Trenchless Tecnology



The no-dig , or  trenchless technologies , of which IATT – a no- profit association, promotes the awareness, experimentation and diffusion in Italy, are also called “ low environmental impact “ technologies.- as they have later been referred to, in virtue of the results of important studies that have demonstrated how their use reduces social(environmental costs by 80%  (TIL lab sources on the model of environmental impact elaborated by the Federation of Swedish Industries) and by 67% in on-site accidents (INAIL sources).

As their name suggests “no-dig” technologies are those which allow the laying, usage and maintenance of sub-soil networks while reducing to a minimum or completely eliminating open trenches and, compared to the latter, have the evident advantage of reducing:

1)    the movement of debris (for example the movement of debris to landfills)

2)    heavy vehicle traffic

3)    the time and area occupation of public soil

4)    road surface, breakage (deterioration)

5)    viability impact

6)    disruption of commercial, residential or leisure activities

7)    major safety of citizens due to the elimination of the dangers connected to open trenches

8)    safeguarding of  the workforce in terms of safety (less dangerous worksite conditions)


Low environmental impact technologies, alternative to traditional trenches – suitable for the laying or rehabilitation of existing infrastructures – are technologies which

are, at the same time, more economical, equally reliable and less invasive above all, in terms of social inconvenience and deterioration of road surfaces; their convenience appears even more evident when, in the global financial evaluation  social costs are

taken into consideration. In fact, the techniques do not only mean reduced costs in terms of feasibility, but also mean the transfer of the costs of non-environmentally, eco-friendly activities.

These technologies can be utilized in various fields and are not new, even though they have been seldom used in Italy where traditional intervention techniques are preferred.

They may be subdivided into five groups according to their characteristics.

° Survey

° Guided horizontal drilling

°  Non guided horizontal drilling

° Associated technologies

° Reclaiming and exploitation of existing infrastructures


The description of each technology is hereafter described, considering its application methodology,  to the advantages of its application restrictions.


1-    SURVEY


1.1  Georadar Systems (Ground Penetrating Radar GPR)


These allow the non destructive or non invasive detection of the presence and position of objects in the sub-soil to a depth of several metres, using the phenomenon of the reflection of electromagnetic waves of particular frequencies.

The system consists in a control and data acquisition unit, and of one or two antennae and permits the acquisition, elaboration and interpretation of data and the restitution of bi-tri dimensional graphs (paper or electronic) in design or in section. According to the number of antennae and frequencies used for introspection, the technique permits the more or less accurate detection of the position and dimension of objects present in the sub-soil.

The use of the technology is preliminary to the use of no-dig techniques which involve reduced drilling or digging and, as well as being useful for the planning of technological networks, allow the execution of stratigraphical profile analysis, archeological, civil engineering and environmental surveys. Its utilization is mainly conditioned by the geological conditions of the soil (the presence of water, in fact,

diminishes the penetrative capacity of the electromagnetic waves) and the type of objects present in the sub-soil (for instance, the presence of metallic chain mail).


1.2  Cameras and CCTV systems


The technology allows the inspection of the internal surfaces of water, sewer and gas pipelines, as well as tanks, wells and cisterns. It is prevalently used to analyse the pipeline conditions and plan their rehabilitation, insomuch as  it allows the evaluation of real dimensions, and the identification of fractures, intrusions or infiltrations, and eventual illegal connections.

The system consists in motorised colour TV cameras or ones mounted on wire guided carriages provided with a 360 ° axial  rotating head with a 270° transversal exploration,with adjustable lights for pipeline illumination and systems to detect the “fault” dimensions and pipeline slant. The camera is connected to an external control monitor and the information registered can be memorized on magnetic or digital supports (bearings); when gas pipelines are involved , the system must be certified

non deflagrating while with water pipelines every caution must be applied in order to avoid leak occurrence. The dimensions and grade of pipeline blockage can limit the use of this technique.


1.1  Tube detector


This allows the detection of sub-soil metallic structures(coils and pipelines) the exploitation of the generation properties of magnetic fields. The technique, preliminary to digging operations, does not, however, provide indications to the depth of objects or underground structures of other materials.


  1. Guided Horizontal Drilling


2.1. Horizontal Directional Drilling


This allows the laying of polyethylene  or steel pipelines of a diameter of 40-1600mm), used in the supply of all types of subservices (including petrochemical products),

The laying is performed by means of horizontal directional drilling from an entry point to an exit point, without digging trenches.

The technique involves various work phases and can be performed “dry” or “humid” (with adjuvant procedure using a fluid stream of water and bentonite),


–         a pilot bore hole is performed by means of a set  of rods, with a guiding front drilling  head .

