appendix a - earthquake commission. residential... · 2015. 11. 1. · of residential properties in...

FINDINGS FROM THE GROUND IMPROVEMENT PROGRAMME

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Appendix AFact Sheets

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• What were the Canterbury Ground Improvement Science Trials?

• What was the Ground Improvement Pilot Project?

• What is a cone penetration test (CPT)?

• What is crosshole geophysical testing?

• What is T-Rex shake testing?

• What is blast-induced liquefaction testing?

• What are stone columns?

• What are Rammed Aggregate Piers?

• What are driven timber poles?

• What are reinforced soil-cement rafts?

• What are reinforced gravel rafts?

• What are Horizontal Soil Mixed (HSM) beams?

• What are standard specifications?

• What is a resource consent and why do I need one for ground improvement?

• What is a building consent and why do I need one for ground improvement?

Appendix AFact sheets

FACT SHEET

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What were the Canterbury Ground Improvement Science Trials?

The Canterbury Ground Improvement Science Trials (the Science Trials) involved the construction and testing of shallow ground improvement methods considered suitable for reducing the liquefaction vulnerability of residential properties. The Science Trials were carried out between April and December 2013 in the Christchurch residential red zone. The work was largely funded by the Earthquake Commission (EQC) and coordinated by Tonkin + Taylor (T+T).

Why were the Science Trials carried out and what were the objectives?

The Science Trials were undertaken to:

1. Develop robust and affordable ground improvement options, which could be constructed on residential properties for varying soil conditions to reduce liquefaction vulnerability

2. Test the effectiveness of each of the ground improvement methods in improving the performanceof liquefaction-vulnerable land.

What ground improvement methods were tested?

The following ground improvement methods were identified as options for cleared sites and tested:

• Rapid impact compaction

• Rammed Aggregate Piers

• Low mobility grout

• Driven timber poles

• Reinforced gravel rafts

• Reinforced soil-cement rafts

• Resin injection.

In addition, the following methods were trialled at properties under existing repairable houses:

• Permeating grouting

• Horizontal Soil Mixed beams – a new, innovative method for strengthening land vulnerable to liquefaction underneath existing houses.

How was the ground improvement tested/measured?

Each method was extensively tested and assessed usinga variety of methods, including:

• Cone penetration testing

• Crosshole geophysical testing

• T-Rex shake testing (truck-mounted vibrating)

• Blast-induced liquefaction testing

• Excavation and observations.

Preparation for blast induced liquefaction testing T-Rex – machine for liquefaction triggering testing

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WHAT WERE THE CANTERBURY GROUND IMPROVEMENT SCIENCE TRIALS?

Rapid impact compaction equipment

Installing steel casings for construction of low mobility grout ground improvement

Construction of Rammed Aggregate Piers

Construction of reinforced gravel raft test panel

What have been the benefits from theScience Trials?

Results from the Science Trials have advanced understanding of the performance of shallow ground improvement for scientists and engineers all around the world.

These effective shallow ground improvement methods are more affordable than deeper methods, giving property owners more ground improvement and foundation options than previously economically viable.

The results have also helped Ministry of Business, Innovation and Employment (MBIE) in updating the 2015 its guidance: Repairing and rebuilding houses affected by the Canterbury earthquakes, Section 15.3 update, Version 3a. This guidance adopted some of the shallow ground improvement methods included in the Science Trials.

Results from the Science Trials have advanced understanding of the performance of shallow ground improvement for scientists and engineers all around the world

FACT SHEET

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One of the main aims of the Pilot was to enhance understanding and acceptance of the ground improvement methods to promote a more holistic approach to buildingon residential land vulnerable to liquefaction

What was the Ground Improvement Pilot Project?

The Ground Improvement Pilot Project (the Pilot) involved the construction of a range of ground improvement methods in Christchurch and Kaiapoi betweenOctober 2013 and January 2015 across 31 residential properties with liquefaction vulnerability. The pilot was the second workstream in the GIP, following the Ground Improvement Science Trials. The work was funded by the Earthquake Commission (EQC) and coordinated byTonkin + Taylor (T+T).

What was the purpose of the Pilot?

The primary objectives of the Pilot were to:

• Establish a market cost for the selected shallow ground improvement methods

• Assess contractor capability to construct the works

• Enhance understanding and acceptance of theground improvement methods to promote a more holistic approach to building on residential land vulnerable to liquefaction

• Assess the practicality of constructing the works

• Establish what consents were required for construction.

What were the benefits of the Pilot?

Key benefits from the Pilot included:

• That shallow ground improvement methods were proven to be practical to construct on residential properties

• An increase in local contractor capability for residential ground improvement works

• Increased knowledge of typical construction costs and affordability of ground improvement

• Development of a standard specification for constructing ground improvement works

• Confidence in design and construction of ground improvement methods on residential properties.

