Video: How to specify ground improvements for properties prone to liquefaction

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Welcome to this Build It Right presentation - Specification of ground improvements in residential areas prone to liquefaction.

This is an introduction to Module 5A of the Earthquake Geotechnical Engineering education series prepared by a partnership between the Ministry of Business Innovation and Employment and the New Zealand Geotechnical Society.

This’ll give you practical assistance in preparing documentation to make your tendering, consenting and building processes more consistent and smoother.

First a bit of background. While this guidance has been developed specifically for Canterbury, we expect it will be useful for other parts of New Zealand prone to liquefaction.  In these other areas you’ll need to take care the guidelines are applied appropriately.

The guidelines will be especially useful  for geotechnical designers and project managers.

If you’re specifying ground improvement works in residential projects, Module 5A provides you with standardised specification clauses.

We want to:

- encourage the use of ground improvement in residential sites when it’s needed
- reduce your contract drafting and construction costs
- support the MBIE residential and repair guidance for Canterbury. 

It is useful to first understand the sequence of events that led to the preparation of this Guidance.

The 2010 and 2011 Canterbury earthquake sequence generated extensive liquefaction damage which affected some 50,000 properties. A large percentage of this damage was related to liquefaction of the upper soil layers.

Due to the shortage of building resource – and the extent of the repair programme - MBIE looked at a number of strategies for the rebuild, including repair and rebuild guidance. Guidance was progressively updated as more information became available. 

MBIE also undertook field trials at QEII park which included ground improvement measures. The results of these trials were incorporated in an update to guidance in 2012.

EQC, with the support of MBIE, undertook a science trials project in 2013/14 – which tested various foundation and ground improvement systems.

They also initiated a pilot project to establish what lessons could be learnt from installing selected ground improvement systems, and published their findings on their website.

The findings from these trials and the lessons from all these projects helped us develop the content of Module 5A.

So how does this all help with your contract documentation?

A contract will typically include five components – Conditions of contract, Specifications, Schedule of Payments, Site Specific information and drawings.

Module 5a will assist you in the specification component of the contract.

The specification included in Module 5A has been structured into several sections, and I will introduce these next then go into more detail later. The first 3 sections which may be common to all ground improvement projects are:

- a preliminary & General section – covering general contractual administration
- a testing section – this is the Quality Assurance testing associated with ground improvement works
- and an Earthworks section covering general earthworks requirements.

These are followed by four Ground Improvement sections covering:

- densified crust
- stabilised crust
- stone columns
- timber driven pile systems.

For most single site residential projects, only one of these improvement methods would normally be used within a set of contract documents. These four were the most commonly used in the Canterbury region – and choosing which one will depend on a number of factors including the site ground conditions, the depth to the  groundwater table and economic considerations. 

Module 5A also includes a Specific Project Requirements section. This section will be completed by the specification writer and should be used to cover things like site location, works area, resource consent conditions and geotechnical information. If you need to make amendments to the standard specification clauses – this is the place to do it.

Guidance notes are also a part of Module 5A, and while not being part of your contract, these will help clarify the intent and purpose of the specification clauses and any options that may exist. 

I’ll now go into more detail on these sections. The Preliminary and General section covers general contract management issues and is not specific to any ground improvement method.

If they are used in conjunction with other specification documents, make sure there is no overlap or conflicting requirements.

In addition, some of the specified contract administration requirements may not be necessary for small projects.

Verification testing is important with any form of ground improvement to measure whether the target ground improvement results have been achieved.

Depending on the type of ground improvement used, common testing methods are:

- nuclear densometer
- SPT and CPT
- dynamic heavy probe. 

These testing requirements specified aim to provide a balance between cost and quality assurance.

The guidance lists mandatory, that is, minimum testing requirements, and discretionary testing – the latter depends on the experience of the contractor, ground improvement systems and familiarity with the materials being used. Its use therefore will depend on the project circumstances and should be reviewed on a case by case basis for each project.

Discretionary testing is usually covered by a specific provisional item in the contract schedule of prices and provided by the person preparing the schedule.  Provisional items and sums are implemented by a contract variation.

This Provisional Sum approach enables comparing tenders on a like-for-like basis.

Earthworks that may be associated with a ground improvement contract are covered in this section. It covers such matters as ground and surface water control, clearing, excavations, subgrade preparation, fill placement, testing, stockpiling and use of geotextiles.

Larger civil works contract documents will commonly have a generic earthwork specification, and this section is more tailored to ground improvement earthwork-related works. Leave this section  out if it’s covered elsewhere or just extract the bits relevant to your particular contract. 

If you have two earthworks specifications in your contract documents, then you will need to double check there are no conflicting requirements.

