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Experimental Investigation of Soil-Pipe Friction Coefficients for Thermoplastic Pipes Installed in Selected Geological Materials

Md. Kamrul Hassan1, Shaurav Alam2, Chris Bartlett3, and Erez N. Allouche4 1 Research Assistant, Trenchless Technology Center, Louisiana Tech University, Ruston, LA, 71270. 2 Research Assistant Professor, TTC, Louisiana Tech University, Ruston, LA, 71270. 3 Research Assistant, Trenchless Technology Center, Louisiana Tech University, Ruston, LA, 71270. 4 Associate Professor and Director, TTC, Louisiana Tech University, Ruston, LA, 71270. 1. ABSTRACT Significant friction forces can develop between the inner surface of the borehole and the pipe wall throughout the pull back (Baumert et al., 2002) stage of HDD installations. A custom testing apparatus was designed and fabricated capable of mimicking the deposition of the filter cake during an installation, while measuring the friction force between the pipe and the surface of the “borehole”. Tests were performed under controlled laboratory conditions for a range of geological materials, with and without the presence of a filter cake. Parameters in the experimental design include the type of geological material (sand, silty sand, clay, silty clay, and pea gravel), magnitude of the normal over burden force (56 lb, 70 lb, and 80 lb) acting on the pipe’s crown, drilling fluid composition (bentonite based fluids for sand and silty sand, polymer based fluids for clay and silty clay, and micro-fiber additives plus bentonite for pea gravel), pulling rate, and the contact surface area between the pipe and the filter cake. Tests performed with and without the presence of a filter cake quantified the benefit associated with the introduction of properly designed drilling fluid formulations in terms of reduced friction coefficients, and subsequently the required pull force. The outcomes of this study are expected to improve the understanding of the relative contribution of various parameters to friction forces developed at the pipe-borehole interface, as well as assist consulting engineers and other industry professionals in estimating the worst and best case scenarios, in terms of pull-load values, for specific installations based on anticipated geological conditions. 2. INTRODUCTION Forces that need to be considered when estimating the stresses developed in the pipe during a HDD pull-back operation (Baumert et al., 2002) include friction between the pipe and the borehole wall, friction between the pipe and the slurry, reaction forces (tension and bending) due to the curved alignment the pipe and drill string must engage, and the normal force, which is the sum of the pipe self-weight, pipe contents, and the uplift buoyancy force. The buoyancy force, which is equal to the volume of slurry displaced by the pipe, acts to push the crown of the pipe, or drill string, upward against the borehole wall, which compresses the filter cake. Bending stresses caused by the pipe negotiating vertical/horizontal curves in the bore path alignment, as well as inadvertent irregular changes in the bore path (‘dog-legs’), also contribute to the resistance to motion during the pull-back operation. Many of these friction forces can be minimized by using proper density drilling fluid, which forms filter cake around the perimeter of the borehole’s inner surface. The static friction between the pipe and the soil/filter cake interface results in an unbalanced pressure around the pipe, resulting in a net normal force that pushes the pipe against the borehole wall (see Figure 1).

North American Society for Trenchless Technology (NASTT) NASTT’s 2014 No-Dig Show

Orlando, Florida April 13-17, 2014

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Figure 1: Pipe-filter cake interface (left) and damage to filter cake as pipe moves upward (right)

This paper examines the resistance to motion resulting from the net normal force acting on thermoplastic pipes, and the friction factor between the pipe and the borehole wall, a principal contributor to the resistance to motion of the pipe in the borehole. An accurate pull-load prediction is important. Overestimating the pull load could result in the specification of a heavier pipe wall/diameter and/or the use of a larger drilling rig than is necessary, while underestimation of pulling load could result in damage to the pipe during the installation due insufficient cross-sectional area (particularly in the case of thermoplastic pipes), and/or the selection of an underpowered drilling rig. Parameters considered in this study include soil type (sand, silty sand, clay, silty clay, and pea gravel), drilling fluid composition (bentonite for sand and silty-sand; polymer for clay and silty clay; micro-fibers and bentonite for pea gravel), normal load (three load levels representing three slurry densities), and diameter of the thermoplastic pipe (6" and 8"). 3. EXPERIMENTAL FABRICATION AND MODIFICATION Two special apparatuses were designed and fabricated to undertake the proposed testing program, namely a soil box with a drilling fluid injection system and a modified direct shear apparatus. The steel soil box (6"× 6" and 2" deep), featuring a 1" flange on all sides and five 1/16" diameter holes at its base, was fabricated with a matching lid. Provisions were made in the lid to attach a pressure gauge, an air release hose, and a drilling fluid injection port (see Figure 2). The lid was attached to the box using twelve ¼" diameter bolts.

