top-up course for tcp t3 on gifw geotechnical content ... · pdf filecounterfort retaining...

Top-up Course for TCP T3 on GIFW and Building Works with Significant

Geotechnical Content

Lecture 5 : Retaining Walls

Presented by: Y C Koo

CONTENTS

• Common Types of Retaining Walls• Design and Construction Considerations• Supervision Requirements

Common Types of Retaining Walls

(Extracted from Geoguide 1)

Common Types of Retaining Walls

(Extracted from Geoguide 1)

(Hand-dug caisson is banned now. Soldier pile walls are now also common.)

Gravity Retaining WallsMass Concrete Retaining Wall

• Particularly suitable for retained height < 3m• Front batter, generally 1 in 50, to be provided.• Expansion joints typical at 22.5m, contraction joints at 7.5m• Drainage blanket can be replaced by proprietary drainage composite.• Mesh reinforcement usually provided to control thermal and shrinkage cracks.

Gravity Retaining WallsCrib Wall

Open-faced

Closed-faced

• Constructed of precast concrete units

• Infill to be free draining granular material

• Drainage provision similar to mass concrete wall

• Geotextile filter to be provided at rear face

• Sensitive to differential settlement. Not suitable on ground liable to settle nor for supporting heavy surcharge.

• Height in general less than 7m.

Gravity Retaining Walls

Crib Wall

Closed-faced

Opened-faced


Gabion Wall• Units typically 1m x 1m x 2m

• Cage commonly constructed of galvanised steel wire, either hexagonal woven or square welded

• Other materials, such as nylon, polypropylene, can be used but need to be protected against fire

• Rock infill preferably 150mm to 300mm and at least twice aperture of mesh

• Free draining, geotextile to be provided at rear face

• Permeable and flexible, hence widely used in river training works.


Gabion Wall


Gabion Wall

Sham Wat Road, Lantau


Reinforced Fill Retaining Structure

• Proprietary product available

• Precast concrete facing panels

• Steel or polymeric reinforcement

• Geoguide 6 – Guide to Reinforced Fill Structure and Slope Design

• More commonly used by road embankments and bridge abutments

Abutment Retaining Wall

Embankment





Reinforced Concrete Retaining Walls

Shear key

• Height in general less than 8m. Counterfort wall to be used for greater height.• Shear key sometimes provided to improve sliding resistance.• Expansion joints typical at 22.5m, contraction joints at 7.5m• Drainage blanket can be replaced by proprietary drainage composite.• Stability of temporary cut for wall construction to be considered.• ELS may be required for space constraint.

R.C. L- or inverted T-Shaped Cantilever Retaining Wall


R.C. L- or inverted T-Shaped Cantilever Retaining Wall


Counterfort Retaining Wall & Buttressed Retaining Wall

• For retaining height greater than 8m

• Reduce wall thickness by T-beam action

• Counterfort wall more common.

Cantilevered Retaining Walls

• Eliminate/reduce temporary cut behind wall

• Heavy construction plant

• Temporary platform required on sloping ground (or provide temporary cut for sitting plant on level ground)

• Can be wholly in soil or socketted into bedrock.

• Retaining height can be up to 15m, depending on pile diameter.

Bored Pile Wall


Caisson Wall / Bored Pile Wall

Caisson wall Bored-pile wall


Bored Pile Wall

Temporary steel platform for bored pile wall construction.


Soldier Pile Wall • Lighter construction plant compared with bored pile wall

• Retaining height up to about 6m.

• Skin wall to be provided.

• Can also be constructed as secant pile wall.

Cantilevered Retaining WallsSoldier Pile Wall

Soldier pile installation by Continuous Flight Auger


Down-the-hole drill head for rock socket

Concentric overburden drill heads

Pile drilling with casing

Soldier piles can also be installed by overburden drilling, similar to socketed H-piles


Secant pile wall

Soldier pile wall with skin wall under construction


Improvement to Existing Retaining WallsSoil Nailing

• Soil nails contribute to the stability of the wall by adding resisting force and moment.

• Though seldom, soil nails can also be used for new retaining walls.


Masonry wall improved by soil nailing


R.C. wall improved by soil nailing

Improvement to Existing Retaining WallsSkin Wall

• The skin wall contributes to the stability of the wall by adding dead weight and increasing moment arm of resisting moment.

