m) on the upper plateau.

Jan 1, 2025

By J. David Frost, Anna Frost, Morgan NeSmith, Boyeong Yoon

The Leaning Tower of Pisa is recognized globally as a testament to resilience in that it has been leaning from early in its 800-year existence and yet is not considered a failure from many, if any, perspectives. If one asks the question, "Could foundation-based architecture have prevented it from leaning from the outset?" then one has to first ask "What is foundation-based architecture?" If you are intrigued by these questions, read on. The word architecture is inherently associated with the portion of a structure that is visible above ground. This is both an omission and a missed opportunity to recognize that the foundation of a structure can contribute as much, if not more, to the realization of a structure. As our planet faces significant growing challenges due to natural hazards such as sea level rise, elevated temperature events, increased precipitation events and more severe wind events, the importance of considering the architecture of a foundation system is likely to become even more critical when the con- sequences of natural hazards such as drought, wildfires, landslides, debris flows, floods, glacial lake outflows and tsunamis, among others, are mani- fested. The above ground physical and societal aspects of infrastructure and the increased demand for below- ground space will require creative problem-solving solutions and con- siderable foresight. Foundation-based architecture will unquestionably play a critical role in helping society adapt to and survive natural hazard stressors that are at best ill-defined and often poorly understood.

Nov 1, 2025

By David Graham

Dedicated by President Franklin Roosevelt in 1940, Chickamauga Lock and Dam is located just upstream of Chattanooga, Tennessee, at Tennessee River Mile 471. The Tennessee Valley Authority (TVA) owns the structure, and the U.S. Army Corps of Engineers (USACE), Nashville District, operates and maintains the navigation lock. Constructed under the direction of the TVA as part of the New Deal, Chicka- mauga Lock and Dam serves the TVA's mandates of flood control, improved navigation and hydroelectric power generation along the Tennessee River and its tributaries. Chickamauga is one of the busiest navigation locks in the country, handling nearly 10 million tons of freight each year. As a critical link in the transportation of goods, particularly bulk commodities like coal, grain and chemicals, its efficiency directly im- pacts the economy. The Chickamauga Lock was designed by the USACE and was based on the design of Guntersville Dam downstream. The Chickamauga Lock is 60 ft (18 m) wide by 360 ft (110m) long. However, the dam was designed to allow a larger 110 by 600 ft (34 by 183 m) lock to be added if needed in the future. That future is now, and the USACE is leading a project to construct a new, larger lock to increase the flow of river freight and address structural issues. Extensive concrete surface cracking was observed soon after the lock and dam was completed in 1940. The cracking is a result of alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR), which cause the concrete to expand. The existing lock suffers from structural deficiencies resulting from ASC and ACR concrete expansion, making a new lock that much more critical. The overall cost of the lock replace- ment project is estimated at $954 million with operational completion estimated in 2028 and final completion estimated in 2031. As part of the project, C.J. Mahan Construction Company was awarded a $61 million contract to con- struct the upstream approach walls that will guide vessels into the new lock. The approach walls were planned to be sup- ported on 28 drilled shafts of 6.5 ft (2 m) diameter. The contract also included a nonproduction technique shaft with a bidirectional load test. The baseline design included four piers, each sup- ported on four drilled shafts, and two nose piers, each supported on six drilled shafts. The drilled shafts included 6 ft (1.8 m) diameter rock sockets ranging in length from 25-45 ft (8-14 m).

May 1, 2025

By Joel Moskowitz, George J. Tamaro

Collaboration, cooperation and - borrowing a phrase from hockey - "changes on the fly" are all aspects that came to mind in the construction of the 30 Hudson Street tower. A former industrial site on the west shore of the Hudson River, offering spectacular views of lower Manhattan, was chosen for the location of this office tower. Available information indicated the presence of multiple existing founda- tions, miscellaneous and potentially contaminated fill, and rock-filled timber bulkheads. The subsurface profile consisted of fill over river bottom organic silt, compact sand and rock at depths varying from 30-55 ft (9-17 m). Existing boring data, some of which dated back as early as 1902, did not include sampling details, nor were precise sampling locations easily. determined. Mueser Rutledge Consulting Engi- neers attended early design discussions with the owner and the rest of the design team and provided foundation alternatives for the following three schemes: . • . No basement with multiple parking levels above grade A shallow basement, still requiring above grade parking A deep basement for all parking, with the most notable functional aspect of keeping the lobby level at grade Due to the presence of known obstruc- tions, a deep seam of decomposed rock, possible contaminants and the neces- sity for deep foundations into rock (for no or shallow basement), the deep basement scheme with a perimeter diaphragm (slurry) wall was selected as the preferred alternative. Although it had a higher installation cost, it pro- vided the owner with many benefits such as below-grade parking, building the lobby at the ground floor level, removal of contaminated soil and the potential for future expansion within the diaphragm wall. The contractor, in consultation with our team, chose to pre-trench the perimeter wall alignment to a depth of about 10 ft (3 m) with a low strength concrete slurry mix, which aided in the removal of obstructions and pro- vided a permanent wall that wouldn't have major intrusions into the site owing to the volume of the removed obstructions.

