Soil Loads on Pipelines

Soil loads on pipelines: the Dutch approach

Kruse, H.M.G. and H.J.A.M. Hergarden

Deltares/GeoDelft, National institute unit geo-engineering

(E-mail: [email protected] and [email protected]) Abstract The pipe stress analysis is very often the most important part of the engineering of pipelines. The different installation techniques cause different soil-pipe interaction after pipeline installation, so that in turn the soil load for the pipe stress analysis is different. In the Netherlands an approach for the determination of soil loads for the different installation methods was developed throughout the years. In case of installation of the pipe in a trench, the occurrence of settlements is an important factor for the soil load on the pipe. Besides macro settlements due to for example heightening at the surface, settlements in the trench cause an initial soil load immediately after installation, which is often normative for the engineering of the pipe. The formation of a borehole in which the pipe is installed by the horizontal directional drilling method causes a serious reduction in soil load on the pipe. Especially in granular soils the reduced soil load allows installation of plastic pipes at large depths. In compressible soils, consolidation causes a less strong reduction of the soil load. The borehole around the pipe induced by microtunnelling is caused by a slight overcut and leads to incomplete vertical deformation of the soil volume above the pipe, which leads to a less developed arching.The less developed arching yields a higher vertical soil load on the pipe, than in case of horizontal directional drilling 1. Introduction Successful operation of a pipeline system on long term is strongly related to the quality of the engineering works carried out before the installation of the pipeline. The pipe stress analysis is very often the most important part of the engineering of pipelines. The pipe stress analysis considers the combination of all loads acting on the pipeline and compares the resulting stresses in the pipeline with the allowable strength of the pipeline. The load on the pipeline caused by the soil-pipe interaction is called soil load or soil reaction [1]. The installation of pipelines is carried out in trenches from times immemorial. After excavation of the trench the pipeline is installed on the bottom of the trench and is subsequently covered by the excavated soil. Since the seventies, last century, other techniques for pipeline installation are introduced. These so called trenchless techniques such as horizontal directional drilling, micro tunneling and other pipe jacking methods are applied on a large scale since the eighties. On one hand they provide a logical alternative when pipelines need to cross roads, railways, dikes, wetlands, rivers and other structures that have to remain intact. On the other hand these techniques minimize the impact of installation activities in densely populated and economical sensitive areas. With the introduction of the trenchless installation techniques the soil reaction forces, which have to be considered in the pipe stress analysis became more complex. The different trenchless techniques cause different soil-pipe interaction so that in turn the soil reaction for the pipe stress analysis is different [1].

5th Pipeline Technology Conference 2010

2. Soil-pipe interaction The magnitude of the soil reaction force on a pipeline surrounded by soil or drilling fluid (or other drilling or tunnelling related substances such as dmmer or grout) is determined by the soil-pipe interaction. In case both the soil and the pipeline are at rest, the soil-pipeline interaction is in neutral condition. The neutral soil load can be calculated for this condition. Due to the soil pipe interaction, induced by either the pipe or the soil, soil deformations and pipe displacement will occur. The deformations lead to increase or decrease of the soil load on the pipe. This soil reaction behaviour is often modelled by using a spring model. By locating springs around the pipe, the displacement and related stress changes can be calculated (figure 1). The displacement in longitudinal direction along the axis of the pipe is modelled by a spring too.

Figure 1 Pipe soil interaction modelled by springs The increase or decrease of the soil reaction stress is usually calculated by linear or bilinear springs. The stiffness of the spring is expressed as a modulus of sub grade reaction. Since the soil which is surrounding the pipe becomes plastic at a certain stress level, this is the maximum stress which can occur. In figure 2 is shown that the depending on the direction of displacement the passive horizontal effective stress and the active horizontal effective stress form the upper bounds of the springs. The calculated soil stress on the pipe using the springs model are in general dependent on:

Sequence of soil layers above the pipeline Soil layer in which the pipeline is installed The water pressure distribution Sequence of soil layers below the pipeline Settlement of the soil layers below the pipeline Horizontal deformation of the soil layers next to the pipe Relative pipeline movement ( in axial or tangential direction, due to for example

temperature variations in the pipeline) Since the soil-pipe interaction is influenced by the installation method, different calculation methods to determine the moduli of subgrade reaction and the maximum and minimum values of the soil loads exist.


