Retaining Wall Pressures
Retaining wall pressures govern the structural integrity of any raised koi pond. The lateral earth pressure exerted by the backfill — and the hydrostatic pressure from the pond water itself — create a complex loading condition that must be resolved through careful structural design. At-rest, active, and passive earth pressure conditions each present distinct demands on the wall, influenced by soil properties, wall movement, drainage, and the presence of water. This page distills the core geotechnical and structural principles behind retaining wall design for raised ponds, covering soil mechanics, drainage solutions, structural analysis, and practical construction considerations.
Proper design accounts for the weight of the wall, the lateral pressure of the soil, the water pressure from the pond, and the bearing capacity of the foundation. Failures typically manifest as overturning, sliding, or bearing capacity failure, each of which can be mitigated through appropriate geometry, reinforcement, and drainage. This guide is not a substitute for site-specific engineering, but it provides the conceptual framework needed to collaborate effectively with a structural engineer or to make informed decisions during the design process.
Test Your Retaining Wall Knowledge
Work through ten scenario-based questions covering lateral earth pressure, hydrostatic pressure, drainage, and structural design. Each answer includes the reasoning behind it.
Retaining Wall Pressures — Quick Facts
Most Asked Questions About Retaining Wall Pressures
On a project in the Pacific Northwest, a raised pond was constructed with a concrete retaining wall that was designed for the lateral earth pressure of the backfill, but the design did not account for the hydrostatic pressure that could build up behind the wall during heavy rain. The drainage system was undersized and became clogged, causing the water level behind the wall to rise to nearly the full height of the backfill. The resulting lateral pressure exceeded the wall’s design capacity, leading to significant cracking and a visible outward tilt of the wall.
Retrofitting the wall with a larger drainage system and installing additional reinforcement saved the structure, but the lesson was clear: drainage is not an afterthought; it is a critical component of the structural design. For any raised pond, the drainage system must be designed to handle the worst-case water level scenario, and it must be easily accessible for maintenance.
Lateral Earth Pressure Theories: Rankine and Coulomb
The lateral earth pressure exerted by soil on a retaining wall is governed by the soil’s properties and the wall’s movement. The two most widely used theories for calculating these pressures are Rankine’s and Coulomb’s theories. Rankine’s theory assumes a smooth, vertical wall and considers the soil as a homogeneous, isotropic material. Coulomb’s theory accounts for wall friction and allows for a sloped backfill surface and a battered wall face. Both theories provide the foundation for designing safe and economical retaining walls.
- Rankine’s Theory: Assumes the wall is smooth (no friction) and vertical. The active pressure coefficient (Kₐ) is calculated as Kₐ = tan² (45° – φ/2), where φ is the effective friction angle of the soil. The pressure distribution is triangular, with the resultant acting at one-third of the wall height.
- Coulomb’s Theory: Accounts for wall friction (δ) and the angle of the backfill (β). The active pressure coefficient (Kₐ) is more complex and depends on δ, β, and φ. Coulomb’s theory is more general and often yields a slightly smaller active pressure than Rankine’s for the same soil, due to the effect of wall friction.
- Application to Raised Ponds: For most raised ponds, the wall is vertical and the backfill is horizontal, making Rankine’s theory a reasonable starting point. However, if the wall is battered, the backfill is sloped, or wall friction is significant, Coulomb’s theory provides a more accurate representation of the pressure distribution.
It is important to note that both theories assume the soil is in a state of plastic equilibrium, meaning the soil has reached its limiting strength. For a properly designed retaining wall, the active pressure condition is the design case, as the wall will move slightly away from the soil to mobilize the active state. The passive pressure is typically only used for the soil in front of the wall (the toe side) to resist sliding.
Hydrostatic Pressure and Uplift Forces
Hydrostatic pressure is the force exerted by a fluid at rest. In the context of a raised pond, this pressure acts on the wall from the inside (the pond water) and, if drainage is inadequate, from the outside (groundwater). The hydrostatic pressure increases linearly with depth, as p = γ_w × h, where γ_w is the unit weight of water (62.4 pcf or 9.81 kN/m³). This pressure acts perpendicular to any surface in contact with the water. The uplift force on the base of the wall is a vertical force caused by hydrostatic pressure acting on the bottom of the footing. If the upward pressure exceeds the weight of the wall, the wall can be lifted or become unstable. To prevent this, the wall must be heavy enough to resist uplift, or the drainage system must be designed to relieve the hydrostatic pressure underneath the footing.
A retaining wall in a raised pond was designed with a reinforced concrete stem and a wide footing, but the contractor neglected to install a drainage system behind the wall. After the first winter, the wall showed signs of distress, with horizontal cracks near the base and a noticeable outward bow. The water level in the soil behind the wall had risen to within a foot of the top, creating a hydrostatic pressure that far exceeded the design’s assumptions.
The solution was to install a drainage system retroactively: a perforated pipe at the base of the wall, wrapped in a geotextile fabric and surrounded by gravel. The pipe was connected to a sump pump to remove the water. After the drainage system was operational, the wall’s movement stabilized, and no further cracking occurred. The lesson: drainage is not a luxury; it is a necessity for the long-term stability of any retaining wall.
Structural Design Checks: Overturning, Sliding, and Bearing Capacity
A properly designed retaining wall must satisfy three fundamental stability checks: overturning, sliding, and bearing capacity. Overturning is checked by ensuring that the moment caused by the lateral earth and water pressures is less than the resisting moment provided by the weight of the wall and the soil above the footing. The factor of safety against overturning (typically 1.5 to 2.0) is calculated as the ratio of the resisting moment to the overturning moment.
Sliding is prevented by the friction between the base of the wall and the foundation soil, as well as any passive resistance from the soil in front of the wall. The factor of safety against sliding (typically 1.5) is the ratio of the resisting horizontal force (friction + passive resistance) to the driving horizontal force. Bearing capacity is checked by calculating the pressure distribution under the footing and ensuring that the maximum pressure does not exceed the allowable bearing capacity of the soil. The pressure distribution is typically trapezoidal or triangular, with the maximum pressure at the toe. The factor of safety against bearing capacity failure is usually 2.5 to 3.0.
During the construction of a large raised pond, the concrete retaining wall was poured with a footing that was slightly narrower than specified. The contractor argued that the wall was “massive” and would not move. However, when the pond was filled and the backfill was placed, the wall began to tilt. The reduced footing width had decreased the resistance to overturning and bearing pressure, allowing the wall to rotate.
The wall was saved by installing tiebacks (soil nails) into the backfill and constructing a buttress wall at the toe. The retrofitting process was expensive and time-consuming, but it prevented a complete collapse. The lesson: every dimension on the structural drawings is there for a reason, and deviations should only be made with the approval of the design engineer.