Improvement to Existing Retaining WallsSkin Wall

Design and Construction Considerations• Building (Construction) Regulations

• PNAP APP-54 (PNAP 142) - Retaining Walls - Building (Construction) Regulations 1992 - Part XIII

• PNAP APP-63 (PNAP 166) - GEOGUIDE 1 (Second Edition) - Guide to Retaining Wall Design

• PNAP ADV-8 (PNAP 168) - Registration of Slopes and Retaining Walls

• PNAP ADV-23 (PNAP 270) - Improvement of Visual Appearance and Landscape Treatment for Man-made Slopes and Retaining Walls

• Geoguide 1 - Guide to Retaining Wall Design (for new walls)

• Geotechnical Manual for Slopes (for upgrading of existing walls)

• Geoguide 6 – Guide to Reinforced Earth Structure and Slope Design

• Geoguide 7 – Guide to Soil Nail Design and Construction (for design of soil nails in retaining walls)

• Codes of Practice for structural concrete, structural steel, foundation, etc.

Building (Construction) RegulationsPart XIII – Retaining Walls

Regulation 64 - InterpretationIn this Part-

"retaining wall" (擋土牆) means a structure retaining earth or fill.

Regulation 65 – General Requirement(1) Retaining walls shall be designed and constructed to support safely the earth

or fill they retain and other loads without impairing the stability of, or causing damage to, any other building, structure, land, street or services.

(2) The design and construction of minor retaining walls shall comply with subregulation (1) but shall be exempted from regulations 66 to 84.

(Refer to PNAP APP-54 for definition of “minor retaining walls”)

Regulations 66 to 84 ( http://www.legislation.gov.hk/eng/index.htm)

Reg 77 – Drainage Measures to Preserve Stability

Reg 78 – Performance of Drainage System

Reg 79 – Surface Channels

Reg 80 – Safety Required during excavation and construction

Reg 81 – Weep holes

Reg 82 – Copings

Reg 83 – Bonding and Foundation Required

Reg 84 – Bond Courses Required

Reg 66 – Filters

Reg 67 – Backfill Material

Reg 68 – Earth Pressure

Reg 69 – Water Pressure

Reg 70 – Minimum Pressure

Reg 71 – Retaining wall design

Reg 72 – Loading Condition

Reg 73 – Site Investigation

Reg 74 – Adequate Factor of Safety

Reg 75 – Validity of Design Earth Pressure

Reg 76 – Masonry and Mass Concrete Walls

PNAP APP-54 (PNAP 142) - Retaining Walls –Building (Construction) Regulations 1992 - Part XIII)

Provides clarification on • Definition on Minor Retaining Walls

• Drainage and Filter

• Backfilling Material behind Retaining Wall

• Rock Faces

• Retaining Wall Design and Stability

• Remedial or Preventive Work to an Existing Retaining Wall

• Monitoring during Construction of Retaining Walls

• Demolition of Existing Retaining Walls

PNAP 142 –Retaining

Walls

Modes of External Instability of Mass Retaining Walls Mass Retaining Wall

Actions affecting wall stability• Self weight W• Active earth pressure PA

• Passive earth pressure Pp

• Bearing pressure N• Base friction S• Hydrostatic pressure at back

of wall U1

• Hydrostatic pressure at wall base U2

• Surcharage

Forces on retaining wall (excluding water forces) Water forces on retaining wall

R.C. Retaining Wall

• Modes of external instability same as mass retaining wall.

• Structural capacity of wall to be designed in accordance with Code of Practice for Structural Use of Concrete.

Virtual Back

R.C. Retaining Walls

Retaining Wall with Piled Foundation

• Minipiles and socketted H-piles commonly adopted.

• Raking piles used to resist lateral forces.

R.C. Retaining Walls

Retaining Wall with Piled FoundationCantilevered Retaining Wall

• External instability by Loss of Overall Stability and Overturning Failure

• Structural capacity of wall to be checked

Earth PressuresActive and Passive Earth Pressure

At-rest Coeff. Ko

Stress – Strain Relationship

• Ka pressure used for stability check

• Ko pressure used for structural design

Earth Pressures Earth Pressure Theories

Active and passive pressures are dependent on soil strength parameters, c’ and φ’.

Planar failure surface

•Rankine

•Rankine-Bell

•Coulomb

•Trial Wedge Method

Planar-Logarithmic Spiral

•Caquot & Kerisel Trial Wedge Method

Water Pressures Gravity and T-shaped Retaining Walls

Flow nets under steady state groundwater seepage

Water PressuresCantilevered Retaining Walls

Flow nets under steady state groundwater seepage

Impermeable wall Bored pile/soldier pile wall

Compaction-induced Earth Pressures

Equivalent weight of rollerRoll width

Ql =

Compaction induced pressure assumed to be clearly stated in drawings.

New Retaining Walls:Geoguide 1 – Guide to Retaining Wall DesignLimit State Approach with partial factors of safety applied to Loads and Material Parameters

Existing Retaining Walls:Geotechnical Manual for Slopes Factor of Safety applied to Sliding, Overturning, Bearing Capacity

Overall Factor of Safety following Geotechnical Manual for Slopes to be observed in both cases.