–         on reaching the exit pit, an appropriate reamer is fixed to the drill head which allows the widening of the hole to the dimensions required to lay the planned pipeline


When pipe line are laid, the reamer phase of the bore hole can be avoided therefore reducing not only the time needed to carry out the job, but also the dimensions of the machinery utilized, and therefore the worksite area . Directional drilling is particularly adapt when obstacles need to be overcome, such as rivers, canals, large road networks and public areas, and can be used in the consolidation of slopes subject to landslides and the containment of polluted sites. The utilization of this technology can be conditioned by the presence of stones  of notable dimensions or of loose ground, such as gravel. Furthermore, according to the diameter of the pipeline to be laid and the length of the plant to be built, the dimensions of the work site are to be restricted in  urban areas,

–    On work completion,  the entry and exit pit are restored.


2.2. Rod pusher


This technology is a particular type of horizontal directional drilling, performed exclusively dry, with machinery of small dimensions and for the laying of pipelines of between 50-160 mm in diameter.

As it requires reduced dimensions work sites, it is particularly used in urban environments, and whenever it is possible to use an existing “mane-hole” as starting point.


2.2. Micro tunnelling


This technology allows the laying of steel, cement or gres ceramic pipes of a diameter of 250-2,500 mm . the laying is performed by pushing the tubes of lengths varying between 1-3 meters from an entry well on an exit one. The front part of the pipe is made up  of a reamer and by a swivel-headed drill, which disintegrates the material while it advances. The debris is brought to the surface by means of a closed circulation system of water and bentonite constantly moved by large pumps.

The orientation of the drill head is controlled by a laser beam sent by the entry well along the direction of the drilling, which activates a support sensor connected to a cutting head, which operates by means of a system of hydraulic jacks. The technology is mainly used for the laying of water and sewer pipelines , generally of

large dimensions, and can be used on all types of ground with good results. The use of this technique can be limited, above all in urban areas, by the necessity to have large areas available for the setting up of work sites as well as economical issues connected to the dimensions of the works to be performed.


2.3 TBM (Tunnel Boring Machine)

The TBM technology is quite similar to  that of the micro-tunnelling and allows the realisation of  large and long-sized tunnels in any soil typology, by means of   a cutting head with the optional insertion of internal linings. The  range varies from  2 m  to 12 m  and can be used for the laying of pipes and for infrastructures  (example highway tunnels). The TBM machine is equipped with  a cutting head,  a guidance unit, pushing units, an electrical engine, conveyor belt for debris removal  , lifting appliances for  the lining units; the driving unit is next to the excavation front;  directional control is performed by means of a laser system.


  1. Non guided horizontal drilling


3.1 Mole

This allows the laying of pipelines of 90-180 mm in diameters which are realized through dry-drilling by means of compressed air thrust systems, from an entry pit to an exit pit. The pipe is laid directly during drilling, by connecting it to the mole extremity using appropriate fittings of small dimensions.

As it is not possible to effectuate significant corrections to the drilling direction, this should be appropriately orientated at the right depth from the onset.

Its utilization is optimal for limited laying lengths and in urban environments due to the reduced dimensions of the machinery,  while it is conditioned by the presence of boulders of significant dimensions compared to those of the machinery.


3.2 Tube pushers


This allows the laying of tubes of 600 – 1500mm in diameter; it is analogue to Micro tunnelling but differs from it for the absence of a cutter positioned on the drill head and for the fact that the drilling can not be directed. This technology is prevalently used for crossing railway lines and roads and is suitable for drilling limited lengths.

Its use is not feasible in the presence of rocky ground or water aquifer layers and can be conditioned in urban environments by the necessity to have a large work site area available.



  1. Associated technologies

4.1. Mini trenches


This technology allows the laying of subservice infrastructures, such as water, electricity and telecommunication services, through the simultaneous execution or non of reduced dimension reaming of the road surface, pipe or cable layout and furrow filling with cemented mortar. The technique is applicable on layouts which involve, generally, asphalted, concrete surfaces, having a foundation of compact material and are usually performed close to the roadside. Its use where foundations, contain medium size boulders, or are of sandy or gravel type, must be appropriately evaluated according to the circumstances. The working phase foresees the road surface reaming (cut)  for a maximum depth of 40 cm, the laying of cables or pipes (up to a maximum of 3 of 40-50 mm in diameter) and the trench filling. For the latter, generally cement mortar   up to 3 cm of the step area is used, completing the filling with the material with which the final wearing course is made.

