p.15
Design against Hydraulic Failure
What minimum factor of safety should be provided due to uncertainties in site conditions?
A minimum factor of safety of 2.0.
p.15
Design against Hydraulic Failure
What should be considered during construction regarding groundwater?
The possibility of piping or heave if the groundwater level rises above foundation level.
p.4
Assessment of Effects of Surcharge
What is the modified Boussinesq formula used for in the context of retaining walls?
To estimate the magnitude and distribution of earth pressures from area loads.
p.3
Design Surcharge Loadings
What is a uniformly-distributed load treated as?
An equivalent height of earth having the same density as the retained material.
p.4
Assessment of Effects of Surcharge
What principle is used to evaluate stress distribution in an isotropic semi-infinite elastic medium due to horizontal loads?
Carother's principle of images.
p.6
Design Water Pressures
How should the design water level be determined during a storm response?
By adding a storm rise corresponding to a 1,000-year return period rainstorm to a typical groundwater level.
p.15
Design against Hydraulic Failure
What should be carefully assessed in the design regarding hydrostatic conditions?
The net hydrostatic head.
p.3
Assessment of Effects of Surcharge
How should earth pressures due to surcharge be treated?
They should be directly superimposed onto active earth pressures and water pressure.
What method can be used to calculate seismic forces on a retaining wall?
The Mononobe-Okabe (M-O) method.
p.6
Effects of Water on Retaining Walls
What effect does the presence of water have on retaining walls?
It has a marked effect on the forces applied to the wall.
p.6
Design Water Pressures
What should design water pressures be based on?
The worst credible groundwater conditions during extreme events.
Which publication provides information on earthquakes for seismic hazard analyses in Hong Kong?
GCO Publication No. 1/91: Review of Earthquake Data for the Hong Kong Region.
p.8
Assessment of Effects of Surcharge
What should be assessed regarding permeability values?
The sensitivity of the results to variations in permeability values.
What are the two main types of surcharges behind a retaining wall?
Permanent and temporary surcharges.
What is the seismicity level of Hong Kong regarding retaining wall design?
Low to medium seismicity, where seismic load is generally not critical.
p.14
Granular and Geotextile Filter Design
What should be ignored in a widely-graded base soil with D90 > 2 mm?
The percentage of particles greater than 5 mm.
p.12
Granular and Geotextile Filter Design
What is the purpose of the permeability criterion?
To ensure that the system permeability reaches an equilibrium level that allows water flow without impediment.
p.7
Design Water Pressures
What risk should be considered in the design of retaining walls in low-lying areas?
The risk of burst water mains causing a local rise in water level.
p.6
Design Water Pressures
When can zero water pressure be assumed for gravity and reinforced concrete retaining walls?
When there are adequate drainage provisions.
p.8
Assessment of Effects of Surcharge
What is recommended to check the results of computer analyses?
Carrying out a quick sketching of flow-nets by hand.
p.14
Granular and Geotextile Filter Design
What is the significance of performing grading tests?
To obtain parameters representative of the soil without dispersants.
p.2
Design Surcharge Loadings
What should be considered in the design regarding heavy lifting equipment?
Heavy lifting equipment and machine foundations exert extreme loads that should be specifically considered in the design.
p.3
Assessment of Effects of Surcharge
What is the basis of the second approach to earth pressures?
Experiments on surcharge effects showing similarity to elasticity theory.
What is required during the construction of drainage provisions?
Close control and adequate supervision.
p.2
Assessment of Effects of Surcharge
How do shallow foundations affect retaining walls?
Shallow foundations are likely to exert much of the foundation load onto the retaining wall.
What is a limitation of the Mononobe-Okabe method?
It grossly simplifies the soil-structure interaction during a seismic event.
p.12
Granular and Geotextile Filter Design
What are the two main performance criteria for filters?
The retention criterion and the permeability criterion.
What are uniformly distributed loads?
Continuous loads acting on the surface or body of the retained ground, such as goods stacked uniformly or traffic on roads.
p.13
Granular and Geotextile Filter Design
What should be used in the design for base soils containing gravel and fines?
A revised base soil grading curve consisting of particles smaller than 5 mm.