Jan 1, 2004

By Adam Hacker

In early 1937, the Ohio River elevated to its highest levels leading to devastating flooding in Louisville, Kentucky. Today, that historical flood, also known as the Great Flood of 1937, is still the flood of record for the area. In an effort to re- duce flooding risk to the population, the Flood Control Act of 1937 was passed to strengthen Louisville's re- silience against future flooding. This triggered the U.S. Army Corps of Engineers (USACE) to construct the Louisville Metro Levee System, which comprises 26.1 mi (42 km) of concrete floodwalls and earthen levees with 16 flood pump stations. One of those flood pump stations was the Paddy's Run Flood Pump Station, which was built in the 1950s, in the western portion of Louisville. That flood pump station was located at an existing stormwater outfall that conveys the stormwater from the western metro- politan area under the levee to the Ohio River. The stormwater flows through the outfall by gravity when the Ohio River is not elevated. When the river is elevated, the flood pump station becomes active after a flood gate at the end of the outfall closes. This is necessary because with- out the flood gate, the Ohio River would backflow into the outfall causing inland flooding. When the flood pump station is active, the stormwater gets pumped over the levee embankment through discharge pipes into the Ohio River. Stormwater can collect in a ponding area and backup during intense rainfall events when the inflow to the flood pump station exceeds the capacity. Recently, Louisville and Jefferson County Metropolitan Sewer District (MSD), which now operates and main- tains the levee system, determined the need to upgrade the existing Paddy's Run Flood Pump Station's capacity to reduce the risk of interior flooding. MSD determined the new flood pump station would need to increase existing capacity from 875 million to 1,900 million gal (4 to 8.6 billion L) a day to account for a 2035 projected storm event, which is a 10-year, 24-hour storm event of 5.0 in (127 mm) in 24 hours. Due to the age and complications associated with up- grading the existing pump station, a new pump station was selected as the preferred approach. In 2021, MSD

Nov 1, 2025

By Frank DiSalvo

As the natural disaster potential escalates and urban coastlines throughout the U.S. continue to bear the brunt of its effects, cities like New York; Jersey City, New Jersey, and Hoboken, New Jersey, are responding with the only effective strategy available: building smarter, stronger and more resilient infrastructure. Nowhere is this more evident than in the vast network of flood protection projects currently transforming the shoreline of the New York-New Jersey metropolitan area. Geotechnical contractor American Pile & Foundation has five major resiliency projects underway, stretching from Hoboken's rail yard to the eastern and western shores of Lower Manhattan. Each of these projects presents unique engineering and construction challenges, particularly given the heavily urbanized, historic and often fragile nature of the terrain. From executing technical foundation work through centuries-old manmade fill to operating adjacent to critical transit infrastructure, these projects demand technical expertise, innovation, con- struction know how and collaboration to help redefine the way the construc- tion industry approaches resiliency. Let's go on a tour throughout the harbor and get a closer look at each site and its unique challenges.

Nov 1, 2025

By Allison Digregorio, Muhammad Ali Naeem, Sam Yao, Joshua Core, Simpson Gumpertz

The Potrero Power Station makes up approximately 1,000 ft (304.8 m) of shoreline on the east side of the San Francisco Peninsula between Pier 70 to the north and 23rd Street to the south. Beginning as early as the 1870s, the site has been home to numerous industrial activities and has not been open to the public in many years. The industrial activities ranged from sugar refining to power generation and gas manufacturing. The Potrero Power Station Project is rehabilitating the waterfront and pro- viding a mixed-use development and public waterfront to reopen the area to the public while also providing robust protection against sea level rise and seismic-induced instability. When transforming such a sizeable post- industrial area, creative design solutions were tailored to each location to increase the grade and stabilize the slope. The solutions implemented ranged from ground improvements to retaining walls.

Nov 1, 2025

By Sarah Banks

"Here and there the grease and filth have caked solid, and the creek looks like a bed of lava; chickens walk about on it...and many times an unwary stranger has started to stroll across, and vanished temporarily." Upton Sinclair offers this revolting - yet accurate - description of the Chicago River in his book The Jungle, originally published in 1906. In the early 1900s, Chicago was a major industrial hub. The Chicago River was not only heavily used for transportation but also for the disposal of waste produced by the meatpacking industry, steel production, rail manufacturing and more. The South Fork of the river was the primary sewer for the Union Stock Yards. The meatpacking industry caused so much pollution, the river became known as Bubbly Creek. Chicago's older buildings were designed to be shut off from the river - no windows, certainly no homes or parks. A river view from an office or hotel was not something to brag about. It wasn't until the 1970s and the passage of the federal Clean Water Act that things began to change. The city now requires public access to the river in all parts and newer architectural designs ensure this is possible. Rather than sewage and industrial waste, the river now carries boat- loads of tourists and kayakers.