How were properties selected for the Pilot?

One of the main aims of the Pilot was to test the selected ground improvement methods on a representative sample of residential properties in Christchurch and Kaiapoi.

After consultation with property owners, properties were selected based on:

• Their soil and groundwater conditions

• The size and location of the various areas vulnerableto liquefaction

• Whether they were roadside or rear propertiesfor ease of access

• Their position next to other selected properties forsome types of ground improvement

• Properties listed on the Hazardous Activities and Industries List (HAIL) or non-HAIL sites

• Whether there were obstructions, such as access bridges, streams and trees

• Health and safety considerations.

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WHAT WAS THE GROUND IMPROVEMENT PILOT PROJECT?

Roller compactor constructing reinforced gravel raft Construction of in-situ mixed soil-cement raft

Construction of stone columns Construction of driven timber poles

Directional drill constructing HSM beams

Which ground improvement methods were constructed?

The ground improvement methods used were:

• Reinforced gravel rafts

• Reinforced soil-cement rafts

• Stone columns

• Driven timber poles

• Horizontal Soil Mixed (HSM) beams.

The Pilot involved ten competitive tenders and onedirect appointment to select the contactors for the construction works.

FACT SHEET

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Tip

Friction

A cone penetration test

A penetrometer cone

A cone penetration test is used to determine geotechnical properties of soils.

The cone penetration test has become internationally one of the most widely used and accepted test methods for determining geotechnical soil properties. In Canterbury the data gained from a cone penetration test can be used to assess whether soil layers are likely to liquefy under different levels of earthquake shaking.

How is a cone penetration test undertaken?

The cone penetration test can be completed from the ground surface.

Cone penetration test rigs vary in size – from small portable rigs to large truck-mounted rigs. Each rig has benefits and limitations but they all conduct the same test. A cone penetration test rig pushes a steel cone (about 32mm wide) into the ground, generally up to 20m below the surface or until the cone reaches a hard layer. The steel cone contains an electronic measuring system that records tip resistance and sleeve friction.

As the cone is pushed into the ground, the soil responds with differing degrees of resistance. This resistance is recorded using force sensors in the tip.

At the same time as the sensors are recording resistance at the cone tip, sensors in the friction sleeve are recording sleeve friction along a 100mm length. Some cones also have a pore water transducer, which records water pressure in the soil. These readings can be used to determine ground water responses as the cone is pushed through the soils.

A cone penetration test typically takes between 30 minutes and three hours. As the cone goes into the ground, measurements are constantly sent back to the rig and recorded on computer.

What is a conepenetration test (CPT)?

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WHAT IS A CONE PENETRATION TEST?

An example of cone penetration test measurements

This data gives a profile of the subsoil layers, often calleda ‘trace’. Examples are shown to the right.

What do the test results tell us?

Cone penetration test results are used by geotechnical engineering specialists to understand the soil properties (the relative density of the soil and the soil behaviour type, both of which are calculated from the cone penetration test cone tip resistance and sleeve friction) and how the ground is likely to behave under different levels of earthquake shaking. This information can help in the design of foundations and ground improvements.

In Canterbury, cone penetration test results are commonly used to determine the liquefaction-triggering resistance of each soil layer. These assessments commonly use computer software to determine if soil layers are predicted to liquefy for different levels of earthquake shaking.

By doing a test before and after ground improvement works, cone penetration test results can also be used to determine how much strength a soil has gained following ground improvement works.

The cone penetration test is internationally one of the most widely used and accepted test methods for liquefaction assessment

Project: Christchurch TC3 Geotechnical Investigations Page: 1 of 1 NNB-POD12-CPT96 Test Date: 15-Jun-2012 Suburb: North New Brighton Operator: McMillanPre-Drill: 0.6m Assumed GWL: 2mBGL Located By: Survey GPS

Position: 2487010.08mE 5746665.62mN 4mRL Coord. System: NZMG

Address: 14 Pegasus Ave Comments:

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FACT SHEET

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Equipment used push probes into the ground Crosshole probe sensor

Crosshole geophysical testing determines the shear wave velocity of soils which provides an indication of it’s stiffness. Until recently this method has not been commonly used for liquefaction assessment and is relatively specialised.

How is it useful and is it required on my site?

Using shear wave velocity measurements from this test, the composite stiffness of the soil – combined stiffness of the soil and ground improvement – can be determined. This is particularly useful for some ground improvement methods, such as Rammed Aggregate Piers, which increase the composite stiffness of the soil to improve the performance of the ground.

Crosshole geophysical testing is useful in two key ways:

• It can be used very soon after ground improvement construction

• It assesses soil properties that are not determined from cone penetration testing. This data is complementary to cone penetration test data and is particularly useful when assessing the effectiveness of the ground improvement construction.

