Abstract
Supported by the Drilled Shaft Committee of Deep Foundations Institute (DFI), the similarities and differences in the bored pile foundations industry between Europe and North America were investigated, and the effort focused on five topic areas: (i) design, (ii) construction, (iii) contracting, (iv) innovations and (v) quality and testing. This paper compares and contrasts Kelly-drilled bored pile construction practices between Europe and North America, and presents the economic state of the marketplace in both Europe and North America at the time of the study. The results of the collaborative work among DFI, the European Federation of Foundation Contractors (EFFC) and numerous practitioners will be used to benchmark current industry practices, to recognise similarities and differences in practices between the two continents and markets, to disseminate solutions to similar or common problems, and to identify areas for potential improvement. By identifying and exploring national, regional and interregional differences on both continents, which made comparisons between North America and Europe more complicated, the basic assumptions and understanding of how best to design, construct, evaluate and contract work for bored piles were evaluated. A number of similarities in construction practice between Europe and North America were identified, which include: (i) the use of larger and deeper bored piles, (ii) greater focus on job site safety, (iii) increased demands imposed on the equipment including cleaner emissions and (iv) increasingly restrictive environmental regulations. Differences in practice exist throughout Europe and North America, which relate to (i) geology (spatial and composition), (ii) local vs. regional vs. national practices, (iii) concrete mix designs, (iv) performance-based vs. prescriptive-based methodology and specifications and (v) borehole stability methods (steel casing vs. bentonite or polymer slurry). The various components of the construction operation and practice contribute to the allowable or comfortable level of risk exposure of an owner, contractor and/or designer, which are typically manifested in the development and execution of the project delivery method, specifications, operating protocols and quality control and assurance programme.
Background
The Kelly-drilled bored pile (or drilled shaft) construction industry is affected by a myriad of factors, including socio-economics, weather, geology, materials and equipment, skilled and unskilled labour, laws and regulations, and biases and experiences. Each factor has influenced the development of national, regional and local practices in the design, construction and contracting of bored pile foundations. Understandably, there are considerable differences in methodology and practice from one area to another (e.g. country to country, state to state and/or city to city).
In order to assess and improve deep foundation design practice on regional and national levels, the bored pile/drilled shaft synthesis research project was initiated during a meeting held during the 2013 DFI Annual Conference, where the scope, goals and work assignments were defined. Among several areas of interest to the foundation engineering community, five main topics were identified for deeper investigation and assessment: design, construction, contracting, developments in technology, and quality and testing. For each of the five main topic areas, two industry practitioners with expertise and strong knowledge from each, Europe and North America, were assigned to determine the similarities and differences in practice. North America is comparable with Europe () in that each is composed of multiple distinct regions (i.e. countries, states or provinces), having a population of millions speaking different languages, and can operate as one unit at times and individually at others. During the meeting, it was postulated that if there were a similarity with a particular method or approach, then it would be assumed the best for the current state of the practice; however, if differences existed, then the focus of the participants should be to determine if modifications could be made to improve the practice.
Table 1 Select statistics about Europe and North America
In late 2013, a survey with 27 questions was distributed to practitioner members affiliated with DFI, EFFC and/or DFI-Europe. Unfortunately, there was a very low response rate, with only 75 surveys being returned reflecting considerable variation in practice and bias toward North American practices (65 and 10 responses from North America and Europe, respectively). A map of North America delineating the different regions used to compare the survey responses is provided in . A map of Europe is shown in , which highlights the countries from where responses to the survey were received.
1 Truncated map of North America. The regions used for the initial survey are delineated (modified from Cartographic Research Lab, University of Alabama Citation2015)
2 Map of Europe highlighting the countries from where responses to the initial survey were received (modified from Cartographic Research Lab, University of Alabama Citation2015)
In May 2014, a workshop was held in conjunction with the DFI-EFFC International Conference on Piling and Deep Foundations in Stockholm, Sweden in which the findings from the survey and from interviews were presented in an open forum. Utilising the input and feedback received during the workshop, the researchers conducted additional interviews and solicitations for information. In October 2014, during the DFI Annual Conference in Atlanta, Georgia, another workshop was held to present a progress report about the preliminary findings. During the open forum portion of the workshop, the project participants received direct feedback from the audience about the survey and the initial findings. Various attendees voiced their concern that there were misunderstandings with the way the survey questions were worded; others expressed that there were not enough European contractors included to draw tangible comparisons; and others mentioned that some topics needed to be explored in more depth (e.g. concrete mix design and additives). As a result, a supplemental survey campaign to solicit additional information from selected contractors in North America and Europe was conducted in January 2015. This supplemental survey, which focused only on construction-related issues, resulted in an additional 16 responses but provided more detailed information than the initial survey. In March 2015, a summary of the pertinent findings and conclusions were presented during a technical session at the International Foundations Congress and Equipment Expo (IFCEE) held in San Antonio, Texas.
Although there was a low response rate from the practitioners in Europe, it was deemed that the additional efforts generated a more representative sampling about European construction practices. However, the information and practice reported hereinafter for Europe will reflect the positions and opinions conveyed by about 25 responders located in France, Germany, Hungary, Italy, Switzerland, Netherlands and the United Kingdom.
