Sheet Pile Retaining Walls in Subway Excavation

You are building a new subway line through a dense city center. The station excavation is 15 meters deep. The buildings on both sides are old and cannot move. The water table is high. You need a retaining wall that is strong, watertight, and can be installed without damaging adjacent structures.

Sheet pile retaining walls are widely used in subway excavation. They provide a continuous, interlocking barrier that holds back soil and water, allowing safe excavation. For deep subway stations, anchored walls with tiebacks or braced walls with internal struts are used. Press-in piling minimizes vibration for sensitive urban sites.

I have supplied sheet piles for subway projects across Southeast Asia and the Middle East. A metro project in Singapore used press-in sheet piles next to a historic mosque. A subway project in Dubai used braced sheet piles for a 15-meter deep station. Let me walk you through how sheet pile retaining walls¹ work in subway excavation.

---

Can sheet piles be used as a retaining wall?

Yes, sheet piles are commonly used as retaining walls for subway excavations, basements, and waterfront structures. They are one of the most common retaining wall systems¹ in construction.

Sheet piles are ideal for retaining walls because they are strong, watertight, and can be installed quickly. The interlocking design creates a continuous barrier that holds back soil and water. For subway excavations, sheet piles are driven around the perimeter before digging begins. As excavation proceeds, bracing or tiebacks are installed to support the wall.

Types of Retaining Walls for Subway Excavation

Let me explain the different types of retaining walls used in subway projects.

Sheet Pile Retaining Walls²

• Description: Interlocking steel sections driven into ground
• Advantages: Fast installation, watertight, reusable
• Depth range: Up to 20 m with bracing
• Best for: Urban sites, high water table

Secant Pile Walls³

• Description: Interlocking concrete piles drilled in place
• Advantages: Very stiff, no vibration during installation
• Disadvantages: Slower, more expensive
• Best for: Very sensitive sites

Slurry Walls⁴

• Description: Cast-in-place concrete walls
• Advantages: Very deep, very stiff
• Disadvantages: Expensive, slow
• Best for: Deepest excavations

Comparison for Subway Excavation

Feature	Sheet Piles	Secant Piles	Slurry Walls4
Installation speed	Fast	Moderate	Slow
Cost	Low to moderate	High	Very high
Water tightness	Good	Excellent	Excellent
Vibration	Low (vibratory)	None	None
Reusable	Yes	No	No

My Experience
For a subway project in Southeast Asia, we used sheet pile retaining walls for the station excavation. The piles were driven with press-in equipment to minimize vibration. The wall held the soil and water perfectly, and the station was built on schedule.

---

What is sheet piling¹ in excavation?

Sheet piling in excavation is the use of interlocking steel sheet piles to support the sides of an excavation, preventing soil collapse and water inflow.

Sheet piling in excavation involves driving steel sheet piles around the perimeter of the excavation before digging begins. The piles form a continuous wall that holds back the soil. As excavation proceeds, bracing (struts or tiebacks) is installed to support the wall. After the permanent structure is built, the piles may be extracted (temporary) or left in place (permanent).

Excavation Process with Sheet Piles

Let me walk you through the excavation process² for a subway station.

Step 1: Sheet Pile Installation

• Drive sheet piles around the station perimeter
• Use vibratory or press-in hammers
• Drive to depth below final excavation

Step 2: First Excavation

• Excavate to first brace level (typically 2-3 m depth)
• Install walers and struts or tiebacks

Step 3: Second Excavation

• Excavate to second brace level
• Install additional bracing

Step 4: Continue to Final Depth

• Repeat until final excavation depth is reached
• Final brace level may be at or near the bottom

Step 5: Station Construction

• Build the subway station (slab, walls, roof)
• Remove bracing as permanent structure gains strength

Step 6: Pile Extraction (if temporary)

• Extract sheet piles using vibratory hammer
• Reuse on another project

Excavation Depth and Bracing

Excavation Depth	Typical Bracing Levels
Up to 5 m	Cantilever (no bracing) or 1 level
5-10 m	1-2 levels
10-15 m	2-3 levels
15-20 m	3-4 levels

My Experience
For a 15 m deep subway station, we used four levels of bracing. The first level was at 2 m depth, the second at 6 m, the third at 10 m, and the fourth at 13 m. The excavation proceeded in stages, with bracing installed before digging deeper.

