The use of hydraulically bonded filling materials for the backfill of utility trenches in sewer construction

Mixtures of construction materials that are applied as a so-called partially liquid, self-compressing (without mechanical compressing works) and self-hardening filling material for the backfill of utility trenches [1, 2, 3, 4] are referred to as hydraulically bonded filling materials. In the European regulations, this filling material is referred to as "soil cement" (DIN EN 1610) [5].

The use of hydraulically bonded filling materials for drains and sewers installed by the open cut method is not explicitly described there. These standards and regulations only deal with conventional trench filling with granular, unbonded materials. For this reason, special constructional and technical aspects have to be considered when hydraulically bonded filling materials are used.

Composition and properties
Hydraulically bonded filling materials generally consist of the following basic materials [7, 8]:

• Initial construction material, e.g. delivered aggregates or (recycling-) materials (about 95%) or native soil (e.g. trench excavation)
• Plasticizer, e.g. mixture of water and swellable clays (bentonite), foam foamers, air-pore forming additives (tensides) or other mineral, vegetable and/or inorganic additives
• Plasticizer, e.g. mixture of water and swellable clays (bentonite), foam foamers, air-pore forming additives (tensides) or other mineral, vegetable and/or inorganic additives
• Stabiliser (binders), e.g. cement or lime (5% together with plasticizer).

The following presentation exclusively deals with the group of filling materials that mainly consists of additives which have been gained artificially at different places or which have broken and been delivered to the manufacturing factory as well as added binders. Native soils that have possibly been recycled or treated with binders on site as well as re-installed soils [9] (then referred to as “hydraulically bonded filling soil”) are not considered here.

The hydraulically bonded filling material assumes a very flowable shape (fluid to mushy substance) after it has been mixed with the above-mentioned components in the (concrete) factory. Thus, it makes it possible to fill cavities that are not or hardly accessible (e.g. spaces at both sides of the bottom half of the pipe [10]) entirely and without mechanical compaction [7]. Further additives known in concrete technology (e.g. solidification accelerators or retarders) can influence the hardening time or other important properties like e.g. the corrosion protection effect, thermal insulation, water permeability, colouring, fluidity etc.

After the inserted filling material has been installed and has reached its necessary compressive strength and load bearing capacity, it hardens permanently only to such a degree that it can be loosened again, if necessary. The hardened filling material is (slightly) pervious to water and corresponds to DIN 18300 [11] after the soil classes 3 and 4 have hardened (‘easily to moderately resolvable types of soil’) so that at any time the pipelines can be opened up again manually using auxiliary tools or mechanically (e.g. hydraulic excavator).

Installation and construction method
After the trench has been excavated and secured, the pipe string is arranged and fixed on the trench base on wooden planks or sand bags (according to [14], the latter should be preferably used for static reasons) in compliance with the planning guidelines (Figure 2a). Before the hydraulically bonded filling material, which is delivered in fluid shape to the construction site in a mixing vehicle of ready-mix concrete (truck mixer), is inserted, so-called “section and load banks” of the same filling material in stiff shape must be installed to guarantee safety against buoyancy and positional stability (Figure 2b and Figure 3a). For ballasting purposes, these are preferably inserted onto the pipe joints with spaces of about 2.00 m to 2.50 m directly out of the concrete mixer via the concrete flume or via tremies or a hose, in exceptional cases via a pump.

The load banks can also serve as a boundary for a filling section within a section (section banks), but have to be dimensioned according to the filling level. The hydraulically bonded filling material is inserted in fluid state via the truck mixer into the individual filling sections (Figure 2b and Figure 3a) and thus it guarantees a complete coating of the pipes (Figure 2c). Before the hardening process starts (up to 3 h after its insertion, depending on the manufacturer), a lining [12] must possibly be installed to secure the trench in the range of the filling. This way the still flowable material fills the lining track entirely and independently and supports the trench walls in a reliable way.

