Retaining wall design principles.

Three pressure states

• At-rest (Ko): wall does not move. Pressure equals geostatic horizontal stress. Ko = 1 - sin(phi') for normally consolidated soil. Used for rigid walls (basement, propped, deeply embedded sheet pile).
• Active (Ka): wall moves AWAY from retained soil (typical retaining wall). Soil mobilizes minimum lateral pressure. Ka = (1 - sin phi) / (1 + sin phi) for level backfill, vertical wall, no friction (Rankine).
• Passive (Kp): wall is pushed INTO the soil. Soil mobilizes maximum resistance. Kp = (1 + sin phi) / (1 - sin phi). Used at the toe of cantilever walls and in anchor passive zones.

Coulomb vs Rankine

• Rankine: simpler, neglects wall friction, vertical wall, level or sloping backfill. Conservative for active pressure (over-estimates), less conservative for passive (under-estimates).
• Coulomb: accounts for wall friction (delta), wall inclination, sloping backfill. More accurate for real walls. Use for design where wall friction can be mobilized.
• Wall friction angle delta: typically 0.5 to 0.67 of soil phi. Use 2/3 phi for sand on concrete, 0.5 phi for cohesive backfill, 0 for design (Rankine simplification, conservative).

Tropical residual soil considerations

• Apparent cohesion (c') from cementation often present in residual soils
• Reduce or zero c' where soil saturates or weathers further over design life
• For ultimate limit state: use phi only (zero c) where saturation possible

Loads to consider

• Self weight (dead): wall mass, retained soil, water
• Surcharge: traffic loads (typical 10 kPa for highway, 5 kPa for residential), structures behind wall, embankment loads
• Hydrostatic: if drainage fails or cannot reduce water level (gamma_w = 9.81 kN/m^3)
• Seismic: pseudo-static (kh = 0.05 to 0.10g for Malaysia Zone 1-2)
• Construction loads: compaction equipment, temporary surcharges
• Earth pressure during compaction: often higher than long-term active pressure

Eurocode 7 design approaches

• DA1-1 (UK common): partial factor on actions only
• DA1-2: partial factor on resistance and material
• DA2 (German): partial factor on actions and resistance
• DA3 (slope stability default): partial factor on actions and material

Working Load Design (WLD) approach

Single global FoS applied to ratio of resisting/driving forces. Still dominant Malaysian practice. FoS targets per JKR Slope Engineering Manual.

Mechanism

Wall slides horizontally along its base. Resistance comes from base friction (delta_b on cohesionless foundation) plus base adhesion (cohesive foundation) plus passive earth pressure at the toe (if mobilized).

Design approach

• Base friction angle delta_b: typically 0.5 to 0.67 of foundation phi
• Base adhesion: typically 0.5 to 0.75 of foundation cohesion
• Passive resistance at toe: mobilize cautiously (significant deformation needed); often neglected in BS 8002 unless specifically detailed
• Inclined base or shear key: increases resistance significantly
• Concrete-on-soil: rough surface preferred (in-situ pour better than precast)

Overturning check

Wall rotates about the toe due to lateral earth pressure. Resistance is the moment of self-weight about the toe.

Eccentricity check (preferred modern approach)

Modern practice (BS 8002, Eurocode 7) prefers eccentricity check over overturning FoS. Eccentricity of resultant force at base must stay within "middle third":

Eccentricity exceeding B/6 means tension at the back of base, which is incompatible with most retaining wall foundations on soil.

Mechanism

Foundation soil punches through under the eccentrically loaded wall. Bearing failure can be local (under one edge) or general (whole base shears down). Critical for tall walls on weak foundation soils.

Design check

• Compute applied bearing pressure at base (eccentric loading): use effective width B' = B - 2e
• Compute ultimate bearing capacity per Terzaghi or Meyerhof using foundation soil parameters
• FoS = ultimate / applied

Bearing capacity equation (Terzaghi simplified)

Common pitfall

• Using full base width B instead of effective width B' (gives unconservative bearing pressure)
• Using foundation soil parameters from a single borehole (use cautious estimate from multiple boreholes)
• Not checking layered profiles (weak layer below stiff crust can fail with general shear)

Mechanism

The entire wall PLUS retained backfill PLUS some foundation soil mass slides as a unit on a deep slip surface that passes BEHIND or BELOW the wall. The wall doesn't fail - the whole system fails. Critical for tall walls on weak foundation soils, sloping ground, or near pre-existing slip surfaces.

