Data presentation in offshore geotechnical site investigations Jens - - PDF document

Site investigations - Data presentation in offshore geotechnical site investigations 1

Data presentation in offshore geotechnical site investigations

Jens Galsgaard GEO, Denmark, jng@geo.dk ABSTRACT In recent years, offshore wind farm projects in several countries around the Baltic and North Sea have made heavy demands on GEO’s data presentation techniques and tools used in reporting

• ffshore site investigations. Different requests from different clients with different backgrounds

make it necessary to report data in many different ways, "tailored" to fit each type of investigation. So a borehole log is no longer a borehole log, several different log layouts are required. This calls for conditional tools that will allow logs to be multi-purpose. – In 2008 GEO began to use gINT software in data reporting. This paper shows examples of this reporting, with data from offshore subsurface investigations presented in multi-purpose borehole logs, cross sections and site maps. Keywords: data presentation, geology, geotechnics, renewable energy, site investigations 1 INTRODUCTION In the field of geotechnics, like in any scien- tific field, data plays a very important role. Data provides the basis for the computations, modellings, classifications, evaluations, etc. that form the main part of a typical geo- technician’s work. Data is the input that later enables the design of a construction or other engineering item to be put out. Data must, of course, be readily available for the geotechnician to work with, both as regards acquisition and presentation: e.g. first carry out boreholes in the field with insitu tests and sampling, then perform geological classification and lab tests on retrieved sam- ples; and finally present all of this data. This paper addresses data presentation, with re- spect to user-friendliness. For various reasons, e.g. lack of time, old habits or insufficient focus on user-friendli- ness, data from different sources or acquisi- tion methods are often being presented in different places in a report, which makes it difficult to get an overview. In fact, the au- thor believes it to be common knowledge – even if not often expressed – that the best way to arrange data is to enable the geotech- nician to view all of the relevant data on one piece of paper, or on one computer screen image, i.e. in one view. Because the best technical overview is often gained, when data from different methods are compared. And the best way to compare data is to arrange them next to one another. 1.1 Presenting offshore data In recent years, GEO has carried out a num- ber of big offshore geotechnical investiga- tions, mainly for wind farms in sea territories

• f both Denmark and her neighbouring
• countries. As the clients of these jobs were
• ften foreign, they often requested sampling,

geological descriptions and laboratory tests to be performed according to their own national

• standards. For the first jobs, this posed some

considerable challenges to data presentation, because GEO’s software used at the time for reporting borehole data did not have options for these foreign approaches. It was therefore decided to instead try and use the internation- ally well-known gINT software for the re- porting of foreign offshore wind farm geo- technical investigations. This paper contains examples of various data presentations, i.e. borehole logs, cross sections and site maps that have been de- signed at GEO with gINT software. The design of all the examples was chosen at user-level, and none of the examples have been modified by other software after the gINT processing. In the printed version the examples are shown at reduced scale, due to the layout, but in the electronic version they may be watched at full scale.

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Figure 1. Example of GEOs new offshore borehole log. Full scale log width (here height) 275 mm.

Site investigations - Data presentation in offshore geotechnical site investigations 3 2 LOG EXAMPLES Like its predecessor, GEO’s new multi-pur- pose offshore log is in A4 landscape format. The log is divided into 11 columns, some of which are further subdivided by thin lines, giving a total of 25 columns, see Figure 1. From left to right the columns display data from the sequence of processes pertaining to a borehole: 4 columns to the left show drill- ing and sampling methods and results, to- gether with a depth scale; the next quite wide column is subdivided into five columns that show both numerical, graphic and text results

• f geological classification; the next five col-

umns, of which the first two are subdivided, all show numerical data, namely test results

• f samples and lab specimens, as well as

from CPT insitu tests; the rightmost columns show the location of tests the results of which can not be displayed on the borehole log, like grain size distributions, consolidation and triaxial tests, and must be found elsewhere. The log is kept in black-and-white, except for the graphic log column in the geology section, where black grain size graphics are underlain by a coloured fill that corresponds to the geological formation, see Figure 1, or the standard Danish deposit-and-age codes, see Figure 3. Whereas these colours are, strictly speaking, redundant on the borehole log – although they do help to present the geological model – they are very useful on cross sections, or fences, where there is no text to explain the graphic log, see Figure 4. Especially with deep boreholes, some cli- ents prefer to view a longer section of the borehole in one page. The log therefore also has an A3 paper size option with 20 m per page, see Figure 2, instead of the normal 6 m, allowing more than three times as many data to be viewed in one view. One of the big challenges of designing the new borehole log was to fit all the columns with the necessary data into the 275 mm width of the log. Here the demand for over- view, i.e. fitting all the different data into one page, easily collides with the demand for legibility, i.e. to enable persons with a slight vision handicap – like most persons over the age of 50 – to also be able to read the log.

Figure 2. Part of A3 size borehole log with 20 m per page. Full scale height 337 mm.

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Figure 3. Various log layouts, full scale width 275 mm. A: Core drilled borehole with competent limestone with fractures & hardness logging, overlain by glacial till. B: Flush drilled borehole with tube sampling through varied lithologies; notice Nk and Dr strength estimates from CPT data. C: Tube sampling in very soft seabed. Compare depth scales and shear strength and moisture content scales for A-C.