What should be done to prevent blockage in drainage systems?
The drain or drainage material should be protected by a suitable filter material.
What is the recommended diameter and spacing for weepholes in retaining walls?
75 mm in diameter, spaced not more than 1.5 m horizontally and 1.0 m vertically.
p.11
Design Water Pressures
What should the permeability of the selected drainage material be relative to the soil being drained?
At least 100 times that of the soil (or rock).
p.14
Design against Hydraulic Failure
What do design charts for estimating depth of penetration prevent?
Heave or piping in isotropic sands and layered subsoils.
p.1
Design Surcharge Loadings
What is a critical consideration when locating a retaining wall?
Avoiding large surcharge loadings near its back.
p.15
Design against Hydraulic Failure
How can hydraulic failure be prevented in gravity retaining walls?
By incorporating adequate drainage measures, such as an inclined drain behind the wall.
p.3
Assessment of Effects of Surcharge
What should be shown for the assumption of active state of stress in soil to be valid?
The deformation of the retaining wall must be sufficiently large.
What is the specified horizontal static force for seismic load design of abutment walls?
An acceleration of 0.07 g applied at the center of gravity of the structure.
p.5
Assessment of Effects of Surcharge
What should a project-specific assessment for seismic load design include?
Analysis of seismic hazard, determination of design earthquake return period, and estimation of amplification effects.
p.4
Assessment of Effects of Surcharge
What is a key consideration when evaluating lateral earth pressures due to horizontal loads?
The behavior of retaining walls subjected to concentrated horizontal loads is not well understood.
p.6
Design Water Pressures
What are examples of extreme events that should be considered in design?
Severe rainfall, flooding, and bursting of water mains.
p.2
Design Surcharge Loadings
What load factor should be allowed around the periphery of all retaining walls during construction?
A load factor of 69 should be allowed to cover incidental loading.
p.7
Design Water Pressures
What is the advised approach for selecting design water levels associated with rare rainfall events?
Designers are advised to exercise judgment and select design water levels conservatively.
p.6
Design Water Pressures
What should be done if groundwater levels exhibit both storm and seasonal response?
Add either a 1,000-year return period storm rise to a typical wet season level or vice versa, whichever gives a higher water level.
What is an example of a permanent surcharge?
Loads due to shallow foundations of an adjacent building.
What should weepholes not be relied upon as?
The sole means of discharge.
p.13
Granular and Geotextile Filter Design
What does Rule 6 restrict in filter design?
The maximum grain size of the filter.
What is a good practice for retaining walls not designed to resist water pressures?
To provide adequate drainage behind the wall.
What are concentrated loads?
Loads that include line loads, point loads, and area loads.
p.7
Design Water Pressures
What is a recommended solution for housing services that cannot be placed outside the active zone?
Using a sealed trench, ducting system, or sleeve connected to a suitable discharge point.
p.14
Granular and Geotextile Filter Design
What should be examined to assess the risk of clogging of geotextiles?
Site conditions, including drainage system performance.
p.11
Design Water Pressures
What governs the rate of seepage flow into the drainage system of a retaining wall?
The design water level and the permeability of the soil (or rock).
p.14
Granular and Geotextile Filter Design
What criteria are appropriate for nonwoven needle-punched fabrics?
Manual criteria (Table 19).
In what situations should seismic loads be specifically considered in retaining wall design?
For abutment walls of highway and railway structures, high-risk structures, and cantilevered walls retaining saturated materials.
What can occur for a wall composed of vertical structural elements spaced apart?
Some through-flow of groundwater.
p.9
Design Water Pressures
What can be assumed about water pressures in the design of certain retaining walls?
Water pressures can be assumed to be zero if the groundwater level will not rise above the retaining wall base.
p.6
Design Water Pressures
What does the 1,000-year return period indicate in design?
It indicates a low level of probability of exceedance that designers should aim for in selecting extreme events.
p.12
Granular and Geotextile Filter Design
What does the retention criterion ensure?
No excessive loss of particles from the base soil initially, and no further loss after the initial period of soil 'piping'.
p.2
Design Surcharge Loadings
What are nominal surcharge loads used for?
