Substructures, Foundations and Hydraulic Design

Introduction and Context

A bridge substructure transfers loads from the superstructure to the ground while maintaining stability under gravity, traffic, wind, seismic action, hydraulic forces, temperature effects, construction loads and accidental events. It also shapes the river channel and interacts with soil, groundwater, flood flows, debris and, in navigable waterways, vessels. Substructure design is therefore an integrated geotechnical, structural and hydraulic problem rather than a series of independent calculations.

The principal components are piers, abutments, wing walls, retaining systems, foundations and protection works. Their design must satisfy ultimate limit states, serviceability limit states, durability requirements and hydraulic performance criteria. A technically adequate design also requires consideration of constructability, inspection, maintenance, environmental impact, uncertainty and applicable national standards. Numerical values and load combinations in practice must be taken from the governing bridge code, geotechnical standard and hydraulic regulations; the equations in this chapter express general engineering principles and do not replace code provisions.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the functions and load paths of bridge piers, abutments and retaining systems.

2. Select and conceptually proportion shallow and deep foundations.

3. Evaluate earth pressures, bearing resistance, sliding, overturning and settlement.

4. Explain river hydraulics, scour mechanisms and hydraulic design controls.

5. Incorporate debris, vessel impact, seismic effects and geotechnical uncertainty into design.

6. Develop an integrated design approach using investigation, analysis, monitoring and risk management.

1. Substructure Systems and Load Paths

A typical bridge substructure consists of:

• Piers, which support intermediate spans and transfer vertical and horizontal actions to foundations.
• Abutments, which support the end spans, retain approach fill and often accommodate bearings and expansion movements.
• Wing walls and return walls, which retain approach embankments and guide water or traffic.
• Foundations, which transfer actions to soil or rock.
• Scour and erosion protection, including riprap, gabions, collars, sheet piles, guide banks and bed armouring.

Loads reach the foundation through bearings, pier shafts, caps, piles or footings. The design must identify both force magnitude and force path. For example, a longitudinal braking force may pass from the deck to bearings, then to a pier cap, shaft, foundation and surrounding soil. A flood-induced drag force may act directly on a pier shaft, while uplift may result from buoyancy, hydraulic pressure or overturning.

Important load categories include permanent loads, traffic loads, braking and traction, wind, thermal movement, seismic action, earth pressure, water pressure, current drag, buoyancy, debris accumulation, vessel collision, construction loads and accidental actions. Load combinations should be established using the applicable bridge code. A force that is insignificant in one combination may govern another; for example, serviceability settlement may control a long viaduct even when ultimate strength is adequate.

2. Piers and Abutments

2.1 Pier configuration

Pier geometry is governed by structural demand, hydraulic obstruction, aesthetics, constructability and inspection access. Common forms include solid wall piers, single-column piers, multi-column bents and hammerhead piers. Rounded or streamlined noses can reduce flow separation and debris accumulation, but their effectiveness depends on alignment with the flow and local channel geometry.

A pier should be proportioned to resist axial load, biaxial bending, shear, torsion where applicable and local effects around bearings or cap connections. Slender piers require stability analysis and second-order effects. Reinforced-concrete design must consider cracking, reinforcement development, confinement, durability and, where relevant, seismic ductility. Steel piers require checks for local buckling, corrosion protection and fatigue.

2.2 Abutments

Abutments perform two functions: they support the bridge end reaction and retain approach soil. Types include gravity, cantilever, counterfort, mechanically stabilized earth (MSE), integral and semi-integral abutments. Integral abutments eliminate conventional expansion joints at the end and transfer cyclic thermal movements into the soil through piles and approach fills. They can reduce maintenance but require careful assessment of pile flexibility, cyclic soil response and approach settlement.

Abutment design must address earth pressure behind the wall, surcharge from traffic and approach embankments, hydrostatic pressure, seismic earth pressure, bearing, sliding, overturning, global stability and drainage. Drainage is essential: a blocked drain can convert a relatively modest lateral soil load into a much larger combined soil and water pressure.

2.3 Retaining systems

Retaining systems associated with bridges include reinforced-concrete walls, sheet-pile walls, soldier piles and lagging, anchored walls, soil nails, MSE walls and mechanically stabilized slopes. Selection depends on right-of-way, groundwater, vibration limits, available equipment, seismicity, construction sequence and environmental constraints.