The time needed for the complete hardening process depends on the particular composition of the hydraulically bonded filling material and has to be stated by the manufacturer. The indications commonly used in this regard, e.g. accessible after one day, ready to be overbuilt after three days (production of the main filling and/or the road superstructure) and trafficable (construction vehicles and/or road traffic), should be verified more clearly and examined by investigations on site. Otherwise there is the danger that the load bearing structure of the artificial soil is destroyed by loading it at an early stage and that loads that have not been considered in the static calculation have negative impacts on the pipe string, which may lead to damages in the worst case.

Moreover, the blocking and securing of the trench after the filling of the embedment or the main filling, respectively, is very important because it may be dangerous for persons and animals when it is still at an early stage, i.e. when the filling material is still fluid and not capable to bear loads (Figure 3b) [16].

A comprehensive static investigation served to examine the effects of the filling of the embedment with hydraulically bonded filling materials on the load and embedment of the pipes [14]. The aim was to develop a calculation method on the basis of ATV-DVWK-A 127 [13] which permits the use of commercially available calculation programmes for pipes that have been installed via trenches by modifying certain parameters, if required. The method was analysed and examined by means of special FEM-calculations and calibrated with the aid of commercially available filling materials in order to verify the correctness of the parameter assumptions for the above-mentioned modification of the calculation and to quantify the effects on the load bearing capacity of the pipe.

Both rigid and flexible pipes have been included into this analysis while the most important installation conditions were varied and compared with a standard trench filling based on a reference system. It was confirmed that the positional stability and serviceability of the pipes that have been calculated by this programme is complied with within a specified area. Depending on the pipe-specific limiting conditions, pipes that have been installed in hydraulically bonded filling materials even have a better load than pipes that have been installed in conventional, granular and unbonded materials (Figure 4).

The used parameter variations as well as the comparisons of the results of the numeric and analytic calculations served to prove that the static calculation can basically be carried out on the safe side according to ATV-DVWK-A 127 [13] for the use of hydraulically bonded filling materials to backfill the trenches. Thus higher securities and cost saving potentials can be achieved. For this purpose, the stiffness and strength values of the cover material above the pipe crown and of the embedment at the side of the pipe (E2) must first be tested and verified on the basis of mechanical suitability tests by the respective manufacturer. Afterwards, the input material data as well as the application range can be determined via reasonable parameter variations and sensitivity analyses.

Conclusion
To sum up, it can be said that the hydraulically bonded filling materials are a promising alternative to the conventional backfill of utility trenches with granular, unbonded filling materials. Their application may involve, for instance, the following advantages:

• Prevention of typical installation failures / damage causes which may occur during the conventional installation (e.g. insufficient filling and sealing of the spaces at both sides of the bottom half of the pipe, point bedding of the pipeline, sealing loads that are too high, application of inappropriate materials etc.)
• Reduction of the trench width according to DIN 4124 [15] or DIN EN 1610 [5] provided that the pipes can be installed by working just in front of them so that the personnel must not necessarily enter the space between the pipeline and the trench wall (admission required by TBG – Technologiebeteiligungsgesellschaft)
• Improvement of bedding conditions for the pipeline
• Reduction of the construction time by changing the construction procedures and clock cycles
• Prevention of surface settlements (especially in the range of the usual lining track)
• Prevention of vibrations, abandonment of vibration energy for mechanical sealing, reduction of noise emissions and vibration emissions
• Prolongation of the service life of the pipeline (reduction of ex-/infiltrations by a certain redundancy (the hardened precipitates generally have a low water permeability, depending on the manufacturer between 10-6 to 10-8 m/s), prevention of root ingress, defined bedding conditions by avoiding typical installation failures/damage causes (see above))

Hydraulically bonded filling materials have been used in the installation of cables and pipelines for only a little while (since the middle of the 1990s). For this reason, the following aspects, among other things, have not yet been clarified for good [7, 16]:

• durability or long-term behaviour
• requirements of the filling material
• impacts on the (pipe-)line embedded into the hydraulically hardened construction material at changing frost and dew conditions
• constant removableness or re-opening of the trench with simple device (so-called “spade removability”).

