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Bridges & Structures

Scourability of Rock Formations

This document is canceled. See HEC 18 Chapter 4, Section 4.6 for current information

Subject: INFORMATION: Scourability of Rock Formations Date: July 19, 1991
From: Chief, Bridge Division
Office of Engineering
Reply to Attn. of: HNG-31
To: Regional Federal Highway Administrators
Federal Lands Highway Program Administrator (HFL-1)

Usually rock is regarded as the best bearing material for structural foundations, however, there are conditions, such as sinkholes in limestone, weathering and scourability which can present problems. Bridge foundation failures have occurred due to scour of rock or rock-like materials. This memorandum presents interim guidance on empirical methods and testing procedures to assess rock scourability until results of ongoing research permit more accurate evaluation procedures. These empirical methods are commonly used by geotechnical engineers and geologists to determine rock mass engineering properties such as, allowable bearing pressures for shallow and deep foundations. Footing elevations on rock should be conservatively selected based on experience and the indirect qualitative interpretation of the methods discussed below. While safety of the travelling public is the primary design consideration, bridge designers should recognize that scour assumptions have a significant impact on the cost and constructibility of foundations and overly conservative assumptions should be avoided.

Academic geologic studies have shown that even the hardest of rocks can scour when exposed to moving water. However, the time for a finite depth of scour, is not possible to predict at this time. Empirical methods can be used to approximate rock scourability within the lifetime of a structure. Several properties contribute to the quality, bearing capacity and soundness of rock. Hence, no single index property will correctly assess the potential for scour. Designers are encouraged to utilize a combination of the following methods to assess rock scourability until a more quantitative procedure becomes available.

  1. Subsurface Investigation

    The objective of a subsurface investigation for shallow foundations on rock should permit an identification of rock type, determination of discontinuity frequency and recovery of high quality rock core samples for testing and evaluation. The number of drill holes per substructure unit should be based on the footing size, structure criticality and variability of subsurface conditions. A minimum of one boring per substructure unit and a 3.3 meter (10 foot) minimum core length below the bottom of footing are recommended. Rock core sample quality is greatly influenced by drilling equipment and technique. Poor drilling techniques will penalize rock quality assessments by lowering core recovery and rock quality designation (RQD). Rock cores should be obtained with NX diameter size core barrels 5.4 centimeters, (2 1/8 inch) or larger. Double or triple tube core barrels should be used for all structural foundation projects.

  2. Geologic Formation/Discontinuities

    Rock type and frequency of discontinuities have a significant impact on engineering properties. The three classes of rock based on geologic origin are igneous, sedimentary and metamorphic. Igneous rocks are formed by solidification of molten material from deep beneath the earth's surface. They are generally uniform in structure and lack stratification and cleavage planes. Examples of igneous rock are granite, diorite, gabbro, basalt and diabase.

    Sedimentary rocks are products of disintegration and decomposition by weathering of preexisting rock. These rocks are formed by mechanical cementation, chemical precipitant and pressure. Examples of sedimentary rock are sandstone, limestone, dolomite, shale and chert. Some common features of sedimentary rock are rounded grains, stratifications, inclination of bedding planes and abrupt color changes between layers.

    Metamorphic rock is formed from igneous or sedimentary rocks which have been altered physically or chemically by intense heat and pressure. Examples are quartzite, marble, slate and schist. Some features include the ease with which parallel layers break into slabs. In general, harder and more sound rock is less susceptible to scour.

    If rocks were free of defects, then the allowable bearing pressure could be taken conservatively as the average compression strength of unconfined rock core samples. However, rock masses are seldom free of imperfections and fractures which have a significant influence on rock behavior. The spacing of discontinuities is an indication of overall rock quality. Spacing is measured as the perpendicular distance between parallel discontinuities. Measurement is easily accomplished for rock outcrops, but is difficult from vertical drill holes. Drill cores with one fracture or less per foot would indicate a good quality rock mass. High fracture frequency (five or six fractures per foot) would indicate a poorer quality rock which would be considerably weaker and more scourable.

