Design and Construction Considerations for Retained Cuttings at Ashford on HS1 High-Speed Rail

Background

High Speed 1 (HS1) consists of 109 km of high-speed track and associated infrastructure linking the Channel Tunnel at Folkestone, Kent to the London terminus at St Pancras. Various measures were adopted to minimize the impact of HS1 in terms of noise and visual intrusion, particularly in urban areas. At Ashford, Kent, the railway runs through approximately 1.8 km of cut and cover tunnels and associated retained cuttings to minimize noise and to avoid crossing existing road and rail routes at grade. The sides of the cut-and-cover tunnels and propped retained cuttings were constructed using contiguous bored pile retaining walls, using piles varying in diameter from 900 mm to 1350 mm. Figure 1 shows a cross section through one of these walls.

It is proposed to extend the length of retained cutting, to provide additional lines running around Ashford station. At one location, a row of brick Victorian terrace houses runs parallel to the line of the cutting, with the nearest part of the houses just 10 m from the retaining wall. A contractor has proposed an outline scheme, including key aspects of the geometry required for the retained cutting (excavation depth, thickness of base slab). Figure 2 shows the dimensions, and soil conditions for, the new proposed excavation. You are asked to do some design calculations to establish the wall embedment depth, wall and prop structural loads, and wall and ground displacements.

Construction Details

The geological succession in the Ashford area is summarised in Table 1, and geotechnical parameters and soil properties are given in Tables 2 and 3 and Figure .

The contractor’s proposed construction sequence has seven stages, which are:

1. Install the diaphragm wall (you will need to determine the design embedment depth)

2. Excavate to a depth of 1.0 m bgl and install the upper, permanent props at 0.5 m bgl

3. Excavate to a depth of 6.5 m bgl

4. Install the temporary props at a depth of 6.0 m bgl

5. Excavate to a depth of 9.5 m bgl

6. Construct the base slab of 1.0 m depth

7. Remove the temporary props

This construction sequence is shown schematically in Table 4

Due to the plant available, the contractor would prefer to build a 1300 mm thick concrete diaphragm panel wall. It is intended that the top props will be of reinforced concrete of cross-section 1000 mm by 1000 mm and spaced at 4.5 m centres, and that the temporary props will be of welded steel construction of 1000 mm diameter and wall thickness 25 mm. The base slab is a 1.0 m-thick reinforced concrete slab.

Assessment Requirements

The assessment focuses on the structural and geotechnical design of retained cuttings along the HS1 high-speed rail near Ashford. Students are required to:

1. Understand the site context – 1.8 km of cut-and-cover tunnels, proximity to Victorian terrace houses, and proposed extension for additional lines.

2. Design calculations – Determine diaphragm wall embedment depth, wall and prop structural loads, and expected wall and ground displacements.

3. Analyze construction sequence – Seven-step sequence from diaphragm wall installation to removal of temporary props.

4. Apply engineering judgment – Consider soil conditions, wall thickness (1300 mm), prop types (reinforced concrete and welded steel), spacing, and base slab design.

5. Produce a structured solution – Clearly present calculations, assumptions, and justifications.

Key Pointers to Cover:

• Diaphragm wall design and embedment depth calculation.

• Structural loads on permanent and temporary props.

• Wall and ground displacement analysis.

• Construction sequence and its impact on stability.

• Consideration of urban constraints (proximity to residential houses).

Approach Taken by Academic Mentor

The mentor guided the student through the assessment step by step:

1. Understanding the Problem:

   • Reviewed background and construction details.

   • Identified key geotechnical and structural parameters from provided tables and figures.

2. Planning the Calculation Process:

   • Advised to first calculate wall embedment depth based on soil pressure and wall stability requirements.

   • Explained how to compute axial and lateral loads on permanent and temporary props.

3. Structural and Geotechnical Analysis:

   • Stepwise analysis of wall forces during staged excavation.

   • Determination of base slab loadings and interaction with diaphragm wall.

   • Prediction of wall and ground displacement using soil parameters.

4. Cross-Checking and Verification:

   • Recommended comparing results with design codes and precedents for retaining walls in similar urban conditions.

5. Documentation and Presentation:

   • Structured the report with introduction, methodology, calculations, diagrams, and conclusion.

   • Emphasized clarity, assumptions made, and proper referencing of soil and structural data.

Final Outcome

• Design Calculations Completed: Embedment depth, prop forces, wall and ground displacements determined.

• Construction Sequence Considered: Analysis reflects staged excavation effects.

• Urban Constraints Addressed: Proximity to residential houses integrated into safety and displacement checks.

• Learning Objectives Achieved:

  • Application of geotechnical and structural principles in real-world scenarios.

  • Integration of construction sequencing with design requirements.

  • Development of professional reporting and engineering justification skills.

  • Understanding the impact of urban infrastructure on civil engineering design.

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