Scott McFarlane & Richard Merifield Ground Improvement Project – Large water storage tanks Carrington NSW Douglas Partners’ Technical Seminar 2019 Scott McFarlane & Richard Merifield

Background Douglas Partners are a trusted consultant to PWCS; PWCS needed to upgrade their stormwater management system; PWCS engaged GHD as the design consultant (civil, hydraulic, mechanical, electrical & geotechnical); PWCS provided GHD previous nearby geotechnical data (by DP) to assist GHD with conceptual geotechnical ground improvement options for tank; PWCS provided DP the concept geotechnical design options by GHD to develop scope of works (i.e. data report).

Stormwater Management System Manage stormwater to minimise off-site uncontrolled discharge; Above ground steel tanks; Supported on concrete slab; Tank 1 – 20 m dia, 11 m high, 5Ml; Tank 2 – 32 m dia, 11 m high, 8 Ml; Tank 3 – 32 m dia, 11 m high, 8 Ml; Pipeline to connect into existing pond. Trench to be excavated adjacent to rail line.

Previous Data Fill Soft Clay VL Sand Clay / Silt and Sand Dense Sand -0 Soft Clay VL Sand Clay / Silt and Sand -10 Dense Sand Stiff Clay -20 Very Stiff to Hard Clay -30 Rock -40

GHD Concept Ground Improvement Options Do nothing Preload CFA piles Driven Piles Cutter Soil Mixing (CSM) Design Details - 8 m high preload (3 month wait) 600 mm dia; Installed to >35 m, 3 m c/c 400 mm sq; 2.5 m c/c CSM to 35 m 15% area replacement ratio Estimated Post Construction Settlement (mm) 700 250 to 300 5 to 10 With preload: 20 mm Without: 100 mm Constraints Settlement Space, time Depth of piles, ASS Depth of Piles PWCS – very risk adverse with any ground improvement (past experience)

DP Difference

DP Revised Scope Investigation – Provide data to GHD to undertake Design; Parallel modelling to compare with GHD design; Review technical specification; Review tenders methodology; Review preferred tenderers design, QA and alternate design.

Investigation Geotechnical Risks Soft clay layer? Sand stratum? Deeper clay? Ground water? Rock strength?

Subsurface Profile Tank 1 – 20 m dia Tank 2 – 32 m dia

Design & Analysis

Tank Design Loads Design Life = 50 years

Serviceability Criteria Max settlement at centre ≤ 100 mm; Max settlement around perimeter ≤ 100 mm; Max settlement between centre and edge ≤ 40 mm; Max edge to edge tilt ≤ 30 mm; Differential settlement ≤ 1 in 500.

Conceptual Time-Settlement Behaviour

Initial Ground Improvement Options No Ground Improvement; Piles with Pile Transfer Layer (PTL); Deep Soil Mixing (GHD preference); Tank Interaction (3D analysis). Max Total Settlement: Case 1 – 71 mm Case 2 – 53 mm Max Total Settlement: Case 1 – 80 mm Case 2 – 60 mm Max Total Settlement: Case 1 – 300 mm Case 2 – 225 mm Max Total Settlement: Case 1 – 76 mm

Initial Tender Review – Mix Soil Option Large QA component in design spec by GHD: Sampling and lab mix design to determine strength properties; Additional CPTs; Trial sites (curing time); Core sampling of mixed soil; Lab testing during mixing; Column Penetration Test or Pull-out resistance test. 1 m preload.

Alternative Tender – Rigid Inclusions Concrete Injected Columns (CIC); Controlled Modulus Columns (CMC); Controlled Stiffness Columns (CSC).

Using the result from b), a larger model was generated. A “unit cell” axisymmetric model consisting of the CSC and its surrounding soils was analysed; Based on the results from a), an equivalent set of soil properties was generated; Using the result from b), a larger model was generated. The analysis of the CSC ground improvement option was undertaken using 2D axisymmetric finite element analyses in Plaxis 2D. More specifically, the following analyses steps were undertaken as part of the CSC assessment: A “unit cell” axisymmetric Plaxis 2D model composed of the CSC and its surrounding soils was created and the settlements versus time estimated. The unit cell model includes the CSC (diameter, spacing) and the load transfer platform (including thickness) as shown in Figure a. Based on the unit cell results from (a) for each tank location and the adopted CSC/Load Transfer Platform properties, an equivalent set of soil properties (stiffness, permeability) for the composite soil/CSC matrix is established that match the consolidation response observed in (a) (Figure b). Using the parameters established in (b) for the composite soil/CSC matrix, a larger 2D axisymmetric Plaxis 2D model composed of the composite soil/CSC zone and the surrounding soils at each tank location was created and the settlements versus time estimated. A sample Plaxis 2D model is shown in Figure 6.   Plaxis 2D Unit Cell Model of CSC

Plaxis 2D axisymmetric model (half model) with equivalent composite CSC/Soil Zone

Sustained Constant Load – 120 kPa Tank Total Settlement mm Sustained Constant Load – 120 kPa Keller Prediction DP Prediction Tank 1 (20 m dia) 74 62 Tank 2 (32 m dia) 86 71

QA During Construction Proof rolling & Plate Load Testing rather than density testing of working platform; Additional CPTs; Concrete testing (by others); Review of concrete takes & penetration depths; Plate load testing of installed columns (similar to pile test)

Plate Load Testing – Working Platform

Installation of Trial Columns

Installation of Trial Columns

Production Rate of Columns Stats No. Metres per day No. of Piles per Day Range 72 – 400 8 – 53 Average 264 35

Column Load Testing (10 MPa – 7 days)

Typical Column Load Test Result Column Diameter – 0.4 m Column Depth – 7.5 m Total No. of Column Tests – 9 Range of Max Deflection – 2.1 to 28 mm

Thanks to All Involved with this Project