Introduction
In addition to their conventional use as structural foundations, bored piles have been used to construct secant or tangent pile walls for earth retention purposes, for cutoff walls for seepage control, and for slope stabilisation purposes to increase the factor of safety (FS) against sliding or overturning. Constructing Kelly-drilled bore piles is becoming ever more challenging given the decreasing supply of ideal site locations, close proximity to adjacent structures, simultaneous work activities by different trades and equipment on the same site, and more restrictive environmental regulations, which all culminate in a frenetic pace of construction within a congested work area. During the design and, at times, construction phases of the project, various factors are evaluated to determine the feasibility of bored piles as the appropriate foundation system.
General observations and factors affecting use
Some of the factors affecting the selection and use of bored piles include site conditions and constraints, budgeting and scheduling concerns, presence of and depth to adequate load bearing soils/rock, environmental and noise restrictions, the magnitude of the imposed loading, reliability and risk and availability of space for construction and for the completed foundation. Especially in urban areas, environmental restrictions and noise limitations may affect the cost, productivity and foundation type/method at a particular site. In general, there seems to be increasingly tighter and more congested project sites, greater demands on drilling equipment, and alternative applications for bored piles other than for structural foundations. An example of a congested and challenging work site is provided in where (a) two hydraulic drill rigs are working in close proximity to each other and to the face of the wall, (b) two concrete trucks are supplying fluid concrete to the drill rigs, (c) a crane is providing lifting services to the operations on the site and (d) an excavator is removing soil and drill spoils adjacent to the drill rig.
3 Photograph of bored piles being constructed in a confined and congested work site (photo courtesy of Soilmec S.p.A.)
In addition, there has been an increase in the magnitude of axial, lateral and flexural loading resulting from updated design codes, larger superstructures and the manner in which extreme event loading (e.g. seismic and scour) is incorporated into design. Monoshaft foundations have recently gained favour where one large bored pile/drilled shaft can replace the need for a group of piles connected to a cap. As such, contractors are requiring greater capabilities from modern drilling equipment to achieve the goals of the project. Within the past decade, there has been a use of and, therefore, a demand for equipment able to drill boreholes in excess of 13 ft (4 m) in diameter and to depths in excess of 260 ft (80 m).
Project type and contracting mechanism
On the basis of the survey responses received, most of the European contractors reported that there was not an appreciable difference in either contracting standards or construction practice between public (i.e. government funded) projects and private/commercial (i.e. non-government funded) projects, where one responder offered that the ‘distinction between public and private practice is not so clear in the United Kingdom’. For North American contractors, about half of the responders reported that bored pile practice was similar for public projects and for commercial projects, while the other half reported that practice varied by project type and by region (especially when comparing state and local projects). Multiple responders stated that there were ‘no major differences’ from a technical standpoint, but the differences were evident in the approach, delivery and execution of the contracts. For projects in Europe, public and commercial projects seemed to be more performance-based, whereby the contractor is allowed to select the method (and foundation type) to achieve a required performance metric. For projects in North America, however, public projects seemed to be more prescriptive in nature, with stricter adherence to specifications, while commercial projects were more performance-based, with a bias towards minimising time to occupancy and cost-based value alternatives.
The differences in practices in Europe and North America may also be attributable to the wide range of contracting methods/risk allocation approaches (e.g. Design-Bid-Build, Design-Build, Design-Build Operate and Quality-Based Evaluation/Selection) utilised in the European and North American markets, which may affect the selection and utilisation of a particular foundation type and construction method (e.g. slurry use vs. casing, monoshaft vs. multiple piles with a cap, bored pile vs. CFA pile vs. driven pile, etc.). Undoubtedly, each of the various contracting methods has its nuances, benefits, limitations and applicability. A detailed discussion and evaluation of the numerous contracting methods is beyond the scope of this paper, but a general commentary regarding the relevancy and pertinence of the different contracting methods to bored pile construction is provided herein. During one of the workshops, one of the attendees mentioned that the contacting methods in Europe appeared to be more sophisticated and refined, but that the methods used in North America appeared to be more streamlined, which allowed faster delivery of the product.
Until the past few decades, the most predominantly used contracting method internationally was the Design-Bid-Build approach. With this contracting method, the owner is responsible and bears the risk for the design, while the contractor is responsible and bears the risk for executing the work (productivity, schedule and cost) and its performance. Currently, Design-Bid-Build is not used as extensively as it once was in Europe, but it is still the predominant method used in North America on public projects (well suited for smaller projects). In the United States, alternative methods (e.g. Design-Build-operate and private–public partnerships) are used to offset the shortfall in federal and state infrastructure spending.
