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

3. 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).

4. 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.

5. 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

6. 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)

7. DP Difference

8. 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.

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

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

11. Design & Analysis

12. Tank Design Loads
Design Life = 50 years

13. 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.

14. Conceptual Time-Settlement Behaviour

15. 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

16. 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.

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

19. 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

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

21. 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

22. 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)

23. Plate Load Testing – Working Platform

24. Installation of Trial Columns

25. Installation of Trial Columns

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

27. Column Load Testing (10 MPa – 7 days)

28. 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

29. Thanks to All Involved with this Project