The growing interest in the utilization of this technology, above all in the telecommunications sector, has led to the development of new filling materials (rapid mortar) which are characteristically superior to classic mortars and allow the mini trench to be filled up to the pedestrian  or vehicle area, therefore avoiding the restoration of the wearing course. In particular, the mortar has recently been awarded certification by the ANAS Study and Research Centre. The equipment used  is of such dimensions as to allow the setup of worksites of extremely limited dimensions, ensuring easy utilization in both urban and extra urban environments.


4.2 Micro trenches

This technology is analogue to the mini trench  but both the drillings and equipment

employed are of extremely reduced dimensions. In particular, the surface drilling has a maximum length of 1.6cm with a maxi  depth of 15 cm-

This technology is particularly indicated, both in urban and extra urban environments, for the laying of subservices under pavements, roads, verges, parking areas or university campus where plant compressive stress is limited when superficially laid.


  1. Rehabilitation and exploitation of existing infrastructures


The rehabilitation techniques of existing infrastructures are several but these can be subdivided into three groups according to whether the installation of the new pipeline requires a reduction, an increase or maintenance of its original dimensions.


5.1. Installation of new pipelines with a diameter inferior to the existing one.


5.1.1 Slip-lining


The technology allows the rehabilitation of an existing pipeline by inserting high density polyethylene pipes with a diameter inferior to the pipeline to be reclaimed, typically made in cast iron. It is possible to realize lengths of 300m with this technology according to the diameter of the pipeline to be laid which could vary from  20-1,000mm,

It is prevalently used for the rehabilitation of water and gas pipelines (up to 630mm) and is utilized in both urban and extra urban environments, allowing ex-novo works or the replacement of pre-existing utilities.

The technology is also conditioned by the existence of bends, multiple or single angle variation to 3°, which therefore mean obligatory interruption of the insertion points.


5.1.2 Compact pipe/ U-liner


The technology allows the rehabilitation of existing pipelines, typically in cast iron by means of the insertion of reduced diameter PEAD pipes, at the time of production, through the formation into a “C” or a “U” shape, and installed through a process of temperature controlled reversion by using steam at 130°.

This technology is part of the so-called close-fit lining, adhering perfectly to the existing pipeline after relining.

It allows, on average, the laying of lengths of pipes up to 250m with a diameter inferior to 250mm and lengths of (100-150) m for diameters superior to (300-350)mm,

It  is prevalently used for the rehabilitation of sewer networks, water networks and circular section gas pipelines, in both urban and extra urban environments, and allows ex novo works or the replacement of pre-existing utilities.

This technology is conditioned by the existence of bends, multiple or single angle variations superior to 225°, which therefore mean obligatory interruption  of the insertion process.


5.1.3. Roll down  (or sewage lining)


The technology allows the rehabilitation of a pre-existent pipeline by means of the on site cold drawing insertion of PEAD pipes of reduced diameter, up to 19%  by means of  axial drawing. After insertion the pipe is brought to standard size through the pressurization with cold air or water, until perfect adherence to the pipe walls is obtained (closed fit lining technique). It allows on average the laying of pipe lengths up to 300 meters, for polyethylene pipes up to 500mm in diameter.

It is mainly used for the rehabilitation of water pipes and circular section gas pipelines, in both urban and extra urban environments, and allows the realization of ex-novo works or the replacement of pre-existing utilities. The technology is conditioned by the existence of bends, multiple or single angle variations superior to 3°, which mean obligatory interruption of the insertion process.


5.1.4. Subline


The technology allows the retionhabilitation of an existing pipeline, of circular section, by means of the insertion of PRAD pipes of reduced diameter by deformation up to n 40%, directly site. The deformation is maintained by plastic clamps. After insertion the pipe is brought back to standard size through the pressurisation with cold water which allows the pipes to adhere perfectly to the existing pipeline (close-fit lining)

The  technology allows the insertion of pipe lengths to 400m for diameters up to 1000mm and its optimum use is for the lining of pipes in concrete-asbestos , in both urban and extra urban environments.

The reclaiming of the existing pipeline can also be performed with the interactive folding “technique”, according to the deterioration of the pipeline to be reclaimed. The technique involves the insertion of pipes of limited thickness while the existing pipeline takes the pressure of the hydraulic load of the fluid flowing on the inside.


5.1.5. Slim liner


This technique allows the reclaiming insertion of an existing pipeline of circular section, by the insertion of PEAD pipes of a limited thickness (2.8-6.2)mm of a reduced diameter even up to 60% by means of a “C” deformation at the moment of production and laid with a controlled reversion process in air or cold water pressure, adhering to the existing pipeline (close. fit lining technique). The technology allows the reclaiming of pipes for lengths up to 300 m of a variable diameter from (75-300mm).