Figure 2 The spring for modelling the soil pipe interaction. 3. Installation in a trench In case of pipeline installation in a trench the interaction between the pipe and the condition of the soil material, which is placed back in the trench plays an important role in the development of the soil load. Besides the condition of the soil material with which the trench is backfilled, the following parameters determine the soil load for a pipeline in a trench:

Dimensions of the trench Soil type in which the trench is excavated Soil type with which the trench is backfilled Unit weight of the soil material with which the trench is backfilled The stiffness of the pipeline

In case there is no relative displacement, both the soil and the pipeline are at rest, the soil-pipeline interaction is in neutral condition.The neutral vertical soil load is defined as (figure 3):

, v,H 00.5 8v nq D

where: v,H Vertical effective stress at depth Hl [kN/m2]

Hl Soil cover above the top of the pipe [m] D0 Outer diameter of the product pipe [m]

Effective unit weight at the top of the pipe [kN/m3]

Figure 3 Schematic diagram for calculation of the neutral vertical soil load

horizontal effective stress

passive effective stress

active effective stress

Pipe towards the soilmass

Pipe from the soilmass

horizontal displacement


In case of construction activities on the surface above the pipeline the soil pipeline interaction changes. In case of very large settlements the soil layers below the pipe will deform more than the relative stiff pipeline. The inability to follow the soil deformation leads to a so called passive soil load on the pipe (figure 4)

Figure 4 Passive stress on top of the pipe, source: [3]. The passive vertical soil load is defined by Marstons formula [2]:

v,H, ,0

v p v n lq q f HD

where: f Factor depending on the soil type [-]

v,H Vertical effective stress at depth Hl [kN/m2] Hl Soil cover above the top of the pipe [m] D0 Outer diameter of the product pipe [m] qv,p Maximum passive vertical soil load [kN/m2] The value f depends upon the soil type, the degree of densification of the soil and the width of the trench bottom. Under normal circumstances with normal degree of densification the factor f is about 0.3. This value decreases when the width of the trench bottom reduces. In case of small settlements, the increase in soil stress can be calculated using the modulus of sub grade reaction. This modulus Ktop is determined by a semi empirical formula which is based on field experiments. Figure 5 shows the results of a large series of upward pulling tests in order to determine the modulus of sub grade reaction of the soil layers above the pipe. The regression line through the measured upward pulling force data was transformed to a linear line in order to define the vertical modulus of sub grade reaction above the pipe:

max

,z

qqk nptopv

For clay and peat, the displacement zmax is determined as follows:

0

5,1

0max

25,0

DHE

Dz

where: E Youngs modulus of the soil above the pipeline [MN/m2] H Soil cover above the top of the pipe [m] D0 Outer diameter of the product pipe [m] zmax Displacement [m]


Figure 4 Results of upward pulling tests with regression line. For sand, the displacement zmax is determined as follows:

0

5,0

0max

20,0

DHE

Dz

The neutral and the passive load (in case of large displacements) and the modulus of sub grade reaction can be used for the long term conditions. In the period directly after the installation of the pipeline in the trench, the compaction of the fill plays an important role in the soil pipe interaction. The compaction of the fill leads to differential settlement of the fill above the pipe and adjacent to the pipe. The differential settlement leads to shear plane with shear forces which are in turn transferred to the pipe.

Figure 5 Compaction leads to initial soil stress, source [3]. The initial soil load in the period after the construction can be calculated using the subsequent formula:

Relative loose fill


0

( )( )

tot p nini n

p ntot

K q qq q q q

KD

Where: qini initial soil load [kN/m2]

displacement coefficient [-] Ktot combined modulus of sub grade reaction of the soil above the pipe, the pipe and the

soil below the bottom of the pipe [kN/m3]:

, ,

1 1 1 1

tot v top pipe v botK K K K

With:

3w

pipey

E IKk r

Where: E Elasticity modulus of the pipeline material [kN/m2] Iw Moment of inertia of the pipe wall [m4] ky Deflection coefficient [-] r radius of the pipe [m] The higher soil load immediately after the installation is very often the normative situation for the design of the pipeline. The spring below the pipe and next to the pipe, which act in horizontal direction are not described in this paper, but can be found in the Dutch guideline for pipeline installation NEN3650 [3]. The axial springs are described in this guideline as well. 3. Installation using horizontal directional drilling In case of pipeline installation using the horizontal directional drilling method, a relative large (compared to the pipeline diameter) borehole is created. The presence of this borehole strongly influences the pipe soil interaction in tangential and axial direction. The tangential spring used to model in the pipe soil interaction is modified (figure 6).

