What Is a Temporary Earth Retaining System, and How Does It Differ from a Permanent Retaining Wall?
A temporary earth retaining system is a structure installed to support the soil around an excavation so it does not collapse during construction, such as sheet piles, a contiguous pile wall (a continuous row of bored piles) or a sloped excavation (cut slope). It is used only during construction; once the underground structure is complete and backfilled, the retaining system has done its job.
It differs from a permanent retaining wall in 2 respects: service life and design criteria.
- Factor of Safety: temporary works resist earth pressure only for a short period, so a lower F.S. than for permanent works can be used (see the F.S. criteria for permanent works in Retaining Wall Safety Factors Under Thai Law)
- Monitoring during excavation: monitoring of actual behavior during excavation must always go alongside, because the highest risk occurs while the excavation is being opened up.
What Was the Brief for This Project?
Design of a temporary earth retaining system for a 4.00 m deep excavation within a construction project in Khon Kaen Province — SPN Soil Engineering was responsible for the design, the stability checks and the movement-monitoring criteria during construction.
The brief had 2 requirements. First, the excavation lay within a project site where other construction was under way, so lateral ground movement had to be kept within safe limits throughout excavation. Second, a sump pit had to be opened inside the excavation to drain water during construction.
What Were the Site Soil Conditions, and Which Design Parameters Were Used?
The soil profile has 3 main layers: clayey sand, hard clay and sandstone. Data from the soil investigation were converted into design parameters for the Mohr-Coulomb model as follows:
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| Soil layer | Unit Weight (t/m³) | c′ / Su (t/m²) | φ′ (°) |
|---|---|---|---|
| Medium Dense Clayey Sand | 1.8 | 0 | 30 |
| Hard Clay | 2.0 | 10 | 0 |
| Sandstone | 2.4 | 15 | 33 |
The analysis includes a surface surcharge of 2.0 t/m² to allow for machinery and construction materials around the edge of the excavation. This step shows that thorough soil investigation (see the criteria for number of boreholes and depth) is the starting point of accurate earth retaining design.
Which Earth Retaining System Was Chosen?
The designer chose a combined system of 2 parts to suit the site: a bored pile wall on the sides where space is limited, and a cut slope on the sides with enough room.
- Contiguous Pile Wall: reinforced concrete bored piles, Dia 0.35 m, in a continuous row at 0.40 m spacing (gap between piles no more than 0.05 m), with a 0.65×0.60 m cap beam; used on sides where space is limited or close to existing structures
- Cut Slope 1:1: a 45° sloped excavation on sides with enough room, covered with 10 cm of lean concrete to protect the slope face from erosion
How Was Stability Analyzed with FEM?
The analysis used the finite element method (FEM) to model the actual construction sequence (staged construction) in 6 stages, to see ground movement and stability at each step of the excavation — not just the final condition.
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| Phase | Construction stage | Details |
|---|---|---|
| 1 | Install Bored Pile Wall | Install retaining bored piles along the excavation edge |
| 2 | Initial Inclinometer Reading | Install inclinometer casing and take initial baseline readings |
| 3 | 1st Excavation | Excavation stage 1 to −1.00 m, with monitoring |
| 4 | 2nd Excavation | Excavation stage 2 to −2.00 m, with monitoring |
| 5 | Final Excavation | Excavate to the base at −4.00 m; prepare to build the underground structure |
| 6 | Open Sump Pit | Open a sump pit inside the excavation to drain water during construction |
What Were the Factors of Safety (F.S.) at Each Stage?
F.S. ranges from 4.57 to 11.32 and decreases with excavation depth, because lateral earth pressure and stress relief increase. However, every stage remains above the minimum criterion for temporary works, F.S. ≥ 1.30, set by the designer. This depends on each project's specification.
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| Analysis Case | Excavation stage | Factor of Safety |
|---|---|---|
| FS 1 | 1st Excavation (−1.00 m) | 11.32 |
| FS 2 | 2nd Excavation (−2.00 m) | 7.51 |
| FS 3 | 3rd Excavation (−4.00 m) | 4.57 |
| FS 4 | Open Sump Pit / Final Stage | 4.57 |
How Stable Is the Cut Slope?
The cut slope is sufficiently stable throughout construction. On the side excavated to a 1:1 slope, slope stability was checked separately using the limit equilibrium method, giving a Factor of Safety = 5.092, well above the criterion for temporary works (see the calculation principles in Slope Stability and Factor of Safety)
How Were the Retaining Piles Reinforced?
The bending moments, shear forces and axial forces from the FEM were used to check the strength of the piles' reinforced concrete section, resulting in the following reinforcement details:
- Bored pile Dia 0.35 m: longitudinal bars 6-DB16 (SD40, fy = 4,000 ksc) + spiral ties RB9 @ 0.15 m, concrete cover 5 cm
- Cap Beam 0.65×0.60 m: longitudinal bars 4-DB20 top and bottom + stirrups RB9 @ 0.125 m
- Concrete strength: the structure uses fc′ = 280 ksc (section checked at a guaranteed strength of 240 ksc)
The section checks passed every criterion, with capacity-to-demand ratios Sn/U = 1.76, Mn/Mu = 1.90 and Pn/Pu = 4.89 (all must exceed 1.00).
Why Monitor with an Inclinometer During Excavation?
Because the FEM results are only a prediction — reality must be proven by measurement. This project therefore required installation of an inclinometer to measure lateral ground movement and compare it against 3 control levels (trigger levels) calculated as percentages of the design movement in each phase (see how the instrument works in What Is an Inclinometer, and other instruments used for this kind of work in 10 Types of Geotechnical Instruments)
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| Trigger Level | Phase 3 (−1 m) | Phase 4 (−2 m) | Phase 5 (−4 m) | Phase 6 (Sump Pit) |
|---|---|---|---|---|
| Alarm (70%) — notify the designer, keep watch | 1.3 mm | 2.4 mm | 4.1 mm | 4.0 mm |
| Alert (80%) — review procedures, increase monitoring frequency | 1.4 mm | 2.7 mm | 4.7 mm | 4.6 mm |
| Action (90%) — stop/slow work, define corrective measures | 1.6 mm | 3.1 mm | 5.3 mm | 5.1 mm |
| Maximum — maximum allowable value | 1.8 mm | 3.4 mm | 5.9 mm | 5.7 mm |
A trigger level system like this lets everyone know in advance what to do at which reading, without waiting for damage to occur before deciding. It is at the heart of modern risk management for deep excavations.
Conclusion: Lessons from Designing a Temporary Earth Retaining System
A complete temporary earth retaining design must cover both the design side and control during construction. This case study reflects the standard workflow: soil investigation → choose a retaining system to suit the site → FEM analysis following the construction sequence → structural strength check → set inclinometer monitoring criteria
Note that all figures in this article (pile size, reinforcement spacing, F.S. values and trigger levels) are design results specific to this project, based on the actual soil conditions on site, and cannot be applied directly to other projects. Every deep excavation must be designed afresh by an engineer based on soil investigation data for that site.
SPN Soil Engineering provides the full service, from soil investigation and geotechnical design (earth retaining systems, foundations, slope stability) to installing instruments and monitoring behavior during construction. With more than 20 years of experience, we work nationwide across Thailand. Consult us about your project for free.