A flexible wall may mobilize active pressure through movement, whereas a restrained wall may experience at-rest pressure. The designer must not use active pressure automatically where the wall cannot move sufficiently. Apparent earth-pressure diagrams may be appropriate for braced excavations, but they are not interchangeable with classical retaining-wall pressure distributions.

3. Earth Pressure and Stability

3.1 Effective stress and groundwater

For saturated soil, total stress is divided into effective stress and pore-water pressure:

\[\sigma' = \sigma - u\]

where \(\sigma'\) is effective stress, \(\sigma\) is total stress and \(u\) is pore-water pressure. Soil shear strength is governed primarily by effective stress. In the Mohr–Coulomb model,

\[\tau_f = c' + \sigma'\tan\phi'\]

where \(\tau_f\) is shear strength at failure, \(c'\) is effective cohesion and \(\phi'\) is the effective friction angle. This idealization is most useful for drained conditions and should be supplemented by undrained analysis for short-term clay loading.

3.2 Rankine active pressure

For a vertical smooth wall retaining level, cohesionless soil, with wall friction neglected and no surcharge, the Rankine active coefficient is

\[K_a = \frac{1-\sin\phi'}{1+\sin\phi'}\]

and the lateral active pressure at depth \(z\) is

\[\sigma_h(z)=K_a\gamma' z\]

where \(\gamma'\) is the effective unit weight below the groundwater table. The resultant active force over retained height \(H\) is

\[P_a=\frac{1}{2}K_a\gamma H^2\]

for dry soil of unit weight \(\gamma\), acting at \(H/3\) above the base. A uniform surcharge \(q\) adds, in the same simplified model,

\[P_q=K_a q H\]

acting at mid-height. These equations assume homogeneous soil, a planar backfill, adequate wall movement to mobilize active conditions and negligible wall friction. Sloping backfill, wall friction, cohesion, layered soil and seismic action require more advanced treatment, commonly Coulomb or numerical methods.

At-rest pressure is often approximated for normally consolidated soil by

\[K_0 \approx 1-\sin\phi'\]

but soil stress history and construction sequence may make this estimate unsuitable. Hydrostatic pressure must be added where drainage does not eliminate water pressure. Using submerged unit weight for soil while omitting water pressure is a common and serious error.

3.3 Stability checks

For a gravity or cantilever retaining structure, basic checks include:

• Sliding: the resisting horizontal forces must exceed driving forces by the code-required margin or satisfy a factored resistance format.
• Overturning: moments about the toe must satisfy the prescribed limit-state condition.
• Bearing: the resultant should lie within the acceptable kern or the soil pressure distribution must be explicitly evaluated.
• Global stability: the entire wall–soil mass must have adequate resistance against deep-seated failure.
• Structural strength: stem, heel, toe, keys and connections require flexural and shear design.

For a simple rectangular footing of width \(B\), if the resultant vertical load \(V\) has eccentricity \(e=M/V\), the linear, no-tension soil-pressure approximation gives

\[q_{max,min}=\frac{V}{B}\left(1\pm\frac{6e}{B}\right)\]

per unit length of wall. This expression assumes a rigid footing, uniform soil response and a resultant within the middle third so that \(q_{min}\geq0\). If the resultant lies outside that range, contact becomes partial and a compression-only analysis is required.

4. Foundation Selection and Design

4.1 Site investigation

Foundation selection begins with geological and geotechnical characterization. Essential information includes stratigraphy, groundwater, density or consistency, shear strength, compressibility, rock quality, cavities, liquefaction susceptibility, aggressivity, scour erodibility and variability across the site. Useful methods include boreholes, standard penetration testing, cone penetration testing, pressuremeter testing, geophysics, laboratory testing and inspection of nearby foundations.

A ground model should distinguish measured facts, interpreted parameters and assumptions. Characteristic values should reflect the uncertainty and failure mechanism rather than simply the mean of test results. Spatial variability is particularly important for long bridges because a weak layer may control one pier even when average conditions appear satisfactory.

4.2 Shallow foundations

Spread footings are suitable where competent bearing strata occur near the surface, settlements are tolerable and scour or erosion can be controlled. Design involves vertical bearing resistance, eccentricity, sliding, overturning, settlement, structural strength and constructability.

A simplified ultimate bearing-capacity expression for a centrally loaded strip footing on homogeneous soil is

\[q_{ult}=c'N_c+qN_q+\frac{1}{2}\gamma BN_\gamma\]

where \(c'\) is effective cohesion, \(q\) is effective overburden at foundation level, \(\gamma\) is unit weight, \(B\) is footing width and \(N_c,N_q,N_\gamma\) are bearing-capacity factors dependent on \(\phi'\). This form assumes a conventional general-shear failure mechanism and requires modification for footing shape, embedment, inclination, eccentricity, slope, groundwater and layered soils. Design codes generally apply partial factors or resistance factors rather than using the unfactored value directly.

