What should be provided behind a retaining wall to prevent failure?
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Adequate drainage.
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What should be provided behind a retaining wall to prevent failure?
Adequate drainage.
What minimum factor of safety should be provided due to uncertainties in site conditions?
A minimum factor of safety of 2.0.
What should be considered during construction regarding groundwater?
The possibility of piping or heave if the groundwater level rises above foundation level.
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.
What is a uniformly-distributed load treated as?
An equivalent height of earth having the same density as the retained material.
What principle is used to evaluate stress distribution in an isotropic semi-infinite elastic medium due to horizontal loads?
Carother's principle of images.
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.
What should be carefully assessed in the design regarding hydrostatic conditions?
The net hydrostatic head.
Where can guidance on groundwater control for excavation and foundation works be found?
In Section 3.6.3.
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.
What effect does the presence of water have on retaining walls?
It has a marked effect on the forces applied to the wall.
What method can be used to estimate lateral earth pressures against a retaining wall due to surcharge loads treated as strip loads?
The trial wedge method.
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.
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.
What is the probability of exceedance for a 1 in 1000 year rainfall event over 120 years?
About 11%.
What should be ignored in a widely-graded base soil with D90 > 2 mm?
The percentage of particles greater than 5 mm.
What is the purpose of the permeability criterion?
To ensure that the system permeability reaches an equilibrium level that allows water flow without impediment.
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.
What is a common cause of retaining wall failures?
The action of water.
When can zero water pressure be assumed for gravity and reinforced concrete retaining walls?
When there are adequate drainage provisions.
What is recommended to check the results of computer analyses?
Carrying out a quick sketching of flow-nets by hand.
What is the significance of performing grading tests?
To obtain parameters representative of the soil without dispersants.
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.
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.
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.
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.
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.
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).
What do design charts for estimating depth of penetration prevent?
Heave or piping in isotropic sands and layered subsoils.
What is a critical consideration when locating a retaining wall?
Avoiding large surcharge loadings near its back.
What is the typical nominal thickness of a drainage layer for walls retaining compacted backfill?
300 mm.
How can hydraulic failure be prevented in gravity retaining walls?
By incorporating adequate drainage measures, such as an inclined drain behind the wall.
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.
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.
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.
What are examples of extreme events that should be considered in design?
Severe rainfall, flooding, and bursting of water mains.
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.
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.
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.
What does Rule 6 restrict in filter design?
The maximum grain size of the filter.
Who provided an outline of a seismic hazard analysis for the Hong Kong region?
Pun & Ambraseys (1992).
What is necessary for removing water from a drainage system?
Drainage pipes.
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.
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.
What should be examined to assess the risk of clogging of geotextiles?
Site conditions, including drainage system performance.
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).
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.
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.
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.
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'.
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.
What happens if the groundwater level does not rise above the retaining wall base?
Positive pore water pressures will not develop behind the wall.
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.
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.
What law is used to calculate the required cross-sectional area of drainage material?
Darcy's law.
What should be used for seepage analysis to obtain a realistic water flow pattern?
Unfactored permeability values selected for design.
What method can be used to determine lateral earth pressures due to a line load?
The trial wedge method.
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.
What can infiltration into the ground cause?
A rise in groundwater level and an increase in pore water pressure in the soil.
When should filtration tests be considered in the design?
When groundwater is continually being removed.
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.
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.
How can water flow patterns affect earth pressure?
They can affect the earth pressure on both the 'active' and 'passive' sides of the wall.
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.
What should be considered when selecting the D85 size for retention criteria?
It should be selected conservatively to allow for soil variability.
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.
What should be avoided in the 'active' zone behind a retaining wall?
Placing service drains, ducts, and other pipes.
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.
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.
What does Rule 3 ensure in the permeability criteria?
That the filter is much more permeable than the base soil it protects.
What should be considered in design regarding infiltration?
The effect of infiltration, especially when the nature of the backfill is unknown.
What is the purpose of the segregation criteria in filter design?
To prevent the separation of materials within the filter.
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.
What does the trial wedge method provide?
The maximum active thrust behind the wall.
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.
What should be provided at suitable locations to facilitate clearing of pipe blockages?
Roding eyes or manholes.
What effect does an impermeable cantilevered retaining wall have on groundwater?
It can 'dam' groundwater flow and result in a rise in groundwater level.
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 partial material factor should be applied to the permeability of granular filter and drainage materials?
What should be ensured regarding the transmittivity of composite drains?
It should be adequate when subjected to the design earth pressure.
What should be ensured when handling granular filter materials?
They should be handled and placed properly to prevent contamination and segregation.
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.
What should be considered in the design if groundwater can rise above the wall base?
Uplift pressure should be determined from a flow-net.
What permeability is needed to achieve steady state seepage?
Around 10^-5 m/s.
What should be considered when designing retaining walls in Hong Kong?
Loads given in the Structures Design Manual for Highways and Railways.
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.
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.
What should be followed for evaluating the magnitude and distribution of earth pressures?
The procedures given in Chapter 6.
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.
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.
How should permeability values be used in seepage analysis?
Directly, without factoring.
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.
What is essential to establish when designing a retaining wall?
The groundwater conditions at the site.
What is the minimum surcharge recommended for retaining walls?
10 kPa.
What methods are often used to evaluate earth pressures?
Limit equilibrium methods, such as the trial wedge method.
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.
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.
What types of geotextiles are Lawson's criteria appropriate for?
Woven geotextiles and nonwoven heat-bonded geotextiles.
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.
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.
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.
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.
