MRWA Specification No 0403 Trench Backfill

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MRWA Specification No 04-03 Trench Backfill

• Presented by
• Max Ervin

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Outline of presentation

• Background
• Principles of compaction
• Selected definitions
• General backfill requirements
• Traffic areas
• Cohesionless soils
• Cohesive soils
• Drives, shafts, tunnels and bores
• Concluding remarks

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Background

• 2002
• CWW introduced mechanical compaction requirement
• flooding in practice was prohibited
• to eliminate risks associated with subsidence and
  collapse of sewer trench backfill
• 2003?
• SEW develop specification
• Higher standards than WSAA previously specified
• Some resistance from industry
• 2004
• SEW ask Golder to review
• Following review, Golder asked (2005) to redraft
  SEW specification
• 2006
• CWW ask Golder to assess whether SEW document can
  be adopted without change by CWW
• Only minor drafting changes
• Some comments re reactive ground and rock
• Three Melbourne Retail Water Agencies adopted
  MRWA Backfill Specification No. 04-03.1 for all
  MRWA Capital Works and Land Development projects
  on 1 September 2006.

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Need For Control of Trench Backfill

• Historically trench backfill often poorly
  controlled
• Negligible compaction, or only at surface
• Watering in of clay (and sand) assumed effective
• The inevitable steps/depressions in pavements
• Heightened OH S issues
• Litigation from trips and damage

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Need For Control of Trench Backfill

• Need to consider requirements early in project
  planning and implementation
• MRWA Specification No 04-03 intended to provide
  guidance, as well as minimum requirements
• Need reliable records
• We live in a litigious world

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Application
Specification No 04-03 only intended for
Trenchfill zone, and not Embedment zone
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Principles of Compaction

• Compaction
• The process whereby the density of a soil mass
  is increased by mechanical (usually dynamic)
  means, by rearrangement of the soil particles and
  expulsion of air, usually with little alteration
  to water content
• Consolidation
• The process whereby the density of a soil mass
  increases under the natural action of gravity (a
  static force) due to self weight or an applied
  surcharge. Water usually expelled, and the
  process is time dependent.
• Two very different processes. Should not be
  interchanged in Specifications or in discussion.
• Note, nevertheless, the dependence of compaction
  on moisture content at time of compaction.

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Compaction

• Specified to provide-
• higher shear strength
• more predictable strength
• lower compressibility
• lower permeability
• reduced susceptibility to water content changes
  (eg softening, swelling)
• Desirable properties are mostly achieved at high
  density.
• May not be the case in relation to reactivity
• Hence there is a need to understand the moisture
  - density relationship

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The compaction process

• At the risk of seeming gratuitous-
• low moisture- shearing resistance to movement of
  soil particles relative to each other is high,
  density increase difficult to achieve
• increasing moisture- progressively easier to
  disturb the soil structure, and for same
  compactive effort higher density can be achieved
  up to a certain point
• get peak in dry density - moisture relationship,
  after which simply increase void ratio, as voids
  fill with water
• Hence the well known compaction curve and
  associated MDD and OMC

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Compaction Curve
Note difficult to achieve better than about 2
air voids
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Compaction

• Maximum dry density and optimum moisture content
  are not unique properties of a given soil.
• Are dependent
• upon the energy input during compaction
• the nature of the compactive effort.
• Different soils will respond differently to the
  same compactive effort.
• Hence, roller optimum will vary from laboratory
  optimum.

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Laboratory Compaction

• Cohesionless soils (clean sands) will not produce
  a compaction curve.
• For these soils can measure a maximum and
  minimum density using standardized procedures,
  and then relate this to field density as Density
  Index, or relative density
• Again maximum and minimum are not unique or
  extreme values.

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Field Density Testing

• To allow comparison of achieved compaction with
  laboratory values, need to be able to reliably
  assess field dry density and moisture content
• Nuclear density gauges most common
• Rapid, reasonably repeatable
• Need to be calibrated
• Moisture content usually in laboratory
• Not non destructive as often assumed
• In trenches need to get background count to
  provide for the confined space and possible
  influence of trench walls.

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Relative Compaction

• Usually reference to minimum dry density ratio
  (AS1289.5.4.1)
• Ratio of FDD and MDD as DDR
• Moisture variation from OMC may also be reported
• Beware!
• 2 OMC is wet for Standard
• 2 variation is dry for Hilf
• Can have moisture ratio also
• Perth Sand Penetrometer may be used in
  cohesionless materials

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Relative Compaction cont.

