## Question

**How to apply soil/water pressure? (Earth/Hydraulic pressure)**

## Answer

**You can apply the soil/water pressure by using Hydrostatic Pressure Loads. (Earth/Hydraulic pressure)**

The 'Hydrostatic Pressure Loads' function allows you to apply trapezoidal loads that vary across multiple elements at once.

Unlike other load functions that allow direct input of load magnitudes, the Hydrostatic Pressure Loads function allows inputting variables involved in the calculation of hydrostatic and earth pressure. In the function menu below, we can see that we define the load direction, load starting position, slope, and starting load magnitude to calculate P.

Input tip. If you know the topographic elevation as shown in the figure below, you can enter either the structure's topmost position of -2 as in figure A or the topographic elevation (HL 0.0) as in figure B for the "Absolute Elevation at Start of Load" H. This is because if there are no elements selected in the range HL 0.0 to HL -2.0, the load at that position will not be applied.

Input Tip. When entering earth pressure on a wall element marked in red as shown below, align the Local z-coordinates of the wall elements in the load direction (or opposite direction) as shown in the figure, and if you enter 0 at the reference level H, you can enter the earth pressure all at once without calculating a separate load value, just by entering the earth pressure coefficient.

- Input seismic loads

When seismic loads are calculated using response spectrum analysis, the resulting soil pressure is curved. In this case, the Hydrostatic Pressure Loads feature allows the user to input the load as a function of position using the Curved Loads option. This allows the user to input the soil pressure as a curved load.

- Input surcharge load

The Hydrostatic Pressure Loads function can be used to apply the surcharge load on tunnel lining.

When applying the surcharge load, input the tunnel height in the reference height H, and input the surcharge load Po, which is calculated by the Terzaghi rock classification method at the top of the tunnel at height H. The slope converges to zero at both the load Po and position H, so by inputting Po/H, the surcharge load can be applied to the tunnel lining.

- When the height of the soil layer varies, input soil pressure

The Hydrostatic Pressure Loads function can input loads that increase over multiple elements, but it can only input loads that increase in one direction. To input loads that increase in both directions due to different heights of the soil layers, you need to use the Plane Load function.

To apply loads that increase in both directions due to the varying height of soil layers, you need to use the Plane Load feature. First, use the 'Define Plane Load Type' function to define the shape of the load to be applied, and then use the 'Assign Plane Loads' function to determine where to apply the defined load. Note that the surface on which the load is applied must be a flat plate.