centre of bore hole

wall of bore hole

pipe line wall

b= gap between pipe and wall

un

spring force F(un)

un

-b

b

k(un)

plastic

centre of bore hole

wall of bore hole

pipe line wall


uncentre of bore hole

wall of bore hole

pipe line wall


centre of bore hole

wall of bore hole

pipe line wall


un

spring force F(un)

un

-b

b

k(un)

plasticspring force F(un)

un

-b

b

k(un)

plasticspring force F(un)

un

-b

b

k(un)

plastic

Figure 6 Pipe soil interaction in a bore hole filled with drilling fluid. Due to arching around the borehole the vertical soil load on the pipe is minimal. The soil load is only in minor extent dependent on the amount of overburden soil. The arching mechanism


of boreholes is described by Meijers and de Kock [4]. Centrifuge tests with soil load measurements on pipes installed by horizontal directional drilling were carried out by Viehofer et. al. [5]. Results of the centrifuge tests showed that the soil load on the pipe in the borehole is minimal due to the mechanism of arching. The results in the centrifuge test (soil stress measurements next to the borehole) are confirmed by finite element calculations, which in turn show that the theory is described by Meijers and de Kock [4] is suitable for the determination of soil loads op pipes installed by horizontal directional drilling. Based on the arching theory the vertical soil load for a pipe in granular incompressible material is calculated as follows:

1

tan11

,

'1

tan

K hB

n r

cBB

q eK

where:

1 01 tan2 4 2

B D

qn,r Reduced neutral vertical soil load [kN/m2] K Neutral earth pressure coefficient [-]

Average angle of internal friction [0] c Average cohesion [kN/m2] Average effective unit weight [kN/m3]

h Height between the top of the borehole and the surface [m] Due to the arching effect, it is possible to install plastic pipelines at a large depth of more than 30 m in granular incompressible soils. In case of compressible soils arching is less strong developed due to the consolidation effect. With increasing time the consolidation process progresses and leads to higher soil loads on the pipe. In compressible soil layers, the reduced neutral vertical soil load is defined as:

,12

rn r

Fq hB

where:

vd

r

BF

CH

hhHBF

F

1

max

1

max

22

ln231

9.0

With:

0

,1max 2 D

QhBF rn

With:

1

tan11

, 01tan

K hB

n r

cBB

Q e DK

c Average cohesion between the surface and the layers above the pipe centre [kN/m2]


C Compression index of the hardened/consolidated drilling fluid [-] Kv Bedding constant of drilling fluid [kN/m3]

d Relative displacement of the soil column [m] H Thickness of the compressible layer [m]

Figure 7 calculated reduced soil load in a soil profile with a compressible clay layer on top of a sand layer. It should be noticed that the arching is taken into account from a safe depth of 8 B1 in case of a compressible layer and a depth of 4 B1 in case of an incompressible layer. In case of large differential displacements in between the pipe and the surrounding soil, a passive soil load may develop on the pipe. For shallow depths the trench approach can be used, but for H< 5 D0 (this limit is determined by a series of finite element calculations), the passive vertical soil load should be determined using the cylindrical elasto-plastic expansion theory:

sin2 1 sin

0,max

0

0.5cot cot0.5p f

Dq p c Q cD h

where:

0 1 sin cosfp c

0 sin coscQG

0 Effective isotrope stress [kN/m2]:

20 HV V Effective vertical stress at the pipe centre [kN/m2] H Effective horizontal stress at the pipe centre [kN/m2]:

VH K G Shear modulus at the pipe centre [kN/m2]


In case of relative small differential movement in between the pipe and the surrounding soil, the increase in soil stress can be calculated using the modulus of sub grade reaction. This modulus Kv is based on Schleichers theory [6]:

21vEk

m A

where: E Average Youngs modulus of the soil [kN/m2] Average Poisson ratio [-]

A Section [m2]:

0A D l l Minimum characteristic length [m]:

l Characteristic stiffness pipeline-soil [m-1]:

