Delft University of Technology
Position estimates for existing trenchless installations
Broere, W; Achterhuis, EJ
Publication date 2015
Document Version
Accepted author manuscript Published in
Proceedings of the international no-dig istanbul 2015, 33rd international conference and exhibition
Citation (APA)
Broere, W., & Achterhuis, EJ. (2015). Position estimates for existing trenchless installations. In Y. Torun, & D. Choi (Eds.), Proceedings of the international no-dig istanbul 2015, 33rd international conference and exhibition (pp. 70-1-70-6). ISTT.
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International No-Dig İstanbul 2015
33rd International Conference and Exhibition
______________________________________________ ________
İstanbul
28-30 September 2015
Paper Ref 70
POSITION ESTIMATES FOR EXISTING TRENCHLESS
INSTALLATIONS
Wout Broere1 and Ernest-Jan Achterhuis2
1 Geo-Engineering Section, Delft University of Technology, Delft, The Netherlands 2 Fodere BV, 's-Hertogenbosch, The Netherlands
ABSTRACT: The position and alignment of existing cables and ducts, previously installed by trenchless technologies, has not always been recorded with sufficient accuracy for new works to be safely conducted close to existing installations. Inaccurate or missing registration in the past, manual data entry, previous activities in the same areas, can all lead to deviations between recorded and actual location. Location detection is often hampered by the depth of installations, especially in the case of HDD.
The Netherlands Society for Trenchless Technologies (NSTT) has conducted a survey of the accuracy of trenchless installation techniques over the past decades, and of external factors influencing the position of existing cables and ducts, to get an estimate of the positional accuracy of older TT installations. This paper gives an overview of estimated accuracies for cables and ducts installed by HDD, micro-tunnelling, impact moling/ramming, auger boring and other TT installation techniques, taking soil conditions and the state of technology at the time of installation into account. The paper also gives an overview of which TT came to the Dutch market in which period.
1. INTRODUCTION
The exact position and alignment of a trenchless installed cable or duct would be known if properly recorded in the past, and if there were no external factors impacting on its position in the ground. However, in many cases the registration in the past was made with a low accuracy, or the technical systems where these registrations have been made have seen conversions over the years, effectively reducing the accuracy of the registration. Also, shallow cables are often no longer exactly located where they were once placed, as they may have been moved due to external causes such as for instance later open trench works at their location. Deeply installed cables may actually deviate from their recorded location if for instance only entry and exit point of an HDD are recorded and low quality as-built data is used.
Therefore, in order to establish the accuracy of a historic location registration of a cable or duct, one needs to take into account
the accuracy of the installation technique and location detection techniques at the time of installation, the accuracy of the location registration systems used, and
the external factors that can have influenced the location of the cable or duct since its installation. If all these factors are taken into account, a zone of uncertainty can be established surrounding existing cables and pipelines, where the installation is within this zone with a high probability. If this zone is accurate and small enough, this would allow the planning of new trenchless installations nearby existing installations, without the
need for trial pits or other pipeline location techniques to accurately establish the factual location of the installation.
In order to determine such zones of probable location, the Netherlands Society for Trenchless Technologies (NSTT) has made an inventory of the various trenchless techniques in use during the past decades, their accuracy and of external factors that can influence the accuracy of the location registration.
Traditionally, the installation of utilities in the Netherlands was by open cut. Even crossing the many waterways was done by placing preformed ducts in shallow trenches in the waterways and dikes (so-called zinkers). The increasing economic and social impact of temporarily closing roads and waterways helped the introduction of trenchless alternatives. Around 1960 unguided auger borings and impact moling made their entry, followed by closed face micro-tunnelling (MT) in the 1970-ies and horizontal directional drilling (HDD) by the mid-eighties. The drive to minimize hindrance and increase safety during works, as well as increasing technical possibilities, meant that from the 1990-ies the use of trenchless technologies steadily increased and boomed in the early years of the 21st century with the mass installation of glass fiber networks.