Settlement has immediate and long-term components. Immediate settlement is associated with elastic distortion; consolidation settlement occurs as excess pore pressure dissipates in fine-grained soil; secondary compression continues after primary consolidation. Differential settlement, not merely total settlement, commonly governs bridge performance because it produces deck irregularity, bearing distress and approach bumps.

4.3 Deep foundations

Piles or drilled shafts are used where surface soil is weak, loads are large, scour removes upper strata, uplift is significant or lateral resistance is needed. Axial resistance can be represented conceptually as

\[Q_u=Q_s+Q_b\]

where \(Q_s\) is shaft resistance and \(Q_b\) is base resistance. For a pile of perimeter \(p

a\) and embedded length \(L\), a simplified shaft-resistance integration is

\[Q_s=\int_0^L p\,f_s(z)\,dz\]

where \(f_s(z)\) is unit shaft resistance. Base resistance is commonly expressed as \(Q_b=A_b q_b\), with \(A_b\) the base area and \(q_b\) the unit base resistance. These are conceptual equations; actual values depend on pile type, installation method, soil state, drainage, plugging, setup, group effects and code rules.

Deep foundations also require analysis for lateral load, group interaction, downdrag, uplift, scour-induced unsupported length, seismic kinematic effects and structural buckling where exposed. Pile load testing, integrity testing and construction records are important because installation can substantially alter ground conditions.

5. River Hydraulics and Hydraulic Design

5.1 Design flood and hydraulic geometry

Hydraulic design begins with a design flood selected according to bridge importance, consequence of failure, catchment characteristics, climate information and regulatory requirements. The analysis should define flood frequency, hydrograph, water-surface elevation, velocity, sediment regime, debris potential and floodplain conveyance. One-dimensional models may be adequate for uniform channels, while two-dimensional models are often preferable where flow is divided, floodplains are complex or bridge contraction produces strong lateral variations.

For steady uniform open-channel flow, Manning’s equation is commonly written in SI units as

\[Q=\frac{1}{n}AR^{2/3}S^{1/2}\]

where \(Q\) is discharge, \(n\) is Manning roughness, \(A\) is flow area, \(R=A/P\) is hydraulic radius, \(P\) is wetted perimeter and \(S\) is energy slope. The equation assumes gradually varied or uniform flow conditions and should not be used uncritically through a bridge opening with strong contraction, separation or rapidly varied flow.

Bridge construction in a floodplain can increase upstream water level through contraction and obstruction. Hydraulic assessment should compare existing and proposed water-surface profiles, velocities, flow distribution, afflux, backwater and impacts on adjacent property. Pier alignment should minimize skew to the dominant current where possible. The foundation must remain stable under the design flood and under credible higher events where risk warrants.

5.2 Scour mechanisms

Scour is removal of bed or bank material by flowing water. It may include:

• Long-term degradation, caused by changes in sediment supply, channel slope or watershed conditions.
• Contraction scour, caused by increased velocity as flow is forced through a reduced opening.
• Local pier scour, caused by vortices around the pier nose and sides.
• Local abutment scour, caused by flow concentration around the embankment and abutment.
• Clear-water scour, where approaching flow does not move the general bed sediment.
• Live-bed scour, where sediment is already moving throughout the channel.

A generalized local-scour relationship often has the form

\[y_s = K\,a\,F(Fr,\,y/a,\,\text{sediment},\,\text{geometry})\]

where \(y_s\) is scour depth, \(a\) is a characteristic pier dimension, \(Fr\) is a Froude-number parameter, and \(K\) represents geometric and alignment effects. Because scour equations are empirical and site-sensitive, the governing bridge guidance should be used rather than treating one equation as universal.

The design bed level should account for long-term degradation, contraction scour, local scour and uncertainty. Foundations should extend below the design scour elevation or be otherwise protected. Riprap sizing, launching aprons and structural countermeasures should be checked for velocity, turbulence, filter compatibility and failure consequences. Protection that merely conceals scour without providing foundation embedment may create a false sense of security.