044v pipe b

DkE I

Epipe Young modulus of the pipe [kN/m2] Ib Moment of inertia of the pipeline [m4]: m Shape coefficient, depending on l/b [-]:

blblm

596.390.5131.52

The results of calculated moduli of sub grade reaction using Scheichers theory are compared with the results of finite element calculations [7]. The comparison shows that values calculated with Schleichers formula are a reasonably well estimation for the stiffness of the spring around the pipe in the borehole. The spring below the pipe and next to the pipe, which act in horizontal direction are not described in this paper, but can be found in the Dutch guideline for pipeline installation NEN3650 [3]. In axial direction the friction is along the pipeline is reduced by the drilling fluid in the borehole. Immediately after the installation the friction in between the pipeline and the drilling fluid is largely determined by the gel strength of the drilling fluid. For a bentonite based drilling fluid this gel strength is approximately 50 Pa. After some time the drilling fluid becomes stiffer and the friction is increasing. In the Netherlands, at some locations long term frictional stresses of about 0,50 kPa are measured. 4. Installation using micro tunnelling The pipe jacking techniques cause a more or less direct soil-pipe contact. The installation of a pipeline using the micro tunneling method yields a soil-pipe interaction, which characteristics are in between the direct soil-pipe contact of pipe jacking techniques and the relative large borehole of the horizontal directional drilling technique. The existence of a borehole due to the so-called overcut of about 1 or 2 cm (on the radius) in combination with the tail injection of lubrication fluid will lead to less ability of vertical deformation, which in turn will lead to a less developed arching. The less developed arching yields a higher vertical soil load on the pipe. The less developed arching can be calculated using the method described in the ATV [8]. This method is based on the incomplete development of shear stresses (arching stresses)


due to a limited vertical deformation of the soil mass above the pipe (figure 8). The limited deformation is in the ATV estimated at about 10 % of the total deformation. The limited deformation yields an approximately 50 % mobilized angle of internal friction. This value can be used in the reduced soil load formulas proposed by Meijer and de Kock [4].

Figure 8 The choice of the angle of internal friction in less developed arching, source [8]. An overview of the springs required for the engineering of a pipeline installed by micro tunnelling can be found in the Dutch guideline for pipeline installation NEN3650 [3]. In axial direction the injected fluid or material around the pipe is of major importance for the friction characteristics. 5. Conclusion The different installation techniques cause different soil-pipe interaction after pipeline installation, so that in turn the soil load for the pipe stress analysis is different. In the Netherlands an approach for the determination of soil loads for the different installation methods was developed throughout the years. In case of installation of the pipe in a trench, the occurrence of settlements is an important factor for the soil load on the pipe. Besides macro settlements, due to for example heightening at the surface, settlements in the trench cause an initial soil load immediately after installation, which is often normative for the engineering of the pipe. The formation of a borehole in which the pipe is installed by the horizontal directional drilling method causes a serious reduction in soil load on the pipe. Especially in granular soils the reduced soil load allows installation of plastic pipes at large depths. In compressible soils, consolidation causes a less strong reduction of the soil load. The borehole around the pipe induced by microtunnelling is caused by a slight overcut and leads to incomplete vertical deformation of the soil volume above the pipe, which leads to a less developed arching.The less developed arching yields a higher vertical soil load on the pipe, than in case of horizontal directional drilling 6. Literature [1] Hergarden, H.J.A.M. (2008) Geotechnical design factors HDD crossings. proc DCA conference, Prien,

Germany [2] Marston (1930), The theory of external loads on closed conduits in the light of the latest experiments,bul 96 [3] NEN (2003), Requirements for pipeline installation Dutch Standard, ICS 23.040.10 NEN Delft, 2003 [4] Meijers, P. and De Kock, R.A.J. (1993), A calculation method for earth pressures on directionally drilled

pipelines, Pipeline conference 1993, Belgium. [5] Viehofer, T., T. Linthof. and A. Bezuijen. (2005), Stability of a borehole during horizontal directional drilling,

Proc. No dig conference Rotterdam [6] Schleicher, F.. (1926). Zur theorie der Baugrundes, Der Bauingenieur, Heft 48/49, 1926 [7] Teunisse, J.A.M., J.P. Pruiksma and H.M.G. Kruse.(2008) Modulus of subgrade reaction for pipelines in a

borehole installed by horizontal directional drilling, Int. No-Dig conf. Moscow [8] Abwassertechnischen vereinigung ATV (1990), Statische berechnung von vortriebrohren, arbeitblad 161


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Soil Loads on Pipelines