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EG7004 Braced Excavation Sample: DPL and FREW Design
For information and reference only – not to be submitted as your own work.
This extract shows how a strong answer to the EG7004 Soil Structure Engineering braced excavation coursework can be laid out: a one-page executive summary, a DPL outline design, prop-load tables, FREW result tables, a wall comparison, dewatering and a recommendation. Everything below uses an illustrative dataset (H = 8 m, W = 12 m, γ = 18 kN/m³). It is not any student’s Moodle dataset, and no number here should be copied into a submission. For the method behind it, read the companion guide on how to design a braced excavation with DPL and FREW.
Table of Contents
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Executive summary (example wording)
A summary should fit one page and let a reader skip the report. State the site, the three options, the numbers that decide between them and your recommendation. Here is a model paragraph written for the illustrative case.
“This report presents the preliminary design of temporary support for an 8 m deep, 12 m wide rectangular excavation in granular soil, with the water table varying across the site. Three options were assessed: a DPL design to CIRIA C517 with 10% penetration, a FREW model with AZ 32-750 sheet piles, and a FREW model with a 500 mm diaphragm wall, all with 273 mm × 10 mm CHS props at the DPL spacing. [State the governing design prop load, peak bending moment and maximum deflection for each option here, with units.] The sheet pile option is cheaper and quicker but deflects more; the diaphragm wall limits movement but costs more and needs a longer lead time. Dewatering by [method] is required to keep the formation dry, and it changes the effective stresses used in design. The recommended option is [option], because [two reasons tied to your results].”
The square brackets are deliberate. Your summary must carry your own computed values, so write it last.
DPL outline design to CIRIA C517
What the method is
CIRIA C517, Temporary propping of deep excavations: guidance on design (Twine and Roscoe, 1999), proposes an empirical method for estimating prop loads, built on field measurements of prop loads from real excavations. The distributed prop load (DPL) is a characteristic value, not a true earth pressure, and it is read for the soil class and wall type (flexible or stiff). Characteristic loads are then multiplied by a load factor of 1.35 to give design values (Katsigiannis et al., 2015, describing C517). Take the exact DPL expression for granular soil from your copy of C517, not from this page.
Prop levels and construction sequence (illustrative)
Wall penetration is set at 10% of total excavation depth, so 0.8 m here. Three prop levels are assumed for illustration.
| Stage | Activity | Level (m below ground) |
|---|---|---|
| 1 | Install wall, install waling and props at level 1 | Prop 1 at 1.0 |
| 2 | Dewater to below next dig level, excavate to 3.5 m | Dig to 3.5 |
| 3 | Install prop level 2 | Prop 2 at 3.5 |
| 4 | Excavate to 6.0 m, keep water below dig level | Dig to 6.0 |
| 5 | Install prop level 3 | Prop 3 at 6.0 |
| 6 | Excavate to formation, blind slab quickly | Formation at 8.0 |
Each dig stage should stop about 0.5 m below the next prop level so there is working room. Say so in your sequence, and explain why you chose the spacing.
Design actions: prop load table
The loads below use an illustrative DPL of 30 kN/m². That figure is an assumption for the worked layout only. It was not read from C517 and must be replaced with the value you derive for your dataset. Tributary heights are a simplified split between prop mid-points, with the base treated as the lower support. Check your own approach against the C517 procedure.
| Prop | Level (m) | Tributary height (m) | Characteristic load (kN/m run) | Design load ×1.35 (kN/m run) | Design load at 3 m spacing (kN per prop) |
|---|---|---|---|---|---|
| 1 | 1.0 | 2.25 | 67.5 | 91.1 | 273.4 |
| 2 | 3.5 | 2.5 | 75.0 | 101.3 | 303.8 |
| 3 | 6.0 | 3.25 | 97.5 | 131.6 | 394.9 |
The critical prop is the lowest one here. Compare it with the axial capacity of a 273 mm × 10 mm CHS over its 12 m span, including slenderness and any temperature or accidental effects. Take section properties from the Tata Steel Blue Book.
