1| Building drainage structures in expansive soils (WQ37) Shane Crawford, Principal Engineer (Structures Management Services)
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Building drainage structures in
expansive soils (WQ37)
Shane Crawford, Principal Engineer (Structures Management Services)
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Agenda
• Background
• Expansive Soil Potential
• Design Methodology
• Investigations and
Management Options
• Main Considerations.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Background
• Numerous culverts present throughout the state
• Culverts most cost-effective structure for the crossing of
minor waterways and floodways
• Typical construction types:
• Reinforced Concrete Box Culverts (RCBC)
• Slab Link Box Culverts (SLBC)
• Slab Deck Culverts (fully cast-insitu)
• Relatively simple structures, easy to construct.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Background
• Instances of unsatisfactory performance observed
• Cracking and separation of aprons and culvert base slabs
• Irregularities in culvert/road alignment (post-construction)
• Cracking and/or rotation of abutments/wingwalls
• Excessive movement in approach embankments
• Often the result of volume changes in expansive soil
foundations and embankments
• Can lead to substantial reductions in service lives of
structures.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Expansive Soil Potential
Expansive soils can cause problems in the following
climates:
• Arid climates, where dessicated (cracked) soils are subjected
to unusually high rainfall, causing soil to swell and expand
• Semi-arid climates, where moisture condition of the soil reflect
the wet-dry seasonal cycle and may be subjected to extremes
of drought and flood
• Predominantly wet soils, which may be subjected to a
prolonged period of drought and exhibit drying shrinkage.
Western soils generally fall into the first two categories.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Expansive Soil Potential
These volume-change characteristics are controlled by three factors:
• Intrinsic expansiveness of the soil (measured by the shrink-swell index)
• Suction change (site specific and tied to atmospheric conditions)
• Applied stress.
Changes in soil moisture lead to suction changes, producing a loading/unloading effect and resulting volume changes within the ‘active depth’ of the soil (generally about 2m).
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Expansive Soil Potential
Expansive soil problems generally tend to occur with soils
which:
• have Linear Shrinkage greater than 8 – 10% and/or swell
strains grater than 5% at OMC
• are located in a region subjected to concentrated rainfall
patterns, followed by prolonged periods of dry weather.
Therefore, when constructing culverts in zones that match
the soil characteristics and climates as described above,
particular design and/or construction considerations need to
be adopted to avoid damage and premature failure.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Design Methodology
TMR standard drawings are based on certain design
assumptions, and are not suitable for all circumstances.
For standard culvert drawings, it is assumed that:
• Foundations are moisture insensitive – differential settlement
due to moisture changes are not considered
• Minimum ultimate bearing capacity under the culvert base slab
is 150 kPa (preferably in excess of 200 kPa).
Sites comprised of expansive soils below the culvert base
are outside of the adopted design methodology for these
standard drawings.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Foundation Investigation
Preliminary testing should be undertaken to determine if
foundation is expansive (linear shrinkage >8% and/or swell
strains >5%).
If confirmed, then additional testing required to determine
extent of the actual expansive zone. This may include: • Trenching or drilling up to a depth of 2m at proposed location
• In-situ moisture content and density testing at intervals of:
• Every 300mm in depth, or
• At each change of soil horizon
• 50mm undisturbed tubes taken from each soil horizon
• Adequate materials taken from each location for additional
laboratory testing.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Management Options
Options for the management of expansive soils in these situations can be categorised as:
• Geotechnical – these may either reduce the expansive potential of the soil, or minimise the seasonal fluctuations of the subgrade moisture
• Structural – typically involving increasing the capacity of the culvert components to accommodate differential movement of the subgrade.
These may be used separately or in combination.
In some cases, short-span bridges may be a feasible option.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Geotechnical Methods
Two main approaches:
• Volume stability – reducing the expansive potential of the substrate material through• Excavation of the foundation and replacement with a low permeability,
granular / non-swelling material
• Chemically treating the existing material (lime stabilisation)
• Ripping, scarifying and compacting the soil with moisture controls.
• Moisture stability – control of foundation moisture fluctuations through• Pre-wetting or ponding foundations prior to construction
• Stabilisation of foundation moisture conditions by introduction of a physical limit, such as vertical moisture barriers
• Extending the concrete apron with a flexible apron (grout filled mattress or rock mattress), with an impermeable membrane placed beneath it.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Structural Methods
Possible options are as follows:
• Improved Layout of Culverts – limiting the lengths of banks of
culverts may provide a better structural solution. Series of
culvert banks rather than a long continuous run
• Stiffened Raft Foundations – used instead of conventional
culvert base slabs. Strengthening may only need to be applied
to the edges/perimeter of the slab
• Short-span bridges or modified culvert strip footings
incorporating micro pile/screw pile foundations.
Site-specific RPEQ advice generally necessary.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Construction Considerations
Construction practices which adversely affect the moisture
content of the soil should be restricted.
• Placement of permeable fill behind the culvert (either granular
or cement-stabilised sand)
• Opening a culvert base up for a long period when moisture
content is low
• Not allowing adequate time for the culvert base to reduce
moisture content after a prolonged wet period.
Where such activities are necessary, specialist advice may
be required.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Managing Existing Structures
• Not covered in WQ37
• Many older concrete structures affected
• Unreinforced concrete components
• Possible options
• Monitor through inspection regime, particularly after flood events
• Minor repairs to resist expansive forces
• Vertical channels at abutments
• Strapping / banding of wingwalls
• Significant strengthening
• Re-casting of concrete components
• Cost vs benefit.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018
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Main Considerations
• When constructing culverts in zones that match the soil
characteristics and climates as described above,
particular design and/or construction considerations need
to be adopted to avoid damage and premature failure
• Design Methodology behind TMR standard drawings for
culverts does not accommodate expansive soils
• When working with expansive soils• Undertake foundation investigations in order to fully understand
the extent of the affected zone
• Management Options – tailored to suit the site
• Geotechnical
• Structural
• A combination of the two.
Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018