  3. Rock Quality Designation (RQD)

    The RQD value is a modified computation of percent rock core recovery that reflects the relative frequency of discontinuities, the compressibility of the rock mass and may indirectly be utilized as a measure of scourability. The RQD is determined by measuring and summing all the pieces of sound rock 10.2 centimeters (4 inches) and longer in length in a core run, and dividing this by the total core run length. The RQD should be computed using NX diameter cores or larger and on samples from double tube core barrels. Figure 1 provides an example of RQD computation and a relationship between RQD and rock quality. Table 1 provides a relationship between RQD, rock type and allowable bearing pressures. Scourability potential will increase as the quality of the rock becomes poorer. Rock with an RQD value less than 50 percent should be assumed to be soil-like with regard to scour potential.

  4. Unconfined Compressive Strength (qu, ASTM D2938)

    The primary intact rock property of interest for foundation design is unconfined compressive strength. Although it is known that strength of jointed rocks is generally less than individual units of the rock mass, the unconfined compressive strength provides an upper limit of the rock mass bearing capacity and an index value for rock classification. In general, samples with unconfined strengths below 1724 Kpa (250 psi) are not considered to behave as rock. As unconfined compressive strength increases, bearing capacity generally increases and scourability decreases. There is only a generalized correlation between unconfined compression strength and scourability.

  5. Slake Durability Index (SDI, International Society of Rock Mechanics)

    The SDI is a test used on metamorphic and sedimentary rocks such as slate and shale. An SDI value of less than 90 indicates a poor rock quality. The lower value of SDI, the more scourable and less durable the rock.

  6. Soundness (AASHTO T104)

    The laboratory test for soundness of rock uses a soaking procedure in magnesium or sodium sulfate solution. Generally, the less sound the rock, the more scourable it will be. Threshold loss rates of 12 (sodium) and 18 (magnesium) percent can be used as an indirect measure of scour potential.

  7. Abrasion (AASHTO T96)

    The Los Angeles Abrasion Test is an empirical test to assess abrasion of aggregates. In general, the less a material abrades during this test, the less it will scour. Materials with loss percentages greater than 40 should be considered scourable.

The above procedures can be effectively utilized to produce a rational screening process to assess rock scourability until more quantitative methods become available.

/s/ original signed by
Stanley Gordon

Figure 1 Rock Quality Designation (RQD) Example

An example is given below from a core run of 152.4 cm (60 inches). For this particular case the total core recovery is 127 cm (50 inches) yielding a core recovery of 83 percent. On the modified basis, only 99 cm (34 inches) are counted and the RQD is 65 percent.
Core Recovery, in Modified Core Recovery, in
10 10
4 4
5 5
4 4
6 6
4 4
5 5
50 39

% Core Recovery = 50/60 = 83%; RQD= 39/60 = 65%

A general description of the rock quality can be made from the RQD value.

RQD (Rock Quality Designation) Description of Rock Quality
0-25 very poor
25-50 poor
50-75 fair
75-90 good
90-100 excellent

Table 1 Recommended Allowable Bearing Pressure for Footings on Rock

Material Allowable Contact Pressure (KPa)
Such igneous and sedimentary rock as crystalline bedrock, including granite, diorite, gneiss, traprock; and hard limestone, and dolomite, in sound condition: RQD = 75 to 100 percent 11491 (120 tsf)
RQD = 50 to 75 percent 6224 (65 tsf)
RQD = 25 to 50 percent 2873 (30 tsf)
RQD = 0 to 25 percent 958 (10 tsf)
Such metamorphic rock as foliated rocks, such as schist or slate and bedded limestone, in sound condition: RQD > 50 percent 3830 (40 tsf)
RQD < 50 percent 958 (10 tsf)
Sedimentary rocks, including hard shales and sandstones, in sound condition: RQD > 50 percent 2394 (25 tsf)
RQD < 50 percent 958 (10 tsf)
Soft or broken bedrock (excluding shale), and soft limestone: RQD > 50 percent 1149 (12 tsf)
RQD < 50 percent 766 (8 tsf)
Soft shale 383 (4 tsf)
Updated: 06/27/2017
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