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Water Surfaces Overview

Water Surfaces are used for the following purposes in RocSlope:

  • Pore Pressure - if the Groundwater Method = Water Surfaces then you can assign a Water Surface (e.g., Water Table, Piezo Surface) to calculate the Pore Pressure for a Material. Pore Pressure is proportional to the vertical distance from the Water Surface. For joints assigned with a Joint Property using Material Dependent Water Pressure Method, any portion of the joint which passes through the material will take on the pore pressure effects of the material.
  • Joint Pressure - if the Water Pressure Method = Water Surfaces then you can assign a Water Surface (e.g., Water Table, Piezo Surface) to calculate the Water Pressure inside the Joints. Pressure is proportional to the vertical distance from the Water Surface. The selected Water Surface is used to compute the water pressure over the entire joint.
  • Ponded Water - if a Water Table is defined above the slope surface, then Ponded Water is automatically created. The weight of Ponded Water is accounted for in the stability analysis, acting on the Free Surface of the block.

Water Surfaces are created and assigned as summarized below.

Adding Water Surfaces

Water Surfaces are created with the Add Water Surface or Add Water by Location options, and may represent a Water Table or a Piezo Surface.

Assigning Water Surfaces

In order for a Water Surface to be used in the RocSlope analysis, it must be either assigned to a Material or to a Joint Property.

Assigning Water Surfaces to Materials

Water Surfaces are assigned to Materials under the Water Parameters tab of the Define Materials dialog or with the Hydraulic Assignments Dialog. See the Assign Water to Materials topic for more details. In order for Material pore pressures to be used in Joint Water Pressure calculations:

  • The Water Pressure Method must be set to Material Dependent for the Joint Property
  • The Joint with the Joint Property must pass through the Material with the assigned Water Surface

Assigning Water Surfaces to Joint Properties

Water Surfaces are assigned to Joint Properties under the Water Parameters tab of the Define Joint Properties dialog. See the Define Joint Properties topic for more details.

Types of Water Surfaces

There are two different types of Water Surface:

The Water Surface Type is selected when you create a Water Surface with the Add Water Surface or Add Water by Location options.

Ponded Water

If a Water Table is defined ABOVE the slope surface, then Ponded Water is automatically created between the Water Table and the ground surface. The weight of Ponded Water is accounted for in the stability analysis, acting on the Free Faces of the blocks. See the Ponded Water topic for more information.

Hu Coefficient

The Hu Coefficient is simply a factor between 0 and 1, by which the VERTICAL distance from a point on the block to a Water Surface is multiplied to obtain the pressure head. The Hu Coefficient is used to calculate the pore pressure as follows:

u= ywhHu

where:

u = pore pressure

yw = the Pore Fluid Unit Weight (entered in the Project Settings dialog)

h = the vertical distance from the base of a column to a Water Surface

Hu = the Hu coefficient for the material (Custom or Auto, see below)

There are two ways of defining the Hu Coefficient - Auto or Custom.

Custom Hu

With the Custom Hu option, the user can enter their own value for Hu. A value between 0 and 1 must be specified. For example:

  • Hu = 1 would indicate hydrostatic conditions. This can be used where the Water Surface is horizontal. Where the Water Surface is inclined, setting Hu = 1 will provide a conservative (low) estimate of the safety factor, since in general this will overestimate the true pore pressure. In most cases, the user will simply set Hu = 1, because this represents the worst-case scenario (maximum pore pressure).
  • Hu = 0 would indicate a dry slope (zero water pressure). Setting Hu = 0 can be used to turn "off" the pore pressure for a material, although this can also be achieved by setting Water Surface = None.
  • Intermediate values of Hu (between 0 and 1) could be used to simulate head loss due to groundwater seepage if water flow is occurring in the rock slope. However, seepage is not explicitly considered in RocSlope. Applying an intermediate Hu value simply applies a fraction to the full hydrostatic condition for calculating the water pressure.
If you are using Piezometric Surfaces, you should, strictly speaking, use the Custom Hu option, with Hu = 1. This is because a Piezometric Surface is usually a direct representation of the pressure head. In RocSlope, Hu = 1 is used for computing ponded water.

Auto Hu

With the Auto Hu option, RocSlope uses Hu = 1, which assumes hydrostatic conditions. This is because RocSlope does not calculate seepage forces and always assumes hydrostatic conditions.

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