Investigation depth for HDD crossings

1Background

Cable routes are usually explored to a uniform depth; the drill path follows later from cover, bending radius and rig. If it lies below the holes, the bore runs through uninvestigated ground: gravel, cobbles or gyttja go undetected, and the homogeneous zones to DIN 18324 [3], [4] lack evidence.

2Normative basis and practice

DIN EN 1997-2 [1] with DIN 4020 [2] does not explicitly mention HDD. Closest are Annex B.3 (8) for pipelines and trenches – at least 2.0 m below the invert – and B.3 (9) for small tunnels, with one to two times the excavation width. For reamed diameters D_B below 1 m, paragraph (8) governs. The invert is the bottom of the reamed borehole:

t_E investigation depth below ground · h_A depth of the governing (lowest) path below the hole’s collar · Δz_E margin below the borehole

Practice recommends 2 to 5 m below the planned bore [5], a US guideline at least 15 ft, approx. 4.6 m [6]. The 2.0 m applies only to a fixed path; if it may still change, we consider 3 to 5 m appropriate.

3The path is not a constant

The depth of cover limits the path from above. It is set by the road, railway or water authority; rule of thumb: 10 to 15 times the borehole diameter [5]. Drill length, radius and rig limit it from below – and, for holes already drilled, Eq. (1). In between lies the entry geometry:

With R = 300 m and α = 12°, this gives L ≈ 62 m and Δz ≈ 6.6 m. Shore holes usually lie at entry and exit, where the path is still shallow; it reaches its lowest position only beneath the obstacle.

4Example: lake crossing

An underground cable route crosses a lake; along the path the water surface is approx. 230 m wide, the bore approx. 500 m long (Figure 1). At entry and exit the window sampling holes reach 8 m. The straight path lies approx. 8.5 m below their collar; with D_B = 0.35 m, Eq. (1) gives:

The holes end above the level of the path; beneath the lake there is no exploratory point at all. The assumed 2.5 m of cover is also below the rule of thumb (3.5 to 5.3 m); the path may move deeper still. Recommended: deepen the shore holes to ≥ 12 m, sound the lake bed, then fix the path.

5Recommendations

• Fix a provisional path before investigating, identify channels and hollows from the DTM, set each hole’s depth by Eq. (1).
• Place holes at most approx. 5 m from the path and seal them (clay pellets, grout) – otherwise an escape path for the drilling fluid [5].
• Mark the ground below the holes as ‘not investigated’.