CONTENTS. Top-up Course for TCP T3 on GIFW and Building Works with Significant Geotechnical Content. Lecture 5 : Retaining Walls

Transcription

1 Top-up Course for TCP T3 on GIFW and Building Works with Significant Geotechnical Content Lecture 5 : Retaining Walls CONTENTS Common Types of Retaining Walls Design and Construction Considerations Supervision Requirements Presented by: Y C Koo Common Types of Retaining Walls Common Types of Retaining Walls (Extracted from Geoguide 1) (Hand-dug caisson is banned now. Soldier pile walls are now also common.) (Extracted from Geoguide 1)

2 Gravity Retaining Walls Mass 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. Crib Wall 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 Open-faced Closed-faced Crib Wall Gravity Retaining Walls Closed-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. Gravity Retaining Walls Opened-faced

3 Gravity Retaining Walls Gabion Wall Gravity Retaining Walls Gabion Wall Sham Wat Road, Lantau Gravity Retaining Walls Reinforced Fill Retaining Structure Gravity Retaining Walls Reinforced Fill Retaining Structure Abutment 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 Retaining Wall Embankment

4 Gravity Retaining Walls Reinforced Fill Retaining Structure Reinforced Concrete Retaining Walls R.C. L- or inverted T-Shaped Cantilever Retaining Wall 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. Reinforced Concrete Retaining Walls R.C. L- or inverted T-Shaped Cantilever Retaining Wall Reinforced Concrete Retaining Walls Counterfort Retaining Wall & Buttressed Retaining Wall For retaining height greater than 8m Reduce wall thickness by T-beam action Counterfort wall more common.

5 Cantilevered Retaining Walls Bored Pile Wall Cantilevered Retaining Walls Caisson Wall / Bored Pile Wall 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. Caisson wall Bored-pile wall Cantilevered Retaining Walls Cantilevered Retaining Walls Bored Pile Wall 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. Temporary steel platform for bored pile wall construction.

6 Cantilevered Retaining Walls Soldier Pile Wall Cantilevered Retaining Walls Soldier Pile Wall Soldier piles can also be installed by overburden drilling, similar to socketed H-piles Concentric overburden drill heads Soldier pile installation by Continuous Flight Auger Pile drilling with casing Down-the-hole drill head for rock socket Cantilevered Retaining Walls Cantilevered Retaining Walls Soldier Pile Wall Secant pile wall Soldier pile wall with skin wall under construction

7 Improvement to Existing Retaining Walls Soil Nailing Improvement to Existing Retaining Walls Soil 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 Improvement to Existing Retaining Walls Soil Nailing Improvement to Existing Retaining Walls Skin Wall R.C. wall improved by soil nailing The skin wall contributes to the stability of the wall by adding dead weight and increasing moment arm of resisting moment.

8 Improvement to Existing Retaining Walls Skin Wall Design and Construction Considerations Building (Construction) Regulations PNAP APP-54 (PNAP 142) - Retaining Walls - Building (Construction) Regulations 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) Regulations Part XIII Retaining Walls Regulation 64 - Interpretation In 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 ( 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 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

9 PNAP APP-54 (PNAP 142) - Retaining Walls Building (Construction) Regulations 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 P A Passive earth pressure P p Bearing pressure N Base friction S Hydrostatic pressure at back of wall U 1 Hydrostatic pressure at wall base U 2 Surcharage Forces on retaining wall (excluding water forces) Water forces on retaining wall

10 R.C. Retaining Wall R.C. Retaining Walls Retaining Wall with Piled Foundation Virtual Back 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. Minipiles and socketted H-piles commonly adopted. Raking piles used to resist lateral forces. R.C. Retaining Walls Retaining Wall with Piled Foundation Cantilevered Retaining Wall External instability by Loss of Overall Stability and Overturning Failure Structural capacity of wall to be checked

11 Earth Pressures Active and Passive Earth Pressure Earth Pressures Earth Pressure Theories Active and passive pressures are dependent on soil strength parameters, c and φ. At-rest Coeff. K o Stress Strain Relationship Planar failure surface Rankine Rankine-Bell Coulomb Trial Wedge Method Ka pressure used for stability check Ko pressure used for structural design Planar-Logarithmic Spiral Caquot & Kerisel Trial Wedge Method Water Pressures Gravity and T-shaped Retaining Walls Water Pressures Cantilevered Retaining Walls Impermeable wall Bored pile/soldier pile wall Flow nets under steady state groundwater seepage Flow nets under steady state groundwater seepage