A widely used alternative to Design-Bid-Build is the Design-Build contracting approach where risk, for both design and construction, can be consolidated to the party/parties most capable to handle it, where an agreed upon and unified focus can be created on the jobsite, and where efficiencies in design and execution can be realised. With Design-Build, the owner typically provides incomplete drawings (about 30% stage) and a technical concept with which the interested parties can provide an offer. Moreover, the risk and responsibility for design, construction and performance are borne by the contractor, who can mitigate the risk, realise increased productivity and flexibility, and decrease the overall cost through an integrated, knowledgeable and efficient team. From responses received of the initial survey, the reported historical usage of different contracting mechanisms on projects incorporating Kelly-drilled bored piles is provided in . While the response rate from European contractors was considerably lower than that from North America, the Design-Bid-Build and Design-Build contracting methods are the most used in these respective areas.
Table 2 Reported historical usage of different contracting mechanisms on projects incorporating Kelly-drilled bored piles – public and commercial projects (data source: initial survey)
As part of the supplemental survey, the responders were asked about the usage of different contracting methods by owners on Kelly-drilled bored pile projects. Contractors in Europe responded that, for both public and commercial projects, the Design-Bid-Build and Design-Build contracting approaches were used the most, but the approach depended upon the owner of the project. From various discussions with different contractors, the sentiment conveyed was that the Design-Build approach was fast becoming the preferred contracting approach. Contractors in North America, however, responded that, for both public and commercial projects, the Design-Bid-Build contracting approach was the method most used. For public and commercial projects, the breakdown of the reported usage of four different contracting mechanisms by owners on Kelly-drilled bored pile projects is shown in .
Table 3 Reported usage of four different contracting mechanisms by owners on projects incorporating Kelly-drilled bored piles – public and commercial projects (data source: supplemental survey)
Along with the risk associated with design and execution of the work, there is also the risk of unknown and unanticipated conditions, and this type of risk is handled differently depending upon contract approach, project type and country/region. In Europe, the handling of risk associated with unknown/unanticipated subsurface conditions differs widely from country-to-country and according to contract form. For public projects using the Design-Bid-Build approach, the owner typically retains the risk with unknown/unanticipated subsurface conditions, but there is no standard differing site conditions clause as there is in North America. For commercial projects using the Design-Bid-Build approach, the handling of risk can be quite different depending on the country where the work is being performed. A European contractor shared that the Netherlands and Germany are typically stricter than Ireland, France or the United Kingdom. For public and commercial projects using the Design-Build, the risk is typically transferred from the owner to contractor.
In the United States, the handling of risk associated with unknown/unanticipated subsurface conditions mainly differs according to project type. For public projects using the Design-Bid-Build approach, the owner typically retains the risk with unknown/unanticipated subsurface conditions, but there is a standard differing site conditions clause included on federal aid projects, which provides implied warranty provisions (via the Spearin Doctrine) that hold that the contractor is not liable for design defects (i.e. constructability). For commercial projects using the Design-Bid-Build approach, the contract may or may not provide protection and compensation (for time and costs) for differing site conditions. For public and commercial projects in North America using the Design-Build approach, there is a greater allocation of risk transferred from the owner to contractor. Although project owners, ideally, would like to transfer all of the risk to the contractor or the Design-Build team, there typically is a shared allocation of risk. This risk sharing results from the inherent nature of Design-Build projects in which the Design-Build team has based its scope of work, costs and scheduling on a partially complete (e.g. 30%) set of project drawings. For example, the Design-Build team may determine that the type, sizes and geometries of the foundation elements proposed in the initial bid proposal may no longer be possible due to information uncovered during the supplemental site investigations and soil/rock testing, which contradicted the initial findings from which the bid proposal was framed. As such, this may result in material (consequential) changes to the initial bid proposal in terms of scope of work, sequencing, etc. (including associated costs) for which the project owner may likely share in the costs associated with the needed changes.
One of the survey questions asked the responders to indicate by percentage the Kelly-drilled bored pile/bored pile work each responder's company performs in its operating region (). For public projects, in general, bored pile construction accounts for more than 50% of the work performed by contractors in both Europe and North America; however, about 10% of responders employed by North American contractors indicated that bored pile construction accounts almost all of the work they perform on public projects. For commercial projects, bored pile construction accounts for about 50% of the work performed by European contractors, while contractors in North America reported that bored pile construction accounts for more than 60% of the work they perform on commercial projects. Indirectly, this survey question also inquired about the range of methods utilised by contractors in Europe and North America, which revealed a noticeable difference in the breadth of technologies performed. In short, contractors in Europe appear to perform multiple technologies, even on the same project, compared to contractors in North America, where specialty contractors seem to limit the range of the work they perform. In Europe, the responders reported that there are many alternatives considered and utilised in addition to Kelly-drilled bored piles (e.g. continuous flight auger (CFA) piles, micropiles, displacement piles, etc.). In North America, the responders noted that there are not that many alternatives considered and utilised in traditional Kelly-drilled bored pile applications; however, similar alternatives were provided (e.g. CFA piles, micropiles and driven piles). Consequently, comparatively speaking, the percentage of the marketplace for Kelly-drilled bored piles is most likely larger in North America than it is in Europe.