What is crossholegeophysical testing?

Owing to the high cost of this test, the cone penetration test method is generally used. However, crosshole geophysical testing may show that the ground has been suitably improved in some cases where cone penetration tests are unable to demonstrate an equivalent level of improvement. In addition, it may be used to verify variations of commonly accepted designs, which cansave construction time and costs.

How is a crosshole geophysical test performed?

A crosshole geophysical test is performed using two special probes that are pushed into the soil using small machines with hydraulic rams. Both probes are pushed in vertically and parallel approximately 1.5m apart and in small depth increments. The probes are mounted at the tip of a rod string and measure the shear wave velocity between the two probes. Shear waves are generated by striking the push rods (attached to the probes in the ground) with hammers. One probe acts as an emitter of the shear waves, the other as the receiver of the waves generated in the first probe.

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WHAT IS CROSSHOLE GEOPHYSICAL TESTING?

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Crosshole geophysical testing of Rammed Aggregate Piers. Number represents testing sequencing


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Who can do these tests?

Currently, there is limited capability to do this testing in New Zealand. A small group of local contractors, consultants and academics have been trained to do this

Crosshole geophysical testing can determine the composite stiffness of ground improvement and the surrounding soils to assess the effectiveness of construction

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test by academic staff from the University of Texas at Austin. Capability is likely to expand if this test method is more frequently used.

FACT SHEET

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‘T-Rex’ – a shaker truck used for liquefaction shake testing

Cross-sectional perspective of the T-Rex in place to shake the Rammed Aggregate Pier ground improved test panel

Liquefaction shake testing, or ‘T-Rex shake testing’, is a specialised method to test the soil behaviour of the natural ground or where ground improvement has been constructed. A T-Rex shake test involves shaking the ground in stages, with shaking becoming stronger after each stage of shaking. During shaking, instrumentation in the ground monitors the response of the soil.

How is T-Rex shake testing done?

To shake the ground, a ‘shaker’ truck is used. This specialist truck, the ‘T-Rex shaker’, was brought to Christchurch from the University of Texas at Austin.

The T-Rex is a 29-tonne truck that can produce large vibrations by shaking. The truck is a large all-wheel drive vehicle with the shaker set between the front and rear axles. It is able to shake test the ground with sufficient energy to trigger liquefaction to a depth of 3m - 4m below the surface.

Why do we use the T-Rex and what does it test?

Researchers used T-Rex shake testing to help determine shear strain and pore water pressure response of the natural unimproved ground to different levels of shaking.

The T-Rex was then used to examine how effective various ground improvement methods were at reducing shear strains and pore water pressure build up, thereby increasing the liquefaction resistance of the soils. By trigger shaking the ground to liquefaction, researchers were able to measure how much each ground improvement method strengthened the ground compared to the unimproved natural soils.

T-Rex shake testing does not adversely affect the ground being tested, so it can be repeated on the same area of constructed ground improvement for many different levels and durations of shaking. The truck uses a staged approach where it increases vibrations to induce liquefaction.

What is T-Rex shake testing?

A T-Rex shake test examines how effective various ground improvements are at developing a thickernon-liquefying crust

RammedAggregate Piers

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Dynamic Shaking

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FACT SHEETFACT SHEET

Blast-induced liquefaction testing was used in the Ground

Improvement Science Trials. The method involves detonating

explosive charges in the ground to liquefy the surrounding soils.

Why undertake blast-induced liquefaction testing?

The blast-induced liquefaction testing was used to assess

whether the shallow ground improvement methods being trialled

were providing a suitable level of ground performance, to test

whether the methods could reduce the effects of differential

ground surface subsidence as a result of liquefaction occurring in

the underlying soils.

How does blast-induced liquefaction testing work?

The method involves installing explosives at multiple depths in the

ground below the constructed ground improvement test areas.

Concrete blocks are put on the ground above to simulate the load

that a house applies to the ground surface. The explosives are

then detonated in a defined sequence to induce liquefaction of the

soils beneath the ground improvement test areas.

How is blast-induced liquefaction testing useful?

The blast-induced liquefaction testing is capable of liquefying the

soils to depths of approximately 10m - 12m beneath the ground

improvement test areas (equivalent to the depth and extent of

liquefaction caused by a large earthquake). Following liquefaction

of the underlying soils, the soil structure consolidates into a denser

arrangement, causing the ground to subside and water and sand

to come to the surface. Because soil layers in the ground are not

uniform, and because liquefaction can result in the ejection of soils

to the surface, often the ground will subside unevenly causing

undulations, tilting and distortions. It is these changes in the ground

surface that cause a lot of the earthquake damage to house and

foundations.

