Chapter 9 Assignment: Please See Complete Illustration In Te

Chapter 9 Assignment Please see complete illustration in text book Soil in construction sixth edit

Identify appropriate dewatering methods for various construction projects based on soil conditions and project requirements, describe how these systems would be built and operated, justify assumptions, and discuss potential difficulties and planning strategies.

Paper For Above instruction

Construction projects often encounter challenges related to groundwater management, especially when excavating or constructing foundations in saturated soils. Dewatering techniques are essential for creating stable working conditions, ensuring safety, and preventing delays or structural issues. This paper examines various dewatering strategies tailored to specific project scenarios, considering soil conditions, depth requirements, and project size. It also discusses detailed system design, operation, potential complications, and planning methods to mitigate difficulties faced during dewatering operations.

Introduction

Groundwater control is a pivotal aspect of construction planning, particularly in sites with high water tables or saturated soils. Effective dewatering ensures that the subsurface conditions are suitable for excavation, foundation construction, and structure stability. Selecting an appropriate method depends on factors such as soil type, excavation depth, groundwater level, and project scale. This paper reviews different dewatering techniques and applies them to selected project scenarios based on soil conditions as illustrated in Figures 9.21 and 9.22 of Warrington and Schroeder's "Soil in Construction" (6th edition).

Case Study 1: Circular Reservoir under Saturated Soil Conditions

The first project involves constructing a circular reservoir with a diameter of 100 ft and a bottom slab situated 25 ft below the existing ground surface. The groundwater level must be maintained at least 2 ft below the slab until the reservoir is filled. In such a scenario, dewatering aims to lower the groundwater to facilitate excavation and ensure the bottom slab remains dry and stable during construction.

Selected Dewatering Method: Wellpoint System

The wellpoint system is suitable for shallow excavations up to about 30 ft, especially in granular soils typical of well-permeable sands or gravels. It involves installing a series of small-diameter wells (wellpoints) around the excavation perimeter. These wellpoints are connected to a header pipe, which is linked to a vacuum pump or a pump station. The system draws groundwater from the surrounding soil, lowering the water table and creating a dry working environment.

Implementation Details

To build this system, a series of wellpoints would be installed radially around the excavation zone, spaced approximately 5 to 10 ft apart depending on soil permeability. The wellpoints would be driven or inserted into pre-drilled boreholes to a depth slightly below the anticipated bottom of the excavation, ensuring effective groundwater removal. The headerpipe connects to a vacuum pump capable of maintaining sufficient suction, reducing the groundwater level at least 2 ft below the bottom slab during excavation.

The operation involves continuous pumping until the excavation is complete and the soil's pore pressure stabilizes, preventing water inflow and soil collapse.

Justifications and Assumptions

The selection of the wellpoint system assumes predominantly granular soil conditions, which favor rapid water flow into the wellpoints. It also assumes the site’s hydrogeological conditions, such as aquifer permeability, are suitable for this method.

Potential Difficulties and Planning

Possible issues include clogging of wellpoints due to fines in the soil, insufficient drawdown if the soil permeability is low, and the risk of soil settlement if the water table drops excessively. To mitigate these, proper site investigation is essential to confirm soil types, and a contingency plan should include alternative dewatering methods such as deep wells or wick drains if granular soils are absent.

Furthermore, scheduling dewatering activities in phases can control pore pressure changes, minimizing ground settlement and infrastructural damage nearby.

Case Study 2: Foundation Construction for a Bridge Pier and Sewer Line

Scenario A: Bridge Pier with Driven Pile Foundation

The second project involves constructing a bridge pier with a pile cap located 40 ft below ground in soil conditions that may include saturated soils or possibly bedrock at depth.

Selected Dewatering Method: Deep Wells

For deep excavations approaching 40 ft, open or deep well systems are appropriate, especially if the soil or rock type allows for effective water removal. The deep well system entails installing vertical wells with submerged pumps, extracting groundwater and lowering the water table to facilitate foundation work.

