How to Design a Braced Excavation: EG7004 DPL and FREW
Few coursework briefs ask you to design the same excavation three ways. EG7004 does, and the hard part is not the software: it is keeping the DPL hand method, two FREW models and a drawing consistent with each other.
Quick answer: Start from your Moodle dataset, set up earth and water pressures, design the DPL scheme (prop levels, spacing, loads, 10% penetration), then build two FREW models using the same props and spacing. Compare the results in a few tables, recommend one wall, and draw it. Keep calculations in appendices.
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What the brief asks you to do
The EG7004 Soil Structure Engineering coursework (UEL, Semester A 2026/27, individual, 60% of the module) gives each student a unique dataset: excavation depth and width, time open and soil parameters, with a varying groundwater level. The soil is granular, loose medium sand over medium dense and dense sand. You design support for a deep rectangular braced excavation under three options: a DPL design to CIRIA C517 with 10% penetration, FREW with AZ 32-750 sheet piles, and FREW with a 500 mm diaphragm wall. Props are 273 mm × 10 mm CHS in the two FREW models, at the spacing found by DPL.
A worked layout for an invented dataset is in our EG7004 braced excavation sample, for reference only. It is not anyone’s real dataset.
Mark map: where the 100 marks sit
| Section | Content | Marks |
|---|---|---|
| a | One-page executive summary | 10 |
| b | Outline design per C517, prop levels, construction sequence | 20 |
| c | Design actions: prop loads per C517 | 10 |
| d | FREW sheet pile: prop loads, moments, deflections | 20 |
| e | FREW diaphragm wall | 10 |
| f | Dewatering approach and effect on design and construction | 10 |
| g | Flexible v stiff wall comparison, recommendation | 10 |
| h | Construction drawing | 10 |
Sections b and d carry the most. Spend time there before polishing the summary.
Step-by-step workflow
1. Read your dataset and write down your assumptions
List excavation depth, width, time open, each soil layer’s unit weight and strength, and the groundwater range. Note which water level governs for each dig stage. A short assumptions table saves repeated explanation later.
2. Set earth and water pressures
Choose partial factors and state the design approach under Eurocode 7 (BS EN 1997-1). Use the effective stress with the water table you will actually have after dewatering, not the natural one, wherever your scheme lowers it.
3. Do the DPL design
CIRIA C517 by Twine and Roscoe (1999) gives an empirical method for prop loads based on field measurements. The DPL is read for the soil class and wall type, treated as a characteristic value and factored by 1.35 for design (Katsigiannis et al., 2015). Check the exact granular-soil expression in C517 itself. Then choose prop levels so each dig stage is workable, work out loads per metre run, and convert them to loads per prop at your horizontal spacing. Check the CHS props for axial capacity and buckling using Tata Steel Blue Book properties.
4. Build the FREW models
Oasys FREW analyses flexible embedded walls through construction stages, including changes in ground level, water pressure and props (Oasys, 2026). Model the sheet pile first, with section properties from the ArcelorMittal data for AZ 32-750, then copy the model and change only the wall for the diaphragm option.
5. Compare and recommend
Put prop loads, bending moments, deflections and penetration for each option side by side. Then pick a wall and justify it with your numbers, not general statements.
6. Draw it
Do the construction drawing last, from your final design values, so the levels on the drawing match the text.
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FREW modelling tips
- Keep one stage per real construction step: install wall, dig, prop, dig again. Skipping stages hides the governing case.
- Use the same prop levels, stiffness and spacing in both models, so differences come from the wall.
- Let the software find penetration, then state how it compares with the DPL 10% value.
- Check that your water pressure input matches the dewatering stages you describe in section f.
- Run a sanity check: do FREW prop loads sit in the same range as the DPL loads? If not, find out why before you write it up.
- Report only governing results in the body. The brief penalises padding with raw software output, so send full printouts to an appendix.
Where students lose time
Most of the lost hours come from three places. The first is the dataset: with unique numbers, there is no model answer to check against, so errors in unit weight, water level or depth travel silently through every section. Re-read the Moodle data against your assumptions table before you start any calculation.
