Trench is connected to very narrow and deep excavation filled with bentonite suspension. This paper presents estimation of its stability in cohesive soil. The stability is assessed by two calculation methods. The first involves the equilibrium of forces acting on the rigid wedge. The second one includes numerical calculations conducted in Plaxis 3D Foundations. A few examples having different dimensions (length and depth) are analyzed in uniform soil conditions. Graphs defining the dependence of length, depth and factor of safety are presented. It is found that for long trenches (L≥6m) the soil kinematics at failure coincides with the literature data. Short trenches are under a large influence of the arching effect and cohesive forces. The limit equilibrium method can be used under the condition of employing a factor, which reduces the value of the earth pressure.