It is mainly used for the reclaiming of water and sewer networks and is suitable for dyking leaks or corrosion while, seeing as  the pipeline is of reduced thickness, (interactive folding). It is the existing pipeline which sustains the pressure of the hydraulic load of the fluid flowing on the inside.

5.2 Reclaiming of existing pipelines using coating, lining and cement


5.2.1 Pipe coating


This technology allows the coating of the existing pipeline through the use of epoxy resin which is sprayed onto the inside of the pipeline itself by means of special robots.


5.2.3 Cured in place pipes


The technology allows the reclaiming of a pipeline or construction through the retroversion of a felt or felt-textile lining impregnated with an appropriate thermo-setting resin and an outer layer in plastic material which is chemically and physically resistant to the fluid to be conveyed. The lining is impregnated on the inside (the part which on insertion comes into contact with the part to be cured) with a particular resin and wound into an extroversion chamber. One of the ends is fixed for cerclage to the mouth of the exit extro flexor  and air pressure produces the reversing and advancing of the lining inside the pipeline to be reclaimed.

For pipelines to be reclaimed superior to 200mm a casing perpendicular to the pipeline is built and the retroversion is performed through the introduction of water inside the resulting crown shaped sack causing through gravity, the reversing and advancing of the lining.

The technology allows the curing of pipelines from 200-2,000mm in diameter and is mainly used for water and sewer pipelines, both in urban and extra urban environment.


5.3 Installation of new pipelines superior in diameter to those already existing

5.3.1 Pipe bursting


This technology allows the substitution of an existing pipe-line made of fragile materials (cast-iron, stoneware, cement, cement-asbestos,PVC) through the insertion from an entry point to an exit point, of a system of  hydraulically operated poles, which in the phase after extraction, are fixed to a cutting instrument which smashes the existing pipeline as well as  a conic head which constipates the fragments, and finally the new pipeline is therefore inserted in the place of the existing one.

The technology allows the laying of lengths from (80 – 100)m and is mainly used for the reclaiming of water, sewer and gas pipelines and allows the replacement of existing utilities. Its utilization is conditioned by the presence nearby of other sub services which could be damaged through compression, as well as by the presence of bends, multiple or single angle variations superior to 3° which mean obligatory interruption of the insertion process.


5.3.2 Pipe splitting


The technology is similar to the previous one, but allows the replacement of an existing pipeline built with ductile materials (spheroid cast iron, steel).


6- Normative aspects.

No dig technologies are in complete harmony with the present legal provisions which place great attention on the environmental theme, orientating towards technical choices which allow to :

–         avoid any possible waste

–         make the recycling of available resources easier

–         rationalize the use of the sub-soil (a limited resource)

–         minimize inconvenience to vehicle traffic and local population

the principal normative reference are :

– nr. 285/1992- Highway Code- which refers to the obligation to drill through road subsoil using horizontal drilling without damaging the road surface.

–         Cabinet Minister’s Directive dated 3 March 1999 – rational replacement of technological plants in the subsoil – which has the aim of giving preference to all those techniques which minimize environmental and vehicle circulation impacts.

Directive 2001/42/CE adopted in the D.lgs nr 52/06 titled “Directives in environmental matters” (more commonly known as “Environmental Code”, at art.53, comma 1, provides that the safeguarding of the soil and subsoil should

–         be ensured, in particular, for soil it means the territory, the soil, subsoil, the inhabitants and infrastructural works


–         Decree dated 10/08/2004 by the Ministry for Infrastructures and Transport with reference to “Technical directives for the crossing and parallelisms of pipelines and canals conveying liquids and gas with railway lines and other transport lines” which introduces the use of horizontal drilling with polyethylene pipes for the crossing of railway lines


–         With particular references to the mini-trench laying technique, while the highway code prescribes a minimum depth of one metre of the extrados, on the

roadway, the above mentioned directive, recently integrated by the L.n.69/09, allows local authorities responsible for road maintenance to derogate such provisions whenever low environmental impact technologies are used.


–         “realization Rules “ (soon to be published) of the new Contract Code (D.lgs nr 163/2006 ) which introduces a new category of special works ( OS  35) called “low environmental impact interventions” which concerns precisely “ the realization and maintenance of any subsoil work involving the use of non-invasive (no-dig) technologies. It includes, for example, guided and non guided horizontal drilling, with the eventual rehabilitation and exploitation of existing structures”


–         local authorities will be able to avail themselves of these techniques providing for them in their call for bids.