Table 4 Reported amount of Kelly-drilled bored pile work performed by contractors on public and commercial projects (data source: initial and supplemental surveys)
Multiple contractors responded that the specifications and quality control/assurance (QC/QA) requirements for public projects are typically more restrictive than building codes used on commercial projects. As one responder from North America noted, ‘public works specifications are more restrictive and stronger than building codes used for commercial projects’. Another responder provided that there were ‘similar design principles but public work was subjected to much tighter construction inspection and therefore provides a more reliable product’. Discussing the findings during one of the workshops, a contractor in attendance postulated that as an unintended consequence of the stronger, more demanding, and restrictive specifications on public projects, ‘when the drilling crew comes off of the public work project and goes to perform commercial work, they will do it right. Sometimes, right means more expense. Ultimately, the crew that does it “right” is less competitive’.
Construction methodology and operations
Selection of foundation type and drilling equipment
Before the survey was developed, there were discussions within the research group about the primary driver of a project from a construction point of view. There were some who postulated that site conditions were the most important consideration when selecting and constructing a foundation type, and others responded that the available equipment or design codes/project specifications were the key considerations. As such, another survey question asked the responders to indicate the primary influencing consideration/driver for decision-making on public and commercial projects: (a) do project specifications influence/drive your company's decision regarding equipment acquisition, (b) does the equipment your company owns/rents dictate the construction method or (c) do the site conditions (e.g. subsurface profile, available work space, etc.) dictate the equipment to be used and the product that can be constructed? For both public and commercial work in both Europe and North America, the responders overwhelmingly indicated that site conditions are the most important driver ().
Table 5 Reported influences or drivers for deciding equipment acquisition and construction method used on projects incorporating Kelly-drilled bored piles – public and commercial projects (data source: initial survey)
The soil, rock and groundwater conditions along with available space, proximity to adjacent structures, etc. dictate construction methods, and, thus, the type of product that can be constructed. In addition to site conditions, the foundation type selected is also a function of the owner's/designer's/contractor's experience and the risks associated with constructing that particular foundation type in those particular conditions. One responder noted that it would be hard for a contractor to own construction equipment suitable for all the various sizes (length and diameter) of bored piles constructed.
Geometry of bored piles
Two of the survey questions inquired about the range of diameters and lengths of Kelly-drilled bored piles contractors in Europe and North America typically constructed. In general, European practice utilises smaller diameter but longer bored piles, while the converse is the typical practice in North America (i.e. larger diameter but shallower piles). Based on the survey responses, the breakdown of the different diameters and lengths of Kelly-drilled bored piles constructed in Europe and North America are presented in and , respectively. With respect to diameter, the practice was similar between public and commercial projects; however, there were slight differences with respect to the length of the bored piles installed on public and commercial projects.
Table 6 Reported usage of different diameters of Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey)
Table 7 Reported usage of different lengths of Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey)
Drilling equipment
To understand the types of equipment being used to construct the bored piles to the diameters and lengths required in the different subsurface conditions, the responders were asked about the type(s) of drilling equipment they used on projects. From the survey responses and discussions with contractors in Europe and North America, it was determined that European-style drilling equipment (i.e. multi-function, hydraulic rotary drill rigs) is very widely used throughout both marketplaces (a). The rotary drill rigs are compact, and transmit full torque and crowd force to the drilling tool. Used to a much lesser degree in Europe, the crane attachment drill rig (b) is well suited for drilling conditions where a hydraulic rotary drill rig cannot be positioned close to the hole (e.g. working from trestles and barges for bridge piling). European equipment manufacturers envisioned the benefit of monitoring both the drilling operation and the equipment performance, and have integrated the monitoring capabilities into the drilling equipment (e.g. Soilmec Drilling Mate System and Bauer B-Tronic System). The onboard systems on the European drill rigs allow real-time monitoring of drilling parameters and equipment performance and status, reduce or eliminate downtime due to preventable equipment malfunction, provide easier machine management and maintenance scheduling, are intended to facilitate an improvement in the quality of the work performed, and allow the preparation and dissemination of jobsite summary reports.
4 Photographs of different styles of drilling equipment: a European-style, hydraulic drill rig and b European-style crane attachment drill rig (photographs courtesy of Soilmec S.p.A.)
In North America, two types of bored pile drilling equipment are used but are rare or absent in Europe: (i) the truck-mounted drill rig (a), and (ii) the excavator-mounted drill rig (b). The truck-mounted rig is well suited when the bored piles are not located in close proximity to each other (e.g. power line foundations) where the mobilisation time between boreholes is more significant than the actual drilling time. The excavator-mounted drill rig is well suited and effective for sites where access to the borehole is restricted or not possible, and the excavator-mount facilitates a long reach even on sloping or uneven terrain. In addition, the conventional crane-mounted drill rig (e.g. manufactured by Watson, Calweld, or Hain, as shown in c) is different than the ones used in Europe in that the ones used in North America are typically larger and heavier.
5 Photographs of different styles of drilling equipment: a truck-mounted, b excavator mounted and c North American-style crane attachment (photographs courtesy of A.C. Kraut at Watson Inc.)