A B C

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Figure 4. Fence with graphic log columns for two boreholes, same project as Figure 1. This matter was further complicated when

new clients from other countries requested

• ther tests on other soil or rock types to be

included on the log. The solution was to allow different tests, not normally carried out together in that country or on those soil or rock types, to share the same column, or, in the case of the three gridded test result fields to the right, share the same column heading field, compare Figures 1 and 3A-C. In this way GEO’s new log developed into a multi- purpose borehole log, which can take many different appearances, depending on choises made by the user. One of the multi-purpose features of the log is it allows the user to change language, see Figure 5. 3 CROSS SECTION EXAMPLES Whereas the borehole log displays data from

• ne borehole in great detail along one dimen-

sion, borehole depth, the cross section dis- plays data from several boreholes in two geographical dimensions, along (the vertical) borehole depth and along a (horizontal) line

Figure 5. Part of Danish language log of vibrocore drilled borehole. The client was mainly interested in grain size properties, and the five columns to the right normally used for core quality and density data were redesigned to instead show a summary of grain size data.

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Figure 6. Series of boreholes through glacial till (beige) down into limestone (purple), separated by transitional zone of glacially disturbed limestone (white). Grey graphic to the right of each graphic log shows fracture degree of the limestone. Same project as Figure 3A. Figure 7. Series of boreholes through soils of varying strength (green and blue soils mainly soft/loose, red and beige soils mainly stiff/dense). Graphics to the right of each borehole show CPT tip resistance and CPT friction ratio (far right, scale reversed). Compare the grain size graphics of the coloured graphic log column with the grey-and-white “soil type” pattern produced by the two CPT curves. Same project as Figure 2.

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Figure 8. Besides black borehole nos, this site map shows seabed levels, coloured according to value: dark green seabed levels are 21-22 m water depth, light green ones are 22-23 m, etc. Figure 9. Sitemap with black borehole nos and percentages of penetrated soil types: blue is Postglacial, red is Glacial, purple is Preglacial.

across the investigation area. In a cross sec- tion, many boreholes are fitted onto the same sheet, and therefore only a few important borehole data may be included, often just the graphic log column of the borehole log. For special technical uses, or in order to support

• r document the geological interpretations of

the graphic log, selected numerical data can be shown next to the graphic log of each borehole, see Figures 6 and 7. 4 SITE MAP EXAMPLES Site maps are used to display data from an entire investigation area in one view. In the case of borehole data, the problem is space. Whereas the borehole log displays data from

• ne point of the investigation area in a broad

band along borehole depth, and the fence displays data from several points in narrow strips along borehole depth, the site map has no depth dimension and only allows a small area around the borehole point for data dis-

• play. So to display borehole data on a site

map always means to select one single, or a few types of data from the borehole, and to disregard the rest of the data, see Figures 8 and 9. In return, a good geographical over- view is gained. Other selected data types may be included on other site maps, to produce a series of site maps that together give a more complete description of the conditions of the investigation area.

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Figure 10. Depth scale with legibility problems,

• cf. the leftmost side of Figures 1, 3, 4 and 5. Full

scale height 102 mm.

5 NOTES ON DESIGN It is important to keep legibility in mind during the design of any type of data

• presentation. Figure 10 is a section from an

early version of GEO’s offshore log, with depth scale to the left, lab specimen depths in the middle and boundary elevations and layer graphics to the right. As regards the depth scale, there are three legibility problems here, namely tic subdivision, tic length and crossing lines. Tic subdivision: Every one in ten of the tics are longer than the others, indicating the full meter depth. With so few long tics, it is difficult to find a particular decimal depth without counting the tics. Of course, e.g. 9.1

• r 9.2 m are easy to find, but already with 9.3

you may have to count the tics one by one, and surely with 9.5, to arrive safely with your eye at the right tic. In later versions, one in five tics is longer than the others, indicating full and half meter depth, Figure 1. Tic length: The long tics are 50 % longer than the short tics (1½ mm instead of 1 mm, at full scale), which makes both tic types seem fairly long, and a little difficult to tell

• apart. In later versions, long tics are the

double length of short tics, Figure 1. Crossing lines: The three boundary lines

• f the graphic log and elevation columns to

the right extends across to the depth scale, in

• rder for boundary depths to be read from the
• log. This, however, introduces some hori-

zontal lines that makes it more difficult to read the depth scale. This was solved in later versions by introducing a boundary depth column next to the boundary elevation col- umn, and equipping these columns with their

• wn depth scale, see Figure 1.

It may be argued that the above observa- tions are somewhat too focused on details, that they are examples of “design overkill”. But if a second or even a split second of reading time is saved with a scale design that is used everywhere, on any log, at any depth, by anyone, then quite some time will be saved by all the people who produce and use the logs, when all the split seconds are added up. 6 CONCLUSION Effort should be made that geotechnical data – like any data – are presented in user- friendly ways. An important aspect here is to find the right balance between data overview (as much data as possible in one view) and data legibility (not too small writing and symbols). Another important aspect is to en- sure that the space available for presentation is used in a suitable way, including any rele- vant data and excluding anything irrelevant

• r redundant.