In the period before the construction of the HSL, the subsoil of the HSL rail infrastructure was examined to learn the geological composition of the soil. This soil investigation served a number of purposes:

• determine the structure of the soil and, based on this knowledge, build in such a manner that subsidence is prevented;
• checking for soil pollution;
• finding out if there are interesting archaeological remains along the track;
• examining if any unexploded bombs from Word War II are contained in the soil. If this were the case, they would of course have to be defused before commencement of construction.        

Soil investigations of various types were conducted: geophysical, geotechnical and land surveying. In addition, the environmental health quality of the soil was determined in an environmental field study.

Geotechnical investigation
The aim of a geotechnical investigation is to gain insight into the types of soil on the route. The soil is investigated by probing and boring: from a probing vehicle, a measuring instrument (conus) is pushed into the ground, collecting information to determine the soil composition and the soil's resistance to pressure. 

Geophysical investigation
In order to obtain certainty about the complete composition of the soil, a second investigation may be conducted: the geophysical investigation. Its aim is to determine the type of soil and the depth of the various layers. A hole is bored to collect the soil. This soil is analysed and tested for bearing power. In addition, a gauge glass can be placed in the bore hole to measure the rise of the grondwater at regular intervals. When the investigation is completed, the bore holes are closed again. 

Land surveying
This determines the exact locations of the above measurements. All activities carried out on land to determine the condition of the soil are marked with a pole. This makes it easy to find probings, bore holes, objects and depth measurements.

Environmental field study
In order to record the environmental health quality of the soil and the groundwater, environmental field studies are conducted. Holes are bored to a depth varying from 0.5 metres to 25 metres. Sometimes gauge glasses are placed to take samples of the groundwater. For the most part, boring is done manually. After the study the holes are closed again. After analysis of the samples, a decision can be made to build on the soil or not. The results of this study are the basis for applying for (building) permits with municipalities. Furthermore, it may be decided whether the earth released during building is clean enough for reuse.

We must handle our archaeological heritage with care. More and more large construction projects, such as railways and roads, are preceded by an investigation that analyses the archaeological richness of the soil. Agreements in this regard have been recorded in the Treaty of Malta. 

For this reason, archaeological investigations were carried out before and during the construction of the HSL (and the road works on the A4 and A16). Archaeologists are involved in the planning, conduct soil investigation before construction starts, and monitor the construction of the railway line.

The work is co-ordinated by the Archaeological Steering Committee. This committee comprises members of the HSL Directorate, the National Service for Archaeological Soil Investigations and municipal archaeologists of Breda and Rotterdam. Along the route, not only (fragments of) earthenware were found, but also the presumed remains of the siege works built by prince Frederik Hendrik in 1637, and a 16th-century well and bones of extinct animals.