Index: Ringtoets/Piping/test/Ringtoets.Piping.Calculation.Test/PipingCalculatorTest.cs
===================================================================
diff -u -rc0a3ced404197fd781c6fcbcfc21c6676592be57 -r8b85f978c7bbe5c29a6ada6509f14d0462b8b9c3
--- Ringtoets/Piping/test/Ringtoets.Piping.Calculation.Test/PipingCalculatorTest.cs	(.../PipingCalculatorTest.cs)	(revision c0a3ced404197fd781c6fcbcfc21c6676592be57)
+++ Ringtoets/Piping/test/Ringtoets.Piping.Calculation.Test/PipingCalculatorTest.cs	(.../PipingCalculatorTest.cs)	(revision 8b85f978c7bbe5c29a6ada6509f14d0462b8b9c3)
@@ -3,6 +3,7 @@
 using System.Linq;
 
 using Core.Common.Base.Data;
+using Core.Common.Base.Geometry;
 using Core.Common.TestUtil;
 using NUnit.Framework;
 using Ringtoets.Piping.Calculation.SubCalculator;
@@ -356,10 +357,31 @@
             var result = calculation.CalculateThicknessCoverageLayer();
 
             // Assert
-            Assert.IsFalse(double.IsNaN(result));
+            Assert.AreEqual(1.0, result);
         }
 
         [Test]
+        public void CalculateThicknessCoverageLayer_WithValidInputWithAquiferAboveSurfaceLine_ReturnsNegativeThickness()
+        {
+            // Setup
+            PipingCalculatorInput input = new TestPipingInput().AsRealInput();
+            input.SurfaceLine.SetGeometry(new []
+            {
+                new Point3D(0, 0, 0.5), 
+                new Point3D(1, 0, 1.5), 
+                new Point3D(2, 0, -1) 
+            });
+
+            var calculation = new PipingCalculator(input, new PipingSubCalculatorFactory());
+
+            // Call
+            var result = calculation.CalculateThicknessCoverageLayer();
+
+            // Assert
+            Assert.AreEqual(result, -3.0);
+        }
+
+        [Test]
         public void CalculateThicknessCoverageLayer_WithExitPointLBeyondSurfaceLineInput_ReturnsNaN()
         {
             // Setup