• 04-03 generally calls up minimum dry density
  ratio
• Not one test to fail criteria (as adopted by
  AS3798) has stood the test of time
• empirically based performance criteria
• allows for inherent variability
• good supervision essential to have confidence in
  representative data
• judgment necessary in interpretation
• 04-03 provides guidance on interpretation of
  results

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Rigour in Control Testing

• If insufficient material able to be recovered
  from FDT site (eg shallow test)-
• Additional material should be recovered from
  around the FDT site
• Square sided hole
• Sampling from elsewhere is unacceptable
• Combining bulk samples is unacceptable

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Rigour in Control Testing

• A reference density should be established for
  each FDT
• unless material is sufficiently uniform in its
  properties and compaction characteristics that
  any variability is solely due to test method
• unlikely except for crushed rock from hard rock
  source
• assigned value AS1289.5.4.2 then may be
  applicable

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Rigour in Control Testing

• AS1289 allows for removal of (and correction for)
  up to 20 coarse material in lab compaction test
• A and B moulds
• Does not recognise possible changes in compaction
  characteristics
• More than this and test will lack relevance. A
  potential issue where ordinary fill being used
  in basalt rock country, for example, and may
  contain gt20 coarser than 37.5 mm

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Properties of Compacted Fill - Reactivity

• Reactive Soils Clay soils, for which a change in
  moisture content may result in sufficient change
  in volume to affect the engineering performance
  of any structures (including pavements)
  influenced by this soil.
• Compacted clay fill may be more reactive than
  natural source material
• Higher DDR, same relative moisture higher
  reactivity
• Drier placement moisture, same DDR higher
  reactivity
• Possibly need to consider specifying maximum DDR
• Not easy to enforce

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Properties of Compacted Fill - Reactivity

• Increased reactivity when placed dry
• Should aim to place at expected long term
  equilibrium
• Not so easy in surface zone of seasonal moisture
  variation
• Sand in embedment zone can allow deeper soils to
  wet up or dry out (swell or shrink)
• Caution required when placing pipes near to
  houses or movement sensitive structures

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Specn. 04-03- Selected definitions

• Cohesionless Soils Poorly graded sand and gravel
  mixtures, generally with less than 5 fines (ie
  finer than 75 µm), which are non-plastic and
  which do not exhibit a well-defined
  moisture-density relationship when tested in
  accordance with AS 1289.5.1.1 or AS 1289.5.2.1.
  These will typically be clean sands.
• Cohesive Soils Those materials which have a
  well-defined moisture-density relationship when
  tested in accordance with AS 1289.5.1.1 or AS
  1289.5.2.1. Whilst cohesive soils are typically
  clayey in nature, for the purpose of this
  definition these may also include well-graded
  granular materials such as crushed rock.
• Ordinary Fill Material obtained from excavation,
  or imported, that contains not more than 20 by
  mass of rock fragments with size between 75 mm
  and 150 mm, with no rock or clay fragments
  greater than 150 mm.
• SettlementSettlement is a downwards displacement
  of the ground surface, relative to either the
  surrounding ground (differential settlement) or a
  stable bench mark (total settlement). It is the
  result of a volume reduction in the ground
  beneath the surface, due to any of several
  possible mechanisms. (e.g. collapse,
  consolidation, (traffic) compaction). Sometimes
  called subsidence

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Section 2. General Backfill Requirements

• Contractor to be responsible for compliant
  backfill methodology, and for consequences (eg
  subsidence)
• Alternative materials or methods subject to
  Superintendent prior approval
• Safe working environment for testing
• Testing by NATA accredited lab
• Tester to choose test locations randomly
• Contractor may only direct where he needs
  additional tests.
• No testing above pipe within 500 mm of top of
  pipe. Otherwise beside pipe.
• Superintendent to record test location. No work
  until tests show compliance

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Section 3 Traffic Areas

• Application- backfill material for trenches in
  all Traffic Areas, including
• the full width of any existing or proposed road
  carriageway plus shoulders, and extending to one
  metre beyond the shoulders/kerb.
• the full width of any property access driveway,
  and extending one metre either side.
• the full length of any constructed footpath
  (including, but not limited to concrete, asphalt,
  crushed rock footpaths).
• the full width of any median strip.
• any other areas that are used as Traffic Areas.
• Road authority requirements govern if they are
  more stringent

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Section 3 Traffic Areas (cont)

• Backfill material
• In traffic areas, other than footpaths
• For trenches less than 1.5 metres deep
• 20 mm Class 2 Plant Mixed Wet Mix Crushed Rock,
  for the full depth.
• For trenches 1.5 metres deep or greater,
• 20 mm Class 2 Plant Mixed Wet Mix Crushed Rock
  for the top 600 mm.
• 20 mm Class 4 (or better) Crushed Rock for the
  remainder, or other backfill material
  specifically approved by the Road Authority.
• For trenches under footpaths
• 20 mm Class 4 (or better) Crushed Rock, or other
  backfill material specifically approved by the
  Road Authority.
• Backfill to be placed and compacted in layers
  200 mm loose thickness

