Field Observation SchemesPublished on January 30 2015
The methods for the collection of geological data have not changed very much over the past 20 years. Due to the high cost of sub-surface exploration, the main investigation is often restricted to field observations. The geo-registrations are mainly based on observation and mapping performed on:
- Outcrops or open cuts
- Underground excavations (tunnels, caverns, shafts) during and after construction, or in adits made prior to construction
- Drill core logging
For the field mapping a hammer, compass with clinometer, camera and a notebook are the basic equipment for the engineering geologist. In addition to this, maps, air photos, pocket stereoscope, knife, hydrochloride acid (for identifying potential calcite) etc., are indispensable tools. Also, GPS-instrument and altimeter may be useful. Good photo documentation is often a good investment.
The Geo-observation scheme contains characterization of all the items in Table 1, similar as used in the classification systems. An example of registrations made is shown here. (See also the example on using the Observation scheme As described in the paper ‘Geo-registrations, Rockmass Conditions and Ground Quality’ the scheme is adapted to an Excel spreadsheet where the rockmass and the ground quality can be calculated.
Sampling is an important part of the field work. A logical first step is to take small hand specimens to get an overview of the distribution of the different rocks and the variations within each type. Later, a programme for the sampling of larger specimens for mechanical testing (often 15 - 20 kg or more) can be worked out. Great care must be taken to collect representative specimens. To avoid taking weathered samples, some blasting is often necessary. This may, however, induce new cracks in the specimens.
Strike and dip measurement
The orientation of joint planes and other planar geological features are described by their strike and dip. A geologist’s compass is needed to make the necessary readings. The strike of a plane is the trace of the intersection of that plane and a horizontal surface, and is measured with the compass held horizontally against the plane. The dip is describing the plane’s inclination to the horizontal, and is measured with the compass held in the vertical plane. Also dip direction is used by many geologists. The relationship between the three terms is illustrated in Figure 1.
Figure 1: Definition of strike and dip.
There are many ways to note the results of strike and dip measurements. The best method is always the one that the respective engineering geologist is most familiar with. If no particular preference exists, it is recommended to measure the strike angle in a clock-wise direction as indicated in Figure 8.3. To make the dip designation unambiguous, it is important to indicate also the direction towards which the plane dips. It is also important to indicate whether a 360o or 400g compass is used.
For the situation in Figure 8.3, the recommended designation of strike and dip thus will be:
strike/dip = N130oE/60oNE or N130o/60oNE
The dip direction and dip are:
dip direction/dip = N40oE/60o
(generally dip direction is found from: strike + 90o or -90o; in this case 130o - 90o = 40o).
The orientation of weakness zones is often best evaluated and calculated from studies of air photos and maps. It is, however, advantageous, and often necessary, with additional observations in the field. A problem in the field is often to find representative planes on which the zone orientation measurements can be made, since close to a tectonic weakness zone there are always joints and fissures of different directions.
Observations and mapping of tunnel conditions
For the owner it is useful to have detailed information of the ground conditions of the project. Many tunnels are difficult to inspect after they have been put in operation. As shotcrete often is applied shortly after the blast, the rockmass conditions must be observed and mapped before the rock surface and the rock conditions are covered and hidden. Tunnel geo-maps should contain all geological elements that influenced on the stability and conditions of the tunnel, such as rock type and character, jointing, faults, water leakage and areas with rock burst problems, in addition to information about support work.
Tunnel mapping may be carried out in various ways, and there are several alternatives for presentation of the results. For additional documentation, the observations may be supplemented by photos.
When the project is completed and all investigations carried out, a final report should be made containing all experience gained during the planning and construction period. Maps and drawings should, as earlier described, be included in this report.