solution 3: the assessment of flow into an open pit with...

CE 566 Deep Excavations and Retaining Structures Department of Civil Engineering

Middle East Technical University, Ankara,Turkey Prof. Dr. K. Önder Çetin

ASSIGNMENT 3: The Assessment of Flow into an Open Pit with RocScience RS2 Software

Solution 3: The Assessment of Flow into an Open Pit with RocScience RS2 Software

1. For a highway cut, a sloped excavation is to be performed. Figure 1 shows a schematic view

of the proposed excavation along with some soil data. Estimate the amount of flow into the

open cut by using either RS2 or Slide soft wares.

Figure 1: Planned excavation

Slide software enables user to make a Steady State FEM Analysis (Under Project Settings –

Groundwater – Method -- Steady State FEM Analysis. Half part of the geometry was

modelled (symmetric geometry advantage). Sınce we are only ınterested with the flow

amount, the soil layers above the water table were ignored. Total head values of the geometry

were shown below. The green line (discharge section) in the model is established to fınd the

flow amount through the slope. (one discharge section also added on the slope surface,

because program calculates flow between higher head and lower heads and these lower value

is not only 10m , shortly program calculates flow between upper 16 m and slope surface

whıch has a linearly varying head of 10-16 m )

Figure 2: Total head values of the boundary




Results of the analysis is shown below.

Figure 3: Interpretation of Analysis (flow lines, flow direction and discharge section values)

As seen on figure, discharge section calculated as 2.413 m3/day flow through the section.

However as seen on figure, program also calculates a 0.350 m3/day flow through the sloped

surface. As a conservative approach, better to assume that this flow will be through the

excavation site. (actually it is not known what percent of this 2.413 m3/day flow is also

responsible for this 0.350 m3/day, so considering them as independent will be conservative).

Therefore, estimated amount of flow that will go into the excavation site will be

Flow estimated = 2 * (2.413+0.350) m3/day = 5.326 m3/day

2. Figure 2 shows the excavation plan for a construction. Considering the project plan;




i. Estimate the amount of flow into the deep excavation

Slide programs was used to model the problem. For the sheet pile wall a very low

permeability value, k=10-50 m/day was assigned. Also in boundary conditions, total flow on

the wall boundaries are assigned as 0 m3/day. Geometry is shown below.

Figure 4: Geometry of the Problem and Total head values with Boundary Cond.

Half part of the geometry used, symmetry of geometry benefitted. Green line shows the

discharge section. Below figure shows the vertical hydraulic gradient line (for part 2) and

discharge section amount.




Figure 5: Flow lines, flow directions, vertical hyd. Gradient and discharge section




Total estimated flow will be 0.01405 m3/day from only one side of excavation. Therefore,

total estimated flow through excavation site is 0.0281 m3/day

ii. Check if boiling is of concern (assume icr=1.0)

Slide program enables the user to check the hydraulic gradient values, too. As seen in the

output, the highest amount of the vertical hydraulic gradient, imax is 0.410 (notice that

hydraulic gradient values are higher close to the wall as seen in Figure 5

𝐹𝑆𝑏𝑜𝑖𝑙𝑖𝑛𝑔 =𝑖𝑐

𝑖𝑚𝑎𝑥=

1

0.410= 2.44

iii. Check if base heave is expected by using Slide or RS2 softwares

Base heave is likely to occur if pore pressure is higher than the total pressure at a point.

Therefore, pore pressures of the downstream part should be checked.

Below the excavation site, Pore pressures were calculated for every 2 intervals, i.e. 2-4-6 so

on. Results are shown in figure 6.

𝐹𝑆ℎ𝑒𝑎𝑣𝑒 =𝑇𝑜𝑡𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒

𝑃𝑜𝑟𝑒 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒=

39

26.18= 1.49

Depth Below, m

Pore Pressure, kPa

Total Pressure, kPa FS

2 26.18 39 1.49

4 52.36 78 1.49

6 78.56 117 1.49

8 104.77 156 1.49

10 130.98 195 1.49

12 157.21 234 1.49

14 183.15 273 1.49

16 208.26 312 1.50




Figure 6: Pore Pressure Output of the Problem

solution 3: the assessment of flow into an open pit with...

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