A survey question asked the responders to indicate the size of the rotary drilling equipment (categorised based on operating weight) typically used (either in their own fleet or rented) to construct Kelly-drilled bored piles on public and commercial projects in Europe and North America. From the responses (), there was not a considerable difference in the range of equipment sizes utilised with the exception that contractors in Europe prefer the larger/heavier drill rigs (>55 tons), while contractors in North America prefer the smaller/lighter drill rigs (<55 tons). Contractors were also asked whether the range of techniques used in their particular region was restricted by the availability of equipment, either owned or rented; most of the contractors responded that equipment availability did not affect the technologies they performed, but the technologies performed were mainly a function of site and subsurface conditions. Multiple contractors in North America did report, however, that transport of the larger/heavier drilling equipment could be cumbersome and difficult depending upon a State's requirements for special permitting.
Table 8 Reported average usage (ownership and rental) of different track-mounted rotary drilling equipment used to construct Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey)
Borehole stability
A survey question asked the responders to indicate for public and commercial projects the different construction methods the contractor used to advance and/or stabilise a borehole during drilling. From the responses indicated in , both open hole and cased hole methods were utilised in both Europe and North America. In the responses and during conversations, contractors in Europe indicated that drilling using unsupported means (e.g. open hole drilling with no casing/slurry and placement of casing in a previously drilled, oversized hole) is not preferred or readily used. As provided, these methods may allow for a relaxation of in situ radial stresses around the borehole, and the designers/contractors prefer direct contact between the borehole and the support means.
Table 9 Reported average usage of different construction methods used to advance and/or stabilise a borehole for Kelly-drilled bored piles on public and commercial projects (data source: initial survey)
Since borehole stability is paramount to the integrity of the bored pile and to the success of the construction project, an additional borehole stability question was posed to the responders inquiring about the selection of different open hole, temporary cased hole and permanently cased hole approaches. As reflected in , there are noticeable variations in practice regarding slurry and casing used during the construction of Kelly-drilled bored piles. In Europe, from the responses and conversations, it was apparent that contractors in Europe prefer the use of full-length temporary casing over the use of open hole drilling methods, and it was reported that it is rare that a contractor will drill a hole with no stability means (i.e. with neither slurry nor casing). Contractors in North America use open hole construction (with or without use of slurry) more than contractors in Europe, and there are areas in the United States where contractors will try to obtain a dry excavation during drilling and concrete placement.
Table 10 Reported average usage of different open hole, temporary cased hole and permanently cased hole approaches used to advance and/or stabilise a borehole for Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey and conversations)
Also, although not very common, contractors in Europe prefer to use mineral-based or bentonite slurries to provide borehole stability (e.g. to maintain sidewall integrity and to prevent bottom heave) during drilling over synthetic-based or polymer slurries. As shared by one contractor, the use of and preference for bentonite slurry in Europe most likely has been driven by international contractors who have significant experience with diaphragm wall construction and deep reverse circulation work. In North America, however, polymer slurry has broad acceptance and widespread use, although, some areas of the United States (i.e. select public transportation agencies) preclude polymer in favour of bentonite. Compared to a typical setup required for bentonite slurry (e.g. hydration and mixing tanks, storage tanks, cleaning system, etc.), polymer slurries typically require a smaller overall footprint, do not require as extensive hydration times, and do not require extensive slurry cleaning equipment (e.g. de-sanders, centrifuges, etc.). Two common issues reported by contractors and designers from Europe and North America relate to (i) cleaning requirements and soundness at the bottom of the hole, especially where high design base resistance is required, and (ii) the time effects associated with slurry and concreting, where the solids suspended in the slurry column may settle out and deposit on the bottom of the hole leaving a potentially soft bottom condition.
In Europe, the majority of Kelly-drilled bored pile construction is performed using full-length temporary casing, which is typically installed to depths needed to stabilise borehole or protect adjacent and/or nearby structures; and the use of permanent casing is not very common throughout Europe. The casing is usually installed using the drilling equipment and/or a casing oscillator, and it was reported that there is very little using of vibratory means to install the casing. In North America, it was reported that the means to install temporary casing is geographically and geologically dependent. That is, in the southeast and southcentral portions of the United States, it is common that the casing is installed in an oversized hole or it is installed using vibratory means. In the northeastern, midwestern and western portions of the United States, it is common that the casing is installed using the drilling equipment and/or a casing oscillator. As indicated in , the use of permanent steel casing is not very common in Europe; however, in North America, the use of permanent casing is more prevalent and depends on the application of the bored pile. As reported, the more prevalent use of permanent casing in Europe is for bored piles installed in bridges over water, whereas, in the United States, permanent casing is used to provide additional lateral resistance against seismic loading, for use in bridge applications over water, and for scour protection. Furthermore, various issues were provided by the responders in the comments section regarding the use of temporary and/or permanent casing in the construction of the bored piles. Common to both European and North American practitioners, these issues can be simplified to two main concerns: (i) the effect of the casing on geotechnical (i.e. axial resistance) and structural design considerations of the bored pile, and (ii) the effect of the casing on the integrity and long-term performance of the bored pile.