In summary, the design of a retaining wall for a raised pond requires a comprehensive understanding of the soil mechanics, the water pressures, and the structural behavior of the wall. Each component — from the wall’s thickness and reinforcement to the drainage system and the footing’s geometry — plays a critical role in ensuring the structure’s longevity and safety.
Retaining Wall Pressures — Full Question Library
Review indexed engineering questions below.
Q1:
What does the effective friction angle (φ’) represent in soil mechanics?
Correct Answer: Option A
The effective friction angle (φ’) is a fundamental parameter that defines the shear strength of a granular soil under drained conditions, where the pore water pressure has dissipated.
Q2:
What is the primary difference between normally consolidated and overconsolidated clay in terms of earth pressure?
Correct Answer: Option B
Overconsolidated clays have a higher at-rest earth pressure coefficient (K₀) due to their stress history, which can significantly increase the lateral pressure on a retaining wall.
Q3:
Which soil property is most critical for determining the active earth pressure coefficient (Kₐ)?
Correct Answer: Option C
The active earth pressure coefficient (Kₐ) is directly a function of the effective friction angle (φ), as expressed in the Rankine equation: Kₐ = tan²(45° – φ/2).
Q4:
What is the effect of increasing the soil’s unit weight (γ) on the lateral earth pressure?
Correct Answer: Option A
The lateral earth pressure is directly proportional to the vertical stress, which is the product of the soil unit weight and the depth. Thus, a heavier soil results in higher lateral pressures.
Q5:
What does the term “effective stress” refer to in the context of soil mechanics?
Correct Answer: Option B
Effective stress is the stress that is transmitted through the soil skeleton and is responsible for shear strength and volume change. It is calculated as the total stress minus the pore water pressure.
Q6:
What is the typical range of the at-rest earth pressure coefficient (K₀) for loose sand?
Correct Answer: Option C
For loose sand, the at-rest coefficient (K₀) typically ranges from 0.4 to 0.5, reflecting a moderate lateral pressure compared to dense sand or overconsolidated clays.
Q7:
What is the effect of soil cohesion (c) on the active earth pressure?
Correct Answer: Option B
Cohesive soils exhibit tensile strength and can sustain a vertical cut; the active pressure is reduced by the cohesive strength, often requiring a tension crack consideration.
Q8:
What does the Mohr-Coulomb failure criterion describe?
Correct Answer: Option A
The Mohr-Coulomb criterion is a mathematical model that describes the shear strength of a soil in terms of effective stress and material constants (cohesion and friction angle).
Q9:
What is the primary cause of lateral earth pressure at rest?
Correct Answer: Option B
At-rest earth pressure is caused by the horizontal stresses that exist in the soil due to its own weight and geological history, without any wall movement.
Q10:
What is the typical relationship between the active, passive, and at-rest earth pressure coefficients (Kₐ, Kₚ, K₀)?
Correct Answer: Option B
For a given soil, the passive pressure coefficient (Kₚ) is the largest, the at-rest coefficient (K₀) is intermediate, and the active coefficient (Kₐ) is the smallest. This is the fundamental ranking of lateral earth pressures.
Q11:
What is the effect of soil particle size on the lateral earth pressure?
Correct Answer: Option A
Finer-grained soils, like silts and clays, tend to have higher at-rest pressures due to their lower permeability and higher water retention, which increases the effective unit weight.
Q12:
What is the significance of the “drained” condition in soil mechanics?
Correct Answer: Option B
A drained condition means that the pore water pressure has had time to dissipate, and the effective stresses are at their maximum, which is the condition assumed for long-term stability analysis.
Q13:
What is the typical value of the active earth pressure coefficient (Kₐ) for a soil with φ = 30°?
Correct Answer: Option A
Using Rankine’s theory, Kₐ = tan²(45° – φ/2). For φ = 30°, Kₐ = tan²(30°) = 0.333, which is a common value for many granular soils.
Q14:
What does the “coefficient of permeability” (k) measure in a soil?
Correct Answer: Option B
The coefficient of permeability (k) is a measure of a soil’s hydraulic conductivity, which determines how quickly water can flow through the pores of the soil.
Q15:
What is the primary effect of soil saturation on the lateral earth pressure?
Correct Answer: Option C
When a soil is saturated, the presence of water adds a hydrostatic pressure component to the lateral pressure, significantly increasing the total load on the wall.
Q16:
What is the “angle of repose” of a soil?
Correct Answer: Option B
The angle of repose is the maximum angle of a slope made of a granular material. It is often used as a rough approximation for the friction angle of the soil.
Q17:
What is the effect of groundwater level on the stability of a retaining wall?
Correct Answer: Option A
Groundwater reduces the effective stress in the soil, which reduces the soil’s shear strength, while simultaneously adding hydrostatic pressure to the wall.
Q18:
What is the typical value of the passive earth pressure coefficient (Kₚ) for a soil with φ = 30°?
Correct Answer: Option B
The passive pressure coefficient is the inverse of the active coefficient; Kₚ = tan²(45° + φ/2). For φ = 30°, Kₚ = tan²(60°) = 3.0.
Q19:
What is the “bearing capacity” of a soil?
Correct Answer: Option A
Bearing capacity is the ability of the foundation soil to support the loads imposed by the structure without experiencing shear failure or excessive settlement.
Q20:
What is the effect of soil compaction on lateral earth pressure?
Correct Answer: Option B
Compaction increases the soil’s unit weight and may induce higher lateral pressures due to the increased stiffness and horizontal stresses from the compaction process.
Q21:
What is the primary assumption of Rankine’s earth pressure theory?
Correct Answer: Option A
Rankine’s theory assumes a smooth, vertical wall and a homogeneous, isotropic soil mass. It is one of the simplest and most widely used theories for calculating lateral earth pressures.
Q22:
What is the primary difference between Rankine and Coulomb’s earth pressure theories?
Correct Answer: Option B
Coulomb’s theory accounts for wall friction (δ) and the angle of the backfill (β) and assumes a planar failure surface, making it more versatile than Rankine’s theory for sloping backfills and rough walls.
Q23:
What is the shape of the failure surface assumed in Rankine’s theory for a vertical wall?
Correct Answer: Option C
Rankine’s theory assumes a planar failure surface that extends from the base of the wall to the ground surface, forming a wedge of soil that is in a state of plastic equilibrium.
Q24:
What is the effect of wall friction on the active earth pressure according to Coulomb’s theory?
Correct Answer: Option B
Wall friction (δ) acts upward along the wall, reducing the vertical stress on the wall and thus decreasing the active lateral pressure.
