Tonkin & Taylor Ltd Environmental and Engineering Consultants

Tonkin & Taylor

Calculation PackageProject: Three Kings Quarry T&T Ref: 25141.003

Originator: Matthew Kent Date: 21 April 2015

Checked: Date:

Subject: Three Kings Quarry - Assessment of Compacted Fill Settlement

Background Purpose:

The Three Kings Quarry is to receive fill materials in order to partially fill the existing open-cast pit. The controlled placement of fill will be undertaken over several years and the final fill surface will be used for a proposed residential development.

Fill materials will bury existing quarry highwalls on the quarry pit floor and walls. The final fill level will be approximately RL 60 m. Maximum fill thicknesses in the order of 27 m are expected.

Groundwater is currently maintained below the quarry floor by a series of pumps. Groundwater levels will be allowed to rise to approximately RL 57 m during the course of the project.

The purpose of this calculation package is to assess the self-weight and inundation settlement of compacted earth fill materials to be placed as part of the project.

On the basis of the above, assessment has also been made for the potential differential settlement over buried quarry highwalls on the quarry floor/walls.

Self-weight settlement

Creep settlement due to self-weight will occur in the compacted fill materials which are placed in a partially saturated (3-phase soil) state. It is expected that the majority of the settlement will occur during the filling period. The magnitude of self-weight creep settlement is expected to reduce with time after the filling operations.

Review of the relevant published literature produced a particularly valid case study (Trow, Carrington and Orpwood 1993)1, which appears to be very similar to the proposed Three Kings Quarry development. This study indicates that total settlements in the order of 3 to 6 % may be expected for a well compacted earth fill. These values are considered to represent the sum of primary compression and self-weight settlements. Residual self-weight settlements post-filling operations for a partially saturated, well compacted clay fill are expected to be around 0.5 % (Leach and Goodger, 1991)2 over a 100 year design life.

It is considered important to maintain the fill in a partially saturated, 3-phase state during the filling phase. This would be achieved by continuing to keep groundwater depressed below the quarry floor level during the

1 Trow, W. Carrington, R. Orpwood, G. (1993); Case History – Monitoring Settlement of 32m Thick Compacted Fill. [paper] Proceedings: Third International Conference on Case Histories in Geotechnical Engineering, St Louis, Missouri, June 1-4, 1993, Paper No. 2.06.

2 Leach, B. Goodger, H. (1991); Building on Derelict Land. Construction Industry Research and Information Association (CIRIA), Special Publication 78, 1991.

T&T Ref: 25141.003 2

Tonkin & Taylor Ltd – Environmental and Engineering Consultants

filling operation, thereby maintaining the maximum effective stress in the earth fill and inducing the maximum

settlements possible during the filling phase of the development.

When filling is completed the groundwater would be raised within the fill materials. The inundation of the fill

by groundwater would reduce the effective stress in the earth fill. It is expected that this will significantly

reduce the rate of settlement. For the purposes of this assessment we have assumed that rates will be halved

to 0.25 % over 100 years.

A key factor in the timeframe required between the end of filling operations and commencement of building

construction will be the time required to fully saturate the fill. Allowance may be required in the design for

drainage layers or similar to increase saturation rates of the fill materials.

Collapse settlement due to groundwater inundation

Collapse settlement caused by the reintroduction of groundwater poses a risk in uncompacted or poorly

compacted fill materials. The risk of collapse settlement can be reduced by achieving a high standard of

compaction within the fill materials as well as ensuring the material is compacted slightly wet of the optimum

moisture content.

Reference to the literature indicates collapse settlement magnitudes of 2-4% should it occur. Timeframes for

this mechanism are reported to be longer than consolidation type settlement. This highlights the need for

careful fill material selection and compaction control.

Differential settlement

Differential settlements assessed include 1:250 (note 1:240 required under NZ building code) and 1:500

(project target for maximum differential settlement beneath building platforms).

Potential differential settlements over the buried quarry highwalls have been assessed using the methodology

by Charles and Skinner (2001)3.

The attached calculations indicate acceptable residual volumetric strain, εv (as a percentage of fill thickness) to

satisfy the differential settlement criteria investigated. The geometry of the quarry floor highwalls and

proposed fill heights result in higher tolerable stain values than the perimeter highwalls.

A summary of the results are provided in the following table. Note that Typical Section 3 is not included in the

results summary below as it is of a particularly shallow slope and generally not representative of the quarry

highwalls at the site:

Location Range of tolerable volumetric strain, εv (as a percentage of fill thickness) for given

differential settlement

1(V):250(H) 1(V):500(H) 1(V):1000(H)

Quarry floor highwalls 0.48 to 0.56 % 0.24 to 0.28 % 0.12 to 0.14 %

Perimeter highwalls 0.32 to 0.40 % 0.16 to 0.20 % 0.08 to 0.10 %

3 Charles, J. Skinner, H. (2001); The Delineation of Building Exclusion Zones over Highwalls. [paper] Ground Engineering Magazine, pg.

28-33, February 2001

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Tonkin & Taylor Ltd – Environmental and Engineering Consultants

The results indicate that a differential settlement of 1:500 (project target) will generally be met over quarry

floor highwalls if the assumed residual volumetric strain, εv of 0.25 % is achieved over the design life of

structures above these areas.

Calculations indicate that residual volumetric strain, εv of between 0.16 and 0.20 % is required for differential

settlements of 1:500 at the perimeter highwalls.

Where the above values are exceeded mitigation measures may be required.

Attachments:

Typical highwall / fill dimensions (2 pages, A4)

Charles and Skinner (2001) calculations (1 page, A4)

21 April 2015

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