Figure 2: Small holes marked by pins at the soil box base (left); Top cover attached to soil box (right)

A mud pump was fabricated by clamping a 16" long and 4" diameter thermoplastic pipe, capped on both ends, to a vertical wooden frame (see Figure 3). Quick connectors were attached to each cap – one connected to an air compressor via a pressure regulator and the other connected to the closed soil box lid through ½" diameter rubber hose.

Provision to Apply

Pressure

Pressure Gauge

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Figure 3: Mud pump connected to pressure source (left) and close view of drilling fluid pump (right)

One of the modifications performed on the direct shear test apparatus (see Figure 4) was the exclusion of the load application frame. The load cell connected to the soil box container was also removed, and a 200 lbf capacity load cell with an accuracy of 0.2 lbf was connected to the end of the pipe. Vertical and horizontal LVDTs were connected to the soil box cover and the pipe, respectively.

Figure 4: Direct shear apparatus prior to (left) and following (right) modifications

4. TEST PROCEDURE Tests were performed on five soil types: sand, silty sand, clay, silty clay, and pea gravel. Every test was performed twice, once with the presence of a drilling fluid and once without it, to investigate the drilling fluid’s effect on the friction force developed at the soil-filter cake-pipe interface. Drilling fluid formulations were prepared in consultation with Baroid Industrial Drilling Products, to ensure compatibility with the soils used. The test matrix is shown in Table 1.

Load Application

Frame

Vertical LVDT

Horizontal LVDT Load Cell

Load Cell Horizontal LVDT

Load Saddle

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Table 1: Test Matrix for Evaluation of Friction Force

Test Series Soil type US

Classification Remarks Filter Cake Deposition

Normal Load, lb

Pipe Diameter

in.

1 Sand SW - Yes 56 70 80 8 6 No

2 Silty Sand SM - Yes 56 70 80 8 6 No

3 Clay CH - Yes 56 70 80 8 6 No

4 Silty Clay MH - Yes 56 70 80 8 6 No

5 Pea Gravel GP Same Size

Aggregates Yes 56 70 80 8 6 No

Prior to commencement of the testing program, a characterization program was undertaken to determine the bulk specific gravity, grain size distribution, cohesion, friction angle, absorption, and optimum moisture content of the five soil materials deployed in the testing program following ASTM standards. Results of the soil tests are summarized in Table 2.

Table 2: Properties of Soil Materials

Soil Type Bulk Specific Gravity

Absorption%

Optimum Moisture Content

Cohesion psi

Friction Angle

°

Sand 2.27 9.7 12.5 0.9 31.3 Silty Sand 1.96 7.0 10.0 0.4 31.4

Clay 1.84 11.6 17.5 11.2 21.8 Silty-clay 1.89 9.9 15.0 12.5 22.2

Pea Gravel 2.60 1.3 - 0.1 35.2 Four 0.75" tall plexiglass guides were placed along the inside edges of the soil box to provide and retain the shape of the soil surface, which simulated the invert of a pipe. The soil was placed on a geotextile fabric and compacted in three 1" lifts using a 2" square piece of wood and a modified Proctor compaction hammer (see Figure 5).

Figure 5: Soil compacted using a modified Proctor hammer (left) and prepared soil surface (right)

After the soil surface was prepared, the lid was closed tight and filtration was performed using the drilling fluid pump. First, drilling fluid was poured into the chamber. The cap connected to the air compressor was then attached

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and the vessel pressurized to 30 psi for a period of 10-minutes. The pressurized drilling fluid migrated into the soil formation, depositing a filter cake on its surface. The water that migrated through the entire thickness of the soil material was drained via the holes at the bottom of the box. Following the pressurization period, the pressure was released and the top cover removed, revealing the filter cake on top of the soil surface (see Figure 6).

Figure 6: Pouring of Drilling Fluid inside the Chamber (left) and Soil Surface with Filter Cake (right)

After the filtration stage, the lid was pulled off and excess drilling fluid removed until an approximately 0.078" (2 mm) thick filter cake remained on the soil surface (see Figure 7). For pea gravel, a significant portion of the pressurized drilling fluid was lost through the holes as the voids in the pea gravel were too large to provide sufficient capillary forces to retain the drilling fluid, resulting in an uneven surface (see Figure 8).

Figure 7: Scraping off of Excess Fluid and Formation of Filter Cake at the End of Filtration

Figure 8: Pointy Pea Gravel Surface Resulting due to the Loss of Drilling Fluid

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It should be noted that, the testing program called for using two pipe diameters, namely 6" and 8"; however, the size of the soil box was fixed at 6" square due to dimensional limitations on the direct shear test apparatus. When the soil surface was prepared for the 8" pipe, the curvature of the soil was modified to compensate for the limited width of the box. Therefore, the soil-pipe contact area for the 8" pipe was about 30% smaller than that of the 6" pipe. This resulted in higher stresses developing at the invert of the 8" pipe.

Next, the soil box with the filter cake was placed on a modified direct shear apparatus comprised of the direct shear box with a provision to apply vertical load onto the pipe. The pipe was placed on the soil surface with vertical load placed on a saddle located at the crown of the pipe, and the bottom box was pulled horizontally at a rate of 1.2 mm/min. A schematic view of the experimental setup is shown in Figure 9 and the actual setup is shown in Figure 10.

Figure 9: Schematic View of Experimental Setup – Front View (left) and Side View (right)

Figure 10: Actual Experimental Setup

5. TEST RESULT Frictional resistance was experimentally measured for geological materials. Results for the 6" diameter thermoplastic pipe pulled over the sand with and without drilling fluid conditions are shown in Figure 11. The curves followed a linear trend during the initial stage of the test, indicating static friction between the pipe and the soil. Once the static friction was overcome and pipe movement was initiated, the dynamic friction between the pipe and the soil resulted in a steady resistance to motion, as depicted by the horizontal portion of the graph. A 30% additional friction force was realized when the normal load was increased from 56 lb to 80 lb. The presence of drilling fluid reduced the friction force by 10% to 50%, depending on the level of normal load applied to the pipe. For the lowest normal load level (56 lb), the resulting reduction in friction force was found around 50% when drilling fluids were present, compared with when drilling fluids were absent. This was attributed to damage inflicted upon the mud cake by the pipe’s wall, as the pipe was pushed deeper into the soil bed by the higher normal load.

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This observation underscores the importance of ballasting the pipe to achieve as close to neutral buoyancy condition as possible during the pullback operation.

Figure 11: Friction Force in Sand for a 6" Pipe without Drilling Fluid (left) and with Drilling Fluid (right)

Maximum friction forces, prior to the initiation of dynamic friction, were compared for the 6" diameter pipe pulled over different geological materials with and without filter cake (see Figure 12). For sandy soil, (SW and SM as per the universal soil classification system) the friction force, when drilling fluids were not used, was found to be approximately 20% to 35% higher when compared to similar test scenarios which deployed drilling fluids. For clayey soils, (CH and MH as per the USCS) the friction force was found to be 10% to 32% higher when drilling fluids were not used. Overall, pulling forces in silty sand (SM) were found to be lower than in the case of sand (SW) when drilling fluids were used, due to the presence of finer particles which assisted in the formation of a better filter cake (i.e., more difficult for the drilling fluid to migrate into the formation). Conclusive results were not obtained in the case of pea gravel due to the difficulty in developing an effective filter cake due to excessive drilling fluid losses. As a general rule, the reduction in friction force due to the presence of drilling fluid was not as prominent in the case of pea gravel, potentially due to displacement of the filter cake with increasing normal load, reducing its effectiveness.

Figure 12: Comparison of Maximum Friction Force for Different Geological Materials – 6" Pipe

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Similar tests were performed using an 8" thermoplastic pipe and the results are shown in Figure 13. It was found that for both sandy soils (SW and SM) the pull force required to initiate motion when drilling fluids were absent was approximately 10% to 70% higher, compared to similar test scenarios which deployed properly formulated drilling fluids.

When drilling fluids were present, pulling forces required for silty sand (SM) at 56 lbf and 80 lbf normal loads were found to be lower than those recorded for sand (SW). As in the case of 6" pipe, failure to construct an effective filter cake prevented reaching conclusive results for pea gravel. Future research work will focus only on the development of an effective filter cake in gravelly soils.

Figure 13: Comparison of Maximum Friction Force for Different Geological Materials – 8" Pipe

The friction force divided by the applied normal load was used to calculate the friction factor for different geological materials. The results are presented in Table 3 for each combination of test parameters. The range of friction factors for each soil type, with and without drilling fluid, is also presented. In all cases, with the exception of pea gravel, the presence of drilling fluids resulted in a significant reduction in the friction factor. The average friction factors for sand (SW) and silty sand (SM), in the absence of drilling fluids, were 0.33 and 0.36, respectively. These values were reduced to 0.24 and 0.23, respectively, when a filter cake was present at the pipe-soil interface. This reduction in friction factor greatly affects the pulling load, as suggested by the equations in ASTM F 1962. For clay (CH) and silty clay (MH), the friction factor values were found to be 0.29 and 0.39 without a filter cake; these values dropped to 0.25 and 0.27, respectively, when a filter cake was present. The friction factor values for silty clay (MH) followed a similar trend as that of the sandy soils; however, the reduction in the friction factor values of clay (CH) were found to be less significant, as expected. In the case of pea gravel the presence of a filter cake reduced the friction factor from about 0.36 to 0.29; however, the observation might not be conclusive due to the formation of an uneven filter cake on the poorly graded pea gravel surface.