5.3 Debris and ice

Flood-borne trees and sediment can accumulate against piers, increasing projected area, drag, upstream water level and scour. Debris can also redirect flow toward an abutment or cause impact loads. Design responses include reducing pier number, increasing opening width, orienting noses to flow, providing debris deflectors where justified and establishing inspection and clearance plans. Ice action may govern in cold regions and should be assessed separately from ordinary hydraulic drag.

6. Accidental, Seismic and Environmental Actions

Vessel collision is a risk-based accidental action. Assessment requires vessel class, traffic frequency, navigation channel geometry, velocity, pier location, warning systems, fendering and consequences of collapse. Protection may involve dolphins, fenders, sacrificial systems, pier relocation or increased structural resistance. The design should distinguish vessel impact energy, contact duration and load path; a static equivalent force is an idealization, not a universal physical law.

Seismic design includes inertial forces, kinematic soil–foundation interaction, liquefaction, lateral spreading, bearing failure, pile ductility and unseating. Abutment backfill and approach slabs can experience large cyclic demands. Deep foundations do not automatically eliminate seismic risk: lateral spreading can impose substantial displacement demands on piles.

Environmental design includes sediment continuity, fish passage, floodplain connectivity, water quality, construction turbidity, noise, wetland disturbance and climate adaptation. Temporary cofferdams and diversion channels may themselves alter flood risk and aquatic habitat. Construction timing, emergency response and regulatory approvals should be integrated into the design rather than treated as post-design matters.

7. Integrated Design Under Geotechnical Uncertainty

Geotechnical parameters are uncertain because soil is naturally variable and investigations sample only limited locations. Uncertainty has three principal sources: measurement error, spatial variability and model uncertainty. A professional design should identify which assumptions are sensitive to each source.

Useful strategies include:

1. Improve information selectively. Additional boreholes or cone tests should target parameters that materially affect the decision.

2. Use observational methods. Establish trigger levels for settlement, pore pressure, lateral movement, scour or vibration, together with predefined responses.

3. Apply robustness. Avoid details that fail suddenly when one parameter is slightly worse than expected.

4. Use redundancy and inspectability. Multiple piles, accessible bearings and scour monitoring may reduce consequences.

5. Separate uncertainty from conservatism. Arbitrarily large factors can produce uneconomic or constructability problems without addressing the actual failure mode.

Reliability-based design can express performance as a limit-state function \(g(\mathbf X)\), where failure occurs when \(g(\mathbf X)\leq0\). The variables \(\mathbf X\) may include soil strength, water level, load, scour depth and resistance. Reliability methods are valuable for comparing alternatives, but they require defensible probability models and should be consistent with code calibration.

Digital terrain models, GIS, remote sensing, LiDAR, drones, structural-health monitoring and numerical finite-element or computational-fluid-dynamics models can improve understanding. Artificial intelligence may assist in anomaly detection, image classification, flood forecasting or prioritizing inspections. It should not replace engineering judgment: training data may be biased, extrapolation may be unsafe and model outputs require physical validation and traceability.

8. Illustrative Applied Case: River Bridge Replacement

Background

A two-lane bridge crosses a gravel-bed river in a rural floodplain. The existing bridge has three narrow openings, visible pier scour and repeated debris blockage. The replacement must carry higher traffic loads, maintain flood conveyance and avoid unacceptable risk to a nearby community.

Problem

Initial planning proposed two large piers founded on shallow footings. Preliminary investigation found dense sand overlying loose silty sand, with groundwater near the riverbed. The design flood produces high velocity and substantial floating debris. A shallow foundation would be vulnerable to scour and differential settlement.

Analysis

Hydraulic modelling compared one-, two- and three-pier arrangements. The two-pier option reduced obstruction and debris collection relative to the existing bridge, but increased local velocities around each pier. Geotechnical analysis indicated that shallow footings could achieve adequate nominal bearing resistance only if founded deeply, which would require extensive cofferdam excavation and still leave uncertainty concerning scour and liquefaction of the loose layer.

The project team assessed driven piles and drilled shafts. Driven piles offered installation verification through driving records but raised concerns about vibration near a buried utility. Drilled shafts provided large lateral capacity and fewer foundation elements, but required temporary casing and strict control of concrete placement in groundwater.

Approach and solution

The selected concept used streamlined reinforced-concrete piers supported by groups of deep foundations extending below the calculated total scour elevation. Pier noses were aligned with the dominant flood direction. A graded riprap apron with a compatible filter system was specified as supplementary protection, not as the sole scour safeguard. Abutments used drained backfill, positive surface-water control and approach slabs to reduce the approach bump.