Table 11 Reported use of permanent casing (excluding for top/surface casing) with Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey)
Concrete – mix design, placement and concerns
During the past 20 or so years, concrete mixes have changed from a straightforward three-component material (aggregate, cement and water) to a complex five-component material (aggregate, cement, water, admixtures and water reducers). The admixtures affect and control the rate of hydration (for workability and set time), and the water reducers (e.g. plasticisers) affect the amount of water needed for fluidity and flowability to ensure the fresh concrete can get to its intended location (e.g. moving around the reinforcing steel) without clogging the lines. Correspondingly, the testing performed on the fresh concrete has also changed; simple slump tests have been supplemented with flow/spread tests, and with tests to check workability, aggressiveness, consistency, etc.
One of the supplemental survey questions inquired about who was responsible to provide the minimum 28-day strength and mix design for the concrete to be used in the bored piles (). Contractors in Europe indicated that the contractor is typically responsible to provide to the owner (for review) the minimum 28-day concrete strength intended for use in the bored piles; the practice is similar on both public and commercial projects, but the owner typically provides more direction/input on public projects. In North America on both public and commercial projects, however, contractors indicated that the owner typically provides to the contractor (for use) the minimum 28-day concrete strength and the recommended concrete design mix design (components or an established mix design). One responder reported that, at times, the owner or designer may refer to the concrete batch plant for its recommended mix design based on the required or specified performance.
Table 12 Reported responsibility to provide the minimum 28-day design compressive strength and the design mix for the concrete used in Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey)
Another supplemental survey question inquired about the minimum 28-day design concrete compressive strength used in Kelly-drilled bored piles on public and commercial projects. As summarised in , for public and commercial projects in Europe and North America, the minimum 28-day design concrete compressive strength typically ranges between 4000 and 5000 psi (27 and 34 MPa). In addition, the design concrete slump ranges between 7 and 10 in. (175 and 250 mm); however, it was reported that higher-flow type mixes are used in certain applications.
Table 13 Reported usage of different minimum 28-day design compressive strengths for the concrete used in Kelly-drilled bored piles on public and commercial projects (data source: supplemental survey)
Along with the answers to the closed-ended questions, the responders also reported various concerns with regard to concrete mix design and placement of concrete into the drilled hole. A common set of concerns from practitioners in Europe and North America included bleeding, use and effect of fly ash, segregation during tremie placement, improper flow of fresh concrete, and set behaviour of the concrete (either extended set or flash set). One responder stated that the bleed water emanating from the concrete during curing and poor rebar cage design resulting in incomplete concrete encapsulation of the rebar can cause problems with the crosshole sonic logging test. Multiple responders offered that the concerns seem to be more exacerbated given that the diameter and length of the bored piles have increased substantially. Others indicated that the owner's engineers and design engineers are not conversant with the latest technologies, especially with admixtures/additives used in the concrete mix designs. Another potential concern in North America but not in Europe is the issue with mass concrete, where North American contractors reported that they are currently struggling with the occasional imposition of mass concreting restrictions (especially since the use of large diameter (10 ft or 3 m) bored piles is becoming increasingly more common).
Responsibility of QC/QA during construction
One of the questions posed in both the initial and supplemental surveys inquired about which party (i.e. owner or contractor) was/is typically responsible to perform the on-site QC and QA of the work pertaining to the bored piles (). In Europe, the responders reported that for both public and commercial projects, the contractor is typically responsible for both QC and QA, and the work is usually performed by the contractor's in-house personnel.
Table 14 Reported responsibility for on-site quality control and quality assurance of the work pertaining to Kelly-drilled bored piles on public and commercial projects (data sources: original and supplemental surveys)
In North America, the responsibility for QC on public and commercial projects is often placed on the contractor with review and oversight provided by the owner's staff/agent. Conversely, the QA on public projects is typically performed by the owner's personnel or by an independent third party agency hired by the owner; and on commercial projects, the QA is typically performed by an independent third party agency hired by the owner. In addition, the owner typically retains a geotechnical engineer to provide construction observation.
Repair work on bored piles
Survey questions inquired about the amount and types of repair work performed by contractors to the bored piles their companies installed. The different types of methods used by contractors to repair Kelly-drilled bored piles on public and commercial projects are provided in .
Table 15 Reported use by contractors of different repair methods for Kelly-drilled bored piles on public and commercial projects (data sources: original and supplemental surveys)
For public projects, contractors in Europe and North America indicated that they typically repair less than 10% of the bored piles they install. For commercial projects, the reported amount of repair work was about half that reported for public projects, where contractors in Europe and North America indicated that they typically repair less than 5% and 3%, respectively, of the bored piles they install. One responder performing work in the western United States stated that in lieu of repair work or installing additional elements, an administrative deduct is applied to the amount to be paid. Another responder indicated that instead of repairing a bored pile containing a serious anomaly or defect, additional bored piles are installed because ‘the replacement is faster with far less argument, and we can get paid faster’. Others reported performing full-scale load tests (typically dynamic tests – e.g. Statnamic) ‘to verify the load carrying capabilities of a questionable shaft rather than repairing it’.