Prior to and following the blast testing, the ground improvement

test areas were surveyed using a variety of high-precision

techniques. The surveying was repeated after the blast test. This

allowed the researchers to measure how well the ground had

performed with different types of ground improvement methods.

Test panels with no ground improvement were also tested to

compare improved ground with non-improved ground. Ground

improvement methods that work well result in a low level of uneven

ground surface subsidence.

What is blast-induced liquefaction testing?

Ground improvement testing area ready for blasting

Diagram showing explosive charges (red) at multiple depths below the area of ground improvement

Blast-inducedliquefaction testing assessed whether ground improvements reduced uneven ground surface subsidence

FACT SHEET

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Stone column ground improvement involves adding vertical columns of stone into the ground to a depth of at least 4m below the ground surface. A layer of compacted gravel can then be put over the top of the columns, ready for the construction of new house foundations. The stone column method is quick to construct and can be done at any time of the year.

How are stone columns constructed?

Stone columns are constructed by experienced contractors using specialist equipment. The construction uses an excavator with a vibrating probe to feed stone into the ground, forming a vertical column of stone.

Some stone column rigs feed stone into the ground through the vibrating probe, exiting at the bottom, and other rigs require the stone to be fed in from the ground surface down the vertical hole in the ground. Both types use a vibrating probe that densifies the surrounding soils to help feed the stone into the ground.

How do stone columns improve the ground?

Stone columns help to limit the amount and consequences of futureliquefaction by:

• Densifying the soil through vibration and introducing stone into the soil

• Reinforcing the soil creating a stiff composite soil mass.

By achieving this, the non-liquefying soil crust is thickened and stiffened to reduce the likelihood of undulations, tilt and uneven ground surface subsidence from liquefaction of the underlying soil layers, therefore reducing damage to the house foundations.

In addition, stone columns may sometimes provide the soil with an increased drainage path to help reduce excess pore water pressure that can lead to liquefaction, so the columns can reduce the consequences of liquefaction when this occurs.

What are stone columns?

The stone column method is quick to constructand can be constructedat any timeof the year

Stone columns Typical triangular grid installation pattern

Non-LiquefiableCrust

Liquefiable Soil

Rammed Aggregate Piers (RAP)

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Specialist stone column equipment and stockpile of stone (gravel)

An advantage of stone columns is that no dewatering or excavation is required for the construction and they typically have a short construction period.

What soils suit stone columns?

Stone columns are best suited to sandy soils. A greater concentration of stone columns are required in siltier soils.

Because of the large equipment required and the requirement for an area to store the stone (gravel), this method may not be practical for smaller properties or those with limited access.

Stone columns are best suited to sites with sandy soils

WHAT ARE STONE COLUMNS?

FACT SHEET

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What are RammedAggregate Piers?

An advantage of Rammed AggregatePiers is thatno dewatering or excavation is required for the constructionand they typicallyhave a short constructionperiod

Geopier Rammed Aggregate PierTM System (RAP) Typical triangular grid installation pattern

The Geopier Rammed Aggregate Pier™ System is a patented proprietary ground improvement system that comprises a series of piers constructed of uniformly graded aggregate (or stone) installed using specialist equipment. The piers are installed vertically in the ground in either a triangular or square grid to at least 4m depth below the ground surface. A layer of compacted gravel is then put over the top of the piers, ready for the construction of new house foundations.

How do Rammed Aggregate Piers improve the ground?

Rammed Aggregate Piers help to limit the amount and consequencesof liquefaction by:

• Displacing soil laterally to densify the soil and increase soil stiffness

• Reinforcing the soil creating a stiff composite soil mass.

By achieving this, the non-liquefying soil crust is thickened and stiffened to reduce the likelihood of undulations, tilt and uneven ground surface subsidence from liquefaction of the underlying soil layers, therefore reducing damage to the house foundations.

How are Rammed Aggregate Piers constructed?

Rammed Aggregate Piers are installed by experienced contractors, using specialist equipment involving an excavator with a vibrating mandrel attachment. An excavator or tele handler continually loads stone into the hopper, feeding it down the mandrel and into the ground. The stone is compacted using a ramming and vibrating action to form a stiff, high-density, vertical aggregate pier within the ground.

An advantage of Rammed Aggregate Piers is that no dewatering or excavation is required for the construction and they typically have a short construction period.


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Rammed Aggregate Piers (RAP)

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WHAT ARE RAMMED AGGREGATE PIERS?

What soils suit Rammed Aggregate Piers?

Rammed Aggregate Piers are best suited to sandy soils. A greater concentration of Rammed Aggregate Piers are required when used in siltier soils.

Because of the large equipment required and the requirement for an area to store the stone, this method may not be practical for smaller properties or those with limited access.

Equipment required to construct Rammed Aggregate Piers

RammedAggregatePiers are best suited to sites with sandy soils

FACT SHEET

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Driven timber poles or driven timber piles are installed vertically in the ground in

a triangular grid pattern to at least 4m depth below the ground surface.

