Geotechnical Problems

Geotechnical Problems

The application of education and experience in geology and other geosciences to solve geological problems posed by civil engineering works. The branches of the geosciences most applicable are surficial geology, petro fabrics, rock and soil mechanics, geo-hydrology, and geophysics, particularly exploration geophysics and earthquake seismology. This article discusses some of the practical aspects of engineering geology.

Terminology

The terms engineering geology and environmental geology often seem to be used interchangeably. Specifically, environmental geology is the application of engineering geology in the solution of urban problems; in the prediction and mitigation of natural hazards such as earthquakes, land slides, and subsidence; and in solving problems inherent in disposal of dangerous wastes and in reclaiming mined lands. Another relevant term is geotechnics, the combination of pertinent geoscience elements with civil engineering  elements to formulate the civil engineering system that has the optimal interaction with the natural environment.

Engineering properties of rock

The civil engineer and the engineering geologist consider most hard and compact natural materials of the earth crust as rock, and their derivatives, formed mostly by weathering processes, as soil. A number of useful soil classification systems exist. Because of the lack of arock classification system suitable for civil engineering purposes, most engineering geology reports use generic classification systems modified by appropriate rock-property adjectives.       

See also:Rock;Rock mechanics;Soil mechanics 

Rock sampling

The properties of a rock element can be determined by tests on cores obtained from boreholes.These holes are made by one or a combination of the following basic types of drills: the rotaryor core drill, the cable-tool or churn drill, and the auger. The rotary type generally is used to obtain rock cores. The rotary rig has a motor or engine (gasoline, diesel, electric, or compressed air) that drives a drill head that rotates a drill rod (a thick-walled hollow pipe) fastened to a core barrel with a bit at its end. Downward pressure on the bit is created by hydraulic pressure in the drill head. Water or air is used to remove the rock that is comminuted (chipped or ground) by the diamonds or hard-metal alloy used to face the bit. The core barrel may be in one piece or have one or two inner metal tubes to facilitate recovery of soft or badly broken rock (double-tube and triple-tube core barrels). The churn-type drill may be used to extend the hole through the soil over-lying the rock, to chop through boulders, occasionally to deepen a hole in rock when core is not required or to obtain drive samples of the overburden soils. When the rock is too broken to support itself, casing (steel pipe) is driven or drilled through the broken zone. Drill rigs range in size from those mounted on the rear of large multi wheel trucks to small, portable ones that can be packed to the investigation site on aperson's back or parachuted from a small plane. See also:Drilling, geotechnical 

The rock properties most useful to the engineering geologist are compressive and triaxial shear strengths, permeability, Young's modulus of elasticity, erodability under water action, and density (in pounds per cubic foot, or pcf).

Compressive strength

The compressive (crushing) strength of rock generally is measured in pounds per square inchor kilograms per square centimeter. It is the amount of stress required to fracture a sample unconfined on the sides and loaded on the ends (Fig. 1 ).

If the load  P of 40,000 lb is applied to a sample with a diameter of 2 in. (3.14 in.²), the compressive stress is 40,000 ÷ 3.14 = 12,738lb/in.2(177,920 N ÷ 0.00203 m2= 87,645 kN/m2). If this load breaks the sample, the ultimate compressive strength equals the compressive stress acting at the moment of failure, in this case 12,738 lb/in.2. The test samples generally are cylindrical rock cores that have a length-to-diameter ratio (L/D) of about 2. The wide variety of classification systems used for rock resultsin a wide variation in compressive strengths for rocks having the same geologic name. The table gives a statistical evaluation of the compressive strengths of several rocks commonly encountered in engineering geology.

Fig. 1

Unconfined compression test. ( a ) Shear failure, showing failure planes QS  . (b) Tension failure. (  After D. P. Krynine and W. R. Judd ,Principles of Engineering Geology and Geotechnics, McGraw-Hill  ,1957) Add to 'My SavedImages'

Most laboratory tests show that an increase in moisture in rock causes a decrease in its compressive strength and elastic modulus; what is not generally known, however, is that there verse situation shown in Fig. 2 has been encountered in certain types of volcanic rocks. Insedimentary rock the compressive strength is strongly dependent upon the quality of the cement that bonds the mineral grains together (for example, clay cement gives low strength) and upon the quantity of cement (a rock may have only a small amount of cement, and despitea strong bond between the grains, the strength is directly related to the inherent strength of the grains). Strength test results are adversely affected by microfractures that may be present in the sample prior to testing, particularly if the microfractures are oriented parallel to the potential failure planes.