Nominal surcharge loads may be used when the effect of surcharge is small compared to earth and water pressures.
p.8
Effects of Water on Retaining Walls
What happens if the groundwater level does not rise above the retaining wall base?
Positive pore water pressures will not develop behind the wall.
p.1
Design Surcharge Loadings
What should be done if a smaller load is deemed appropriate for a structure?
Obtain agreement from the relevant authorities.
What should be provided in retaining walls with level backfill to avoid ponding?
A suitable material of relatively low permeability in the top 1.5 m layer and a gradient towards a drainage channel.
p.1
Design Surcharge Loadings
How should foundation loads be assessed in relation to retaining walls?
By considering dead and live loads, wind loads, and other types of loading.
What should be done for impermeable retaining walls to prevent hydrostatic pressure build-up?
Provide drainage holes (weepholes) through the wall.
p.7
Evaluation of Water Pressures
What should be used for seepage analysis to obtain a realistic water flow pattern?
Unfactored permeability values selected for design.
What effect should be assessed and allowed for in the design of walls with vertical structural elements?
The rise in groundwater level due to the 'damming' effect.
p.8
Effects of Water on Retaining Walls
What can infiltration into the ground cause?
A rise in groundwater level and an increase in pore water pressure in the soil.
p.14
Granular and Geotextile Filter Design
When should filtration tests be considered in the design?
When groundwater is continually being removed.
p.13
Granular and Geotextile Filter Design
What are the conditions necessary for geotextile filter criteria to be applicable?
Flow through the geotextile filter is unidirectional, hydraulic gradients are moderate to low, and effective normal stresses are static.
p.7
Evaluation of Water Pressures
What factors influence water flow patterns and pressures behind a retaining wall?
Nature of the ground, groundwater conditions, topography, surface cover, land use, and drainage layout.
p.8
Effects of Water on Retaining Walls
How can water flow patterns affect earth pressure?
They can affect the earth pressure on both the 'active' and 'passive' sides of the wall.
p.9
Design Water Pressures
What may act on a retaining wall if a relatively impermeable soil layer is present below its toe?
A hydrostatic water pressure may act on the wall.
p.14
Granular and Geotextile Filter Design
What should be considered when selecting the D85 size for retention criteria?
It should be selected conservatively to allow for soil variability.
p.13
Granular and Geotextile Filter Design
What is the purpose of Rule 5 in filter design?
To ensure a reasonably uniform grading of the filter.
When is flow-net construction unnecessary?
For gravity and reinforced concrete retaining walls with a proper inclined drain system.
p.7
Design Water Pressures
What should be avoided in the 'active' zone behind a retaining wall?
Placing service drains, ducts, and other pipes.
p.9
Design Water Pressures
What happens to water pressure in permeable walls like gabion walls?
No positive water pressure can develop at the rear face unless the wall is submerged.
p.2
Assessment of Effects of Surcharge
What two approaches are commonly used to assess lateral pressures due to surcharge?
The first approach assumes the retained earth is in an active state, and the second uses the trial wedge method.
What is the infiltration rate limited by in compacted fill of low permeability?
The permeability of the fill.
p.12
Granular and Geotextile Filter Design
What does Rule 3 ensure in the permeability criteria?
That the filter is much more permeable than the base soil it protects.
p.8
Design against Hydraulic Failure
What should be considered in design regarding infiltration?
The effect of infiltration, especially when the nature of the backfill is unknown.
p.12
Granular and Geotextile Filter Design
What is the purpose of the segregation criteria in filter design?
To prevent the separation of materials within the filter.
p.7
Evaluation of Water Pressures
What methods can be used to evaluate design water pressures?
Flow-net construction or numerical seepage analysis.
What should be ensured for the drainage system in terms of discharge points?
It should be provided with sufficient discharge points.
What theory is the Mononobe-Okabe method based on?
Plasticity theory, extending Coulomb earth pressure theory.
p.3
Design Surcharge Loadings
What does the trial wedge method provide?
The maximum active thrust behind the wall.
p.2
Assessment of Effects of Surcharge
What is the effect of deep foundations on retaining walls?
Deep foundations transmit loads to depth and have less effect on the retaining wall.