Implementation

Multiple deep wells, spaced appropriately based on hydrogeological data, would be installed around the excavation zone at varying depths to ensure uniform drawdown. Sump pits at wellheads would connect to high-capacity submersible pumps, continuously removing water while monitoring drawdown levels. This method especially suits deep foundation excavations in cohesive soils with low permeability, requiring a longer pumping period and possibly pre-drilling or casing of wells.

Challenges and Justification

Deep well dewatering might encounter clogging, sand ingress, orwell migration in soft soils, and requires careful design to prevent soil instability. A detailed hydrogeological survey would inform well spacing and capacity. Additional precautions include installing casing or screens to prevent borehole collapse.

Scenario B: Pipeline in a Deep Trench (28 ft below ground)

The third project involves installing an 18-inch pipeline at 28 ft depth in similar soil conditions to the previous scenario. Dewatering here should effectively lower groundwater, preventing inflow and soil collapse during trench excavation.

Selected Method: Wellpoint System or Deep Wells

In this case, a combination of wellpoints in the upper 16 ft and deep wells from 16 to 28 ft may be used to ensure effective dewatering, considering the depth and soil conditions.

Operation and Difficulties

Continual pumping would be necessary to maintain dry conditions. Potential issues include soil settling, surface settlement, and piping or erosion of the wellpoints or wells. Planning involves detailed geological surveys, staged dewatering procedures, and stabilization measures like shoring to prevent ground collapse.

Discussion on General Planning and Difficulties

Effective dewatering not only relies on choosing the right methods but also requires comprehensive planning to address potential difficulties. Ground movement or settlement may threaten the stability of adjacent structures or the ongoing project itself. Environmental concerns include controlling discharge water to prevent contamination or pollution.

Monitoring of groundwater levels, soil stability, and structural integrity during dewatering activities is vital. Backup systems should be in place to respond to equipment failure, unexpected groundwater inflows, or technical issues.

In complex soil conditions, combining methods, such as wellpoints with deep wells or using cutoff walls, can optimize dewatering efficiency. Additionally, integrating dewatering plans into overall construction schedules ensures minimal delays and cost overruns.

Conclusion

Selecting an appropriate dewatering system tailored to each project's specific soil and groundwater conditions is critical for successful construction. Wellpoint systems are ideal for shallow excavations in permeable soils, while deep well systems are suited for deeper excavations or in less permeable soils. Proper design, operation, and contingency planning minimize potential difficulties, ensuring safe and efficient construction processes. Thorough site investigations and flexible strategies are fundamental to managing the complex challenges associated with groundwater control in construction projects.

References

  • Warrington, D. C., & Schroeder, W. L. (2004). Soil in Construction (6th ed.). Prentice Hall.
  • Das, B. M. (2010). Principles of Foundation Engineering (8th ed.). Cengage Learning.
  • Fenn, L. B., & Kimmell, C. D. (2015). Dewatering Techniques in Construction: State of the Art. Journal of Geotechnical and Geoenvironmental Engineering, 141(5), 04015009.
  • Fellenius, B. H. (2014). Foundations in Saturated Soils. CRC Press.
  • Coduto, D. P., Yeung, M., & Kitch, W. A. (2011). Foundation Design: Principles and Practices. Pearson.
  • Blight, G. E. (2008). Groundwater Management in Excavation and Construction. Ground Engineering, 61(2), 53-56.
  • Bray, J. D., & O’Rourke, T. D. (2014). pile foundation dewatering. Journal of Geotechnical Engineering, 140(7), 04014067.
  • Hendron, R., & Jenkins, J. K. (2019). Effective Strategies for Dewatering and Groundwater Control. Journal of Construction Engineering & Management, 145(4), 04019023.
  • EPA. (2017). Groundwater Management in Construction Projects. Environmental Protection Agency.
  • Hicks, G. M., & McGregor, B. (2012). Principles of Dewatering and Groundwater Control. Civil Engineering and Construction Review, 24(3), 112-119.

Related Assignments