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Soil friction along HDD’s – the influence of a refined model on expansion by Frigco Kwaaitaal In the article the aspects influencing the built up of soil friction along HDD’s are introduced. Based on these aspects a proposal is done how to deal with soil friction modeling along HDD sections in a day to day engineering practice. After definition of this approach a quantitative assessment on the proposed model itself and its impact on longitudinal elongation (expansion) along the HDD section are presented. This is done for three typical HDD sizes: DN100, DN600 and DN1200. The assessment shows that applying soil friction significantly influences the expansion built up along the HDD and in the end leads to a reduction of stress in the upper bends. By applying soil friction, the need of taking mitigating measures to divert expansion (e.g. expansion cushions, expansion loops) is lower for some cases.

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1. INTRODUCTION

2. CURRENT SOIL FRICTION MODEL

The reason for setting up the investigations on modeling of soil friction was that in the new revision of the Dutch pipeline code a friction value of zero was introduced as a worst case approach. A literature survey showed that research generally was done on friction during installation (of relevance for the pull back of the pipe) but not for the operational phase of the pipeline. For pipelines under elevated temperature axial displacements (as a result of expansion) in longer HDD configuration increase quadratically and thus the loads on the upper bends of the HDD section also increase. In specific cases this may lead to stresses above the acceptable limits. To mitigate these stresses either the pipeline routing can be changed by incorporating expansion loops or other expansion measures can be taken such as placement of expansion cushions behind the bends. Taking into account any friction (versus no friction) along the HDD section seems to be crucial for a sound and structural reliable design of the pipeline section. Next to this, minimizing or even preventing mitigating measures will lead to a reduction of material and construction costs.

To model the soil friction a linear elastic behavior of the soil springs along the HDD is assumed. The maximum friction (Wmax) is reached at small relative displacements. Main aspects influencing the soil friction Wmax are in general: . ■ intergranular pressure around pipe; ■ adhesion between pipe and soil; ■ angle of friction between pipe and soil (dependent of soil friction angle and pipe wall roughness).

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To determine Wmax the following basic relationship is available for pipelines in open trench:




































(1)

With: W soil friction along the pipe N/m m πDo outside pipe circumference K ratio horizontal/vertical intergranular pressure, in case of neutral horizontal soil pressure K equals Ko=1 – sinφ' φ' angle of internal friction of soil °

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Figure 1. Soil friction acting on pipes in open trench compared to pipes in an HDD.

σk vertical intergranular pressure at pipe axis level N/m2 tanδ friction coefficient between soil and pipe wall (depending on internal friction of soil and pipe wall roughness) a adhesion (only for clay or peat, equals the undrained cohesion parameter, cu) N/m2 Qeg deadweight of the pipe N/m Qvul weight of medium in the pipe N/m N/m Qop buoyancy capacity of pipe

  




 


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ite shell is present around the pipe. The bentonite shell around the pipe will stiffen and will act as a highly compressible layer. The combination of this compressible layer and the thickness of the soil column above the pipe leads to arching of the soil. In compressible soil types such as clay the arching effect fades out after installation. For non-compressible soil types (sand) the arching effect stays present and leads to a reduced vertical intergranular pressure on the pipe (see Figure 2).

The build up of friction in a HDD differs significantly from a pipe in open trench. In HDD crossings the borehole is filled with drilling fluid (bentonite slurry) which stiffens after installation. After a period of time the drilling fluid around the pipe acts as a shell. The friction distribution on this shell acts on two interfaces (shear planes): the pipe-bentonite interface and the bentonite-soil interface. Another difference is that due to the installation at large depth soil arching occurs (depending on the soil type) which can reduce the vertical intergranular pressure in the soil surrounding the pipe. The modelling of Wmax differs for a pipeline in open trench from a pipeline in a HDD section. In Figure 1 an overview is given how Wmax is built up in both cases.

2.1 Arching in soils and determination of intergranular pressure in HDD’s Under normal circumstances the vertical intergranular soil pressure is determined by the weight of the soil column above the pipe. Just after drilling the pilot hole arching occurs and after installation of the pipe a benton-

Figure 2. Arching around borehole.

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Based on the theory of Terzaghi it is considered that arching occurs when the thickness of the soil mass extending above the pipe is larger than 4 times the width of the soil column in shear (2B1). The width B1 is defined as:

qr











kv y λ














°





(2)

With: B1 Do φ R

is half the width of the soil column in shear is the outside diameter of pipeline is the internal angle of friction is the radius of the borehole

m m ° m

2.2 Friction due to curvature of pipeline During pull back of the pipe along the HDD borehole the elastic bend is cold formed by applying a torque. Since the pipe has an axial stiffness which cannot be neglected mechanically equilibrium reaction forces will occur in the soil, see Figure 3. The distribution of the soil reaction forces is based on the theory for beams on elastic foundations by Hétenyi. The soil reaction forces will occur at the ends of each bend and induce additional frictional effect acting on the pipe. The additional normal force in the elastic bend as a result of this torque can be calculated by:





















(3)


With:

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is the additional normal force for one bend in the borehole N

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EI Do R f3
















is the vertical modulus of sub grade reaction is the maximum displacement is the pipe-soil stiffness characteristic



A reduction of the vertical soil load due to arching results in a lower vertical intergranular pressure (σk) on the pipe and thus reduces the overall friction. For the friction model in HDD sections is assumed that when arching occurs the contribution of the vertical intergranular pressure is totally neglected. At soil covers H ≤ 8B1 (when arching is assumed) the actual vertical intergranular pressure at top of pipe level is used. For larger soil columns arching will occur and the vertical intergranular pressure then is assumed to be zero. For pipes in multiple layer soils the approach for determination of the vertical intergranular pressure is similar. It should be noted that according to the theory arching will only occur in a sand layer at a depth of H≥8B1 below the layer separation. In the new approach arching is assumed to be effective as of the layer separation. Immediately taking into account arching over this layer and thus neglecting the build up of vertical intergranular pressure over the layer depth for H