Engineering Geology Rock Properties for Engineering • Hussien aldeeky 1
Engineering Geology Rock significant Rock are significant for two major reasons in engineering: (1)As building materials for constructions; (2)As foundations on which the constructions are setting For the consideration of rocks as construction material the engineers concern about: (a)Density to some extent (for calculating the weight, load to the foundation, etc.); (b)Strength; (c)Durability; 2
Engineering Geology For the consideration of rocks as the construction foundation the engineers concern about: (a)Density (b)Strength (c)Compressibility So the major difference is for material we want the durable (not have to be hard) rock, and for foundation we want hard rock (hard ones usually durable especially in a stable subsurface condition).
Engineering Geology • The same rock properties measured in lab and in field may have different values • . This is because that in lab the rock properties are measured on small sized samples, but rocks in situ usually contains weakness 4 planes (foliations, joints, cracks and fractures, etc.) • In general the measured values on density, strength, seismic velocity, etc usually have a smaller value in field than in lab. Rock mass ( in situ) properties are fundamentally controlled by weakness planes . • So it is a common practice to have a room for the variability when use lab measured value to field engineering projects. 4
Engineering Geology Factors influence the deformation and failure of rocks – 1-Mineralogical composition and texture; – 2-Planes of weakness; – 3-Degree of mineral alteration; – 4-Temperature and Pressure conditions of rock formation; – 5-Pore water content – 6-Length of time and rate of changing stress that a rock experiences. 5
Engineering Geology 1-Mineralogical composition and texture -Very few rocks are homogeneous, continuous, isotropic (non directional) and elastic. - Generally, the smaller the grain size, the stronger the rock. -Texture influences the rock strength directly through the degree of interlocking of the component grains 2-Planes of weakness -Rock defects such as micro fractures, grain boundaries, mineral cleavages, twinning planes and planar discontinuities influence the ultimate rock strength and may act as “ surfaces of weakness ” where failure occurs. -When cleavage has high or low angles with the principal stress direction, the mode of failure is mainly influenced by the cleavage .
Engineering Geology 3-Degree of mineral alteration . - Anisotropy is common because of preferred orientations of minerals and directional stress history. - Rocks are seldom continuous owing to pores and fissures (i.e. Sedimentary rocks). - Despite this it is possible to support engineering decisions with meaningful tests, calculations, and observations. 4-Temperature and Pressure conditions of rock formation - All rock types undergo a decrease in strength with increasing temperature, and an increase in strength with increasing confining pressure. -At high confining pressures, rocks are more difficult to fracture as incipient fractures are closed
Engineering Geology 5-Pore water content - The presence of moisture in rocks adversely affects their engineering strength. -Reduction in strength with increasing H 2 O content is due to lowering of the tensile strength, which is a function of the molecular cohesive strength of the material. -6-Length of time and rate of changing stress that a rock experiences - Most strong rocks , like granite show little time-dependent strain or creep .
Engineering Geology Different degrees of rock weathering (from Johnson and DeGraff, 1988 9
Engineering Geology Density, specific gravity, porosity, and void ratio Porosity: Proportion of void space given by n = p / t , where p is the pore volume and t is the total volume. Typical values for sandstones are around 15%. In Igneous and Metamorphic rocks, a large proportion of the pore space (usually < 1-2 %) occurs as planar “fissures”. With weathering this increases to > 20%. Porosity is therefore an accurate index of rock quality. Density: Rocks exhibit a greater range in density than soils. Knowledge of the rock density is important to engineering practice. A concrete aggregate with higher than average density can mean a smaller volume of concrete required for a gravity retaining wall or dam. Expressed as weight per unit volume.
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Engineering Geology . Schematic representation of porous medium indicating relationship between air (A), solid skeleton (B) and water (C).
Engineering Geology Porosity types 13
Engineering Geology VUGGY FRACTURE FENESTR AL FRACTURE INTRAPARTICLE MOLDIC 14
Engineering Geology Porosity does not give any information about 1- Pore size 2-pore distribution 3- pore connectivity Pock of the same porosity can have widely different physical properties Porosity depends on stress conditions 15
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Engineering Geology THE MOHO Pulse Sonic Velocity Sonic Velocity : Use longitudinal velocity V l measured on rock core. Velocity depends on elastic properties and density, but in practice a network of fissures has an overriding effect. Can be used to estimate the degree of fissuring of a rock specimen by plotting against porosity (%). The value of the compressional wave velocity can serve as an indicator of the degree of weathering. For instance, Dearman et al. (1978 ) have tabulated . ranges of velocity for various degrees of weathering in granites and gneisses: fresh, 3050-5500 m/s; slightly weathered, 2500- 4000 m/s; moderately weathered, 1500-3000 m/s; highly weathered, 1000-2000 m/s; completely weathered to residual soil, 500-1000 m/s. Note that an empirical upper limit for the velocity of 2000 m/s is often used in practice to define geologic materials that can be ripped without difficulty .
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Engineering Geology Permeability due to open joints and fractures : As well as the degree of interconnection between pores / fissures, its variation with change in normal stress assesses the degree of fissuring of a rock. Dense rocks like granite, basalt, schist and crystalline limestone possess very low permeabilities as lab specimens, but field tests can show significant permeability Permeability related to the following • volume of pores • degree of openness or connection between pores and fractures • Grain size • Sorting of grains
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Engineering Geology
Engineering Geology Abrasion resistance test: Sample weight 5 kg, specific size gradation specific number of steel spheres, interior projecting shelf, 500 revolutions, then use #12 sieve with d=0.141 mm. Percent loss = (material finer than #12 sieve) / (original weight) For highway construction, we need percent loss less than 35 – 50 %.
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Engineering Geology Example: Mass placed in abrasion machine 5,000 g Mass of intact particles left after test 3,891 g (1 pound = 454 grams) Common Values Basalt » 14% Limestone » 30% Granite » 40%, Marine Limestone » 53% 26
Engineering Geology THE MANTLE . slake durability test Approximately 500 g of broken rock lumps (~ 50 g each) are placed inside a rotating drum which is rotated at 20 revolutions per minute in a water bath for 10 minutes. The drum is internally divided by a sieve mesh (2mm openings) and after the 10 minutes rotation, the percentage of rock (dry weight basis) retained in the drum yields the “slake durability index (I d )”. A six . step ranking of the index is applied (very high-very low). as shown in tables 1 and 2. Used to evaluate shales and weak rocks that may degrade in service environment. D: the mass of the empty dry drum. A: The initial dry mass of rock plus drum C: dry mass of the drum and the rock after two cycles of wetting and drying, From a practical point of view, slaking of clay-bearing rocks requires protection of all outcrops. Shot crete or any other form of protective layers are usually adequate. After slaking for 10 minutes the rock samples were then dried in an oven at a temperature of 105 C for up to 6 hrs
Engineering Geology THE CORE
Engineering Geology Durability test For durability test there are two major methods: . 1) sulfate soundness: Example -Soaking the material under test into sulfate solution and put it into oven for drying to crystal for 5 cycles, then use the same sieve and get the percent loss; -Provides a measure of the aggregates durability when exposed to the elements • measures resistance to rapid weathering • important in frost -susceptible regions Sulfate Soundness Test Example Original mass of sample = 2,175 g Shale Mass of particles after test = 1,847 g
Engineering Geology 2) freezing-thawing test: Freezing and thawing the material for 25 cycles, then use the same sieve and get the percent loss; For highway construction material, the maximum loss for concrete aggregate is 12-15%, and for base course this number is 15-18%. 30
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