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The starting data is the file 'topo_nlc.dat', that is a text file with xyz of a grid (5800 points)

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We want to have only the region inside the circle with radius=4000 (4 km assuming that probably the model represent meters) and 2D center at (-3850,-2877732rectangle with 2D corners (33,-2865760,0) to (-8281,-2870880,0)

Create a new layer 'circlerectangle' and set it as 'layer to use' to separate the new circle rectangle of the topography

Geometry->Create->Circle->Object->Rectangle

33,-2865760,0

-38508281,-28777322870880,0

normal of its plane in 'Positive Z' direction

radius=4000

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Topography (transparent layer) and circle rectangle in XY and XZ views

To trim the terrain we will extrude lateral surfaces and use them for intersection

Utilities->Copy...

A translation of 3000 m in z, extruding surfaces

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Copy with extrusion 

And calculate numerically the intersection of surfaces with:

Geometry->Edit->Intersection->Surfaces

Try to select the minimum surfaces possible, because will find the intersection of the selected surfaces all against all, and it could be very expensive in time.

(select in XY view first with a box a little bigger than the rectangle, and again with a box a little smaller to remove surfaces from the selection)

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Selection of surfaces to be intersected

Note: Don't worry if the advance bar show more than 100%, this is because the final number of surfaces to be intersected grown because when a cutted surface is splitted the new surfaces are added to the collection to find more intersections.

If everything end successfully all surfaces will be divided. Some intersection could not be done because of tolerances (too small cuts, etc), and some manual extra operation could be required.

Now we can delete the unwanted part, e.g. to have only a volume of terrain under the rectangle for some geotechnical simulation.

Delete the unneeded surfaces.

Geometry->Delete->Surfaces

In the contextual menu click LowerEntities (to delete also its dependent lines and points)

and select the surfaces

Then the model must have only the surfaces that enclose a volume ,create it with

Geometry->Create->Volume->By contour

and select all surfaces

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Final volume and detail of cutted surfaces