A layer of compacted gravel can then be placed over the top of the installed

poles, ready for the construction of new house foundations.

Construction of driven timber poles ground improvement is fairly quick and

simple, generally taking less than one week for most residential properties.

The installation of timber poles can only be constructed on land clear of buildings.

How do driven timber poles improve the ground?

Driven timber pole ground improvement helps to limit the consequences

of future liquefaction by:

• Displacing soil laterally to densify the soil, and increase soil stiffness between

poles. This increased stiffness improves the soils resistance to liquefaction

and its damaging effects

• Redistributing the vertical stresses, thereby resulting in more even settlement

of the ground surface.

How are driven timber poles installed?

Timber poles may be driven into the ground using either a pile-driver or a

vibrating plate mounted on a tracked excavator. The vibrating plate method

causes less noise and potentially damaging vibrations and is therefore the

preferred option when working in residential areas.

Timber pole installation is quick to construct and can be carried out at any

time of the year.

What are driven timber poles?

Construction of driven timber pole ground improvementis fairly quick and simple, generallytaking less than one week for most residentialproperties

Liquefiable Soil

Ground water table

TopsoilCompactedgravel fill

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Driven timber poles Driven timber poles with a new house constructed above

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WHAT ARE DRIVEN TIMBER POLES?

Left: Driven timber pole installation during the Ground Improvement Pilot Project using a pile-driver mounted on an excavator

Above and below: Timber pole installation during the Ground Improvement Pilot Project, using a vibratory plate fixed toan excavator

Where can driven timber poles be used?

Driven timber poles can be used in almost all soil types.

Timber poles may be difficult to install in dense soil layers. Dense soils can cause unacceptable vibration on neighbouring properties and stress on the piling rig and may damage the top and base ends of the poles. However, as contractors become more experienced (and more suitable installation equipment is used) this is likely to become less of an issue.

In some cases pre-drilling holes (for the timber poles) may be needed. However, pre-drilling requires careful design and construction monitoring to ensure sufficient densification of soil between the timber poles and sufficient contact between the surrounding soil and the timber pole is achieved.

One advantage of driven timber poles it that they can easily be installed on smaller properties or those with restricted access, close to property boundaries and against fences and other obstacles.

FACT SHEET

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Non-LiquefiableCrust Cement stabilised soil

GeogridLiquefiable Soil

What are reinforced soil-cement rafts?

Reinforced soil-cement rafts involve the construction of a

1.2m thick compacted crust made of cement-stabilised soil

with reinforcing mesh called a geogrid.

Once constructed the raft provides a stable platform to

build a house. These rafts can be used in areas vulnerable

to liquefaction. The platform is designed to reduce the

effects of liquefaction in a future earthquake.

Reinforced soil-cement rafts can only be constructed on

land clear of buildings.

How do reinforced soil-cement rafts work?

Reinforced soil-cement rafts are designed to limit the

consequences of liquefaction. They are designed to be

stiff enough to limit undulations, tilt and uneven ground

surface subsidence, therefore reducing damage to house

foundations. A raft will also prevent the ejection of soils

beneath the house foundations when the underlying soils

are liquefied from earthquake shaking, therefore reducing

the likelihood of localised ground surface subsidence.

How are reinforced soil-cement rafts constructed?

To form a reinforced soil-cement raft, soil is excavated to

a minimum depth of 1.2m. Excavated topsoil is usually

kept and later spread across other parts of the site, as it is

unsuitable for cement stabilisation. The excavated suitable

soil is progressively mixed with approximately 8% cement (by weight). This has traditionally been done using a machine called a pug mill but has also recently beendone using a rotovated soil mixing method.

A pug mill is a specialist machine that mixes soil and cement, however, it is relatively expensive and oftentoo large for use in residential areas. It is commonlyused on commercial or road construction projects.

Rotovated soil mixing was trialled in the Ground Improvement Pilot Project. Rotovated soil mixing uses a small tractor with a rotovator (like a rotary hoe) attachment to mix cement spread on the ground intolayers of placed soil.

To construct the raft, a layer of soil (typically no more than 200mm thick) is mixed and spread around the base of the excavated area and a roller compactor is used to compact the soil-cement mixture. Near the base of the raft one or two layers of geogrid are rolled out across the raft at different depths. Additional soil-cement layers are then put over the top and compacted to the ground surface.

Construction in wet weather should generally be avoided, particularly when working at the upper layers of the raft. Delays from surface water ponding can result fromheavy rainfall.

Non-Liquefiable CrustGround water table

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Reinforced cementstabilized raft

Note: Two layers of geogrid are required in areas of “major” lateral spread

Geogrid

Topsoil

Soil-Cement Raft under construction A Soil-Cement Raft beneath a rebuilt house

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WHAT ARE SOIL-CEMENT RAFTS?