Moreover, it has to be considered that their application in utility trenches with strong slopes (>10%) requires – if at all possible – special devices [16]. The Road and Transportation Research Association (FGSV) has not yet issued a general admission of hydraulically bonded filling materials.

If they are applied, however, a competent engineering company should be consulted in order to determine the requirements of each application case (general, mechanical (static), chemical, thermal etc.) as well as to carry out corresponding tests to verify the seviceability, if these have not yet been provided by the manufacturer.

Literature

[1] Berger, W., Krausewald, J., van Heyden, L.: Boden-Mörtel: Anwendungsfragen und Wirtschaftlichkeit für den Tiefbau der Gasverteilung. gwf Gas & Erdgas 140 (1999), vol. 8, pp. 513-518.

[2] Just, A.: Einsatz von Flüssigboden in Braunschweig. bi-umweltbau (2003), vol. 1, pp. 42-44.

[3] ONR/FW 110A: Fernwärmeversorgung: Stabilisierte Rohrgrabenverfüllmaterialien. Technische Spezifikation für stabilisierte Rohrgraben-Verfüllmaterialien – SVM für den Einbau von Fernwärme-Kunststoffmantelrohren – KMR. Date of issue: 01 April 1999.

[4] Kiesselbach, G.: Projektstudie über die Verfüllung von Künetten. Im Auftrag des Magistrats der Stadt Wien, Magistratsabteilung 22 – Umweltschutz in Zusammenarbeit mit ÖkoKaufWien – working group Civil Engineering. Vienna, 1999.

[5] DIN EN 1610: Construction and testing of drains and sewers (10.1997) DIN EN 1610 supplement 1: Construction and testing of drains and sewers – List of relevant standards and guidelines (status as of 02.1997).

[6] Worksheet ATV-DVWK-A 139: Installation and testing of drains and sewers (01.2002) (ed.: German Association for Water, Wastewater and Waste e.V. – ATV-DVWK, Hennef).

[7] Worksheet FW 401 part 12: Verlegung und Statik von Kunststoffmantelrohren (KMR) für Fernwärmenetze. Bau und Montage; Organisation der Bauabwicklung, Tiefbau (02.1999).

[8] Stolzenburg, O.: RSS®-Flüssigboden im Kanalbau: Ein Praxisbericht. Dokumentation 18. Oldenburger Rohrleitungsforum, 05-06 February 2004.

[9] Kronenberger, E. J.: Bodenrecycling im Rohrleitungs- und Kanalbau: Wiedereinbau in trockener und flüssiger Form möglich. bi umweltbau (2002), vol. 2, pp. 44-46.

[10] Stein, D.: Instandhaltung von Kanalisationen. 3rd revised and extended edition, Ernst & Sohn, Berlin 1998.

[11] DIN 18300: VOB German construction contract procedures – part C: General technical specifications for building works (ATV); Earthworks (12.2000).

[12] Stein, D., Möllers, K.: Grabenverbau: Einflussfaktor auf das Ingenieurbauwerk Rohrleitung. Tiefbau (1988), vol. 3.

[13] Worksheet ATV-DVWK-A 127: Standards for the structural calculation of drains and sewers (08.2000).

[14] Static calculation concept by Prof. Dr.-Ing. Stein & Partner GmbH, Bochum (www.stein.de).

[15] DIN 4124: Excavations and trenches – Slopes, planking and strutting, breadths of working spaces (10.2002).

[16] CETE Normandie Centre (CETE = Centre d'Etudes techniques de l'Equipement.) (ed.): Verfüllung von Gräben – Verwendung von selbstverdichtenden Materialien. 3rd edition 03.1999. Standard of knowledge: 31 December 1997 (German translation).