Rock-Socketed Shafts for Highway Structure Foundations (2006) / Chapter Skim
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Pages 7-31
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From page 7... ...
properties of a rock mass are determined by the combined properties of intact rock and naturally occurring discontinuities, such as joints, bedding planes, faults, and other structural features. 8 SITE GEOLOGY Understanding the geologic environment provides information used to plan the more detailed, subsequent phases of exploration.
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From page 8... ...
Results of load tests on drilled shafts in mica schist of the Wissahickon Formation, commonly encountered in Philadelphia and other parts of eastern Pennsylvania, are given in Koutsoftas (1981)
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Because most rock masses depart significantly from this assumption, elastic modulus values calculated from seismic wave velocities are normally larger than values measured in static field load E v v v Vd d d p= −( )
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From page 10... ...
Changes in measured apparent resistivity are used to deduce lateral variations in material type. Electrical resistivity methods are inexpensive and best used to complement seismic refraction surveys and borings.
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From page 11... ...
data reduction for depth to hard layer. FIGURE 6 Rock mass modulus versus shear wave frequency by Bieniawski (Goodman 1980)
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From page 12... ...
core recovery and (2) the ability to determine the orientation of rock mass structural features relative to the core.
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From page 13... ...
Large-diameter holes provide access for obtaining high-quality undisturbed samples and may be used for performing in situ plate load tests to measure rock mass modulus. If a full-size excavation is made by a drilled shaft contractor, information of value to both engineers and contractors is obtained.
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From page 14... ...
(2005) , use of optical and acoustic televiewer equipment is gaining popularity over oriented coring techniques because it is generally less labor intensive and is particularly useful where access or ability to drill inclined holes is limited or where local drilling companies lack the equipment necessary to collect oriented cores.
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Rock Mass Description -- i. Spacing, j.
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Weathering and alteration reduces shear strength of both intact rock and discontinuities. TABLE 6 TERMS TO DESCRIBE STRATUM THICKNESS Descriptive Term Stratum Thickness Very thickly bedded Thickly bedded Thinly bedded Very thinly bedded Laminated Thinly laminated >1 m 0.5 to 1.0 m 50 mm to 500 mm 10 mm to 50 mm 2.5 mm to 10 mm <2.5 mm FIGURE 11 Key for rock core description (sheet 1)
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From page 17... ...
is the direction of the horizontal trace of the line of dip measured clockwise from north, in degrees. Determination of dip and dip direction from core samples is possible using oriented coring techniques, borehole televiewers, downhole cameras, or other devices capable of establishing orientation of the discontinuity relative 18 to the core.
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From page 18... ...
The number of sets of intersecting discontinuities has a major effect on RMS and compressibility. As the number of sets increases, the extent to which the rock mass can deform without failure of intact rock also increases.
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From page 19... ...
specifies tolerances on smoothness over the specimen length, flatness of the ends, the degree to which specimen ends are perpendicular to the length, and length-to-diameter ratio. Uniaxial compressive strength of intact rock is used in empirical correlations to evaluate ultimate side and base resistances under axial loading; ultimate limit pressure under lateral loading; and, by contractors, to assess constructability.
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From page 20... ...
(1992) for drilled shafts in Florida limestone.
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From page 21... ...
TABLE 8 IN SITU TESTS WITH APPLICATIONS TO ROCK-SOCKET DESIGN Method Procedure Rock Properties Limitations/Remarks Pressuremeter (includes devices referred to as rock dilatometer) Pressuremeter is lowered to the test elevation in a prebored hole; flexible membrane of probe is expanded exerting a uniform pressure on the sidewalls of the borehole Rock mass modulus; rock mass strength in weak rocks ASTM D4719 Test affects a small area of rock mass; depending on joint spacing, may or may not represent mass behavior; limited to soft or weak rocks Borehole jack Jacks exert a unidirectional pressure to the walls of a borehole by means of two opposed curved steel platens Rock mass modulus; rock mass strength in weak rocks ASTM D4971 Measured modulus value must be corrected to account for stiffness of steel platens; test method can be used to provide an estimate of anisotropy Plate load test Load is applied to a steel plate or concrete foundation using a system of hydraulic jacks and a reaction frame anchored to the foundation rock Rock mass modulus; rock mass strength in weak rocks Loaded area is limited, so may not be effectively testing rock mass if joints are widely spaced; modulus values corrected for plate geometry, effect of rock breakage, rock anisotropy, and steel plate modulus; not common for deep foundations Texas cone penetration test Steel cone is driven by a drop hammer; number of blows per 300 mm of penetration is TCPT N-value; depth of penetration per 100 blows is penetration resistance (PR)
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From page 22... ...
The draft 2006 Interim AASHTO LRFD Bridge Design Specifications recommends determination of RMR for classification of rock mass in foundation investigations. Seventeen states reported using RMR either always or sometimes for rock mass classification associated with drilled shaft design.
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From page 23... ...
8. Engineering Properties of Rock Mass Shear Strength Geotechnical evaluation of foundation ultimate capacity under axial and lateral loading is calculated on the basis of shear strength along assumed failure surfaces in the rock or at the concrete–rock interface.
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From page 24... ...
Stability analyses of rock sockets governed by massive rock are normally evaluated directly in terms of qu. When rock core is not sufficient for uniaxial compression testing, or sometimes for convenience, qu is correlated to results of point load tests.
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From page 25... ...
where σ'1 and σ'3 = major and minor principal effective stresses, respectively; qu = uniaxial compressive strength of intact rock; and mb, s, and a are empirically determined strength parameters for the rock mass. The value of the constant m for intact rock is denoted by mi and can be estimated from Table 11.
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From page 26... ...
Rock mass modulus can vary from less than 1 MPa to greater than 100 GPa and depends on intact rock modulus, degree of weathering, and characteristics of discontinuities. Compiled values provide guidance for very preliminary evaluations, but should not be relied on for final design.
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From page 27... ...
Notes: ER = intact rock modulus, EM = equivalent rock mass modulus, RQD = rock quality designation, RMR = rock mass rating, Q = NGI rating of rock mass, GSI = geological strength index, qu = uniaxial compressive strength.
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From page 28... ...
The modulus of intact rock from laboratory uniaxial compression tests on core samples is consistent with the observation of Heuze (1980) that field rock mass modulus values range from 20% to 60% of intact rock modulus and serve as an additional upper-bound check.
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From page 29... ...
. This figure shows the relationship between unit side resistance determined from field load tests on drilled shafts and one-half of the unconfined compressive strength.
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From page 30... ...
and empirical correlations with GSI to establish representative values of rock mass modulus for foundation design. Site and geomaterial characterization are interrelated with design, construction, and load testing of drilled shafts in rock.
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From page 31... ...
X Geological Strength Index (GSI) X X Rock Mass Characteristic Axial LoadDisplacement Unit Base Resistance Unit Side Resistance Design Applications Axial Loading Lateral Loading Load-Displacement Ultimate Resistance Notes: X = property is used directly in equations that are currently applied widely in practice.
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