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Section 3 Traffic Areas (cont)

• Compaction Testing
• All road crossings to be tested
• Base Course and Sub-base
• For every 50 metre length of trench (or part
  thereof) three tests shall be taken
• Within the top 100 mm (ie within the base course)
  of an existing road.
• In the depth range 100 mm to 300 mm depth (ie
  within the pavement sub-base).
• Backfill Under Roads, Road Shoulders, Median
  Strips (below sub-base)
• For every 40 metre length of trench (or part
  thereof) one test shall be taken per two layers
  of backfill
• Layers to be tested shall be selected randomly,
  in accordance with Clause 2.6. The position of
  the layer to be tested, and the location of the
  test, shall vary from test to test.
• Adjacent layers shall not be selected for
  testing.
• Backfill Under Footpaths
• As for roads, except that the testing shall be
  for every 100 metre length of trench (or part
  thereof).
• Note overall Contractor responsibility for
  performance (Cl. 2.2)

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Section 3 Traffic Areas (cont)

• Required Relative Compaction
• Modified compaction for reference density
• Minimum dry density ratio (AS 1289.5.4.1)
  required-
• The top 100 mm of pavement 98 (existing roads
  only)
• Below 100 mm from surface, where crushed rock
  95
• Below 300 mm from surface, where permitted to be
  sand min 85 Density Index, or 10 blows/300 mm
  PSP
• Local Authority requirements over-ride if more
  stringent

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Compaction

• Interpretation of test results
• Action to be taken according to following

• If a layer is rejected it is to be re-excavated,
  backfilled with new material, compacted and
  re-tested for compliance

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Cohesionless Soils (Sand)

• Sand fill, except beneath Traffic Areas
• Use of water
• Flooding not permitted
• Jetting only permitted where surrounding soil is
  also cohesionless
• Approved ITP in place
• Must achieve required compaction
• Pay for the water used
• Contain sediment
• 24 hour delay before testing
• For mechanical compaction, layers 400 mm, but
  as required to achieve compaction over full depth
  of layer

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Cohesionless Soils (Sand)

• Required Relative Compaction
• Road Reserves (other than Traffic Areas)
• Minimum Density Index of 65, or
• Minimum PSP resistance of 8 blows per 300 mm
• Other Areas
• Minimum Density Index of 60, or
• Minimum PSP resistance of 7 blows per 300 mm
• Review of test outcome (Section 4.6) similar to
  above
• Note possible need to static roll surface layer,
  and to test penultimate layer

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Cohesionless Soils (Sands)

• Testing Frequency
• Where testing using PSP
• Where using conventional testing

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Cohesive Soils (clays and clayey soils)

• Application
• Soils which will behave like a clay when placed
  and compacted
• Anywhere except Traffic Areas
• Flooding or jetting not permitted (ever)
• Fill to be moisture conditioned as necessary to
  allow compliant compaction
• In any event within range 85 to 115 of SOMC
• Contractor may need to trial layer thickness,
  etc.
• Some Ordinary Fill (e.g. from basalt rock
  excavation) may be acceptable backfill, but be
  unable to be tested
• Contractor to develop method specification for
  placement and compaction, and submit to
  Superintendent for approval prior to use

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Cohesive Soils (clays and clayey soils)

• Required Relative Compaction

Note that test means one field test plus one
laboratory test

• Note Contractor still responsible for overall
  performance

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Cohesive Soils (clays and clayey soils)

• Test result outcomes
• Test results shall be reviewed by the Contractor,
  and action taken in accordance with the following
  table

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Drives, Shafts , Tunnels and Bores

• Section 6 refers not part of Golder review
• Drives and tunnels must be refilled above the
  embedment zone using the following materials and
  methods
• 20 mm Class 4 crushed rock, or embedment concrete
  sand or 5 mm minus, pneumatically placed (blowing
  backfill into the excavation using compressed air
  through temporary piping that can be retracted as
  the excavation fills).
• Grouting, either by gravity, or under pressure.
  (Gravity grouting should only be used for tunnels
  where there is sufficient head.)
• etc

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Concluding Remarks

• Control of trench backfill important
• Watering in is not appropriate compaction,
  despite folklore
• Specification 04-03 represents a higher but
  necessary standard
• Keeping of records important
• Beware possible influence of granular fill in
  embedment zone, in reactive clay areas

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Thank You for your attention
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• Discussion?