Next up are specific ground improvement specification sections. Method G1 Densified Crust (as identified in the MBIE guidance Chapter 15.3) details a densified block beneath the future building foundation elements.  This can be achieved through excavation and recompaction of existing soils, dynamic compaction alone of soils in situ or excavation and replacement with new higher quality fill with geogrid reinforcement.

Imported material, if strong enough, can reduce the thickness of the ground improved raft.

Choosing the particular densified raft system is dependent on the site. Some are better suited to sites with a low water table or if they can easily be dewatered and there is also space on the site to stockpile fill material.

You also need to be aware of the effects of vibrations and dewatering on adjacent land.

MBIE guidance Method G2 Stabilised Crust is achieved by mixing cement with existing soil. There are three methods commonly used: in-situ mixed, ex-situ mixed and rotovated. These methods are described in detail in the MBIE guidance.

The cement dosage rates required to achieve the specified strengths depend on a number of factors including silt/clay content, water content, the proposed mixing method and degree of compaction. It is considered that specialist Contractors are likely to have the best experience in judging the amount of cement to be added, as the construction methodology and the particular plant and equipment proposed to be used will have a large influence on the ultimate strength gain achieved.

In order for the Contractor to be able to control their risk, however, a certain level of pre-tender soil testing should be undertaken to allow the Contractor to make an informed decision on the cement dosage rate.

This testing will lower the risk carried by the Contractor and is likely reduce the construction costs.

Again, you’ll need to be aware of the effects of vibrations and also possible dewatering depending on the particular method used.

And if the soil has a high organic content you’ll need to import new material.

MBIE Methods G4 and G5, which are methods involving either stone columns, or inclusions such as Rammed aggregate piers or timber piles, are best suited to clean sands as these are easier to densify than silty soils.

Densification is commonly verified by CPT and sometimes with shearwave testing. Information on both is provided in the MBIE residential document. It is worth noting that currently there are a limited number of contractors with appropriate equipment and experience to perform crosshold shearwave testing as this method for verification testing is quite new in New Zealand.

You will need to watch out for vibration effects on adjacent properties as these can be significant with this method. 

With regard to vibrations it may mean the works need to be set back from affected structures or other stabilisation measures used.  You’ll need to discuss this with experienced geotechnical engineersand the contractor so the soil conditions and construction method can be assessed in relation to construction effects. 

Also, depending on the ground heave and associated loosening effect on the surficial soils after the ground improvement, you may need to re-compact surface soils.

MBIE Method G5b is listed as timber piles however other materials such as precast driven concrete piles may also be considered.  Different specification requirements would apply for concrete piles.

This method is often chosen where you have clay or peat materials where other methods such as stone columns wouldn’t be suitable. In sandy sites where densification between piles is expected then this method can be verified with CPT testing before and after ground improvement.

Again vibration effects can be significant and will need to be monitored closely particularly if there are sensitive structures nearby.

Vibration effects can be reduced by setting the pile driving back from that structure.  Predrilling the upper portion of the pile holes is an option or you could use a vibration installation method rather than a drop hammer. 

You may also need to re-compact surface soils after ground improvement.

Next up is a Project Specific Requirements section to cover items that are not specifically addressed in the template specification.

This may include:

- site location and description
- consent conditions
- variations to  existing standard clauses
- new specification clauses
- project specific site inspections and recording requirements
- background information such as geotechnical data, known site hazards, expected site conditions, ground contamination, archaeological conditions and services.

Module 5A also includes guidance notes. It’s important to note that these are not part of a contract specification. They are provided to help explain the principles and background to specific clauses, and to provide options and levels of detail that may be required.

There are some great additional resources available online that I recommend.

MBIE’s residential guidance is available at Building.govt.nz and in particular chapter 15.3 and Appendix C4. Module 5a is also available on the NZGS website. 

The EQC website has videos on ground improvements systems and a report on the ground improvement trials project that they ran. 

Template documents are available on the MBIE website, including pricing schedules, basis for payment documents and template drawings.

Finally, the earthquakes in Christchurch on 14 February 2016 gave us an early indication of how the ground improvement regime performed.

The event was almost exactly an SLS event in North New Brighton with ground shaking levels of approximately 0.2 g.  Liquefaction was widespread in this area.  This EQ event is important as it provides an opportunity to assess the performance of ground improvement against a specific load case.

Two interesting examples are:

- a stone column site – the ejecta was evident immediately adjacent to the ground improved portion of the site but not within the ground improvement area
- and in a gravel raft site, the ejecta was also immediately beside the raft.

The early indications at both sites demonstrate that the ground improvements performed well.  A study has been commissioned to confirm or otherwise these preliminary findings.

This concludes our presentation. Feedback on this module can be emailed to modulefeedback@nzgs.org.