Q25:
What is the “critical failure surface” in the context of Coulomb’s earth pressure theory?
Correct Answer: Option A
The critical failure surface is the one that maximizes the lateral pressure on the wall, which is determined by taking the derivative of the pressure function with respect to the failure angle.
Q26:
What does the coefficient of earth pressure at rest (K₀) represent?
Correct Answer: Option C
K₀ is the ratio of the effective horizontal stress to the effective vertical stress when the soil is in a state of rest (no lateral strain).
Q27:
How does the angle of the backfill (β) affect the active earth pressure coefficient (Kₐ) in Coulomb’s theory?
Correct Answer: Option B
A steeper backfill slope (β) increases the lateral pressure on the wall because the weight of the soil wedge is larger, leading to a higher Kₐ.
Q28:
What is the effect of soil cohesion (c) on the active earth pressure coefficient in Rankine’s theory?
Correct Answer: Option A
Cohesion provides tensile strength to the soil, which can reduce the active pressure. In the case of a vertical cut, it can create a tension crack that modifies the pressure distribution.
Q29:
What is the “passive state” in soil mechanics?
Correct Answer: Option B
The passive state occurs when the wall is pushed into the soil, compressing it and increasing the horizontal stress to its maximum value.
Q30:
What is the effect of wall movement on the lateral earth pressure?
Correct Answer: Option A
If the wall moves away from the soil, the horizontal stress decreases from the at-rest condition to the active condition, which is the minimum pressure.
Q31:
What is the typical shape of the active pressure distribution on a retaining wall?
Correct Answer: Option B
For a homogeneous soil, the active pressure is zero at the ground surface and increases linearly with depth, forming a triangular pressure diagram.
Q32:
What is the effect of a surcharge load on the active earth pressure?
Correct Answer: Option A
A surcharge load (e.g., from equipment or soil) increases the vertical stress, which translates into an additional horizontal pressure (Kₐ × q) that is uniform with depth.
Q33:
What is the “Rankine active state” characterized by?
Correct Answer: Option B
The active state is the condition where the horizontal stress has been reduced to its minimum value, and the soil is on the verge of failure.
Q34:
What is the primary limitation of Rankine’s earth pressure theory?
Correct Answer: Option C
Rankine’s theory assumes a smooth, vertical wall, which is not always realistic. In practice, wall friction and batter can significantly affect the pressure distribution.
Q35:
What does the “coefficient of active earth pressure” (Kₐ) depend on?
Correct Answer: Option A
The active pressure coefficient (Kₐ) is solely a function of the soil’s effective friction angle (φ) for a smooth, vertical wall in Rankine’s theory.
Q36:
What is the effect of a sloped backfill on the distribution of active pressure?
Correct Answer: Option B
When the backfill is sloped, the pressure distribution becomes more complex and is not strictly triangular, requiring the use of Coulomb’s theory or graphical methods.
Q37:
What is the “point of application” of the resultant active earth pressure?
Correct Answer: Option A
For a triangular pressure distribution, the resultant force acts at the centroid of the triangle, which is one-third of the height above the base of the wall.
Q38:
What is the effect of wall friction on the passive earth pressure in Coulomb’s theory?
Correct Answer: Option B
Wall friction acts downward on the soil wedge during passive failure, increasing the vertical stress and thus the passive resistance.
Q39:
What is the primary use of the passive earth pressure in retaining wall design?
Correct Answer: Option A
The passive pressure developed in front of the wall (on the toe side) provides additional horizontal resistance to prevent the wall from sliding.
Q40:
What is the relationship between the active and passive earth pressure coefficients?
Correct Answer: Option C
For a smooth, vertical wall, the passive coefficient is the reciprocal of the active coefficient, i.e., Kₚ = 1 / Kₐ, reflecting the maximum and minimum horizontal stresses.
Q41:
What is the hydrostatic pressure at a depth of 10 feet below the water surface?
Correct Answer: Option A
Hydrostatic pressure is calculated as p = γ_w × h, where γ_w = 62.4 pcf. At 10 feet, p = 62.4 × 10 = 624 psf.
Q42:
What is the effect of water on the lateral earth pressure on a retaining wall?
Correct Answer: Option B
When the backfill is saturated, the total lateral pressure is the sum of the effective earth pressure and the hydrostatic pressure, which can be a significant increase.
Q43:
What is the “uplift pressure” under a retaining wall footing?
Correct Answer: Option C
Uplift pressure is the upward force exerted by groundwater on the base of the footing, which can reduce the effective weight of the wall and lead to instability.
Q44:
How does a drainage system reduce the lateral pressure on a retaining wall?
Correct Answer: Option A
A properly designed drainage system prevents the build-up of hydrostatic pressure behind the wall, which is often the largest component of the lateral load.
Q45:
What is the typical unit weight of water used in hydrostatic pressure calculations?
Correct Answer: Option B
The unit weight of fresh water is 62.4 pounds per cubic foot (pcf), which is a constant value used in all hydrostatic pressure calculations.
Q46:
What is the effect of a high water table on the stability of a retaining wall?
Correct Answer: Option C
A high water table adds hydrostatic pressure to the wall and reduces the effective weight of the wall, increasing the overturning moment and decreasing the bearing capacity.
Q47:
What is the purpose of a “weep hole” in a retaining wall?
Correct Answer: Option B
Weep holes are small openings at the base of the wall that allow water to drain from the backfill, relieving hydrostatic pressure.
Q48:
What is the resultant force of hydrostatic pressure on a vertical wall?
Correct Answer: Option A
The hydrostatic pressure distribution on a vertical wall is triangular, with the resultant force acting at one-third of the height above the base.
Q49:
What is the effect of a “drainage layer” on the hydrostatic pressure?
Correct Answer: Option B
A drainage layer (e.g., gravel or a drainage mat) provides a path for water to flow to the weep holes, preventing the build-up of hydrostatic pressure.
Q50:
What is the “hydrostatic uplift force” on a submerged footing?
Correct Answer: Option C
The uplift force is the hydrostatic pressure acting on the base of the footing, which reduces the effective weight of the structure and must be resisted by the wall’s weight.
Q51:
What is the typical factor of safety against uplift for a retaining wall?
Correct Answer: Option B
The factor of safety against uplift is typically 1.2 to 1.5, ensuring that the weight of the wall and any soil above the footing is sufficient to resist the uplift force.
Q52:
What is the effect of a drainage system on the uplift pressure under the footing?
Correct Answer: Option A
By lowering the water table or preventing the build-up of water, a drainage system reduces the hydrostatic uplift pressure on the footing.