The increase use also drove innovation in safety and control. For HDD this led to the more widespread use of steering tools such a Tru-track and mechanical gyroscopes by 2000, followed a few years later by optical gyroscopes. As a new MT alternative, Directpipe was first used in the Netherlands in 2010 and around that time also adapted auger techniques became more wide-spread. These include guided auger borings and systems with a water lock, in order to safely operate below the water table. An overview is given in Figure 1.
Figure 1. Time-line of the introduction of trenchless techniques in the Netherlands.
2. LOCATION ACCURACY
To get an indication of the accuracy that was be obtained normally with the various trenchless techniques over the years and of other practices normally impacting the recorded accuracy of locations, a series of interviews with senior industry experts was conducted (Achterhuis et al., 2013). The main factors identified from these interviews are the trenchless installation method used, the date of installation, the material of the pipe or duct, the guiding system used, whether the pipeline has been surveyed after construction, the general geotechnical description of the main soil layers, which permit authority was involved and whether the as-built data has been digitally collected and stored or digitized afterwards.
Other factors that may influence the accuracy of the current location data are the common use of local reference points for surveying operations in the past, instead of using the national reference grid (RD-net) and the common practice to store (hand-drawn or CAD-drawn with manual data entry) maps containing the as-built data only, which have been converted to digital storage after the introduction of laws on underground location information (WION) in 2008. More details are given in section 2.4.
The results of the interviews have been condensed to a number of tables, which summarize the location accuracy and the impact of the various factors using a number of color-coded classes, as listed in Table 1. In order to estimate the location accuracy, one would look up the base accuracy depending on the year and type of installation. Additionally, the influencing factors are listed and color-coded as well, and the estimated accuracy is equal to the lowest class encountered for the installation type or influencing factors. If multiple factors with a class below IV (no impact) are encountered, it is suggested to take the combined effect into account in that
multiple class IV factors have no effect,
3 class III factors are converted to 1 class II factor,
Technique
Open face → steered auger
→ auger → auger w/water lock
Closed face → closed face M-TBM → directpipe
HDD → HDD → steering tool
→ gyro → optical gyro Impact moling → impact moling
→ manual excavation at the face
The accuracy classes have been defined such that class V (excellent) includes the accuracy limits as set by WION and that class I (poor) encompasses all deviations of more than 5 m.
Table 1. Accuracy classes used in the location estimate tables below.
2.1 Open Face, Auger Boring, Impact Moling and Impact Ramming
Even in Dutch soft soils with high water tables, a substantial number of open face and auger borings have been made over the years, either at locations above the groundwater table or with temporary lowering of the water table. The accuracy of the manually excavated open face borings is excellent, as they are limited to man-accessible pipe sizes and the position is normally surveyed on an ongoing basis during excavation. Smaller diameter auger borings are not normally surveyed, but the relatively short lengths installed and the relatively high stiffness of the pipes means that the position is relatively well known after construction. The steered augers have an even better positional control, which matches that of open face excavations. The recently introduced augers with a water lock between casing and starting pit allow limited operation below the water table with limited impact on the accuracy. Their accuracy is estimated as equal to augers without water locks.
The accuracy of ducts installed by impact moling and impact ramming is estimated as mediocre. This is also evidenced by the high number of damages to nearby utilities reported in recent years which are attributed to impact moling. Especially when impact moling installations pass through soil layers with significant differences Table 2. Accuracy estimates for open face, auger and impact moling.
Class Description Deviations from the recorded location Color
IV Excellent / no impact Less than 1m
III Good Less than 2 m
II mediocre Less than 5 m
I Poor More than 5 m
No information / not applicable
Asbestos-cement Steel
Glass-fibre reinf. polymer Concrete
Polymer reinf. concrete
PVC * * Ty pe of fac e Auger Steered auger Impact moling So il ty pe
Peat (applies to vertical deviations only)
Clay Sand Gravel Lo ca tio n d ata As built by hand Digital recorded & stored
Du ct / pip elin e m ate ria l
* only applies to diameter < Ø200 mm
Manual or mechanical excavation at the face
Reference system set locally
RD-net used as reference system 1960 1970 1975 1980 1985 1990 1995 2000 2002 2004 2006 2008 2010 2012 2014 Bouy ant Tens ioned Pass ing la yers with differ ent s tiffne ss
in stiffness or soil density, the positional accuracy degenerates and large deviations have been observed. Similar considerations hold for unguided auger borings.