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FREW sheet pile analysis: how to present outputs
Oasys FREW is Oasys software for analysing flexible embedded retaining walls such as sheet pile and diaphragm walls, working through construction stages (Oasys, 2026). For AZ 32-750, ArcelorMittal lists a section modulus of 3200 cm³/m and a mass of 156 kg/m² of wall. Use the manufacturer’s published values for stiffness and moment capacity in your model. Report the results as a table, one row per stage or envelope, and keep raw output in an appendix.
| Result | Governing stage | Value (your FREW result) | Check against |
|---|---|---|---|
| Prop 1, 2, 3 loads (kN/m) | [stage] | [ ] | DPL design loads above |
| Maximum bending moment (kNm/m) | [stage] | [ ] | Moment resistance of AZ 32-750 with partial factors |
| Maximum wall deflection (mm) | [stage] | [ ] | Project movement limit you state |
| Penetration depth required (m) | Final dig | [ ] | 10% DPL penetration (0.8 m here) |
Do not leave the table to speak for itself. One paragraph should say which stage governs, why (usually the last dig before the final prop), and whether FREW loads sit above or below the DPL loads. Differences are normal, and the explanation is where the marks are.
FREW diaphragm wall and wall comparison
Repeat the same table for the 500 mm diaphragm wall, using the same props and spacing so the comparison is fair. Then compare the two walls directly.
| Aspect | AZ 32-750 sheet pile (flexible) | 500 mm diaphragm wall (stiff) |
|---|---|---|
| Deflection | Usually larger | Usually smaller |
| Bending moment | Often lower, as the wall moves and redistributes load | Often higher, as stiffness attracts moment |
| Prop loads | Compare your FREW values | Compare your FREW values |
| Water cut-off | Interlocks give partial cut-off | Good cut-off with sound panel joints |
| Programme and cost | Quick to install, can be extracted and reused | Slower, needs plant, usually permanent in the ground |
| Vibration and noise | Driving may be an issue near neighbours | Low vibration |
The first two rows describe typical trends, not guarantees. Your own results decide, so say plainly if they differ.
Dewatering approach and its effect on design
| Option | Suits | Main risks | Effect on design |
|---|---|---|---|
| Sumps and ditches | Shallow groundwater inflow, coarse soils with low drawdown | Loose sand boiling or piping at the base, instability of the excavation face | Weak control, so the wall must resist full water pressure |
| Wellpoints | Sands, drawdown of a few metres | Limited lift, so several stages may be needed | Lower water pressure behind and in front of the wall during each dig |
| Deep wells | Deeper drawdown, higher permeability | Settlement of nearby structures, pumping costs, discharge consent | Larger pressure reduction, but monitor settlement |
State your assumed groundwater level, the dig level it must be below and the pressure condition you modelled in FREW at each stage. Flag that a toe that is too short, or a dewatering failure, risks base heave or piping in sand.
Recommendation (example wording)
“On the evidence of the FREW analyses, the [flexible/stiff] wall is recommended. [It meets the stated movement limit with a maximum deflection of X mm / it exceeds the limit by Y mm and so is rejected.] Governing design prop load is X kN per prop at level 3, which the 273 mm × 10 mm CHS can carry with a utilisation of Y. The DPL method gave a value Z% higher/lower, which is explained by [reason]. The dewatering scheme must be in place before each dig stage.”
Notice the structure: a decision, evidence, a check against the empirical method and a condition. Four short sentences do the work of a page.
Please Write Fresh Non Plagiarized Assignment on this Topic
If you want help with the underlying topic, see our soil structure engineering assignment help page or the wider geotechnical engineering assignment help service. An earlier EG7004 DPL design question is also available for comparison.
About the expert
Eleanor Lawrence completed a BTech in Civil Engineering at the University of Oxford, as stated on her profile, and has solved assignments on fluid mechanics, soil mechanics, strength of materials, structural analysis and construction management.
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).
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