The design included debris loading, inspection access, scour monitoring and a flood emergency plan. Construction sequencing required seasonal low-flow work, turbidity controls and verification of foundation depth. Sensitivity studies varied design flood level, scour depth, soil friction angle and pile stiffness. Trigger levels were established for unexpected pore pressure, pile movement and riverbed lowering.

Outcome

The final arrangement reduced predicted backwater and debris blockage compared with the existing bridge while improving foundation robustness. Its cost exceeded the shallow-footing concept, but life-cycle evaluation showed lower expected disruption, reduced inspection difficulty and lower consequence of scour-related failure.

Learning points

The case demonstrates that the cheapest foundation based on nominal bearing capacity may not be the least-cost or safest system. Hydraulic opening, pier geometry, foundation depth, debris, construction method and monitoring must be selected as one system. It also illustrates the value of sensitivity analysis and explicit treatment of uncertainty.

9. Best-Practice Design Workflow

A defensible workflow is:

1. Establish functional, hydraulic, geotechnical, seismic, navigation and environmental requirements.

2. Develop a site investigation plan based on likely failure modes.

3. Build a ground and river model, recording uncertainty and data quality.

4. Generate alternative layouts for piers, abutments and foundations.

5. Analyse hydraulic effects, earth pressures, foundation resistance, settlement and structural actions.

6. Check construction stages, temporary works, access, cofferdams and dewatering.

7. Assess scour, debris, vessel impact, seismic hazards and accidental events.

8. Apply code load combinations and independent design verification.

9. Specify drainage, filters, erosion protection, inspection access and monitoring.

10. Update the design using construction observations and as-built records.

Quality assurance should include independent review of hydraulic assumptions, geotechnical parameters, scour elevations, groundwater treatment and load paths. All critical calculations should state units, assumptions, data sources and limitations.

Challenges and Limitations

No simplified equation fully represents the interaction of soil, water, structure and construction. Classical earth-pressure theory may be unreliable for layered, cemented or highly structured soils. Bearing-capacity equations can overstate performance when settlement or weak layers govern. Empirical scour equations contain uncertainty outside their calibration range. One-dimensional hydraulic models may hide local flow paths, while sophisticated numerical models can give false precision when boundary conditions are poor.

Other limitations include climate non-stationarity, incomplete debris records, undocumented existing foundations, inaccessible riverbeds and changes in channel morphology. Accordingly, design should combine analysis with field evidence, conservative but rational assumptions, monitoring and an emergency management plan.

Chapter Summary

Bridge substructure design integrates structural actions, soil behaviour, groundwater, river hydraulics, scour, accidental hazards and construction realities. Piers and abutments must transfer loads safely while limiting hydraulic obstruction and retaining approach soil. Earth-pressure selection depends on wall movement, drainage, soil properties and construction sequence. Foundation choice requires assessment of bearing, settlement, sliding, overturning, lateral action, scour and uncertainty; deep foundations must also consider group effects, downdrag, seismic demands and installation.

Hydraulic design evaluates design floods, flow distribution, backwater, debris and channel change. Scour protection should supplement, not replace, adequate foundation embedment. Vessel impact, seismic hazards, environmental obligations and climate-related uncertainty require explicit risk-based treatment. The most reliable designs use a site-specific ground and river model, sensitivity analysis, independent verification, inspectability and monitoring throughout the asset life cycle.

Key Terms

Effective stress: Total stress minus pore-water pressure, governing much of soil strength and deformation.

Active earth pressure: Lateral pressure mobilized when a retaining wall moves sufficiently away from retained soil.

At-rest pressure: Lateral pressure in soil where wall movement is insufficient to mobilize active or passive conditions.

Abutment: End support of a bridge that also commonly retains approach soil.

Pier: Intermediate vertical or inclined support between bridge spans.

Bearing capacity: Ability of foundation soil to resist applied load without shear failure.

Settlement: Downward movement of a foundation or soil mass.

Differential settlement: Difference in settlement between locations, potentially causing structural or riding-surface distress.

Pile: Deep foundation element transferring load through shaft resistance, base resistance or both.

Drilled shaft: Large-diameter cast-in-place deep foundation constructed by excavation and concreting.

Scour: Erosion or removal of bed or bank material by flowing water.

Contraction scour: Scour caused by increased flow velocity through a reduced waterway opening.

Local scour: Erosion concentrated around a pier, abutment or other obstruction.

Afflux: Increase in upstream water level caused by a bridge or other obstruction.