Q53:
What is the “hydrostatic paradox”?
Correct Answer: Option B
The hydrostatic paradox states that the pressure at a given depth is independent of the shape or size of the container, depending only on the depth and fluid density.
Q54:
How does the presence of water affect the effective stress in a soil?
Correct Answer: Option C
Water in the pores of the soil carries part of the total stress, reducing the effective stress and thus the soil’s shear strength.
Q55:
What is the purpose of a “geotextile fabric” in a drainage system?
Correct Answer: Option B
A geotextile fabric acts as a filter, allowing water to pass through while retaining the fine soil particles, preventing the drainage system from becoming clogged.
Q56:
What is the effect of a “drainage mat” on the lateral pressure?
Correct Answer: Option A
A drainage mat provides a continuous drainage path behind the wall, effectively eliminating hydrostatic pressure and reducing the total lateral load.
Q57:
What is the typical pressure exerted by water on a vertical wall at a depth of 5 feet?
Correct Answer: Option C
Using p = γ_w × h, at 5 feet, p = 62.4 × 5 = 312 psf.
Q58:
What is the effect of a “subsurface drain” on a retaining wall?
Correct Answer: Option A
A subsurface drain, such as a perforated pipe, intercepts groundwater flow, preventing it from building up pressure behind the wall.
Q59:
What is the “hydrostatic pressure diagram” for a wall with a water table at the surface?
Correct Answer: Option B
When the water table is at the surface, the hydrostatic pressure is zero at the surface and increases linearly to its maximum at the base, forming a triangular diagram.
Q60:
What is the effect of a “cutoff wall” on the seepage path and pressure?
Correct Answer: Option A
A cutoff wall extends into the soil, increasing the length of the seepage path and reducing the hydraulic gradient, which reduces uplift and seepage forces.
Q61:
What is the primary cause of overturning failure in a retaining wall?
Correct Answer: Option A
Overturning occurs when the moment caused by the lateral earth and water pressures exceeds the moment resisting the rotation, causing the wall to tilt outward.
Q62:
What is the typical factor of safety against overturning for a retaining wall?
Correct Answer: Option B
A factor of safety of 1.5 to 2.0 against overturning is common practice, ensuring that the wall has adequate resistance to lateral forces.
Q63:
What is the primary cause of sliding failure in a retaining wall?
Correct Answer: Option C
Sliding occurs when the horizontal force from the lateral pressure exceeds the frictional resistance between the base of the wall and the foundation soil.
Q64:
What is the effect of a “key” or “shear key” on the sliding resistance of a retaining wall?
Correct Answer: Option A
A shear key is a protrusion at the base of the wall that extends into the soil, creating a passive resistance that increases the wall’s resistance to sliding.
Q65:
What is the primary cause of bearing capacity failure in a retaining wall?
Correct Answer: Option B
Bearing capacity failure occurs when the maximum pressure under the footing exceeds the soil’s ability to support it, causing excessive settlement or a shear failure.
Q66:
What is the typical factor of safety against bearing capacity failure?
Correct Answer: Option C
A factor of safety of 2.5 to 3.0 against bearing capacity failure is typically required to account for uncertainties in soil properties and loading conditions.
Q67:
What is the effect of increasing the width of the footing on the stability of a retaining wall?
Correct Answer: Option A
A wider footing increases the resisting moment and reduces the bearing pressure, improving both overturning and bearing capacity stability.
Q68:
What is the effect of a “buttress” or “counterfort” on a retaining wall?
Correct Answer: Option B
Buttresses or counterforts are vertical or inclined structural members that tie the stem to the footing, reducing the bending moment in the stem and allowing for a thinner wall.
Q69:
What is the primary purpose of reinforcement in a concrete retaining wall?
Correct Answer: Option B
Concrete is strong in compression but weak in tension. Reinforcement (rebar) is placed in the tensile zone of the wall to resist the bending moments caused by lateral pressure.
Q70:
What is the “moment arm” in the context of overturning stability?
Correct Answer: Option A
The moment arm is the perpendicular distance from the point of rotation (the toe) to the line of action of the force, used to calculate the resisting and overturning moments.
Q71:
What is the effect of a “drainage system” on the structural design of a retaining wall?
Correct Answer: Option B
By reducing the hydrostatic pressure, a drainage system significantly reduces the lateral load, allowing for a more economical structural design.
Q72:
What is the “center of pressure” for a triangular pressure distribution?
Correct Answer: Option C
The center of pressure for a triangular load is located at one-third of the height from the base, which is the point of application of the resultant force.
Q73:
What is the purpose of a “toe” in a retaining wall footing?
Correct Answer: Option A
The toe is the portion of the footing that extends in front of the wall, increasing the bearing area and improving stability against overturning and bearing failure.
Q74:
What is the effect of a “heel” in a retaining wall footing?
Correct Answer: Option B
The heel is the portion of the footing that extends behind the wall, providing additional weight (from the soil above it) that increases the resisting moment.
Q75:
What is the primary load combination for designing a retaining wall?
Correct Answer: Option C
A retaining wall is designed for a combination of vertical loads (dead and live) and horizontal loads (earth and hydrostatic pressures) to ensure stability.
Q76:
What is the “Euler buckling” check for a retaining wall?
Correct Answer: Option A
Euler buckling is a stability check for slender walls under axial compression, ensuring that the wall does not buckle under its own weight and vertical loads.
Q77:
What is the effect of “temperature and shrinkage” reinforcement in a retaining wall?
Correct Answer: Option B
Temperature and shrinkage reinforcement is placed in the concrete to control cracking caused by thermal expansion and contraction and drying shrinkage.
Q78:
What is the purpose of a “construction joint” in a retaining wall?
Correct Answer: Option C
Construction joints are placed at predetermined locations to allow the concrete to be poured in stages, with a rough surface provided for bonding between pours.
Q79:
What is the primary purpose of a “coping” or “cap” on a retaining wall?
Correct Answer: Option A
A coping is a protective cap that sheds water away from the wall, preventing water from entering the wall and causing damage, especially in freeze-thaw climates.
Q80:
What is the effect of “soil reinforcement” (e.g., geogrids) on the lateral pressure?
Correct Answer: Option B
Geogrids and other soil reinforcements increase the shear strength of the backfill, effectively reducing the lateral pressure acting on the wall.
Q81:
What is the primary purpose of a drainage system in a retaining wall?
Correct Answer: Option A
The primary function of drainage is to prevent the build-up of water pressure behind the wall, which is a major cause of wall failure.
Q82:
What is a “weep hole” and where is it typically placed?