Design Standards

Major Inspection Items for Retaining Walls(1) Requirements on Approved Drawings and imposed by BD

(2) Contractor’s Submissions

Method Statement, Drawings, Precautionary & Protective Measures, Materials submissions, etc.

(3) Temporary Site Works

Formwork, Shoring, Excavation, Temporary Cut Slope etc.

(4) Permanent Site Works

Founding Condition, Reinforcement Fixing, Concreting, Backfilling. For Cantilevered Walls, founding level of piles, pile verticality, pile structural steel section/pile reinforcement, grouting/concreting

Sampling & Testing(1) Concrete and Steel Reinforcement

• CS1 : 1990 – Testing Concrete

• CS2 : 1995 – Carbon Steel Bars for the Reinforcement of Concrete

(2) Formwork and Finishes to Concrete

• Falsework and Formwork Design

(3) Founding Material

• Description of Soil/Rock, Plate Load Test, GCO Probe

(4) Backfill Material

• PNAP APP-15 (PNAP 55) – Site Formation Temporary & Permanent Filling Work

• PNAP APP-54 (PNAP 142) – Retaining Walls

•PNAP APP-64 (PNAP 167) – Methods for Testing Hong Kong Soils (GEOSPEC 3 -Model Specification for Soil Testing)

(5) Drainage and Filter Material

• Section 8.5 of the GEOGUIDE 1 (2nd Edition)

• GEO Publication No. 1/93 – Review of Granular & Geotextile Filters

Founding Materials

• Description of Soil / Rock

• Plate Loading Test

• Dynamic Probe Test

• Founding material to be protected from ingress of water after inspection

• If founding on piles, then design and supervision requirements of piling to be followed.

Reference

Code of Practice for Foundations

GEOGUIDE 2 – Guide to Site Investigation

Code of Practice for Foundation

Code of Practice for Foundation Reinforcement

Submissions• Particulars of Bar Reinforcement, e.g.

certificate• Samples of Materials• Bending SchedulesTesting• Tensile Test• Bend Test, Rebend Test

Inspection• Steel Reinforcement Free of Scale• Bar Size, Spacing, Lap• Cover

Reinforcement - TestingPNAP APP-45 (PNAP 122) Testing of Reinforcement for Concrete• Purchaser’s test referred to in CS2:1995 performed by HOKLAS laboratory

Class 1 Reinforcement from QA manufacturers, handled by a QA Stockistand retaining manufacturer’s lot traceability

Class 2 Reinforcement from QA manufacturers, handled by a QA Stockistbut without manufacturer’s lot traceability

Class 3 Reinforcement other than Class 1 and Class 2

Reinforcement - TestingTensile Test• Individual test result ≥ 0.93 x characteristic strength (250MPa/460MPa)• Mean of test results ≥ Characteristic strength + 10MPa• Not necessary to assess mean value if all individual result ≥ characteristic strength

Bend Test• Withstand bend through 180° with no sign of cracks on visual inspection

Rebend TestWithstand bend through 45° and rebend without breaking

Reinforcement - Inspection• Loose rust, excessive flaky rust or mill

scale removed by wire-brushing.

• Sufficient number of spacers, chairs (spacing ≤ 1.5m in general).

• Spacers to be constructed of concrete or of proprietary plastic or concrete type. Strength should not be less than surrounding concrete.

• Sufficient number of intersecting and lapping bars tied by tying wire or clips

• Access over reinforcement to be well supported to avoid displacement of reinforcment

• Laps in reinforcement to be provided at specified positions with design lap length.

Formwork & Finishes to Concrete

Submissions• Falsework and Formwork Design• Time for removal of Falsework &

Formwork• Cover Spacers, Release Agents,

Formwork Tie, Surface Retarders• Class of Formed, Unformed, & Treated

Finishes

Inspection• Falsework• Sealing of joints• Cover to reinforcement bars

Formwork & Finishes to Concrete

Formwork tied to existing wall

Checking of thickness of wall

Concreting

Testing• Testing for Workability - slump• Testing for Compressive Strength -

cubes

Submissions• Particulars of Concrete Mix• Particulars of Construction Joints

Inspection• Preparation of joints• Clearing of debris• Compaction

Concreting – Slump Test• Test of workability• Test procedure and details: CS1 Section 2• Sampling from each concrete truck• Slump cone 300mm high• Measured to nearest 5mm

Note: Concrete with high workability, i.e. slump in excess of 150mm, may sometime exhibit a slump pattern similar to that of a collapse slump. In such case, the slump shall still be