Manning roughness: Empirical coefficient representing boundary resistance in open-channel flow.

Debris loading: Hydraulic and impact action caused by material accumulated against a bridge element.

Liquefaction: Loss of effective stress and strength in susceptible saturated soil during cyclic loading.

Mechanically stabilized earth wall: Retaining system using reinforced soil mass and facing elements.

Observational method: Design and construction approach using monitoring and predefined responses to manage uncertainty.

Review Questions

Short Answer Questions

1. What are the principal functions of a bridge abutment?

2. State the distinction between active and at-rest earth pressure.

3. Define contraction scour and local pier scour.

4. Why is drainage important behind an abutment?

5. Name three sources of geotechnical uncertainty.

Long Answer Questions

1. Explain the complete stability assessment required for a retaining abutment.

2. Compare shallow foundations, driven piles and drilled shafts for a river bridge.

3. Discuss how hydraulic modelling influences pier arrangement and foundation design.

4. Explain the mechanisms of total and differential settlement and their bridge impacts.

5. Describe a risk-based approach to debris, vessel impact and scour.

Analytical / Application Questions

1. A retaining wall is 5 m high and supports dry, cohesionless soil with \(\phi'=30^\circ\), \(\gamma=18\,\text{kN/m}^3\), and no surcharge. Using the simplified Rankine active equation, calculate the active force per metre length and its point of application. State the assumptions.

2. A proposed pier reduces the river opening by 20%. Identify the hydraulic, geotechnical and structural investigations required before accepting the arrangement.

3. Develop a foundation-selection matrix for a bridge site with loose saturated sand, high debris potential, seasonal flooding and restricted construction access. Justify the criteria and preferred alternative.

Multiple Choice Questions with Answers

Question 1

Which condition is required for the simplified Rankine active-pressure equation to be directly applicable?

A. The wall must be fully restrained against movement

B. The wall must move sufficiently to mobilize active conditions

C. The retained soil must be saturated and undrained

D. The backfill must contain only cohesive soil

Correct Answer: B. The wall must move sufficiently to mobilize active conditions

Question 2

In the effective-stress relationship σ′ = σ − u, u represents:

A. Unit weight

B. Pore-water pressure

C. Ultimate bearing pressure

D. Wall friction

Correct Answer: B. Pore-water pressure

Question 3

For a rectangular footing under the linear no-tension approximation, the condition qmin ≥ 0 requires:

A. e ≤ B/6

B. e ≤ B/2

C. e ≥ B/6

D. e = B

Correct Answer: A. e ≤ B/6

Question 4

Which mechanism is specifically associated with increased velocity through a reduced bridge opening?

A. Secondary compression

B. Contraction scour

C. Downdrag

D. Passive earth pressure

Correct Answer: B. Contraction scour

Question 5

In Manning’s equation, hydraulic radius is defined as:

A. Wetted perimeter divided by flow area

B. Flow area divided by wetted perimeter

C. Discharge divided by velocity squared

D. Channel width divided by depth

Correct Answer: B. Flow area divided by wetted perimeter

Question 6

Which foundation component is represented by Qs in the conceptual equation Qu = Qs + Qb?

A. Shaft resistance

B. Base resistance

C. Wind resistance

D. Scour resistance

Correct Answer: A. Shaft resistance

Question 7

Why should hydrostatic pressure be considered behind an abutment?

A. It always reduces lateral pressure

B. Blocked or inadequate drainage can add water pressure to soil pressure

C. It eliminates settlement

D. It increases the soil friction angle

Correct Answer: B. Blocked or inadequate drainage can add water pressure to soil pressure

Question 8

Which action is most directly associated with floating trees accumulating against a pier?

A. Debris loading and increased local scour

B. Reduction of projected area and drag

C. Reduction of upstream water level in every case

D. Elimination of hydraulic turbulence

Correct Answer: A. Debris loading and increased local scour

Question 9

What is a major advantage of an observational design method?

A. It removes the need for investigation

B. It uses monitoring and predefined responses to manage uncertainty

C. It guarantees that no movement occurs

D. It replaces all code requirements

Correct Answer: B. It uses monitoring and predefined responses to manage uncertainty

Question 10

For a 5 m wall retaining dry soil with γ = 18 kN/m³ and φ′ = 30°, the simplified Rankine active force per metre is approximately:

A. 45 kN/m

B. 75 kN/m

C. 150 kN/m

D. 225 kN/m

Correct Answer: B. 75 kN/m