---

What are the disadvantages of using sheet piles?

Sheet piles are excellent for subway excavation, but they have limitations that engineers must consider.

The main disadvantages of sheet piles for subway excavation are noise and vibration during installation (mitigated by press-in piling), potential for ground movement if not installed carefully, difficulty driving through cobbles or rock, and the need for bracing that can obstruct the excavation. In urban areas, noise restrictions may limit working hours.

Addressing the Disadvantages

Let me discuss each disadvantage and how to mitigate it for subway projects.

Noise and Vibration¹

• Disadvantage: Pile driving is loud and creates vibration
• Mitigation: Use press-in (silent piling) for sensitive areas
• Mitigation: Use vibratory hammers with noise blankets
• Mitigation: Work during permitted hours only

Ground Movement²

• Disadvantage: Pile driving can displace soil, causing settlement
• Mitigation: Use press-in piling (minimal ground movement)
• Mitigation: Pre-drilling to reduce displacement
• Mitigation: Monitor adjacent structures during installation

Hard Driving Conditions³

• Disadvantage: Cobbles, boulders, or rock make driving difficult
• Mitigation: Pre-drilling through hard layers
• Mitigation: Use impact hammers for hard driving
• Mitigation: Consider alternative wall types if rock is near surface

Bracing Obstructions

• Disadvantage: Internal struts block access inside excavation
• Mitigation: Use tiebacks instead of internal struts
• Mitigation: Design struts to allow equipment access
• Mitigation: Use rakers (diagonal braces) that take less space

Corrosion⁴

• Disadvantage: Steel corrodes in groundwater
• Mitigation: Use corrosion allowance (extra thickness)
• Mitigation: Use marine grade steel for aggressive conditions
• Mitigation: Apply protective coatings

My Experience
For a subway project adjacent to a hospital, we used press-in piling to eliminate noise and vibration. The installation was silent, and the hospital experienced no disruption. The press-in method cost more, but it was essential for that site.

---

What is a pile retaining wall?

A pile retaining wall is a structure that holds back soil and water using vertical piles driven into the ground. Sheet piles are the most common type of pile retaining wall.

A pile retaining wall consists of vertical piles (steel sheet piles, concrete piles, or timber piles) that are driven into the ground to form a continuous or semi-continuous barrier. Sheet pile walls are the most common because the piles interlock to create a watertight barrier. Other types include soldier pile walls (piles with lagging between them) and secant pile walls (interlocking concrete piles).

Types of Pile Retaining Walls

Let me compare the different types of pile retaining walls used in subway excavation.

Sheet Pile Walls¹

• Piles: Interlocking steel sections
• Water tightness: Excellent (interlocks)
• Installation: Driven or pressed
• Best for: Urban sites, high water table
• Cost: Low to moderate

Soldier Pile Walls²

• Piles: Steel H-piles with timber or concrete lagging
• Water tightness: Poor (gaps between piles)
• Installation: Drilled or driven
• Best for: Dry sites, temporary excavations
• Cost: Low

Secant Pile Walls³

• Piles: Interlocking concrete piles
• Water tightness: Excellent (interlocking)
• Installation: Drilled
• Best for: Very deep excavations, sensitive sites
• Cost: High

Contiguous Pile Walls⁴

• Piles: Concrete piles with gaps between
• Water tightness: Poor
• Installation: Drilled
• Best for: Dry sites, temporary works
• Cost: Moderate

Comparison for Subway Excavation

Wall Type	Water Tightness	Vibration	Cost	Best Depth
Sheet pile	Good	Low (press-in)	Low-moderate	Up to 20 m
Soldier pile	Poor	Low	Low	Up to 10 m
Secant pile	Excellent	None	High	Over 20 m

My Experience
For most subway excavations, sheet pile walls are the best choice. They provide good water tightness, can be installed with low vibration using press-in methods, and are cost-effective. For the deepest stations (over 20 m) or extremely sensitive sites, secant piles may be used.

---

Conclusion

Sheet pile retaining walls¹ are ideal for subway excavation. They are strong, watertight, and can be installed with minimal vibration using press-in methods. For deep stations, anchored or braced walls² are used. Disadvantages include noise, vibration, and bracing obstructions, but these can be mitigated.

---