Loads and seismic loading

External loads are applied on the ground surface (a surcharge over a stretch of boundary, or a force at a point), and an earthquake is represented by a pseudo-static coefficient that acts on the soil of every slice. The Loading menu has the commands for both, and for the acceleration records that a Newmark displacement analysis integrates.

2.5.1Distributed loads#

A distributed load is a pressure on a stretch of boundary: a surcharge, a foundation, the thrust of water or fill on a face. To add one:

1. Choose Loading → Add Distributed Load… and fill in the dialog (Table 2.5.1).
2. Press OK, then click the start point and the end point of the load on the boundary; the clicks are snapped to the boundary. Esc cancels.

Click the two points from left to right. The orientations relative to the boundary are built from the order of the clicks, and only a load drawn from left to right along the surface has its normal pointing into the ground.

The magnitude is a pressure on the horizontal projection of the loaded segment: on a slice of width $b$ under the load the force is $p\,b$, in the direction of the load. A load on a vertical segment, such as the face of a wall, has no horizontal projection; it is integrated along the segment and applied as one force at the centroid of the pressure diagram.

2.5.2Line loads#

A line load is a force per metre of slope at a point: a wall, a strip footing seen in section, the reaction of a structure. Choose Loading → Add Line Load…, fill in Magnitude (kN/m), Orientation, Angle and the excess pore pressure box, which mean the same as for a distributed load, press OK and click the point.

• Normal to Boundary and Angle to Boundary are taken from the ground surface at the point: the normal points into the ground, and the angle is measured from the ground surface running left to right, so −90° is that normal. The point must be on the ground surface (within 0.1 % of the extent of the ground profile) or the load is not added and the status bar says why.
• Vertical, Horizontal and Angle from Horizontal are absolute and can be placed anywhere; with an angle, a line load can act upwards.

A line load enters the slice that contains its abscissa. The status bar confirms the point and the magnitude.

2.5.3Modifying and deleting loads#

• Right-click a load on the canvas and choose Modify… to reopen its dialog with every field, or Delete.
• Loading → Modify Load… lists the loads (Distributed n, Line n), then asks only for the magnitude and, for a distributed load with an end value, the magnitude at the far end.
• Loading → Delete Load lists every load with its magnitude, position and orientation; select one or more and press OK.

Changes to loads are not on the undo stack. Loads are not moved when the geometry is edited (Change Slope Angle, for instance, leaves them where they were), so check their position after reshaping the slope. With the data tips on, hovering over a load shows its magnitude, orientation and distribution. With a design standard switched on, every distributed and line load is multiplied by the variable-action factor (Section 2.1.8).

2.5.4Seismic load#

The pseudo-static method represents an earthquake by an inertial force proportional to the weight of the soil of each slice (Kramer, 1996). Choose Loading → Seismic Load…:

• $k_h$ is a magnitude. Its force always acts in the direction of failure of each surface (out of the slope, the unfavourable sense), so a negative value is refused rather than read as a force into the slope.
• $k_v$ keeps its sign: positive downwards, so that the soil weighs $W(1 + k_v)$; negative upwards. Both coefficients must stay below 1 in magnitude.
• The forces act on the weight of the soil of each slice. Distributed and line loads and ponded water carry no seismic force.

The coefficients come from the seismic code that applies to the project, for slopes for instance EN 1998-5. After OK the status bar repeats the values in force. To report, for every surface, the coefficient that would bring it to failure (the critical acceleration $k_y$) or a Newmark displacement, use the Seismic page of Project Settings (Section 2.1.10). The methods are described in Section 3.9.

2.5.5Seismic records#

A Newmark analysis integrates an acceleration record over the part of each pulse that exceeds the critical acceleration of the sliding mass (Newmark, 1965). Records belong to the project, like materials: they are defined once in Loading → Seismic Records… and then chosen on the Seismic page of Project Settings.

The file is plain text in either of the two layouts used for Newmark integration programs (Jibson, 1993): pairs of time and acceleration, one pair per line, or one acceleration per line at the constant interval set in Time interval (s). The layout is decided by the number of values on the first line of data. Lines that are not numbers (headers, and lines starting with #, ; or %) are skipped, and commas are read as separators, so decimals must use a point. With paired data the interval is taken from the first two times; a record whose time step is not constant is imported with a note saying so, because the integration assumes a constant step and the program does not resample.

Choose File units before importing. Deleting a record that the Seismic page had selected leaves the selection empty, and the next Newmark run says that no record is chosen.

2.5.6Loads and excess pore pressure#

With Calculate Excess Pore Pressure (B-bar method) chosen in Project Settings, a distributed or line load whose box Load creates excess pore pressure (B-bar method) is ticked raises the pore pressure of the undrained materials beneath it by $\bar B\,\Delta\sigma_v$. Only the vertical component of the load counts. The procedure, and why a line load is a poor model of a surcharge for this purpose, are explained in Section 2.4.6.