In-situ cement stabilisationA pug mill as sometimes used to mix soil and cement together

A small tractor and rotovator attachment used to mix cement with soil

What soils suit reinforced soil-cement rafts?

Reinforced soil-cement rafts are generally suitable for most soils found in Canterbury, including silts and sands. However, very weak and highly-compressible peat/organic soils are unsuitable for cement stabilisation or for founding a raft on. These unsuitable soils may be disposed of and more suitable soil for cement stabilisation used. An advantage of these rafts is that they can be used in areas with lateral spread vulnerability.

Soil-cement raft construction

What about in-situ mixing?

An alternative method for constructing soil-cement rafts is ‘in-situ mixing’. This method uses specialised machinery to mix cement into ‘in-situ’ soil, without having to excavate the soil.

The strength of a mixed in-situ raft relies almost entirely on the binding effect of the cement. The soil is not compacted (like it is in a mixed ex-situ raft) and does not have any installed geogrid. For this reason an in-situ mixed raft needs more cement and needs to be thicker (2m deep) to offset the expected lower strength/consistency. Experience has shown that high level of quality control is required to construct rafts with suitably well-mixed soil to the required strengths and consistency.

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FACT SHEET

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Reinforced gravel rafts involve the construction of a 1.2m-deep compacted ‘raft’ of engineered aggregate (gravel).

Once constructed the shallow gravel raft provides a stable platform on which a house can be built on land vulnerable to liquefaction. The platform is designed to reduce the effects of liquefaction in a future earthquake.

Reinforced gravel rafts can only be constructed on land clear of buildings.

How do reinforced gravel rafts work?

Reinforced gravel rafts are designed to limit the consequences of liquefaction. They are designed to

be stiff enough to limit undulations, tilt and uneven

ground surface subsidence, therefore reducing damage

to house foundations.

How are reinforced gravel rafts constructed?

Construction of reinforced gravel rafts is relatively

straightforward and can be completed with standard

earthworks machinery. Using excavators (commonly 14 to

20 tonne), soil is excavated down to the designed depth of

the rafts, typically around 1.2m thick.

Excavated soil is removed. Existing topsoil may be stored

for gardens and lawns. Layers of crushed gravel are spread

A shallow reinforced gravel raft provides a stable platform on which a house can be built in areas vulnerable to liquefaction. The platform is designed to reduce uneven settlement caused by liquefaction in a future earthquake

What are reinforced gravel rafts?


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Gravel Raft (M4/40 gravel)

Geogrid

Note: Three layers of geogrid are required in areas of “major” lateral spread

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Reinforced gravel raft under construction A reinforced gravel raft beneath a rebuilt dwelling


GeogridLiquefiable Soil

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WHAT ARE REINFORCED GRAVEL RAFTS?

Reinforced gravel raft – geogrid and gravel installation Roller compactor used to compact gravel

over the base of the excavation and compacted using a 1.5 to 10 tonne roller compactor.

Near the base of the raft two or three layers of geogrid are rolled out at different depths. Additional gravel layers are placed into the excavation and compacted, all the way up to the ground surface.

Construction in wet weather should generally be avoided as flooding can result.

Reinforced gravel raft construction

What soils suit reinforced gravel rafts?

Reinforced gravel rafts are suitable for most soil conditions encountered in Canterbury, including silts and sands. However, very weak and highly-compressible deep organic soils are unsuitable for the raft as these soils make gravel placement and compaction difficult. In addition, the future weight of the raft and house may cause excessive consolidation settlement of these weak or compressible underlying soils. The raft may be thickened so it is founded on a more suitable soil. An advantage of these rafts is that they can be used in areas with lateral spread vulnerability.

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FACT SHEET

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Horizontal Soil Mixed (HSM) beams are made from the soil present under an existing house and introduced grout (a mixture of water and cement) which is mechanically mixed together, creating cylindrical-shaped, horizontal beams of cement-stabilised soil.

How do HSM beams improve the ground?

HSM beams are significantly stiffer and stronger than the natural ground that surrounds them. The beams are constructed at the uppermost ground elevation where there is liquefiable soil, typically near to the groundwater surface. The HSM beams are non-liquefiable and do not lose their strength during earthquake shaking. Testing has shown that at lower and moderate levels of shaking, the soil between the beams also does not liquefy. The net result is that the house has a thicker and stiffer ‘crust’ of non-liquefying soil to support it during an earthquake. This means the improved ground and house foundations are likely to suffer less damage during earthquakes.

Testing showed that a double row of beams provided significantly better protection against uneven ground surface subsidence than a single row of beams.

