CHAPTER 15 ROCK SLAKE DURABILITY

Transcription

1 PENNSYLVANIA DEPARTMENT OF TRANSPORTATION PUBLICATION GEOTECHNICAL ENGINEERING MANUAL CHAPTER 15 ROCK SLAKE DURABILITY DRAFT 15-1

2 Contents 15.1 INTRODUCTION AND DESCRIPTION OF SLAKING Introduction Purpose Terms and Definitions Overview of Slaking SLAKING CONCERNS IN HIGHWAY CONSTRUCTION Conditions Where Slaking May Affect Foundation Design Spread Footings Driven Piles Micropiles and Drilled Shafts Weathering of Rock Cut Slopes SLAKING ROCK TYPES IN PENNSYLVANIA Identifying Slaking Rock Stratigraphy and Geologic Mapping Rock Identification and Description Rock Type Rock Composition Rock Hardness Rock Fissility Laboratory Testing Selecting Samples for Laboratory Testing SLAKE TESTING METHODS, RESULTS AND ITERPRETATION Introduction Jar Slake Test Slake Durability Test FOUNDATION DESIGN GUIDELINES FOR SLAKING ROCK Mitigation Strategies and Foundation Design Recommendations for Slaking Rock REFERENCES

3 15.1 INTRODUCTION AND DESCRIPTION OF SLAKING Introduction This chapter provides: standard procedures to identify where slaking rock exists; guidance to assess the slake durability of these rocks through simple test methods; direction for the interpretation of the test results; and discussion of appropriate mitigation strategies for the design of structural elements such as spread footings, driven piles, and drilled shafts in slakeprone, low-durability rock Purpose Fine-grained sedimentary rocks which include shales, claystones, mudstones, and siltstones are very common within the surface/near surface geology of Pennsylvania and are frequently encountered during highway design and construction. Some of these rock types, when exposed to air and water, can begin to slake or disintegrate. These slaking or lowdurability rocks can present adverse geotechnical conditions during highway design and construction. Consequently, clear guidelines which assist the designer in identification of these materials, provide testing procedures, and present a method for the interpretation of results to support geotechnical highway design are necessary. These guidelines are presented in the subsequent sections Terms and Definitions The following definitions are commonly associated with evaluating the slake durability of finegrained sedimentary rock types. 1. Cementation Process by which dissolved minerals become deposited in the spaces between individual sediment grains. These dissolved minerals act as a cement to bind sediments and mineral particles together thereby lithifying the sediments into rock. 2. Compaction The densification of soil particles by application of static or dynamic pressure that results in an increased unit density due to a reduction of inter-particle air voids. 3. Durability The ability of the material mass to resist physical breakdown into smaller aggregate sizes, particularly due to the effects of abrasion and cyclical wetting and drying. 4. Fissility The property of sedimentary rock to split easily along planes of weakness into thin layers along closely spaced, parallel surfaces. 5. Lithification The conversion of unconsolidated sediments to rock by three primary processes: compaction, cementation and recrystallization. 6. Slaking The disintegration of rock under wetting and drying conditions and when exposed to air. This behavior is related primarily to the chemical composition of the rock. 15-3

4 Overview of Slaking Fine-grained sedimentary rock which remains underground, confined and undisturbed, maintains its inherent composition and strength, even when natural moisture conditions fluctuate. When some fine-grained sedimentary rock types are excavated and exposed, subsequent fluctuations in moisture content can begin to cause degradation. This degradation is generally termed slaking. These rocks can quickly degrade from an apparent hard, blocky, rock mass into some combination of fractured rock pieces, rock chips, thin rock flakes, granular mineral particles, and/or mud paste. Depending on the susceptibility of the rock to slaking, the breakdown can occur within minutes of exposure, or could happen progressively over a period of days, weeks, months, or years as various cyclical environmental stressors act upon the exposed rock. Figure shows results of weathering of intact rock samples in the natural environment that were part of a research study conducted in Ohio (Gautam, 2012). 15-4

