SRA 2D: ground settlement analysis for tunnelling

Workflow

Analysing a tunnel route in SRA 2D, step by step

1. Prepare the alignment drawings in AutoCAD

SRA 2D reads the alignment from DXF files, so tidy the drawings first:

• Bring the start of the route to an even station (+00). The plan alignment must start at the same station as the existing alignment; if that starts at 940+00, erase everything on the plan line before 940+00.
• If the line is broken, use JOIN to make it one polyline.
• Place a POINT at the start, then use a Path ARRAY to spread points along the route every 100 ft.
• If there is a cut-and-cover segment at the start, extend the line back by about half its length. For a 1,900 ft segment (0+00 to 19+00), step back about 9+50.
• For the existing alignment, select the whole drawing and MOVE it to the origin (0,0).
• Save As AutoCAD 2000/LT2000 DXF (*.dxf).

2. Create a project

The start page offers NEW PROJECT and Open existing project; recent projects are listed on the right, double-click one to open it. For a new project, pick one of the four analysis types, then enter the project name, the folder (Browse) and the units: U.S customary (ft, in) or Metric SI (m, cm). Click OK.

3. Enter the tunnel section

Open Bored Tunnel Section and fill in:

• Radius: from the tunnel centre to its edge.
• Distance Z: vertical distance from the tunnel centre to the top of rail.
• Distance Y: horizontal distance from the tunnel centre to the rail; Distance of two rails: the rail spacing.
• Twin tunnels: tick Twin tunnel, fill in Tunnel 2 and the distance between the tunnel centrelines.

Click Apply to draw the section, and Save once it looks right.

4. Define the soil layers

In Soil Profile, click Material to open the soil library. Each soil type has a K value, a colour and a note; for example Well-Graded GRAVEL has K = 0.2, with the note that sand and gravel above the groundwater table has K = 0.2–0.3 and below it K = 0.2–0.6. Add your own types with Add and give each layer its own colour.

Then build the soil profile: enter the layer, its top and bottom, whether it is above or below groundwater, and click Add Layer; Insert Layer puts a new layer above the selected one. When the profile is complete, click Add Profile and Save.

5. Load the alignment, ground line and top of rail

• Alignment Plan: choose the direction of subdividing, open the alignment DXF, pick the layer that holds the route and enter the starting station. For twin tunnels, pick a layer for each tunnel.
• Existing Ground Line and Top Of Rail Line: open a DXF, XML or Excel file, pick the layer, enter the starting station and click Apply. Both lines can be saved back to XML, DXF or Excel.
• Open Alignment Overview to check both lines before calculating.

6. Station excavations and cut-and-cover

In Station excavations, each station has a name, the excavation depth below top of rail (TOR), the settlement to excavation depth ratio and the lateral movement to excavation depth ratio. Enter the X, Y coordinates of the four corners of the main or secondary excavation, click Add for each point, then Add Station and Save. View all stations in plan shows every station on plan so you can check its position.

Cut-and-cover tunnels are entered as a Start Segment and an End Segment, each with coordinates and depth.

7. Create cross sections and assign soil

In Cross Section Data, enter the spacing between sections (for example 100 ft), the first and last cross-section stations, and the stations where the bored tunnel starts and ends. The diagram on the right explains each value. Click Create All Section, or Create a Section for a single one.

Click Assign soil profile. In Auto mode each row is one segment of the route: start station, end station, volume loss V_L (%) and soil profile (which carries K). Twin tunnels can have a different V_L for each bore: switch to Manual, enter V_L for Tunnel 1 and Tunnel 2 for the selected sections and click Apply. The list on the right shows the soil profile, VL1 and VL2 of every section.

8. Enter buildings and utilities

• Buildings: open the Excel Building table and click Open Excel file... to read the list (.xlsx). Columns are matched by header name, so both the Single Bore and Twin Bore templates work; you can also copy rows in Excel and paste them with Ctrl+V.
• The grey columns are filled in by the software: maximum tensile strain, damage category and distance between tunnel centrelines. They update every time the contours are run.
• Each building has a representative section (ground elevation, depth of tunnel springline, V_L for both tunnels, K) that can be edited for that building alone. Pile-founded buildings also get a list of piles.
• Utilities (Utility Input): choose parallel or perpendicular to the tunnel, enter the station of the representative section and click Create section, then fill in the pipe and soil parameters (deformation modulus E_s, Poisson's ratio). The pipe list also imports from and exports to Excel.

9. Run the analysis and read the trough

Open Settlement Contours and click Run Analysis. SRA 2D calculates settlement at every section, draws the five contours, then updates the maximum tensile strain and damage category of every building. With the Hsieh method for open excavations, the trough is drawn as straight lines through the control points 0, 0.5H_e, 2H_e and 4H_e (H_e is the excavation depth) to set it apart from the Canadian method.

To inspect a section, double-click it in the list: the settlement trough (red) and lateral movement (blue) appear on the right, with the maximum settlement S_max.

10. Smooth and export the contours

Click Smoothing contours. In Smoothy Input, each contour line has its own segment length; Subdivision breaks lines into smaller pieces. The Station / C&C ends group shapes the contours around the ends of stations and cut-and-cover boxes: rounding zone (default 15 ft), flare zone, cap outside box (default 4 sections) and gradual bend at ends (0 = off). Click Apply.

To export, choose File > Export Dxf File in the Plan View and pick where to save.

11. Read building and utility damage

Open Building Damage Level and pick a building. The Slope, Curvature, Horiz Strain and Building Strains and Distortions tabs show the slope, curvature, horizontal strain and where the building falls on the damage chart. Excel exports the summary for the whole project.

For utilities, open Utility Damage Level and click a pipe: its settlement and tensile strain are calculated and drawn at once. Names end in [Transverse] or [Longitudinal] to tell perpendicular and parallel pipes apart.