Building drainage structures in expansive soils...

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Building drainage structures in

expansive soils (WQ37)

Shane Crawford, Principal Engineer (Structures Management Services)

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About us

Building Drainage Structures on Expansive Soils (WQ37) | 30 May 2018

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Agenda

• Background

• Expansive Soil Potential

• Design Methodology

• Investigations and

Management Options

• Main Considerations.


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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.


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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.


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Truck componentsCracked Base Slab/Apron

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Abutment/Wingwall Interface

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Abutment Cracking

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Approach Embankment/Protection

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Swelling/Expansion of Substrate

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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.


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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).


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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.


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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.


15 |15 |Demolition of the Angellala Creek Bridges | 28

November 2017

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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.


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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.


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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.


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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.


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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.


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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.


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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.


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Thank you and stay connected

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LinkedIn Department of Transport and Main Roads

Blog blog.tmr.qld.gov.au

Date post:	27-Mar-2020
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Building drainage structures in expansive soils...

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