Impact of environmental and safety requirements on construction
Safety requirements
As part of the surveys, questions were posed to the responders querying about how safety practices and safety regulations affect bored pile construction. Similar from both Europe and North America, about half of the responders indicated that safety regulations and practices directly affect production and increase costs, and the other half responded that construction practices improved thereby lowering costs. A contractor from North America stated that ‘safety is considered a part of the work and is generally not driven by regulations, and most utilities have a high desire for safe work environments and have their own safety requirements’. One of the survey questions inquired about which safety regulations (e.g. hoisting, in-hole cleaning and inspection, hours of work, etc.) affect cost, construction technique selection and/or time of installation of bored piles. From the responses provided in , various aspects of the construction operations and inspection are affected by safety regulations, but the contractors did not indicate one aspect was more important or prevalent than the others.
Table 16 Reported effect of safety regulations on cost, technique selection and/or installation of Kelly-drilled bored piles on public and commercial projects (data sources: original and supplemental surveys)
Improvements in onboard monitoring systems on construction equipment and inspection techniques have largely eliminated the necessity of manual downhole inspection and cleaning. However, there are areas along the east coast of the United States where visual inspection and cleaning are still often performed manually by sending someone into borehole (e.g. downhole entry). Furthermore, the advent of downhole cameras has improved the safety and speed of inspection so that the impact of inspection on installation time and cost is minimised. One contractor reported, ‘we used to do downhole hand cleaning, and the inspector would do downhole inspection … these processes allowed us to use higher end-bearing capacity for the bored pile. Currently, a majority of contractors do not carry out downhole hand cleaning, and this has resulted in longer shafts, which are more costly’.
From the workshops and other conversations with contractors, compared to North America, jobsite requirements in Europe for safety can be much more restrictive and difficult to manage for bored pile construction. Undoubtedly, as claims and lawsuits have become more prevalent and common throughout the construction industry, a contractor's safety record is becoming increasingly important to owners. As such, this focus on safety may affect a contractor's ability to bid or perform work if the contractor's safety record and protocol do not satisfy an owner's standards and/or requirements.
In most of Europe and Canada, drill rig operator training is mandatory, and incorporates both safety and competency concerns into the training; meanwhile, in the United States, drill rig operator training focuses mainly on safety aspects of the operations. Drill rig manufacturers in Europe are subjected to the safety requirements of the European Execution Standards (EN 16228, Drilling and Foundation Equipment – Safety), which must be followed in order for the rigs to be sold and utilised in Europe. Given that many of the rotary drill rigs used in North America for Kelly-drilled bored pile construction come from Europe, practices in Europe may eventually affect the North American jobsite as well.
To stress the importance of safety, some countries in Europe have enacted laws and regulations with explicit language and provisions to ensure the proper practices are followed. In Germany, as one specialty contractor shared, ‘the proper application of construction standards is considered a matter of public order and safety, thus certain governmental regulations have to be obeyed … The state authorities in charge refer to comprehensive lists of standards, which are mandatory for design and construction’. In the United Kingdom, officials have enacted laws ensuring the proper design, construction and maintenance of working platforms used to support construction equipment and personnel. The safety of working platforms (e.g. constructed using earth materials, crane mats or suspended by a crane) is a very serious issue that affects onsite personnel on an continuing basis. The issues associated with problematic platforms, who owns the risk, who is responsible for design/construction/maintenance of the platforms, and how to increase the safety of the platform(s) through the life of the platform are of utmost importance and should not be left to chance. In the United Kingdom, the British Standards are explicit on platform stability concerns, especially concerning design/construction issues and ground pressures beneath the construction equipment. Moreover, it is mandated by law that a Health and Safety Certificate is required for each working platform on a jobsite. The certificate must be signed by a geotechnical engineer, who is also responsible for the design and for ensuring maintenance is performed on the platform throughout its life. As allowed by law, should the platform not be maintained and the engineer feels the safety of the platform has been compromised, the engineer is allowed to cancel the certificate, which could result in a shutdown of the project site. Highlighting the concern and importance for jobsite safety, there is a line item on each bid sheet explicitly separating the costs of the platforms (design/construction/maintenance), and the cost is borne by the owner of the project.
Environmental regulations
Based on the discussions during the workshops and conversations with contractors, environmental restrictions are more demanding and/or restrictive in Europe than in North America. As an example, select examples of the environmental restrictions from the Cityringen project in Copenhagen, Denmark are provided in .
Table 17 Select environmental restrictions on the Cityringen project in Copenhagen, Denmark (source: M. Chiarabelli, Soilmec S.p.A.)
The reported influence and effect of select environmental restrictions on construction techniques used with Kelly-drilled bored piles on public and commercial projects are provided in . From the survey responses, the contractors indicated that environmental restrictions drive up the cost and sometimes decrease the productivity at the site. Moreover, from the responses, contractors in both Europe and North America listed the same environmental restrictions (noise/dust, drill spoil/slurry disposal and ground vibrations) as the ones having the most influence on their construction techniques and operations.
