Influence of bedding and backfill soil type on deformation of buried sewage pipeline

Nowadays, especially in heavily urbanized areas, it is necessary to conduct construction works in the direct surroundings of the existing underground installations. This can result in exceeding the limit states of underground pipeline construction, for example, excessive deformation of the pipeline wall.

Failure analyses of sewage pipes available in the literature, for example, [1] and [2] usually concern the cases of symmetrical loads exerted on the pipe, mainly the loads resulting from the dead weight of backfill and the traffic live loads. In such cases, the lateral pressure exerted on the pipe reduces its deformation and, thus, the internal forces in the pipe. Furthermore, one can find in researches that verify the influence of dynamic loads on ducts, for example, [3]. However, designers and researchers rarely consider adverse consequences of soil lateral pressure. Common methods of designing underground sewage ducts, that is, the Scandinavian method [4] and Standard ATV-DVWK-A127 [5], consider only vertical loads acting on the pipe. None of the above mentioned documents deal with asymmetrical pipe loadings. Such a situation can occur when a new structure is built nearby and asymmetrically against the existing pipelines.

The case study presented in this paper considers the asymmetrical loading of pipes resulting from the construction of embankments in the vicinity of existing pipelines. This kind of loading was proved to be the direct cause of a failure in the form of excess pipe deformation. However, it is also presented that the excessive deformation could be avoided by the use of proper bedding and backfilling soil.

In 2016 Wroclaw University of Science and Technology prepared the expert opinion on the causes of excessive deformation of underground sewage PE pipe placed a few metres under the ground surface [6]. In the real situation, the embankment was erected near the pipe after its installation in the subsoil. The construction of embankment was proved to be the direct reason of horizontal deformation. However, the aim of the analyses was to check whether the other factors had a significant influence on the existing state of deformation. Eventually, it was concluded that proper bedding and backfill soil is of crucial importance. According to the existing requirements [4] for bedding and backfill, in the case of soft soils, it is necessary to provide a layer of gravel with thickness of at least 0.2 m and a layer made of cohesionless soil with thickness of 0.15 m. The backfill has to reach 0.3 m above the pipe. The original soil can be used to fill the remaining part of excavation if the pipe is settled under green areas.

Nevertheless, under the roads, the same material must be used for backfilling the entire excavation. Furthermore, each layer of the backfill needs to be properly compacted. In this paper, the influence of different types of bedding soil on the pipe deformation was investigated. The numerical calculations were carried out for three variants:

During the preparation of expert opinion, the validation of geotechnical parameters was performed based on the comparison of results from inspection carried out in real object and results from numerical simulations. The validation was carried out for the 2^nd variant. In addition to that, the calculations accounting increased load were executed in order to observe the potential failure mechanism, which – in the framework of numerical analyses – is determined by the factor of safety (FOS) reaching the value of 1.0. Eventually, it allowed to evaluate pipe deformation in failure. FOS values were evaluated at every stage of construction. The shear strength reduction method [10, 11, 12] was used for this purpose. FOS was also computed for the additional 4^th variant, that is, without the pipe.

In the area of the analysed object, three main geological layers were determined:

At the depth of 5.5 m b.g.l., a sewage PE DN600 pipe was installed. Before setting the pipe, an open area excavation was performed and the narrow one between two, six metre long retaining walls spaced at 3.5 m. The retaining walls were connected by struts. The excavation was filled in four stages and each layer of backfill was properly compacted. Next, the 4.6 m high embankment reinforced with geosynthetics was formed in additional six stages. The angle of embankment slope was 43°. On the embankment top surface, the parking area was designed.

Calculations were performed using the finite element commercial software ZSoil [7], especially intended for performing numerical analyses in geotechnics [8, 9]. Plain strain and elastic-perfectly plastic constitutive model of soil were assumed in the considered case. Following stages of construction and loading were distinguished:

Elastic-perfectly plastic Coulomb-Mohr model was used for soils. Non-associated plastic flow rule was defined by the angle of dilatancy with values set down according to ZSoil manual [7]:

where: ψ – angle of dilatancy, φ – internal friction angle. The interfaces between soil and pipe as well as between soil and retaining wall were modelled with the contact elements, maximum shear stress value is limited by Coulomb condition:

where: adhesion a=0 , friction angle δ=0.6φ, and φ is internal friction angle of adjacent soil.

Parameters of soils, used in numerical calculations, are shown in Table 1.

The sewage pipe was modelled with beam elements. Long-term Young modulus E_L=200 GPa, Poisson’s ratio υ=0.2 and the moment of inertia I=1.75∙10^-6 m⁴/m were assumed. The circle, describing sewage pipe geometry, was discretized in 30 segments. The beam elements were also used for modelling the retaining walls. Nodes of truss elements representing struts were separated from soil continuum nodes. Reinforcement of the embankment made of geosynthetics was modelled with membrane elements, which can withstand tensile forces only. Compacting of backfill layers of the trench and forming the embankment were modelled by short-term load of intensity 5.0 and 30.0 kPa, respectively. The load on the crown of the embankment amounted 20.0 kPa.

In Figures 1, 2, 3, the numerical model at the selected stages of construction and in Figures 1, 2, 3, 4, 5, 6, the different types of bedding and backfill were displayed.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Summary of the results, that is, the values of displacements and changes of pipe diameter, are shown in Table 2. Additionally, in Figure 7, the changes of pipe diameter are represented for different types of bedding and backfill. In Figure 8, the development of pipe deformations at subsequent stages is shown. Comparison of FOS values is presented.

Figure 7

Figure 8

Analysis of results presented above leads to the following observations:

In this paper, the results of numerical calculations of deformation PE pipe were presented. Three different types of surrounding soils were considered: gravel bedding with full gravel backfill of excavation (1^st variant), gravel bedding with partial gravel backfill (2^nd variant) and bedding and backfill made of original soft soils (3^rd variant). Appropriate soil around the pipe provides proper interaction between the pipe and the soil, which emerge in lower deformation. More specific conclusions are following: