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Predicting Settlements of Existing Utility Lines under Surface Loadings

A DeepEX Case Study


A. Introduction

The intricate web of utility networks, including electricity, telephone lines, fiber optics, and water pipelines, underlies every residential area, crucially supporting our daily activities and well-being. Maintaining the functionality and structural integrity of these lines is paramount. In urban environments where space for new construction is scarce, the necessity to perform public works or install heavy structures above or adjacent to existing networks is common. However, such soil surface disturbances can induce settlements, potentially compromising the integrity of the utility lines, typically situated just below the surface, at depths ranging from 10 to 15 feet (3 to 5 meters).


B. Finite Element Analysis and DeepEX Software

Predicting the effects of surface surcharges on the soil mass, which may comprise various soil materials, poses a significant challenge. Finite Element Analysis (FEA) emerges as a powerful tool for accurately simulating soil conditions in such scenarios. DeepEX, a comprehensive shoring and tunnel design software, integrates advanced FEA engines to facilitate detailed soil mass simulation and utility line creation. With DeepEX, users can manipulate tunnel dimensions, positions, and linings using various materials such as reinforced concrete, steel plates, ribs, and corrugated sheets. Utilizing FEA within DeepEX allows for the calculation of stresses induced by tunnel construction and monitoring of stresses, displacements, settlements, and more caused by surface loads applied to the model.


FEM mesh and soil mass settlements shadings in DeepEX

Figure 1: Utility lines - FEM mesh and soil mass settlements shadings in DeepEX


C. Case study: Predicting Settlements for Two Existing Utility Lines

In the subsequent sections, we demonstrate how DeepEX tunnel generation tools can be utilized to simulate two utility lines within the soil mass. Additionally, we present settlement results obtained from DeepEX when subjecting the area to a 1 ksf surface load. The table below outlines the assumed soil properties for this case study.

Table 1: Assumed soil properties and stratigraphy

Soil

Top El.

Description

Unit Weight

Friction Angle

C’ or Su

Soil Behavior

Eload

exp

(-)

(ft)

(-)

(pcf)

(deg)

(psf)

(-)

(ksf)

(-)

F

0

Fill - Sand

120

30

0

Exponential

300

0.5

C

-8

Undrained Clay

125

0

3000

Exponential

420

1

GT

-20

Glacial Till

135

36

150

Exponential

600

0.4

 We will simulate the presence of two utility pipelines, each with an external diameter of 3 feet, installed at a depth of 12 feet below the surface. For the first pipeline, we will assume a 4-inch thick reinforced concrete lining, while for the second pipeline, we will assume a 1-inch thick steel plate lining. Using the DeepEX tunnel wizard, we will create simulations for each pipeline within our soil mass. The images below depict the model and lining settings, as well as the generated model.


 Concrete pipe position, structural section and lining in DeepEX

Figure 2: Concrete pipe position, structural section and lining in DeepEX


Steel pipe position, structural section and lining in DeepEX

Figure 3: Steel pipe position, structural section and lining in DeepEX


Utility lines in a DeepEX model

Figure 4: Utility lines in a DeepEX model


DeepEX software encompasses all construction stages necessary for tunnel installation. As the utility lines we simulated already exist within the soil mass, we can presume that the soil has settled during the pile installation phase. By accessing the analysis settings, we can reset the model displacements to account for this settling. Subsequently, we can add another stage to apply the external surcharge we wish to examine on the soil surface and monitor the effect solely of the new load, disregarding settlements due to installation. The image below illustrates the surcharge applied in the new construction stage.


External surcharge position and magnitude in DeepEX software

Figure 5: External surcharge position and magnitude in DeepEX software


We will conduct the model analysis using the DeepEX FEM analysis engine, employing a very fine mesh density. The images below depict the generated vertical displacement contours, as well as the developed moments and displacements on the pipeline linings resulting from the surface surcharge. Our observations indicate that in the soft fill layer above the pipelines, the predicted settlement is approximately 0.3 inches, while the general settlement around and below the pipes is approximately 0.1 inch.


Utility lines under surface load in DeepEX - Vertical displacement arrows

Figure 6: Utility lines under surface load in DeepEX - Vertical displacement arrows


 Pipe lining moments - DeepEX

Figure 7: Pipe lining moments - DeepEX


Horizontal and vertical pipe lining displacements – DeepEX

Figure 8: Horizontal and vertical pipe lining displacements – DeepEX


D. Conclusion

In conclusion, the integrity of existing utility lines amidst surface loadings poses significant risks and challenges for geotechnical engineers. Settlements induced by such loads can compromise the functionality and structural stability of vital infrastructure. However, with the advanced capabilities of DeepEX software, engineers can effectively address these challenges. Through its comprehensive suite of tools, DeepEX empowers engineers to accurately assess and mitigate risks associated with surface disturbances. We invite all geotechnical engineers to explore the capabilities of DeepEX and experience firsthand how it can streamline the analysis and design processes, ensuring the resilience of utility networks in the face of dynamic construction environments.


 


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