Correct Answer: Option B
Weep holes are small-diameter pipes or openings located at the base of the wall that allow water to escape from the backfill, relieving hydrostatic pressure.
Q83:
What is the purpose of a “drainage layer” or “filter fabric” in a retaining wall?
Correct Answer: Option B
Filter fabric allows water to pass through while retaining the soil particles, preventing the drainage system from becoming clogged with fines.
Q84:
How does a “perforated pipe” function in a retaining wall drainage system?
Correct Answer: Option A
A perforated pipe is placed at the base of the backfill, and the holes allow water to enter the pipe and be conveyed away, effectively lowering the water table.
Q85:
What is the effect of a “French drain” on the stability of a retaining wall?
Correct Answer: Option B
A French drain is a trench filled with gravel and a perforated pipe that intercepts groundwater and directs it away from the wall, reducing hydrostatic pressure.
Q86:
What is the primary cause of failure in an improperly drained retaining wall?
Correct Answer: Option C
Without proper drainage, hydrostatic pressure builds up behind the wall, often exceeding the design load and causing overturning or structural failure.
Q87:
What is the recommended spacing for weep holes in a retaining wall?
Correct Answer: Option A
Weep holes are typically spaced 4 to 8 feet apart horizontally, and they are placed at the base of the wall to ensure adequate drainage.
Q88:
What is the purpose of a “drainage aggregate” (gravel) behind a retaining wall?
Correct Answer: Option B
Gravel or crushed stone placed behind the wall acts as a drainage layer, allowing water to flow quickly to the drainage system and preventing hydrostatic pressure.
Q89:
How does a “drainage system” affect the lateral earth pressure?
Correct Answer: Option C
By removing water, a drainage system ensures that only the effective earth pressure acts on the wall, significantly reducing the total lateral load.
Q90:
What is the effect of a “clogged” weep hole on a retaining wall?
Correct Answer: Option A
A clogged weep hole prevents water from draining, leading to a build-up of hydrostatic pressure that can overload the wall and cause failure.
Q91:
What is the purpose of a “cutoff wall” in a retaining wall drainage system?
Correct Answer: Option B
A cutoff wall extends below the base of the wall to block the flow of groundwater, preventing it from building up pressure behind the wall.
Q92:
What is the typical diameter of a weep hole pipe?
Correct Answer: Option C
Weep hole pipes are commonly 2 to 4 inches in diameter, providing a sufficient opening for water to drain without being easily clogged.
Q93:
What is the effect of a “drainage system” on the factor of safety against overturning?
Correct Answer: Option A
By reducing the hydrostatic pressure, the total lateral load is reduced, which increases the factor of safety against overturning.
Q94:
What is the purpose of a “drainage swale” or “ditch” at the top of a retaining wall?
Correct Answer: Option B
A swale or ditch at the top of the wall intercepts surface runoff, preventing it from flowing down the face of the wall and potentially saturating the backfill.
Q95:
What is the primary benefit of a “geocomposite drainage sheet”?
Correct Answer: Option C
Geocomposite drainage sheets (drainage mats) consist of a core and a filter fabric, providing a reliable and efficient drainage path behind the wall.
Q96:
How does a “drainage system” affect the soil’s effective stress?
Correct Answer: Option A
By removing water from the soil, the drainage system reduces the pore water pressure, which increases the effective stress and the soil’s shear strength.
Q97:
What is the effect of a “drainage system” on the wall’s bearing capacity?
Correct Answer: Option B
By reducing the hydrostatic uplift pressure on the footing, the drainage system increases the effective weight of the wall and improves the bearing capacity.
Q98:
What is the primary purpose of a “weep hole cover” or “screen”?
Correct Answer: Option C
A cover or screen prevents debris, insects, and small animals from entering and clogging the weep hole, ensuring it remains functional.
Q99:
What is the effect of “groundwater recharge” on a retaining wall?
Correct Answer: Option A
Groundwater recharge (e.g., from heavy rain or melting snow) can raise the water table, increasing the hydrostatic pressure behind the wall and reducing stability.
Q100:
What is the purpose of a “drainage channel” at the base of a retaining wall?
Correct Answer: Option B
A drainage channel collects water from the weep holes and carries it away, preventing it from pooling at the base of the wall and saturating the foundation soil.
Q101:
What is a “surcharge load” in the context of a retaining wall?
Correct Answer: Option A
A surcharge load is any additional load (e.g., from equipment, soil, or traffic) that is applied to the ground surface above the backfill, increasing the lateral pressure on the wall.
Q102:
How does a surcharge load affect the lateral earth pressure on a retaining wall?
Correct Answer: Option B
A surcharge load (q) creates an additional horizontal pressure of Kₐ × q, which is uniform with depth and adds to the active earth pressure.
Q103:
What is the typical surcharge load from a concrete slab or pavement?
Correct Answer: Option C
A typical concrete pavement or slab can impose a surcharge of 150 to 300 psf, which must be considered in the wall’s design.
Q104:
What is the effect of a “live load” (e.g., traffic) on a retaining wall?
Correct Answer: Option A
Live loads, such as traffic or parked vehicles, act as surcharges and increase the lateral pressure on the wall, especially near the surface.
Q105:
What is the “equivalent height of soil” concept for surcharge loads?
Correct Answer: Option B
The equivalent height of soil (hₛ) is calculated as q / γ, where q is the surcharge pressure and γ is the soil unit weight, and it represents the additional soil height that causes the same pressure.
Q106:
What is the effect of a “point load” on the lateral pressure distribution?
Correct Answer: Option C
A point load (e.g., from a column) creates a localized stress increase in the soil, which translates into a localized increase in lateral pressure on the wall.
Q107:
What is the effect of a “line load” (e.g., from a wall footing) on the lateral pressure?
Correct Answer: Option A
A line load, such as a footing parallel to the wall, transmits a stress into the soil that results in a lateral pressure on the wall that is maximum near the surface.
Q108:
What is the “Boussinesq method” used for in retaining wall design?
Correct Answer: Option B
The Boussinesq method provides a solution for the stress distribution in a soil mass due to a point load on the surface, which can be used to determine the lateral pressure on a wall.
Q109:
What is the effect of a “seismic load” on a retaining wall?
Correct Answer: Option C
Seismic loads are evaluated using the pseudostatic method, where a horizontal acceleration is applied to the soil wedge, adding to the lateral pressure on the wall.
Q110:
What is the purpose of a “surcharge reduction factor” in design?
Correct Answer: Option A
In many design codes, a surcharge reduction factor is used to account for the likelihood that the surcharge load is not always present, allowing for a more economical design.
Q111:
What is the effect of a “surcharge load” on the factor of safety against sliding?