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Table 3: Summary of Friction Factors for all Normal Loads and Different Geological Materials

Soil Type Pipe

Diameter (in.)

Applied Normal Load, lb Friction Factor Range

W/O Drilling Fluid W/ Drilling Fluid 56 70 80 56 70 80 W/O

Drilling Fluid W/

Drilling Fluid Friction Factor Sand (SW)

8 0.31 0.30 0.36 0.18 0.20 0.32 0.30 - 0.36 0.18 - 0.32

6 0.33 0.33 0.33 0.24 0.24 0.27 Silty Sand

(SM) 8 0.30 0.30 0.34 0.17 0.21 0.23

0.30 - 0.43 0.17 - 0.30 6 0.39 0.40 0.43 0.21 0.26 0.30

Clay (CH)

8 0.27 0.26 0.36 0.24 0.24 0.27 0.26 - 0.36 0.24 - 0.27

6 0.28 0.28 0.27 0.26 0.24 0.27 Silty Clay

(MH) 8 0.34 0.33 0.36 0.20 0.20 0.24

0.33 - 0.45 0.20 - 0.35 6 0.44 0.45 0.41 0.35 0.35 0.30

Pea Gravel (GP)

8 0.38 0.36 0.44 0.29 0.29 0.33 0.27 - 0.44 0.26 - 0.33

6 0.27 0.29 0.34 0.26 0.27 0.29 The subtle movements of the pipe prior to breaking the frictional resistance between the soil and the pipe are given in Table 4 for each combination of test parameters. As a general rule, the smaller the movement of the pipe is, the lower the friction at the soil-pipe interface. The presence of drilling fluids resulted in a significant reduction in the movement needed to overcome the static friction at the soil-pipe interface in the case of sandy soils and silty clay, and a moderate reduction in the case of clay and pea gravel.

Table 4: Pipe Movement Prior to Overcoming Frictional Resistance

Soil Type

Pipe Diameter

in

Applied Normal Load, lb

W/O Drilling Fluid W/ Drilling Fluid

56 70 80 56 70 80

Pipe Movement Prior to Overcoming Frictional Resistance, in

Sand (SW)

8 0.0450 0.0150 0.0170 0.0050 0.0080 0.0139

6 0.0200 0.0241 0.0310 0.0030 0.0072 0.0185

Silty Sand (SM)

8 0.0037 0.0158 0.0230 0.0030 0.0103 0.0127

6 0.0042 0.0077 0.0280 0.0010 0.0018 0.0142

Clay (CH)

8 0.0088 0.0056 0.0240 0.0030 0.0029 0.0204

6 0.0086 0.0129 0.0190 0.0060 0.0096 0.0189

Silty Clay (MH)

8 0.0117 0.0128 0.0160 0.0050 0.0068 0.0092

6 0.0166 0.0094 0.0290 0.0110 0.0036 0.0016

Pea Gravel (GP)

8 0.0102 0.0221 0.0170 0.0040 0.0020 0.0170

6 0.0232 0.0230 0.0260 0.0022 0.0211 0.0248 6. SUMMARY The parameters (e.g., soil type, filter cake formation, contact surface, and normal load) that affect the soil-filter cake-pipe interface were identified and incorporated into a laboratory-based testing program. The effect of soil types on friction at the soil-pipe interface was quantified by experimentally simulating borehole conditions during HDD installations. Proper formulations of drilling fluid were used to form filter cakes for different soil types; the formation of an effective filter cake was found to significantly reduce the friction factors at the soil-pipe interface,

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particularly in the case of sandy and clayey soils. A friction factor values for thermoplastic pipes installed in five common were derived to assist industry professionals in estimating pull-load values for specific installations based on anticipated geological conditions and estimated normal force (i.e., uplift force). Test data also suggested that the magnitude of the normal load acting on the pipe has considerable effect on the friction response at the pipe-soil interface.

8. REFERENCES Baumert M.E. and Allouche E.A. (2002). Methods for Estimating Pipe Pullback Loads for HDD Crossings. Journal of Infrastructure Systems, ASCE, Vol. 8, No. 1, pp. 12-19.

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