The material of the pipeline has no noticeable impact, except for small diameter PVC ducts which are relatively flexible. If these are installed below groundwater without correction for their buoyancy, if they are (partly) air filled below groundwater during their lifetime and subsequently experience buoyancy behavior during their lifetime or if the pipeline is tensioned after installation, these may deviate more than anticipated. And note that asbestos-cement (AC) pipelines have not been newly installed after 1995. The impact of soil conditions is generally limited, except in peat layers where buoyancy and a lack of soil resistance may give rise to vertical uncertainties.
The final factor to consider is the way the local reference points are setup, to which the position of the pipeline is referenced. If the contractor or owner sets up his own local reference system, instead of using the nation RD-net coordinates, a loss of accuracy may occur. The same holds if as-built data is collected by hand instead of recorded and stored digitally immediately.
2.2 Micro-tunnelling and Directpipe.
For micro-tunnelling and directpipe projects the positional accuracy is normally excellent. The only case where unexpected deviations may occur is for pipelines in peat layers, where the bouyant behavior of the pipe, the relatively high weight of the TBM and the lack of soil resistance may lead to deviations. If properly surveyed, which is certainly normal for larger diameters, these deviations should be recorded in the as-built data.
Table 3. Accuracy estimates for closed face micro-tunnelling and direct pipe.
2.3 Horizontal Directional Drilling.
When estimating the accuracy of HDD installed pipelines, more influencing factors have been identified, compared to auger boring or micro-tunnelling. Compared to other installation methods, the possible accuracy of HDD is very high, but the final location of the pipeline is influenced by volumetric weight differences between pipeline and surrounding medium leading to buoyant behavior of the pipeline and by a post-installation tensioning of the pipeline.
In all soil types, the difficulties in steering when passing the interface between layers with different stiffnesses or different densities may lead to larger uncertainties in the location. Also, small diameter pipelines in peat layers have been observed to drift, most notably due to overburden loads of nearby earth works. In such conditions pipelines have been found more than 5 m from their original location, although the effects of location errors during installation and drift due to external loads could not be separated.
Asbestos-cement Steel
Glass-fibre reinf. polymer Concrete
Polymer reinf. concrete
So
il ty
pe
Peat (applies to vertical deviations only)
Clay Sand Gravel Lo ca tio n d ata As built by hand Digital recorded & stored
Du ct / pip elin e m ate ria l
Reference system set locally
RD-net used as reference system
1960 1970 1975 1980 1985 1990 1995 2000 2002 2004 2006 2008 2010 2012 2014 Bouy ant
Tens ioned
Table 4. Accuracy estimates for horizontal directional drilling.
The most influential factor, however, is the steering system used during installation. The walk-over system is widely used and has improved its accuracy oer time. For deep borings, below 10 m and below the water table, the accuracy drops to poor, and the same can be said of the reliability in the vicinity of large bodies of steel such as sheet pile walls or near high-voltage utility lines.
Where such magnetic disturbances are problematic, the TruTrack and Paratrack systems provide a higher accuracy and greater reach. They remain sensitive to magnetic disturbances from large steel bodies such as sheet pile walls. The mechanical gyroscope and especially the optical gyro do not suffer from these downsides and have seen widespread acceptance since the early 2000's.
Table 5. Accuracy estimates for horizontal directional drilling (continued). 2.4 Location Data Handling, Storage and Conversion.
The accuracy of the location of a cable or pipeline as drawn in maps does not only depend on the accuracy of the installation technique used, the soil conditions and the pipe material. The way that location data is collected and stored storage also plays an important role.
In the early days of pipeline installation, contractors and owners often used local reference points as the basis for location surveying (e.g. a local landmark or the front of the HDD rig). The results of the survey were recorded on hand-drawn maps in the field, and might include notes on the reference point used (e.g. 'entry point of the duct is 1 m out from the building corner'.). When redrawn or digitized in the office these might introduce errors, especially if years later the reference points have changed (e.g. the building has been remodeled or demolished.)