Correct Answer: Option B
A surcharge load adds to the driving horizontal force (Kₐ × q), which can reduce the factor of safety against sliding unless the wall’s weight is also increased.
Q112:
What is the typical surcharge load from a soil stockpile?
Correct Answer: Option C
A soil stockpile can impose a significant surcharge, often exceeding 300 psf, depending on its height and density, and must be considered in the wall design.
Q113:
What is the effect of a “surcharge load” on the bearing pressure under the footing?
Correct Answer: Option A
The surcharge load contributes to the vertical load applied to the footing, increasing the bearing pressure and potentially affecting the factor of safety.
Q114:
What is the purpose of a “surcharge barrier” or “berm”?
Correct Answer: Option B
A berm or barrier can be placed at the top of the wall to prevent loads from being placed near the wall, effectively eliminating a potential surcharge.
Q115:
What is the effect of a “surcharge load” on the wall’s reinforcement?
Correct Answer: Option C
The additional lateral pressure from a surcharge increases the bending moment in the wall, requiring additional reinforcement to resist the tensile stresses.
Q116:
What is the “influence line” method used for in retaining wall design?
Correct Answer: Option A
The influence line method is used to find the critical position of a moving load (such as a vehicle) that produces the maximum lateral pressure on the wall.
Q117:
What is the effect of a “surcharge load” on the wall’s overturning moment?
Correct Answer: Option B
The additional lateral pressure from a surcharge acts to increase the overturning moment, which must be resisted by the wall’s weight and geometry.
Q118:
What is the purpose of a “surcharge load” in the design of a retaining wall?
Correct Answer: Option C
Surcharge loads are a critical part of the design to ensure the wall can safely support all anticipated loads, including those from future construction or usage.
Q119:
What is the effect of a “surcharge load” on the soil’s passive resistance?
Correct Answer: Option A
A surcharge load increases the vertical stress, which in turn increases the passive resistance of the soil, which is a beneficial effect for sliding stability.
Q120:
What is the typical surcharge load from a construction vehicle (e.g., a backhoe)?
Correct Answer: Option B
Construction equipment can impose very high surcharge loads, often in the range of 500 to 1000 psf, which must be considered during construction and in the design.
Q121:
What is “soil-structure interaction” (SSI) in the context of retaining walls?
Correct Answer: Option A
SSI refers to the complex interaction where the soil supports the wall and the wall transmits loads to the soil, influencing the stress distribution in both.
Q122:
What is the effect of wall stiffness on the lateral earth pressure distribution?
Correct Answer: Option B
A flexible wall can move more, allowing the soil to reach the active state (minimum pressure), while a very stiff wall may remain at or near the at-rest state.
Q123:
What is the “arching effect” in soil behind a retaining wall?
Correct Answer: Option C
Arching is the phenomenon where stresses are redirected around less stiff zones, which can reduce the lateral pressure on a flexible wall compared to the theoretical active pressure.
Q124:
What is the effect of wall roughness on the shear stress at the soil-wall interface?
Correct Answer: Option A
The roughness of the wall interface influences the shear stress (friction) developed between the soil and the wall, which is an important parameter in Coulomb’s theory.
Q125:
What is the “active state” in the context of soil-structure interaction?
Correct Answer: Option B
The active state is reached when the wall moves sufficiently away from the soil, allowing the soil to expand and reach its minimum lateral pressure.
Q126:
What is the “passive state” in the context of soil-structure interaction?
Correct Answer: Option C
The passive state is reached when the wall is pushed into the soil, compressing it and increasing the lateral pressure to its maximum value.
Q127:
What is the effect of “soil settlement” on a retaining wall?
Correct Answer: Option A
Differential settlement of the backfill can create additional lateral pressures or drag forces on the wall, potentially overloading the wall.
Q128:
What is the purpose of a “geotechnical investigation” for a retaining wall?
Correct Answer: Option B
A geotechnical investigation provides the critical soil parameters (e.g., friction angle, unit weight, bearing capacity) required for a safe and economical wall design.
Q129:
What is the effect of “soil compaction” on the lateral pressure on a retaining wall?
Correct Answer: Option C
Compaction of the backfill introduces horizontal stresses that can exceed the at-rest pressure, increasing the load on the wall during and shortly after construction.
Q130:
What is the “coefficient of lateral earth pressure” (K) a function of?
Correct Answer: Option A
The coefficient K is primarily a function of the soil’s effective friction angle and the state of wall movement (at-rest, active, or passive).
Q131:
What is the effect of “wall deflection” on the active earth pressure?
Correct Answer: Option B
For the active pressure to be fully mobilized, the wall must deflect (move away from the soil) by a sufficient amount, typically about 0.1% to 0.5% of the wall height.
Q132:
What is the effect of “soil creep” on a retaining wall?
Correct Answer: Option C
Soil creep is a time-dependent deformation that can occur in certain soils, potentially leading to an increase in lateral pressure on the wall over time.
Q133:
What is the purpose of a “tieback” or “anchor” in a retaining wall?
Correct Answer: Option A
Tiebacks are tensioned anchors that extend into the soil behind the wall, providing an external stabilizing force to resist lateral loads.
Q134:
What is the effect of “soil variability” on the design of a retaining wall?
Correct Answer: Option B
Soil properties can vary significantly across a site, necessitating the use of conservative soil parameters and higher factors of safety to account for uncertainty.
Q135:
What is the “failure plane” in the context of a retaining wall?
Correct Answer: Option C
The failure plane is the critical surface within the soil mass along which the soil wedge tends to slide, causing the wall to fail.
Q136:
What is the effect of “seismic loading” on soil-structure interaction?
Correct Answer: Option A
Seismic loads add significant dynamic lateral forces and can cause liquefaction in loose, saturated soils, which can lead to catastrophic wall failure.
Q137:
What is the purpose of a “geogrid” in a retaining wall?
Correct Answer: Option B
Geogrids are used to reinforce the backfill, creating a composite soil mass that can support larger loads and reduce the lateral pressure on the wall.
Q138:
What is the effect of “soil consolidation” on a retaining wall?
Correct Answer: Option C
In clay soils, consolidation (the gradual expulsion of water) can cause long-term settlement and an increase in effective stress, which may increase the lateral pressure on the wall.
Q139:
What is the effect of “frost heave” on a retaining wall?
Correct Answer: Option A
In cold climates, frost heave of the backfill can apply large uplift and lateral forces to the wall, requiring special design considerations and drainage.
Q140:
What is the purpose of a “soil nail” in a retaining wall system?
Correct Answer: Option B
Soil nailing is a technique where passive reinforcement bars are installed into the ground, creating a reinforced soil mass that acts as a retaining structure.