Analysis method

• Slope stability software: Slope/W, Slide, PLAXIS LE
• Methods: Bishop, Janbu, Spencer, Morgenstern-Price (use rigorous method for non-circular surfaces)
• Model: include wall, reinforcement (if any), foundation soil, retained backfill, surcharge, groundwater
• Search for critical slip surface across all candidate geometries

FoS target

When global stability governs

• Tall walls (greater than 8 m) on weak foundation
• Walls on sloping ground (below or above a slope)
• Sites with pre-existing slip surface
• Soft alluvial foundations under embankment loads
• MSE walls with limited reinforcement length

Why drainage matters

Hydrostatic pressure behind a retaining wall can DOUBLE the lateral earth pressure if drainage fails. A 5 m wall with full hydrostatic head adds approximately 125 kN/m at the base (0.5 x gamma_w x H^2 = 0.5 x 10 x 25). This often overstresses RC cantilever wall stems and shears MSE wall reinforcement.

Drainage components

• Granular drainage layer at the back of wall (typically 300 to 500 mm thick of free-draining gravel)
• Filter geotextile between drainage layer and retained soil (prevents fines migration)
• Weep holes through wall stem (75 to 100 mm at 1 to 3 m horizontal x 1 to 2 m vertical spacing)
• Subsoil drain at toe (slotted PVC or HDPE pipe in granular envelope)
• Discharge collection at toe (lined ditch, manhole, outfall)

Drainage by wall type

• RC cantilever: all components above; weep holes critical
• MSE wall (concrete panel): chimney drain at back of reinforced fill, weep at toe panels
• MSE wall (block face): drainage layer + chimney drain; the open back face provides natural drainage path
• Crib wall: inherent drainage through cell fill; no separate system needed
• Gabion wall: inherent drainage through stone fill; provide filter behind
• Sheet pile: typically cohesive backfill with controlled drainage if designed for water retention

Settlement modes

• Foundation settlement: wall and backfill settle together as foundation consolidates. Larger walls and softer foundation = more settlement.
• Differential settlement: wall and backfill settle differently. Causes face cracking on rigid walls, joint opening on segmental walls.
• Wall-backfill settlement differential: backfill compacts under self-weight; wall does not. Creates downdrag on wall back, increasing apparent earth pressure.
• Lateral wall deflection: wall translates / rotates outward. MSE walls flex; RC walls are rigid (low deflection). Tieback walls: very low deflection.

Tolerable movement

Malaysian seismic context

Malaysia is in low-seismic zones (Zone 1 to 2 per Eurocode 8 / Malaysian National Annex). Pseudo-static analysis with horizontal seismic coefficient kh = 0.05 to 0.10g is typical for retaining wall design. Federal infrastructure may require site-specific seismic hazard analysis. Vertical coefficient kv usually neglected or taken as +/- 0.5 kh.

Mononobe-Okabe method

Modified Coulomb active pressure with pseudo-static body forces. Gives dynamic active pressure coefficient K_AE that is higher than static Ka.

Seismic FoS targets

Seismic-induced wall pressure increment

Seismic increment Delta P_AE = P_AE - P_a is applied at 0.6H above the base (higher than static line of action at H/3). This increases overturning moment.

Compaction zones

• Light compaction (within 1 m of face): avoid damaging wall facing or displacing precast units
• Heavy compaction (rest of fill): 95 to 98 percent modified Proctor
• Hand-tamped at face: for modular block, gabion, RE wall panel zones

Lift heights

• Granular fill: 200 to 300 mm loose, compacted to 150 to 250 mm
• Cohesive fill: 150 to 200 mm loose
• Match facing unit height where possible (e.g., 600 mm for some MSE panels)

Sequencing considerations

• Wall face placement first, then fill - never the reverse
• Reinforcement (geogrid, strip) placed on compacted lift, then next lift placed
• Drainage system installed concurrently with wall (filter, drain pipe, granular layer)
• Coping and final finish at top after fill complete and settled
• Surcharge loads NOT applied until wall has cured / consolidated