PE **
Steel *
Glass-fibre reinf. polymer ** **
Cast iron So il ty pe Peat Clay Sand Gravel Lo ca tio n d ata As built by hand Digital recorded & stored
Du ct / pip elin e m ate ria l
* (applies to vertical deformations) ** (applies to vertical deformations, count as 2 class III events)
Reference system set locally
RD-net used as reference system 1960 1970 1975 1980 1985 1990 1995 2000 2002 2004 2006 2008 2010 2012 2014 Bouy ant Tens ioned Pass ing la yers with differ ent s tiffne ss diam eter pipe line
depth below surface
5 10 15 20 & > Ste erin g s ys tem Walk over
Accuracy of the system Relaton to reference grid Deviations during borig Handling as-built data Accuracy of the system Speed
Deviations during boring Gyro
Accuracy of the system Speed
Deviations during boring Sensor-TruTrack and Paratrack vicini ty of powe r lines sh eet p ile w alls 1960 1970 1975 1980 1985 1990 1995 2000 2002 2004 2006 2008 2010 2012 2014
Also, in several cases, the line-width in hand-drawn maps is so wide that it would indicate ducts of decimeters to a meter wide if taken to scale.
The depth of the installation was often not recorded accurately at this time. The exception would be HDD installations, where the as-built data would include the vertical curvature and depth, but horizontal curvatures were often not required and subsequently many HDD installations ended up as straight lines between entry and exit point even if a horizontal curve had been bored.
Other sources of errors stem from the manual recording of location data on notepads during the execution of works. This was for instance often the case between 1990 and 2000 when walk-over systems were used for HDD. Writing errors, lost notepads or smeared note due to bad weather conditions are notable sources of errors. These practices change with the introduction of the Wet Informatieuitwisseling Ondergrondse Netwerken (WION), which came onto law in 2008. This national law on the Information Exchange of Underground Networks required, amongst other things, pipeline owners to digitally record the location of all pipelines and cables at the Cadastre (NEN, 2004). The location data contained in hand-drawn maps was converted at this time to digital data, but the conversion was mostly done by external parties, without any knowledge on pipelines or trenchless installation techniques. Additional data in manually added notes (e.g. noting the installation method used for a particular pipeline) was mostly discarded. Where the local reference points described in the maps had changed and no global reference grid was used, the converted location of installations would easily differ from its actual location.
With the introduction of WION, the surveying and handling of location data has improved. Most surveys are now based on the national reference grid (RD-net) and digitally recorded and directly stored at the Cadastre. The remaining issues stem from the fact that the Cadastre records data with the accuracy given by the owner, but WION only requires a 1m accuracy and excludes depth of location from the required data. As a result, normally no depth registration is stored and the actual location could be 1 m off in the horizontal plane without raising any flags from the system.
3. CONCLUSIONS
The recorded location of pipelines installed by trenchless techniques in the past decades in the Netherlands has been converted from hand drawn location mads held by the owners to a digital storage at the Cadastre. Here an accuracy of 1m in the horizontal plane is required by law. Due to the accuracy of the original installation techniques and the circumstances by which the data was handled and stored in the intervening years, the accuracy can be less than 1 m.
A series of tables has been devised to estimate the accuracy of older installations, based on the technical standards and common practice at the time. These position estimates can be used at the design stage of a project to check if sufficient space is available to install new trenchless installations in the vicinity of existing ducts and pipelines, without the risk of hitting the existing infrastructure.
It appears that ducts installed by impact moling and HDD with more than 10 overburden hae the highest probability of deviating from their projected location, and that the manual handling of data and the use own self-selected local reference points instead of the national reference grid are common causes of uncertainty for all technqiues.
In all cases a new survey of the actual location of the pipeline would yield more accurate information, but such a survey may not always be possible, certainly not without taking a cable or pipeline (temporarily) out of operation.
4. REFERENCES
Achterhuis, E.J., Snikkenburg, G.H., Ballintijn, O., and Meinen, K. (2013). Nauwkeurigheid van boortechnieken voor de aanleg van kabels en leidingen. Netherlands Society for Trenchless Technologies (NSTT).
