Reference no: EM132501471

7303ENG Advanced Foundation Engineering - Griffith University

Project Assignment

Ground improvement using sand drains

Two soft clay layers A and B, 10m and 8m thick, respectively, sandwich a 1m thick sand layer. Assume ground water level is at the ground surface.

Section A: Design calculations

Part 1: Design without using sand drain

1.1) What will be the time taken to achieve an average Degree of Consolidation (Uv) of 90% for Clay A and Clay B, respectively?

1.2) What will be the corresponding total ground surface settlements due Clay A and Clay B with respect to Q1.1) above?

1.3) If time of 3 years is desired to achieve average Uv of 90%, what will be the additional required surcharge pressure (p) (in kN/m2)?

1.4) If the sand used for permanent fill and surcharge has a bulk unit weight, γs of 18 kN/m3, what will be the total permanent fill + surcharge height?

Part 2: Design using sand drain (based on Carillo's Theorem of combined vertical & horizontal drainage)
Verify that Clay A is the more critical layer in terms of having a lesser average Degree of Consolidation (Uv) than Clay B based on a 9-month accelerated ground treatment program.

Subsequently from Q2.1) above, determine the sand drain spacing, S, in Clay A to achieve Combined Degree of Consolidation (Uv,h) = 90% if a square grid pattern is required, assuming smearing occurs during installation.

If Clay B were to be also treated using sand drains, evaluate the Combined Degree of Consolidation (Uv,h) of Clay B after 9 months if the spacing of sand drains in Clay B is 1.414 times the spacing in Clay A in Q2.2) and triangular pattern is required. Consider smearing occurred during installation.

Part 3: Engineering evaluation

In about half a page, discuss the outcomes of each design philosophy; i.e. without sand drains (Part 1) and with sand drains (Part 2) focusing on the impact on construction timeline and safety.

Section B: Slope stability analysis using Slope/W software

You do NOT need the answers from Section A Part 1 and Part 2 to complete Section B. However, general concept and understanding from Section A are required in order to meaningfully complete Section B.

A proposed road project requires a 5 m high sand embankment to be constructed on the same site as in Section A above. The underlying soil layers are represented in Fig. 1 (attached) above.

The given design requirements are:

a) Use Bishop Method when performing slope stability analyses
b) Use ‘Grid and Radius' method to define the slip circles
c) For Cases 1 and 2, Factor of Safety (FOS) required is 1.5
d) For Case 3, FOS required is 1.7
e) In the slope stability output, the FOS cannot be located at the edge of the FOS grid. If this happens, you have to move or enlarge your FOS grid and re-run your analysis.
f) Selected slip circle must have lowest FOS as well as cutting through and cutting underneath the main embankment body.
g) Assume ground settlements are not of interest in this study.

Case 1: Before PVD treatment

1.1) Using the Slope/W software, perform 1 (one) slope stability analysis (without stability berm for now) for Case 1 and determine the Factor of Safety (FOS). It is expected the FOS will be less than the required of 1.5.

Case 2: 1 week after PVD installation

As such, you are now required to construct a stability berm to try to increase the FOS. Using the Slope/W software, perform 1 (one) slope stability analysis with a stability berm and determine the FOS.

Provide your responses to the following:
(i) Why has the FOS increased as compared to Case 1? What is the role of the stability berm?
(ii) What do you notice about the undrained shear strength at this stage? Why did the undrained shear strength value change over this relatively short time (1 week) after PVD is installed?
(iii) Is the FOS acceptable at this stage? Why?

Case 3: 9 months after PVD installation

Using the Slope/W software, perform 1 (one) slope stability analysis (with stability berm) for Case 3 and determine the Factor of Safety (FOS).

Provide your responses to the following:
(i) Why has the FOS increased as compared to Case 2? Discuss with reference to the relatively much longer time (9 months) after PVD is installed.
(ii) Is this FOS acceptable? Why?

Attachment:- Advanced Foundation Engineering.rar