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GEOTECHNICS

For the identificaton of the ground-rock medium in a detailed way there are performed geotechnical surveys. The survey is done at the characteristic points so that its localization goes with the terrain condition and specific engineering tasks. It is often situation that the geophysical surveys are done first and their results determine the best place where to do 1D geotechnics (e.g. drillings). The most frequent Geotechnical surveys performed for engineering jobs are:

Geotechnical manual and mechanical drillings

these are basic methods to acquiring information about the ground or bedrock, depending on the medium and possibility of getting to the appointed place we use equipment for manual drilling or mechanical drillings. Manual drillings could be performed to a few meters deep assuming favourable type of the ground. Their advantage is mobility and possibility of drilling in every terrain conditions. But it often happen that identified ground are so solid or include addition of stones and rubble that it is not possible to do manual driling. In that case we can use mechanical derricks mounted at the vehicles with car or crawler chassis. Mechanical derricks have no depth limitations – the bigger and more powerful derrick is the higher maximum depth it can achieve. For such drillings are used different techniques depending on the requirements.

auger drillings

Geotechnical dynamic soundings

These are field tests of the leading parameter for non-cohesive natural soil, ie the degree of compaction ID or density index IS for embankments. The test consists of dropping a mass of known mass from a certain height and counting the number of such impacts to a certain depth of the normative cone. Depending on the depth of investigation, probes of different weights and drop height are used. The results of the probing are graphs of the number of impacts on the depth compartment. On the basis of the correlation between number of impacts and thickness of soil layer the mean value of the specified parameter can be counted.

Geotechnical static soundings

is supplement for the geotechnical drillings. Static soundings are performed with e.g. CPT probes. These devices are made for the assessment of deformation and shear parameters of the tested ground. The survey consists in pressing into the ground special tool with the measurement ending. CPT probes have two different constructions: mechanical and electronic, which provides continuous measurement. There are also expanded CPTU probes that have several pore pressure sensors. That modern and advanced methods can be supplement for classic drillings because they can obtain parameters such as liquidity index and density index considering the humidity or water influence on certain soil, rock or other material. Static probings itself are also used for the verification of the geophysical surveys when voids and other weaknesses of the ground are searched.

Static sounding with pore preassure measurement

Geotechnical laboratory analysis

if there appear complex conditions and if there is need to obtain more parameters of the surveyed ground and rocks, collected samples go to the laboratory for respective analysis. These can be different research depending on the needs (e.g. endurance test when samples are checked with different forces to obtain different static modulus). For the calibration purpose there are often made density analysis which are essential e.g. for geophysical engineering method MASW/MAM.

The result of the geotechnical surveys is borehole card which includes complex informations about certain 1D profile. Geotechnical data, that are presented on the borehole cards are the correlated data from drillings, probings and laboratory tests.

Plate load test VSS

is commonly used at engineering works to determine the state of the anthropogenic soil. This survey allows to quick and simple determination of carrying capacity and compressibility of the ground – parameters that impact its density. Thanks to plate load test VSS we can also get the subsidence curves as well as the deformation index because of the measured primary and secondary deformation modules.

plate load test

Lightweight dynamic plate test

is commonly used at engineering works to determine the state of the anthropogenic soil. This survey allows to quick and simple determination of carrying capacity and compressibility of the ground – parameters that impact its density. Thanks to plate load test VSS we can also get the subsidence curves as well as the deformation index because of the measured primary and secondary deformation modules.

Stability analysis

calculations of the embankments and slopes stability – Designed investments on leaning terrain use geotechnical surveys with stability analysis in order to eliminate mass wasting like landslides and other unfavourable phenomena. Stability calculations are made to help the designers in choosing suitable technique to preserve landslides while building works are done as well as they are finished.

Methods of results presentation

During 2D interpretation of the data from borehole cards, there are often used other surveys like geophysical surveys, which make geotechnical borders continuously between performed boreholes. The result of final interpretation is a geotechnical or geological-engineering cross-section.

geotechnical drilling sheet

Sample of geotechnical drilling card

Geological-engineering cross-section for subsurface layers made using hand-held drilling

Geological-engineering cross-section for deeper layers of bedrock using mechanical cored drilling

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