5 Elapsed Time Siltstone Shale Claystone Initial Sample condition After 1-month of weathering* After 6-months of weathering* After 12-months of weathering* *Note: Not all claystone, siltstone, and shale deposits produce the same type and magnitude of slaking. Figure Natural Weathering of Fine-grained Sedimentary Rock Samples (from Gautam, 2012) The extent of slaking or degradation shown in Figure is not guaranteed for a given rock type, and is more variable in shales than claystones and siltstones. The severity of slaking is dependent upon the individual rock type and environmental conditions. The slake durability or resistance to softening or flaking of a given sedimentary rock is to a large extent dependent upon its degree of lithification. The type of clay mineral and cement bonding that binds the silt and clay particles together are also key factors in the resistance to slaking exhibited by a given rock. 15-5

6 Physical weathering conditions in Pennsylvania include: wetting-drying cycles, freezethaw cycles, and heating-cooling cycles. Each of these cycles can serve to breakdown rock material with time by exerting various internal stresses near exposed surfaces of rock. It should be recognized that while each form of physical weathering tends to degrade intact rock and reduce the overall rock strength, that strictly speaking, slaking is a result of the moisture fluctuations and exposure to air. Evaluating the effect of wetting-drying cycles is the focus of this chapter SLAKING CONCERNS IN HIGHWAY CONSTRUCTION Conditions Where Slaking May Affect Foundation Design Slaking rock conditions can have negative impacts on both deep and shallow foundations. Deterioration of rock due to slaking can cause severe reductions in shear strength (Schaefer, 2013). Two important considerations are the degree of slake deterioration and the proximity of the suspected slaking rock to the foundation elements. For example, if slaking rock is providing direct support to driven piles, moderate or severe slaking can result in significant reduction of pile resistance. Therefore, any suspect slaking rock that will provide geotechnical resistance for a structure foundation must be evaluated for slake potential. Table provides a list of commonly used foundations for Department projects, and a list of concerns if slaking rock is identified within the bearing material. Section , Mitigation Strategies and Foundation Design Recommendations for Slaking Rock, provides guidance on mitigation strategies for handling slaking rock during foundation design. 15-6

7 Table Geotechnical Concern for Foundation Design in Slaking Rock v Foundation Type Spread Footings Driven Piles Micropiles and Drilled Shafts Geotechnical Concern Softening of slaking rock due to exposure to air and moisture during construction, resulting in reduction of bearing resistance. Driven pile creating pathway for water to reach slaking rock, resulting in softening and reduction of axial and/or lateral resistance. Softening of slaking rock due to exposure to air and moisture during construction, resulting in reduction of axial and/or lateral resistance Spread Footings Slaking presents two primary risks to spread footings that bear directly on rock which are the loss of shear strength and reduced resistance to scour. This loss of strength increases the risk of bearing and/or sliding failure. Footings in open water environments can be vulnerable to erosion/scour if the slaking potential of the founding rock is determined to be high. Slaking rock can become significantly less resistant to the erosive energy of flowing water, and any such bearing strata that is exposed to stream flow can lead to eventual undercutting of the foundation Driven Piles End-bearing that are driven into slaking rock may relax and may not maintain their axial resistance obtained during initial driving. The disturbance and breakage of the rock caused by driving the pile reduces the rock confinement pressure. Additionally, pile driving can create a pathway for water to reach the pile tip, facilitating slaking Micropiles and Drilled Shafts It is generally not acceptable to bear micropiles or drilled shafts in slaking rock because they may fail to maintain their axial and/or lateral load resistance. Some exceptions for drilled shafts include lightly loaded foundations for light poles or retaining structures Weathering of Rock Cut Slopes Chapter 8 of this publication, titled Rock Cut Slope and Catchment Design, provides guidelines, recommendations, and considerations for the design of new rock cut slopes and rehabilitation of existing rock cut slopes. Chapter 8 includes discussion of slaking rocks with respect to rock cuts. 15-7