Oct 1, 2025

By Shawna Munn, Katie Castelo, Dave Wiley

Risk is inherent to work in the field of subsurface engineering, and many contract approaches have become so focused on shedding risk that they leave no room for one of the best tools in our industry, the observational method. Its development in subsurface engineering owes its foundation to Karl Terzaghi and Ralph Peck through their respective works dating back as early as Soil Mech- anics in Engineering Practice published in 1948 and "Advantages and Limitations of the Observational Method in Applied Soil Mechanics" in 1969. Throughout their fundamental works, a strong argu- ment for the observational method is made: "... the potential for savings of time and money without sacrifice of safety is so great that every engineer who deals with applied soil mechanics needs to be informed." (Peck 1969) Not every project will benefit from its use, but in today's risk management focused world, projects and contracts should be set up to allow for its use, while sharing the risk clearly and equitably between all parties. Without a proper contractual mechanism to leverage its use, perceived manage- ment of "risky unknowns" will over- ride substantive application of the method, and significant benefits may be lost as a result. As Terzaghi said, "A vast amount of labor goes into securing only roughly approx-imate values for the physical constants that appear in equations Therefore, the results of computations are not more than working hypotheses, subject to confirmation or modification during construction." It is our collective view that sub- surface engineers should take this task of confirmation and modification seriously. We should use the observa- tional method to derive performance- based designs that deliver design economy, constructability, cost and schedule advantages without sacrificing safety or quality. Collaborative style contracts provide a framework to approach shared risk and opportunity with appropriate incentives for the individual parties.

Aug 1, 2025

By Marya Gorczyca, Damian Siebert

Air rights projects are inherently challenging as they involve building in the "air space" above an existing structure. From a foundation engineering and installation standpoint there are numerous constraints and additional complexities to consider. This article describes how these issues were overcome for deep foundation design and construction at Lyrik Back Bay - Air Rights Parcel 12, the first successful air rights project over the Massachusetts Turnpike/Interstate I- 90 through this neighborhood in downtown Boston in 40 years. The project is unique in that it is both an infrastructure and commercial development project undertaken as a collaboration between a private development team S&A P-12 Property led by Samuels & Associates, and the state transportation agency, the Massachusetts Department of Transportation (Mass DOT), owner of the land and transportation infrastructure.

Oct 1, 2025

By Erik Westerberg, K. Rainer Massarsch

The paper was presented 30 years ago at DFI's 20th Annual Conference in Charleston, South Carolina, in 1995. Its main objective was to describe the operation and performance of vibrators with variable frequency and to illustrate their practical application in foundation engineering. Since then, powerful vibrators with variable frequency have been increasingly used in the foundation industry for installing/extracting piles or sheet piles, treating granular soils to decrease total and differential settlement, or mitigating the liquefaction hazard. However, the potential of variable frequency vibrators for pile installation. or ground compaction has not yet been fully realized. By adjusting the operating frequency of the vibrator, the penetration/extraction speed of piles or sheet piles can be increased signifi- cantly. At the same time, the emission of ground vibrations can be minimized. Some recent important developments for efficiently using vibrators are briefly summarized below. An important aspect of vibratory driving is "system resonance," at which the vibrator and the probe oscillate "in phase" with the surrounding soil. The relative displacement between the probe and the soil decreases during this phase. Probe penetration slows down, and ground vibrations increase. To effectively install piles or sheet piles in granular soils, the vibrator should be operated at a high frequency (typically above 30 Hz). The vertically oscillating pile emits horizontal stress pulses that are transmitted to the ground. This mechanism reduces the shaft resistance during driving in granular soils. Con- sequently, the shaft resistance of vibrated piles is often lower than that of impact-driven piles, the impact of which, by definition, occurs at res- onance of the pile-soil system. However, the shaft resistance of vibrated piles can be enhanced if, during the final phase of installation, the operating frequency is gradually lowered to achieve com- paction of the soil surrounding the pile shaft. It is also desirable that the vertical movement amplitude of the pile be gradually reduced to eliminate the formation of a gap below the pile toe. Electronic monitoring systems are available to assist the machine operator in determining system resonance dur- ing driving. Vibration sensors mounted on the vibrator or on the ground can determine, monitor and adjust the resonance frequency. Nowadays, ro- bust electronic systems can display the measured values to the machine operator and the geotechnical engineer on-site or in the office.

Jan 1, 1996