Q141:
What is the typical concrete strength (f’c) used for a retaining wall?
Correct Answer: Option A
Retaining walls are typically constructed with concrete with a compressive strength (f’c) of 3,000 to 4,000 psi, which is adequate for the stresses involved.
Q142:
What is the typical yield strength (fy) of reinforcing steel (rebar) used in a retaining wall?
Correct Answer: Option B
The most common reinforcing steel used in retaining walls has a yield strength of 60,000 psi (Grade 60), which is the standard for structural concrete.
Q143:
What is the purpose of a “construction joint” in a retaining wall?
Correct Answer: Option C
Construction joints are necessary when the concrete pour cannot be completed in one continuous operation, allowing for a controlled bond between the pours.
Q144:
What is the minimum cover for reinforcing steel in a retaining wall?
Correct Answer: Option A
For concrete cast against soil, the minimum clear cover for reinforcement is typically 2 inches to protect the steel from corrosion.
Q145:
What is the purpose of a “backfill” material in a retaining wall construction?
Correct Answer: Option B
The backfill is the soil or granular material placed behind the wall, which is the source of the lateral pressure and must have adequate drainage properties.
Q146:
What is the recommended material for drainage backfill (filter material) behind a retaining wall?
Correct Answer: Option C
Clean, well-graded gravel or crushed stone is the preferred material for drainage backfill because it provides high permeability and prevents clogging.
Q147:
What is the purpose of a “geotextile fabric” in a retaining wall?
Correct Answer: Option A
Geotextile fabric acts as a filter, allowing water to pass through while retaining the soil particles, preventing the drainage system from clogging.
Q148:
What is the effect of “improper compaction” of the backfill on a retaining wall?
Correct Answer: Option B
Poor compaction can lead to settlement of the backfill, which can increase the lateral pressure on the wall and cause uneven loading.
Q149:
What is the typical slope of the backfill surface (drainage slope) away from the wall?
Correct Answer: Option C
The backfill surface is typically sloped at 2% to 5% away from the wall to direct surface water away from the wall and prevent saturation.
Q150:
What is the purpose of a “curing compound” on a retaining wall?
Correct Answer: Option A
Curing compounds are applied to the surface of fresh concrete to retain moisture, allowing the cement to hydrate fully and achieve its design strength.
Q151:
What is the effect of “cold weather” on the construction of a retaining wall?
Correct Answer: Option B
In cold weather, concrete must be protected from freezing during curing, which can be achieved through the use of heating blankets or insulation.
Q152:
What is the purpose of a “formwork” in a retaining wall construction?
Correct Answer: Option C
Formwork is the temporary structure (usually made of wood or metal) that holds the concrete in place while it cures, giving the wall its final shape.
Q153:
What is the effect of “sulfate attack” on a concrete retaining wall?
Correct Answer: Option A
In soils with high sulfate content, the concrete can be chemically attacked, leading to expansion, cracking, and loss of strength. Sulfate-resistant cement or coatings may be required.
Q154:
What is the purpose of a “waterstop” in a retaining wall?
Correct Answer: Option B
A waterstop is a flexible strip (often made of PVC or rubber) placed in a construction joint to block the flow of water through the joint.
Q155:
What is the effect of “improperly placed rebar” on a retaining wall?
Correct Answer: Option C
Incorrect placement of reinforcement (e.g., insufficient cover or wrong location) can reduce the wall’s capacity to resist bending moments, leading to cracking or failure.
Q156:
What is the purpose of a “curing blanket” in cold weather?
Correct Answer: Option A
Curing blankets are insulating covers that trap heat from the concrete’s hydration process, maintaining a temperature that allows proper curing in cold weather.
Q157:
What is the effect of “hot weather” on the construction of a retaining wall?
Correct Answer: Option B
In hot weather, the concrete may lose moisture too quickly, leading to plastic shrinkage cracks. Proper curing and protection are essential.
Q158:
What is the purpose of a “tremie” in concrete placement?
Correct Answer: Option C
A tremie is a pipe used to place concrete underwater or in deep excavations, ensuring the concrete is placed without being washed out by water.
Q159:
What is the effect of “aggregate” quality on the concrete strength?
Correct Answer: Option A
The quality, shape, and gradation of aggregates directly affect the compressive strength and durability of the concrete.
Q160:
What is the purpose of a “rebar tie” (wire tie) in a retaining wall?
Correct Answer: Option B
Rebar ties are used to secure the reinforcing bars at their intersections, maintaining the correct spacing and position as specified in the design.
Q161:
What is the most common sign of a failing retaining wall?
Correct Answer: Option A
The most visible signs of a retaining wall in distress are cracks (especially horizontal or diagonal cracks) and outward bulging of the wall face.
Q162:
What is the purpose of a “periodic inspection” of a retaining wall?
Correct Answer: Option B
Regular inspections help identify issues such as clogged weep holes, cracks, or leaning before they lead to serious structural problems.
Q163:
What is the effect of “clogged weep holes” on a retaining wall?
Correct Answer: Option C
Clogged weep holes prevent water from draining, causing hydrostatic pressure to build up behind the wall, which can lead to overturning or structural failure.
Q164:
What is the most effective way to clean a weep hole?
Correct Answer: Option A
Cleaning a weep hole typically involves using a metal rod, a drill, or a high-pressure water jet to remove the blockage and restore drainage.
Q165:
What is the primary purpose of a “maintenance plan” for a retaining wall?
Correct Answer: Option B
A maintenance plan includes regular inspections, cleaning of weep holes, and timely repairs to prevent deterioration and extend the wall’s life.
Q166:
What is the effect of “tree roots” on a retaining wall?
Correct Answer: Option C
Tree roots can grow behind the wall, exerting significant pressure and causing the wall to crack, bulge, or tilt.
Q167:
What is the most common cause of retaining wall failure?
Correct Answer: Option A
Inadequate drainage is the single most common cause of retaining wall failure, leading to excessive hydrostatic pressure behind the wall.
Q168:
What is the effect of “settlement” on a retaining wall?
Correct Answer: Option B
Settlement of the backfill or the foundation can cause differential movement, leading to cracking, leaning, and potential failure of the wall.
Q169:
What is the purpose of a “repair” in a retaining wall?
Correct Answer: Option C
Repairs are made to fix damage (e.g., cracks, displacement) and restore the wall’s load-carrying capacity and stability.
Q170:
What is the most common repair method for a cracked retaining wall?
Correct Answer: Option A
Epoxy injection is a common method used to seal cracks and restore the structural integrity of the wall by bonding the concrete back together.
Q171:
What is the effect of “efflorescence” on a retaining wall?