8 15.3 SLAKING ROCK TYPES IN PENNSYLVANIA Identifying Slaking Rock It is necessary to identify rock types that are possibly slake prone for any project requiring structure foundations. The sequence of methods to identify suspected slaking rock is, as follows: 1. Locate the project on geologic mapping that may indicate weak sedimentary rock types. 2. Check for published geotechnical documentation of known slake-prone geologic formations of concern. 3. Examination the rock samples by the Project Geotechnical Engineer to determine the general rock composition, apparent hardness, and fissility, and to assess the need for slake testing. 4. Perform laboratory testing, if deemed appropriate. These individual methods are described in further detail in the following sections Stratigraphy and Geologic Mapping Initial identification of potentially slaking rock should begin with consulting the most detailed published geologic map and stratigraphic columns available for the project area, and determining the geologic unit or units underlying the project area. If the geologic map indicates the project site is underlain by limestone, sandstone, igneous or metamorphic rock, concerns for slaking are minimal and may often be eliminated. A majority of slaking rock formations in Pennsylvania were formed around the Pennsylvanian and Permian time period, and are located within the Appalachian Plateaus physiographic province. This province is dominated by alternating sedimentary rock sequences of shale, siltstone, coal, claystone, mudstone, limestone, and sandstone. These fine-grained sedimentary rocks (i.e., shale, siltstone, claystone, and mudstone) are less indurated, and more likely to slake and disintegrate when exposed to air and water. Table lists geologic units which are well known and documented to contain slaking rocks. Table is not an all-inclusive list of slaking rocks located within Pennsylvania but is intended to preliminarily alert the designer of known stratigraphic units which have a high potential for slaking. For each project, the drilling inspection logs and rock core samples must be carefully examined and considered for suspected slaking rock types. An attempt should be made to correlate the rock core samples to published stratigraphic maps or geologic studies of the area. Any potential slaking stratigraphic zones that may have a potential impact on project design should be examined closely and targeted for laboratory testing. 15-8

9 Geologic Group Dunkard Monongahela Conemaugh Table Slake-Prone Geologic Units in Pennsylvania Geologic Formation (1) Greene Washington Waynesburg Uniontown Pittsburgh Casselman Glenshaw Slaking Potential (2) Moderately High to Severe Moderately High Moderately High Moderately High to Severe, various red beds, including the Clarksburg and Schenley Moderately High to Severe Pittsburgh red beds Allegheny Low to Moderate - Pottsville Low Notes: (1) Table is not inclusive of all geologic formations having slaking rock. (2) Conditions are typical and may vary. The central and eastern portions of Pennsylvania, more specifically the Ridge and Valley, Piedmont, and New England physiographic provinces are comprised of sedimentary, igneous, and metamorphic rock types as shown in Figure Shale and siltstone units are present within these physiographic regions, however, they are generally more indurated and less slake prone than similar lithologies located in the Appalachian Plateaus Province. The presence of slaking rock formations often coincides where there are high occurrences of landslides. As shown in Figure , the Waynesburg Hills Section and the Pittsburgh Low Plateau Section of the Appalachian Plateaus Province, have the highest landslide susceptibility within Pennsylvania. Consequently, slaking, sedimentary rock sequences are also prevalent within these areas. 15-9

