diff --git a/ProcessLib/DeactivatedSubdomain.cpp b/ProcessLib/DeactivatedSubdomain.cpp
index 1669ad4941356b63d7f9d3f7a306a6d742a77dce..5422fa378132503c16270ecb1f4295ea0c4f0b9c 100644
--- a/ProcessLib/DeactivatedSubdomain.cpp
+++ b/ProcessLib/DeactivatedSubdomain.cpp
@@ -50,4 +50,20 @@ bool DeactivatedSubdomain::includesTimeOf(double const t) const
            t <= time_interval.getSupportMax();
 }
 
+bool DeactivatedSubdomain::isDeactivated(MathLib::Point3d const& point,
+                                         double const time) const
+{
+    // Line from a to b.
+    auto const& a = line_segment.first;
+    auto const& b = line_segment.second;
+    // Tangent vector t = (b - a)/|b - a|.
+    Eigen::Vector3d const t = (b - a).normalized();
+
+    // Position r on the line at given time.
+    Eigen::Vector3d const r = a + t * time_interval.getValue(time);
+    Eigen::Map<Eigen::Vector3d const> const p{point.getCoords(), 3};
+
+    // Return true if p is "behind" the plane through r.
+    return (p - r).dot(t) <= 0;
+}
 }  // namespace ProcessLib
diff --git a/ProcessLib/DeactivatedSubdomain.h b/ProcessLib/DeactivatedSubdomain.h
index f48b71c63fae2418514b55ad4b3d030d439cdf14..a2832b63dd470a6580024df06e21bcd2adb21c17 100644
--- a/ProcessLib/DeactivatedSubdomain.h
+++ b/ProcessLib/DeactivatedSubdomain.h
@@ -18,6 +18,7 @@
 #include <vector>
 
 #include "MathLib/InterpolationAlgorithms/PiecewiseLinearInterpolation.h"
+#include "MathLib/Point3d.h"
 
 namespace MeshLib
 {
@@ -49,6 +50,12 @@ struct DeactivatedSubdomain
 
     bool includesTimeOf(double const t) const;
 
+    /// \returns true if the point is in the deactivated part of the subdomain.
+    /// The domain is split into two parts by a plane defined as a normal plane
+    /// of the line segment and the position on the line segment, where the
+    /// latter is defined by the time curve.
+    bool isDeactivated(MathLib::Point3d const& point, double const time) const;
+
     MathLib::PiecewiseLinearInterpolation const time_interval;
 
     /// Line segment along which excavation progresses. Represented by start and