Correct Answer: Option B
Efflorescence is a deposit of salts on the concrete surface, caused by water moving through the concrete. While unsightly, it is generally not a structural concern.
Q172:
What is the purpose of a “survey” to monitor a retaining wall?
Correct Answer: Option C
Surveying instruments can be used to monitor the wall’s position over time, detecting any movement that could indicate a stability problem.
Q173:
What is the effect of “freeze-thaw” cycles on a retaining wall?
Correct Answer: Option A
Freeze-thaw cycles can cause the water in the concrete’s pores to expand and contract, leading to surface cracking, spalling, and deterioration over time.
Q174:
What is the purpose of a “drainage inspection” during maintenance?
Correct Answer: Option B
Regular inspection of the drainage system is crucial to prevent hydrostatic pressure build-up and ensure the wall’s stability.
Q175:
What is the effect of “animal burrows” on a retaining wall?
Correct Answer: Option C
Animal burrows can create voids in the backfill, leading to settlement and a loss of support for the wall, potentially causing it to fail.
Q176:
What is the purpose of a “safety factor” in the maintenance of a retaining wall?
Correct Answer: Option A
Factors of safety account for uncertainties in loads, material strengths, and construction quality, ensuring the wall is designed to remain stable under varying conditions.
Q177:
What is the effect of “water infiltration” through the wall surface?
Correct Answer: Option B
Water entering through the surface can lead to efflorescence and, when it freezes, can cause the concrete surface to flake or spall.
Q178:
What is the purpose of a “structural assessment” of a retaining wall?
Correct Answer: Option C
A structural assessment is a comprehensive evaluation of the wall’s integrity, often using visual inspection, testing, and analysis to determine its safety and serviceability.
Q179:
What is the effect of “vibration” (e.g., from nearby construction) on a retaining wall?
Correct Answer: Option A
Vibrations from heavy equipment or blasting can cause densification of the soil (settlement) and can crack or displace the wall.
Q180:
What is the purpose of a “record drawing” (as-built) for a retaining wall?
Correct Answer: Option B
As-built drawings are a record of the wall as it was actually built, including any modifications, which are essential for future maintenance and repairs.
Q181:
What is “finite element analysis” (FEA) used for in retaining wall design?
Correct Answer: Option A
FEA is a numerical method used to analyze complex geometries and soil-structure interaction, providing detailed insights into stresses and deformations.
Q182:
What is the purpose of a “case study” in retaining wall engineering?
Correct Answer: Option B
Case studies analyze actual projects, providing valuable lessons on what works, what doesn’t, and the practical application of design principles.
Q183:
What is the effect of “climate change” (e.g., increased rainfall) on retaining walls?
Correct Answer: Option C
Climate change can lead to more intense and frequent rainfall, increasing the risk of water infiltration and hydrostatic pressure on retaining walls.
Q184:
What is the purpose of a “risk assessment” for a retaining wall?
Correct Answer: Option A
A risk assessment identifies potential failure modes, their likelihood, and the consequences, informing decisions on design, monitoring, and repair.
Q185:
What is the effect of “leaching” on a concrete retaining wall?
Correct Answer: Option B
Leaching is the process where water dissolves and removes calcium hydroxide from the concrete, which can reduce its pH and lead to reinforcement corrosion.
Q186:
What is the purpose of a “life cycle cost analysis” for a retaining wall?
Correct Answer: Option C
Life cycle cost analysis considers construction, maintenance, and repair costs over the wall’s lifespan, allowing for a more informed design decision.
Q187:
What is the effect of “seismic liquefaction” on a retaining wall?
Correct Answer: Option A
Liquefaction, in saturated loose soils, can cause the soil to behave like a liquid, causing the wall to sink, tilt, or collapse.
Q188:
What is the purpose of “instrumentation” (e.g., piezometers, inclinometers) on a retaining wall?
Correct Answer: Option B
Instrumentation allows for the monitoring of key parameters, providing early warning of problems and data for verifying design assumptions.
Q189:
What is the effect of “surcharge load from a building” on a retaining wall?
Correct Answer: Option C
The load from an adjacent building is a major surcharge that can significantly increase the lateral pressure on the wall and must be accounted for in the design.
Q190:
What is the purpose of a “peer review” in retaining wall engineering?
Correct Answer: Option A
A peer review is an independent evaluation of the design by another engineer, helping to identify any potential errors or oversights.
Q191:
What is the effect of “soil creep” on the lateral pressure over time?
Correct Answer: Option B
Soil creep is a slow, time-dependent deformation that can lead to a gradual increase in lateral pressure on the wall, potentially overloading it in the long term.
Q192:
What is the purpose of a “forensic investigation” of a failed retaining wall?
Correct Answer: Option C
A forensic investigation is a systematic analysis of a failed structure to identify the root cause(s) of the failure, providing lessons for future designs.
Q193:
What is the effect of “climate change” on the design of retaining walls?
Correct Answer: Option A
With climate change leading to more extreme rainfall events, engineers are increasingly designing drainage systems with higher capacity to manage the increased water loads.
Q194:
What is the purpose of a “construction monitoring” program for a retaining wall?
Correct Answer: Option B
Construction monitoring involves regular inspections and testing during construction to ensure compliance with the design and to catch any issues before they become critical.
Q195:
What is the effect of “vibration” from construction on the backfill?
Correct Answer: Option C
Vibrations can cause the soil particles to rearrange and densify, leading to settlement and an increase in the horizontal stress on the wall.
Q196:
What is the purpose of a “geophysical survey” in retaining wall engineering?
Correct Answer: Option A
Geophysical methods (e.g., seismic refraction, ground-penetrating radar) can provide a non-invasive way to map the subsurface, aiding in the design of the wall’s foundation.
Q197:
What is the effect of “surcharge load from a slope” on a retaining wall?
Correct Answer: Option B
An upslope surcharge can be a major load on the wall, and the stability of the upslope itself is a critical consideration in the design.
Q198:
What is the purpose of a “sustainability” assessment for a retaining wall?
Correct Answer: Option C
Sustainability assessments consider the environmental footprint of the wall, including material sourcing, construction energy, and long-term maintenance.
Q199:
What is the effect of “tree roots” on the drainage system of a retaining wall?
Correct Answer: Option A
Tree roots can grow into and clog perforated drainage pipes, rendering the drainage system ineffective and leading to hydrostatic pressure problems.
Q200:
What is the primary lesson from the failure of the Teton Dam and how does it apply to retaining walls?
Correct Answer: Option B
The Teton Dam failure, caused by piping/seepage, underscores the critical importance of managing water pressures, which is the most common cause of retaining wall failure.