The second is FREW itself. A model that runs is not the same as a model that is right. Look at the deflected shape, the bending moment diagram and the prop forces at each stage. If a prop force changes sign, or the moment diagram looks odd, check the stage definitions and water levels first, then the soil layers.
The third is scope. Sections a to h are marked separately, and an examiner scanning for the stage that governs wants to find it in one table, not in forty pages of output. Write for that reader. Say what you found, why it matters for the design, and move on.
A final practical point: keep a single spreadsheet of input values and results, and build every table in the report from it. It makes changes after a late model re-run painless, and it keeps the executive summary, the body and the drawing in agreement.
Dewatering: what to write
Compare sumps, wellpoints and deep wells for your soil and drawdown. Say what each does to groundwater levels around the wall, and what it risks: boiling or piping at the base of loose sand, settlement of neighbouring ground, and discharge consents. Then link back to design: the water pressure on each side of the wall at each stage must match what you modelled.
Construction drawing checklist
- Plan and a section through the excavation, with dimensions and a scale.
- Ground levels, soil layers and groundwater levels.
- Wall type, length and penetration.
- Prop levels, sizes and horizontal spacing, with the waling.
- Construction sequence notes and dig levels.
- Dewatering layout in outline.
- Title block, drawing number, revision and units.
If you need to sharpen CAD skills, see our AutoCAD assignment help.
Common mistakes
- Copying software printouts into the body of the report instead of the appendix.
- Using different prop spacing in DPL and FREW, which makes comparison meaningless.
- Ignoring the groundwater range and using one convenient level.
- Recommending a wall without citing a number from your results.
- Leaving drawing levels out of step with the calculations.
- Quoting AZ 32-750 or CHS properties from memory rather than the manufacturer’s tables or Blue Book.
- Running past the page limit on the executive summary.
FAQs
Is the executive summary written first or last?
Last. It must carry your actual governing loads, moments and deflections, which you will not have until the end.
Why does the brief fix penetration at 10% for DPL but not for FREW?
DPL is an empirical prop-load method, so penetration is an input. FREW is a staged analysis, so the software shows the penetration needed for stability. Compare the two in your discussion.
Do I need to show every calculation in the report?
No. Put workings in appendices and quote the results and the logic in the body, as the brief warns against padding.
Where do I get AZ 32-750 section properties?
From ArcelorMittal’s published AZ-750 data. For the CHS props, use the Tata Steel Blue Book.
Which wall should I recommend?
Whichever your results support against your stated movement limit, cost, programme and water cut-off needs. There is no automatic right answer, and a justified choice scores better than a default.
Can I use another student’s numbers as a check?
No. Each dataset is unique, and the sample linked above is invented. It shows layout only.
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If you want subject support, see soil structure engineering assignment help, geotechnical engineering assignment help or our wider engineering assignment help. Our guide to report writing help is useful for the summary. Subject guidance comes from Eleanor Lawrence, whose profile lists Soil Mechanics and Structural Analysis among her topics.
References
- ArcelorMittal (n.d.) AZ sections – AZ-750. Available at: https://sheetpiling.arcelormittal.com/products/az-sections/az-750 (Accessed: 24 October 2026).
- British Standards Institution (2004) BS EN 1997-1:2004+A1:2013 Eurocode 7: Geotechnical design – General rules. London: BSI.
- Katsigiannis, G., Schweiger, H.F., Ferreira, P. and Fuentes, R. (2015) ‘Design of deep supported excavations: comparison between numerical and empirical methods’, in Geotechnical Safety and Risk V. Amsterdam: IOS Press, pp. 482–488. Available at: https://doi.org/10.3233/978-1-61499-580-7-482 (Accessed: 24 October 2026).
- Oasys (2026) Oasys Frew. Available at: https://www.oasys-software.com/frew/ (Accessed: 24 October 2026).
- Twine, D. and Roscoe, H. (1999) Temporary propping of deep excavations: guidance on design (C517). London: CIRIA. Available at: https://www.ciria.org/SOILCOP/iCore/Store/StoreLayouts/Item_Detail.aspx?iProductCode=C517&Category=BOOK (Accessed: 24 October 2026).