By preventing liquefied soils ejecting from the ground, the likelihood of localised ground surface depressions occurring is reduced. Such localised depressions can cause damage to house foundations.

HSM beams decrease the vulnerability of the existing building to liquefaction-related damage. However, in the event that future building work is done for an extension or rebuilding of the existing house, further assessment and work may be required to ensure resilience of the extension or new building to liquefaction-related damage.

How are HSM beams constructed?

To construct an HSM beam, directional drilling equipment is used to pilot a horizontal borehole beneath an existing house. This equipment ‘day-lights’ in a receiving trench on the opposite side of the house.

A 500mm diameter mixing tool is then attached to the end of the drill. This is progressively dragged back along the alignment of the horizontal borehole. Grout is pumped through the drill rods to the mixing tool, which mixes grout into the surrounding soil.

This process leaves a horizontal beam of cement-stabilised soil in the ground. The beam-formation process is repeated to make layers of HSM beams below the house.

What soils suit HSM beams?

HSM beams can be constructed in silty and sandy soils but not in peat/organic soils. Construction of HSM beams in gravels has not been trialled.

Not every house on suitable soils can have HSM beams constructed. The construction method requires space around a house for equipment and some small excavation trenches.


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Horizontal Soil Mixed beamsbeneath a repairable house

The improved ground and house foundations are likely to suffer less damage during future earthquakes

What are HorizontalSoil Mixed (HSM) beams?

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WHAT ARE HORIZONTAL SOIL MIXED (HSM) BEAMS?


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Construction methodology of Horizontal Soil Mixed beams


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The positioning of the beams in the ground is typically determined by the depth of the groundwater surface. Where this is too high, the beams would need to be constructed too close to the house foundations, which could cause damage. Where it is too low, construction becomes difficult or unachievable because of the depth at which the beams would need to be formed.

HSM beams are considerably more expensive compared to cleared-land solutions that can be constructed on properties without houses. HSM beams are considered suitable for improving the resilience of an existing house in future earthquakes. However, while they will improve the performance of the existing houses, they are unlikely to provide the same level of performance as cleared-land ground improvement methods.

Constructed HSM beams beneath a house Exposed HSM beam

This stabilised soil is much stiffer and stronger than the surrounding soil

FACT SHEET

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What are standard specifications?

A specification is a document that gives contractors guidance during construction works. During 2014 and 2015, a group of geotechnical specialists prepared some standard specifications for small-scale ground improvement works typically required for single residential properties.

How does a specification fit into the contract agreement?

The flowchart below shows where a standard specification document aligns with other documents required for a contractual agreement before works begin on site. These documents are provided so contractors can estimate a construction cost.

The standard specifications are intended to:

• Provide a guidance document for use by individuals andorganisations involved in designing and constructingground improvement works

• Lower overall industry costs to design works

• Lower overall construction costs as techniques,specifications and materials are standardised

• Standardise and improve consistency in groundimprovement design and construction

• Support the MBIE guidance document on Repairingand rebuilding houses affected by the Canterburyearthquakes.

Contract Agreement

Drawings

Specification

Schedule of Prices/Basis of Payment

Site Specific Information

Not part of the standard specification

• Including certification requirements (PS3, PS4 etc.)

Currently not part of the standard specification

Preliminary and General Requirements – Section 1 of the standard specification

• Requirements particular to the repair method i.e. Sections 2, 3, 4or 5 of the standard specification

• Other project/site specific particular requirements

Currently not part of the standard specification

Not part of the standard specification

• Geotechnical investigation data

• Consent conditions

• Contamination test data etc.

Use of the Standard Specification within a contractual agreement

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WHAT ARE STANDARD SPECIFICATIONS AND WHAT ARE THE BENEFITS?

The standard specifications cover several sections of a tender document for proposed ground improvement works. These include the preliminary and general requirements, particular repair methodology and other project or site-specific requirements.

This may be used throughout the Canterbury region during reconstruction and in the future. Although the document is being written for the Canterbury recovery, it was recognised that it may also be a useful guide for other residential areas within New Zealand that are vulnerable to liquefaction.

Who was involved in developing the standard specifications?

Specialists from the following companies/organisations were involved in developing the standard specifications:

• Tonkin + Taylor

• Beca

• Coffey Geotechnics

• Golder Associates

• Aurecon

• Brian Perry Civil

• Hiway Geotechnical

• Betterground

• Canterbury Earthquake Recovery Authority

• Earthquake Commission

• Ministry of Business, Innovation and Employment.

What ground improvement methods have standard specifications?

Standard specifications are available for the following shallow ground improvement methods:

• Densified crusts

• Stabilised crusts (ex-situ / rotovated or in-situ mixed)

• Stone columns

• Driven timber poles.

Where can I find the standard specifications?

The standard specifications are expected to be available from the New Zealand Geotechnical Society in late 2015.

