Sheet Pile Walls | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Introduction

Sheet pile walls are thin, interlocking wall elements driven into the ground to provide lateral support against soil or water. They are commonly used for quay walls, river training works, temporary excavations and retaining structures where space, speed of construction or economy favour prefabricated steel, concrete or composite sections. A sheet pile wall resists lateral earth pressure by a combination of cantilever action and passive resistance in the embedded portion.

Components and classification

• Crest - top edge of the sheet pile wall.
• Web - vertical plate resisting lateral pressures.
• Interlock - joint between sheet piles that transmits bending and shear.
• Toe - lowest edge of the driven sheet pile section.
• Types - cantilever (free-standing), anchored (tied-back), propped (strutted), and embedded sheet piles (fixed at depth by passive soil resistance).

Basic principles of design

• Determine lateral pressure distribution on the retained and excavated faces.
• Compute resultant active thrust above ground and passive resistance below ground.
• Locate the line of action of resultant forces and check equilibrium of forces and moments.
• Check structural strength (bending, shear) of the sheet pile and connections.
• Include appropriate factors of safety for bearing, sliding and passive resistance.

Sheet pile walls embedded in sands

Behaviour in cohesionless (sandy) soils

For cohesionless soil with unit weight γ and internal friction angle φ, the lateral earth pressure at a depth z under active or passive conditions is given by

p(z) = K · γ · z

where K is an earth pressure coefficient; use K_a for active pressure and K_p for passive pressure. The standard Rankine (or Coulomb) coefficients are

K_a = tan²(45° - φ/2)

K_p = tan²(45° + φ/2)

The resultant triangular force per unit length on a retained height H is

P_a = 1/2 · K_a · γ · H²

P_p (for an embedded depth d below ground) = 1/2 · K_p · γ · d²

The centre of pressure for a triangular distribution lies at one-third the height from the base of the triangle.

Design procedure (general method for cantilever embedded sheet pile)

Calculate the active resultant above ground and its line of action:

P_a = 1/2 · K_a · γ · H²

Distance of its resultant from the ground line depends on the assumed origin; for a triangular active diagram the centroid is at H/3 from the toe of that triangle.

Calculate the passive resultant for the embedded depth d and its line of action:

P_p = 1/2 · K_p · γ · d²

Locate the centroid of the passive triangle at d/3 from the toe of the passive triangle.

Determine d by equilibrium of moments (and forces) about a convenient point so that resisting moment of passive pressure equals overturning moment of active pressure and external loads. Iterative solution or graphical methods are commonly used.

Practical note on factor of safety

Designs normally apply a factor of safety to passive resistance because passive resistance is mobilised by movement and its full value may not be reliable. The passive force is often divided by a factor of safety (FS) or the active pressure is multiplied by FS depending on the code practice. The two sketches below illustrate the typical conceptual difference:

without factor of safety

with factor of safety

Sheet pile walls embedded in clays

Behaviour in cohesive soils (clays)

When cohesion c (or undrained shear strength c_u) is significant, lateral pressure distributions include an additional term due to cohesion. For cohesive-frictional soils the lateral total pressure at depth z may be expressed as

p_a(z) = K_a · γ · z - 2 · c · √(K_a)

p_p(z) = K_p · γ · z + 2 · c · √(K_p)

The resultant lateral forces over a depth H are

P_a = 1/2 · K_a · γ · H² - 2 · c · √(K_a) · H

P_p = 1/2 · K_p · γ · H² + 2 · c · √(K_p) · H

Special case: pure cohesive soil (φ = 0)

If the soil behaves as an ideal cohesive material with zero friction angle (φ = 0), then K_a = K_p = 1.

At depth z, the active and passive pressures become

p_a(z) = γ · z - 2 · c

p_p(z) = γ · z + 2 · c

The resultant active and passive forces over depth H are

P_a = 1/2 · γ · H² - 2 · c · H

P_p = 1/2 · γ · H² + 2 · c · H

Therefore the net difference (passive minus active) for equal depths is

P_p - P_a = 4 · c · H

In the input notation sometimes q = γ · H is used as a shorthand for the vertical stress at depth H; using that symbol, expressions involving q and c can be related.

Determination of embedment depth d in cohesive soils

The same equilibrium principle is used as for sands: compute P_a above ground and P_p below ground (to depth d). Include the cohesive terms in P_a and P_p. Apply moment equilibrium about a suitable point to determine the required embedment d. Reduce passive resistance by an appropriate factor of safety when checking design.

Worked illustrative calculations (summary form)

Example 1 (cohesionless sand): Compute the active resultant for a retained height H in sand.

P_a = 1/2 · K_a · γ · H²

The line of action is at H/3 from the base of the triangular distribution.

Example 2 (cohesive soil, φ = 0): Express the active and passive resultants for the same depth H.

P_a = 1/2 · γ · H² - 2 · c · H

P_p = 1/2 · γ · H² + 2 · c · H

Net resisting force = P_p - P_a = 4 · c · H

Checks and detailing

• Check bending moment and shear in the sheet pile section using the computed pressure diagram and selected section modulus.
• Check serviceability - permissible deflection and movement depending on significance of wall (permanent vs temporary).
• Provide corrosion protection and connection detailing for interlocks; consider construction sequence and driving criteria.
• Apply appropriate factors of safety for passive resistance, bearing capacity, and sliding according to code or good practice.

Applications and limitations

• Sheet piles are economical and fast for temporary excavations, river works and quay walls in moderately soft to medium soils.
• Very stiff or rock-like strata near the required embedment depth may make sheet piles impractical.
• Excessive cyclic loads or corrosive environments require special material selection or protective measures.

Summary

Design of sheet pile walls requires calculation of lateral pressure distributions (active and passive), determination of embedment depth by force and moment equilibrium, and checks for structural strength and serviceability. For cohesionless soils use the standard K_a, K_p relations; for cohesive soils include cohesive terms (±2c√K) in the pressure expressions. Always adopt suitable factors of safety, particularly when relying on passive resistance.