Concreting – Slump Test

Concreting – Concrete Cube for Compression Test• Test of compressive strength• Test procedure and details: CS1 Section 12• Sampling and testing: B(C)R Reg 58• At least one sample from each grade of concrete produced on any one day• Two 150mm cubes from each sample of concrete

• Serial number to be given to each concrete cube• Cubes to be adequately cured• Test for 28-day compressive strength at HOKLAS laboratory• Average of compressive strength of a pair of cubes taken as test result

Concreting – Concrete Cube for Compression Test

Concreting – Concrete Cube for Compression Test• Acceptance Criteria: B(C)R Reg 59

• If the difference in compressive strength of a pair of cubes exceeds 20% of the test result (i.e. average of the pair), that test result shall be disregarded and investigation shall be made to establish whether the concerned concrete is acceptable or note.

Backfilling behind Retaining WallsAccording to APP-54 (PNAP 142)

1. Backfill material behind retaining walls should have properties that meet designrequirements. It should also meet the requirements in Table 1 at Appendix A attached.

2. Material selected for use as backfill generally must not contain :

(a) peat, vegetation, timber, organic or other degradable materials;

(b) dangerous or toxic material;

(c) material susceptible to combustion;

(d) metal, rubber, plastic or synthetic material;

(e) material susceptible to significant volume change – e.g. marine mud, swelling clays and collapsible soils; or

(f) soluble material.

3. In addition, the backfill should not be chemically aggressive : e.g. the presence ofexcessive sulphate in soils can cause accelerated deterioration of concrete and steel.

PNAP APP-54 (PNAP 142) Appendix A Table 1

Backfilling behind Retaining Walls

According to PNAP APP-15 (PNAP 55)

The in situ field dry densities of compacted materials forming the peripheral portion of an earth fill slope shall be not less than 95% of the maximum dry density

Relative Compaction =In situ dry density

Max. dry density> 95%


PNAP APP-15 (PNAP 55) – Testing FrequencyBackfilling behind Retaining Walls

In situ Field Tests :

Sand Replacement Method => determine in situ bulk density

Laboratory Tests :

Oven-drying => determine moisture content

Geospec 3 Test Method 10.1 to 10.8 => determine max. dry density

Nuclear Densometer Method => determine in situ bulk density

Speedy Test => obtain in situ moisture content (advance information)

(Geospec 3 Test Method 11.1 or 11.2)

(Geospec 3 Test Method 11.3)

(Geospec 3 Test Method 5.1 to 5.3)

(Proctor Test) & optimum moisture content


Sand Replacement TestBackfilling behind Retaining Walls

Relative Compaction TestBackfilling behind Retaining Walls

PNAP 167 – Methods for Testing Hong Kong Soils (GEOSPEC 3 - Model Specification for Soil Testing)

Where soil tests are to be carried out, only laboratories accredited under the Hong Kong Laboratory Accreditation Scheme (HOKLAS) for the relevant tests shall be employed. The BA will only accept results issued on HOKLAS-endorsed test certificates or reports.

Backfilling behind Retaining Walls Drainage and Filter Material

Two Types :

(i) Granular Filters

(ii) Geotextile Filters

• Section 8.5 of the GEOGUIDE 1 (2nd Edition)

• GEO Publication No. 1/93 – Review of Granular & Geotextile Filters

GEO Publication No. 1/93 – Review of Granular & GeotextileFilters

Drainage and Filter Material

(Extracted from GEO Publication No. 1/93)

GEO Publication No. 1/93 – Review of Granular & GeotextileFiltersStability – pores small enough to prevent excessive migration of base soil

Permeability – more permeable than base soil

Segregation – Not segregated/contaminated during and after installation

Min. thickness – 300mm

Drainage and Filter Material

Drainage and Filter MaterialGEO Publication No. 1/93 – Review of Granular & GeotextileFilters

PredrillingFor piles designed to be socketted into rock (Grade III Rock with TCR>85%, UCS>25MPa +&PL50>1MPa)

Pre-drilling Rig

Measuring Depth of Drillhole Drillhole Core Box

Construction of Cantilevered Pile Wall

Confirmation of Founding Level

Inspection of rock sample at founding level

Measuring the depth for the founding level

Construction of Cantilevered Pile Wall Construction of Cantilevered Pile Wall

Koden Test – Checking of bored pile section and verticality


Koden Test – Checking of bored pile section and verticality

Checking of cleanliness of water before concreting

Air-lifting Inspection of Water Sample


Checking of Steel Cage


Concreting of bored pile by tremie method

Use of Pump Truck Concreting by Hopper



Checking of pile integrity• Full Core• Sonic Test


Sonic Test – Checking of pile integrity

• Verticality Check for Soldier Pile Wall

• Test of grout (Flow cone test, bleeding test, cube strength)


The End

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