Table 18 Reported influence and effect of select environmental restrictions on construction techniques used with Kelly-drilled bored piles on public and commercial projects (data sources: initial and supplemental surveys)
Economic state of the marketplace
In 2013 and 2014, construction spending in North America was strong and sustained due to a variety of factors. The total construction spending in North America (United States and Canada) increased from about US$1.11 trillion to US$1.15 trillion, a 3.9% increase, from 2013 to 2014 (FMI Corporation Citation2014; FMI Corporation Citation2015). In the U.S., the increase in construction spending and capital expenditures, especially for commercial construction (e.g. office, educational, residential) in various cities in the U.S. (especially the large cities such as Manhattan, Miami, Dallas and Las Vegas), was propelled by continued jobs growth, improved business confidence, increased consumer spending, low interest rates and low fuel prices. There remained, however, an uneasiness and uncertainty in long-term forecasts given the serious shortfall in federal and state infrastructure spending, funding difficulties, partisan gridlock in Congress, and the reduced investments and activity in the energy sector due to low oil prices. To offset the shortfall in government spending and traditional financial lending, there had been an infusion of commercial/private funding into public infrastructure markets, but the U.S. has been especially slow to embrace (but has been improving) alternative project delivery methods (e.g. private–public partnerships (P3s) and construction manager general contractor (CMGC)).
In Canada, there was an increase in infrastructure spending, especially at provincial and local levels, but this was more than offset by both decreased spending in the residential and nonresidential structures markets and by reduced investments and activity in the energy sector due to low oil prices. In the U.S., the total construction spending increased from about US$900 billion to US$947.8 billion, a 5.4% increase, from 2013 to 2014, while the total construction spending in Canada decreased from about US$211 billion to US$206.2 billion, a 2.3% decrease, from 2013 to 2014 (FMI US Construction Market Outlook Overview, 2014; FMI Construction Outlook – 1st Quarter 2015 Report; IHS Economics, Global Construction Outlook 2015).
In the Eurozone, the economy expanded from 2013 to 2014 and was expected to grow slightly in 2015 and will be driven mainly by low fuel prices and a weaker Euro, despite current financial and unemployment concerns. The total construction spending in Europe increased from about US$2.1 trillion to US$2.19 trillion, a 4% increase, from 2013 to 2014 (IHS Economics, Global Construction Outlook 2015). The increase in construction spending had been realised in mostly Northern European Countries, especially in the United Kingdom, Poland, Ireland and Germany, where year-over-year growth from 2013 to 2014 was about 13.2%, 8.9%, 8.3% and 6.1%, respectively. For the seven countries represented by the responders (i.e. France, Germany, Hungary, Italy, Switzerland, Netherlands and the United Kingdom), the total construction spending increased from about US$1.39 trillion to US$1.46 trillion, a 5.1% increase, from 2013 to 2014 (Pruell and Blanford Citation2015).
Affecting both Europe and North America was the concern with the skilled labour talent shortage. Extensive debates addressed the potential skilled talent shortage, the difficulty of finding talent, and the difficulty developing talent with specialised skills for both office and field personnel. This remain remains an continuing issue across all of the countries evaluated.
Summary
As part of a larger research effort, this paper presents and discusses the similarities and differences in the Kelly-drilled bored piles industry between Europe and North America. There is a wide variety of practice throughout Europe and North America that is attributed to geology, local practices and project type (public infrastructure or commercial practice). Practice in both Europe and North America continues to evolve, especially toward larger, deeper, higher capacity bored piles and with greater incorporation of extreme event (e.g. seismic) considerations. In Europe, bored pile practice typically follows performance-based methodology and specifications, whereas, in North America, traditional prescriptive-based methodology and specifications are more widely used. In Europe, full length, temporary casing and/or mineral-based slurry is used to provide support to the borehole during drilling; in North America, temporary casing and/or polymer-based slurry is used more often. The use of permanent casing is not common in Europe, but there is a more widespread use in North America, especially for bridge structures over water and for scour protection. A contractor's safety record is becoming increasingly more important to owners in both Europe and North America, and, as such, a poor safety record may preclude a contractor from bidding and/or performing work on some projects. Lastly, environmental restrictions and safety requirements are becoming more restrictive in both Europe and North America, and are producing a change to the approach used to construct the bored piles (e.g. equipment and inspection capabilities).
Acknowledgements
The authors would like to express their thanks to Mary Ellen Large (DFI), Dr Anne Lemnitzer (UC-Irvine) and the graduate students at UC-Irvine for their assistance in performing the surveys, gathering the data used in this paper, and input in the preparation and review of this paper. In addition, the authors would like to acknowledge the other collaborators on this project (and associated publications), whose work and contributions will serve to advance the state-of-practice of Kelly-drilled bored piles: Dr Michael Arnold (Bauer Foundation Corp.), Maurice Bottiau, ir (Franki Foundations), Dr Dan Brown, P.E. D.GE, (Dan Brown and Associates, P.C.), David Coleman (Skanska), Ray Fasset (Condon-Johnson & Associates Inc.), Bernard H. Hertlein (GEI Consultants), James Johnson (Condon-Johnson & Associates), Alan Macnab (Macnab Consulting), Arthur Tipter, Dipl. Ing. (Züblin), Gerald Verbeek (Allnamics, Inc.) and Thomas Wulleman, ir (Franki Foundations).
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