Are there any limitations?

It is intended that the standard specifications are used as general guidance for technical specification for the four ground improvement methods. Specifications can be varied by a project engineer as appropriate. Such variation would depend on the type, size, quality and performance requirements of any particular project.

Other general engineering considerations, such as ground surface subsidence, slope stability, lateral spreading and flooding, still need to be taken into account where appropriate. This may require additional site-specific geotechnical investigations and additional ground improvement not covered by the MBIE guidance document or the standard specifications.

FACT SHEET

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The Resource Management Act 1991 (RMA) promotes the sustainable management of natural and physical resources and seeks to protect land and the environment. District and regional councils are responsible for preparing the suite of plans and policy documents that provide a framework for implementing the RMA purpose and principles. These plans set out the resource consent requirements for different activities.

Some ground improvement activities are ‘permitted’ under the relevant regional and district plans. You are required to apply for resource consent if an activity is not permitted in the relevant plans.

Who can grant resource consents?

In Canterbury, resource consents are granted by the district/city council (Christchurch City Council [CCC], Waimakariri District Council [WDC] or Selwyn District Council [SDC]) and/or the regional council, Environment Canterbury (ECan), depending on which rules trigger the need for consent.

Even if an activity is permitted under the relevant planning rules, documentation may need to be submitted to the local council both before and after works have taken place, for example, a statement of professional opinion or producer statement, and as-built plans from a suitably qualified and experienced geotechnical engineer.

How do I get a resource consent?

Applications for a resource consent can be made to either the district/city or regional council, depending on rules that trigger the need for consent. They must be accompanied by an assessment of effects on the environment (AEE) report. Information provided in the district and regional

What is a resource consent and why do I need one for ground improvement?

plans, and guidance from council staff and RMA planning consultants, can help in determining what information is needed. In some cases, specialist advice may be required to prepare an AEE report.

It is recommended that a RMA planning consultant is engaged to assess any proposed ground improvement works against the land repair provisions to confirm the resource consent requirements. Even if resource consent is not required for ground improvement works, compliance with the performance standards set out in the land repair rules developed by ECan, CCC, and WDC is required for the works to be permitted. These performance standards include measures for erosion and sediment control, construction noise and vibration and hours of work.

Other consents and approvals

It is important to note that additional consents and approvals may be required prior to doing the works, such as if the site is potentially contaminated. Ground improvement works on contaminated sites may require consents under the provisions of the Resource Management (National Environment Standard for Assessing and Managing Contaminants in Soil to Protect Human Health) Regulations 2011 (NES Soil).

Where the proposed works have the potential to disturb sites of archaeological or cultural interest an ‘Authority to Modify’ may be required from Heritage New Zealand under the Heritage New Zealand Pohere Taonga Act 2014. Approvals may also be required where equipment or machinery necessary to undertake the works occupy the public open space or the road reserve or where dewatering discharges are pumped to the stormwater networks.

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FACT SHEET

A building consent is a requirement of the Building Act 2004, which aims to ensure that construction, alteration, demolition and maintenance of new and existing buildings is of a suitable standard. Because ground improvement is related to building foundations, a building consent or a building consent exemption is required, to ensure that the ground improvement works are suitable for the structure that is intended for the site.

Who can grant building consents?

It is recommended to apply for a building consent for the construction of the ground improvement and for a new house.

In Christchurch city, a building consent must be granted by the Christchurch City Council (CCC) prior to undertaking ground improvement. If ground improvement is intended to be constructed prior to planning the construction of a new house, CCC may grant a building consent exemption. This exemption may only be granted as long as the planned ground improvement is in accordance with the April 2015 update of the MBIE guidance on Repairing and rebuilding houses affected by the Canterbury earthquakes.

Waimakariri District Council (WDC) and Selwyn District Council (SDC) do not require building consents prior to undertaking ground improvement works. The constructed ground improvement and house plans are assessed to ensure that the ground improvement and house foundations are compatible with the Building Act 2004.

What is a building consent and why do I need one for ground improvement?

To ensure compatibility, it is recommended that ground improvement and the new house design be done prior to the application for building consent and any construction works begin on site.

How do I get a building consent?

Application for a building consent can be made to the local council. For a building consent to be granted for ground improvement, a Chartered Professional Geotechnical Engineer will need to provide the local council with site investigation information and a suitable design of the proposed ground improvement.

The design may be in accordance with the MBIE guidance, or may be a bespoke design that the engineer can show to be suitable for the ground conditions on the property. The ground improvement design must also be compatible with the proposed new house foundations (an engineer and/or licenced building practitioner will provide the details).

Other consents and approvals

It is important to note that additional consents and approvals may be required before undertaking the works.

appendix a - earthquake commission. residential... · 2015. 11. 1. · of residential properties in...

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