12 Compaction-induced Earth Pressures Design Standards New Retaining Walls: Geoguide 1 Guide to Retaining Wall Design Limit State Approach with partial factors of safety applied to Loads and Material Parameters Q l = Equivalent weight of roller Roll width Compaction induced pressure assumed to be clearly stated in drawings. 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. 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 (2 nd Edition) GEO Publication No. 1/93 Review of Granular & Geotextile Filters

13 Founding Materials Code of Practice for Foundation 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 Reinforcement Submissions Particulars of Bar Reinforcement, e.g. certificate Samples of Materials Bending Schedules Testing Tensile Test Bend Test, Rebend Test Inspection Steel Reinforcement Free of Scale Bar Size, Spacing, Lap Cover

14 Reinforcement - Testing PNAP APP-45 (PNAP 122) Testing of Reinforcement for Concrete Purchaser s test referred to in CS2:1995 performed by HOKLAS laboratory Reinforcement - Testing Tensile 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 Test Withstand bend through 45 and rebend without breaking Class 1 Reinforcement from QA manufacturers, handled by a QA Stockist and retaining manufacturer s lot traceability Class 2 Reinforcement from QA manufacturers, handled by a QA Stockist but without manufacturer s lot traceability Class 3 Reinforcement other than Class 1 and Class 2 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

15 Formwork & Finishes to Concrete Formwork tied to existing wall Submissions Particulars of Concrete Mix Particulars of Construction Joints Testing Testing for Workability - slump Testing for Compressive Strength - cubes Concreting Inspection Preparation of joints Clearing of debris Compaction Checking of thickness of wall Concreting Slump Test 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

16 Concreting Concrete Cube for Compression 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 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 Walls According to APP-54 (PNAP 142) 1. Backfill material behind retaining walls should have properties that meet design requirements. 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 of excessive sulphate in soils can cause accelerated deterioration of concrete and steel.

17 Backfilling behind Retaining Walls 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% Backfilling behind Retaining Walls PNAP APP-15 (PNAP 55) Testing Frequency Backfilling behind Retaining Walls In situ Field Tests : Sand Replacement Method => determine in situ bulk density (Geospec 3 Test Method 11.1 or 11.2) Nuclear Densometer Method => determine in situ bulk density (Geospec 3 Test Method 11.3) Speedy Test => obtain in situ moisture content (advance information) Laboratory Tests : Oven-drying => determine moisture content (Geospec 3 Test Method 5.1 to 5.3) Geospec 3 Test Method 10.1 to 10.8 => determine max. dry density (Proctor Test) & optimum moisture content

18 Backfilling behind Retaining Walls Sand Replacement Test Backfilling behind Retaining Walls Relative Compaction Test Backfilling 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. Two Types : (i) Granular Filters (ii) Geotextile Filters Drainage and Filter Material Section 8.5 of the GEOGUIDE 1 (2 nd Edition) GEO Publication No. 1/93 Review of Granular & Geotextile Filters

19 Drainage and Filter Material GEO Publication No. 1/93 Review of Granular & Geotextile Filters Drainage and Filter Material GEO Publication No. 1/93 Review of Granular & Geotextile Filters (Extracted from GEO Publication No. 1/93) Stability 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 GEO Publication No. 1/93 Review of Granular & Geotextile Filters Construction of Cantilevered Pile Wall Predrilling For piles designed to be socketted into rock (Grade III Rock with TCR>85%, UCS>25MPa +&PL 50 >1MPa) Pre-drilling Rig Measuring Depth of Drillhole Drillhole Core Box

20 Construction of Cantilevered Pile Wall Construction of Cantilevered Pile Wall Koden Test Checking of bored pile section and verticality 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 Checking of cleanliness of water before concreting Air-lifting Inspection of Water Sample

21 Checking of Steel Cage Construction of Cantilevered Pile Wall Construction of Cantilevered Pile Wall Concreting of bored pile by tremie method Use of Pump Truck Concreting by Hopper Construction of Cantilevered Pile Wall Checking of pile integrity Full Core Sonic Test Construction of Cantilevered Pile Wall Sonic Test Checking of pile integrity

22 Construction of Cantilevered Pile Wall Verticality Check for Soldier Pile Wall Test of grout (Flow cone test, bleeding test, cube strength) The End