What happens to soil shear strength under seismic excitation?
It degrades, especially in loose fill and saturated materials.
p.9
Design Water Pressures
What effect does an impermeable cantilevered retaining wall have on groundwater?
It can 'dam' groundwater flow and result in a rise in groundwater level.
p.2
Assessment of Effects of Surcharge
When should Rankine earth pressure theory be used?
It is used in the case of a uniformly-distributed surcharge, but at-rest pressures should be used for stiff walls.
What should be omitted in watertight basement walls?
Weepholes, but suitable waterproofing measures should be provided.
What should be ensured regarding the transmittivity of composite drains?
It should be adequate when subjected to the design earth pressure.
p.12
Granular and Geotextile Filter Design
What should be ensured when handling granular filter materials?
They should be handled and placed properly to prevent contamination and segregation.
p.3
Design Surcharge Loadings
What is the significance of the modifications to the Boussinesq solution?
They are based on experimental evidence and assume a rigid, non-yielding retaining wall.
p.9
Design Water Pressures
What should be considered in the design if groundwater can rise above the wall base?
Uplift pressure should be determined from a flow-net.
p.1
Design Surcharge Loadings
What should be considered when designing retaining walls in Hong Kong?
Loads given in the Structures Design Manual for Highways and Railways.
p.14
Design against Hydraulic Failure
What is a common method to prevent hydraulic failure of a retaining wall?
Designing the wall to penetrate a sufficient depth below the formation level.
p.9
Design Water Pressures
What is the simplified water pressure distribution for impermeable walls in homogeneous isotropic soil?
It assumes that the hydraulic head varies linearly down the back and up the front of the wall.
p.3
Assessment of Effects of Surcharge
What should be followed for evaluating the magnitude and distribution of earth pressures?
The procedures given in Chapter 6.
p.13
Granular and Geotextile Filter Design
Why is it important to check individual batches of the filter against Rule 5?
Because compliance with design limits does not ensure that the filter will satisfy Rule 5.
p.12
Granular and Geotextile Filter Design
What is the aim of Rule 1 in the design criteria for granular filters?
To limit the size of pore channels within the filter to restrict the movement of fine particles from the base soil.
p.11
Design Water Pressures
How should permeability values be used in seepage analysis?
Directly, without factoring.
p.14
Design against Hydraulic Failure
What can cause instability or gross loosening in front of a retaining wall?
When the vertical seepage exit gradient equals about unity in dense sands or when uplift force exceeds the weight of the overlying soil column in loose sands.
p.7
Evaluation of Water Pressures
What is essential to establish when designing a retaining wall?
The groundwater conditions at the site.
p.8
Assessment of Effects of Surcharge
What methods are often used to evaluate earth pressures?
Limit equilibrium methods, such as the trial wedge method.
p.9
Design Water Pressures
What should be done if soil hydraulic properties vary at a site?
A proper flow-net analysis should be carried out to evaluate the effect of the water flow pattern on earth pressures.
p.11
Design Water Pressures
What should be taken into account in design if a static water head builds up behind a vertical wall-back drain?
The resulting hydrostatic water pressure.
What type of drainage composites have become increasingly popular?
Prefabricated drainage composites made up of a synthetic core wrapped with a geotextile filter envelope.
p.13
Granular and Geotextile Filter Design
What types of geotextiles are Lawson's criteria appropriate for?
Woven geotextiles and nonwoven heat-bonded geotextiles.
p.2
Assessment of Effects of Surcharge
What is the trial wedge method applicable for?
It is applicable when the surcharge is in the form of a uniformly-distributed load, line load, or strip load.
p.13
Granular and Geotextile Filter Design
What is the significance of the minimum thicknesses of filter layers?
To ensure integrity and consistency in performance appropriate to the chosen method of construction.
p.13
Granular and Geotextile Filter Design
What should be checked in addition to internal requirements of a filter?
Compatibility with the particular base soil, in accordance with Rules 1 and 3.
p.9
Design Water Pressures
How should design water pressures be assessed for impermeable walls?
By conducting a seepage analysis after establishing groundwater levels on the 'active' and 'passive' sides.