10 Figure Landslide Susceptibility Map of PA (PA-DCNR) 15-10

11 Rock Identification and Description Rock type, rock composition, rock hardness, and rock fissility are four primary rock characteristics that are used to assist in anticipating slake susceptibility. They are discussed in the following subsections in more detail Rock Type The slake-prone rock types listed in Table are based on characteristics such as predominant grain-size and laminations. Table Slake Potential based on Rock Type Rock Type Description Slake Potential Claystone A fine-grained sedimentary rock formed of predominantly clay-sized particles. Claystone is comprised of lithified clay possessing the texture of shale but lacks laminations and fissility of a shale. Exhibits a massive, blocky appearance and often possesses slickensides. Siltstone Shale A fine-grained sedimentary rock formed of predominantly silt-sized particles. Siltstone is comprised of lithified silt and lacks lamination and fissility. Exhibits a fine gritty texture. A fine-grained sedimentary rock formed of clay, or claysized and silt-sized particles. Shale exhibits a laminated structure which gives it fissility along which the rock readily splits. Generally exhibits low slake durability (i.e., high slaking potential) Generally medium slake durability (i.e., high to moderate slaking potential) Generally exhibits low to medium slake durability (i.e., moderate to high slaking potential) Rock Composition Rock composition is an important factor in slake assessment, but it cannot be used alone to accurately predict material behavior. Although clay is known to be the main constituent of many slaking rock types, the amount of clay, type of clay, degree of compaction, cementation, fracturing, and organic content all have a potential influence on the physical properties of the rock. A relatively minor amount of clay content can have a significant effect on the slaking behavior of the rock. If the rock sample has a clayey feel, it likely has ample clay mineral content to be considered a highly suspect slaking rock. Regardless of the perceived clay content by visual inspection, any rock that has the potential for slaking should be considered for slake testing

12 Additional rock composition properties that should be considered are unit weight and natural moisture content. Research by Masada (2013) and ODOT (2011) suggests rock having a unit weight less than 140 pcf has been shown to commonly have a low slake durability. Conversely, rock having a unit weight greater than 160 pcf has been shown to commonly have a high slake durability. Research by Bryson (2011) suggests that rock deposits having a natural moisture content below 4% were expected to be slake resistant with a corresponding Slake Durability Index value greater than 85%. Laboratory determination of unit weight and natural moisture content is discretional, but recommended, for the foundation design. These values may assist in determining the extent of slake testing needed, and also confirm the overall results. If this information is obtained, it should be used in conjunction with the slake durability test results to assist in the evaluation of the anticipated slaking performance of the foundation rock. When testing the natural moisture content of a rock sample, field preservation of the rock s natural moisture content is important, and should be completed immediately upon removal of the rock from the core barrel Rock Hardness The apparent hardness of a freshly cored rock sample is a material characteristic that is always determined and documented by the PennDOT Certified Drilling Inspector. The apparent hardness could be misleading if the possibility of rock softening over time due to slaking is not considered. This is the underlying basis for completing the slake testing. Determining the apparent hardness of a rock sample is used to give an initial indication of possible slake susceptibility and helps to determine which rock interval should be selected for further testing. Slake-prone rock tends to be weakly bonded due to weathering, weak cements, and/or significant clay content, and will often appear less hard than a rock type that is bonded by harder mineral cement types. Rock identified as very soft or soft is generally much more suspect to slaking than a medium hard rock. Sedimentary rock types identified as hard or very hard likely contain durable mineral cements and are typically much less prone to slaking when exposed to weathering Rock Fissility Any rock type identified by the drilling inspector as shale or described as shaley is expected to be fissile to some degree, which by definition splits along planes of weakness. It is possible for a rock to have visual laminations but not physically split easily or weakly along laminations. Siltstones and claystones that are stratified may be visually similar to fissile rock types (shales), but may differ in percent silt and clay composition, and in how readily the laminations tend to part or split. When the strength of the lamination bonds are equal to that of the rock layers, the horizontal and vertical shear strength are equal (i.e., the rock is isotropic). In this case, the rock is not fissile, and is likely not shale. When a rock sample has closely spaced lamination interfaces that are only slightly weaker than the rock layers themselves, the layers can be mechanically broken into closely spaced, parallel layers, and the rock could be identified as shaley or fissile. Since rock fissility enhances the potential surface area of the rock mass that is exposed to weathering effects (air and water), the intensity of the fissility is thought to have some 15-12

13 correlation to the degree of slake potential (i.e., the more fissile the rock, the more likely it is to slake) Laboratory Testing Laboratory testing is the final step in the identification and confirmation process of slaking rock. This step is normally completed only if the previous steps (i.e., mapping, stratigraphy, rock identification/description) indicate that the rock may be slake prone. However, laboratory testing can be conducted as a precautionary measure if there is doubt or concern regarding the slake potential of the rock. There are two different slake index tests used by the Department to assist in qualitatively and quantitatively assessing the slaking behavior of rock: the Jar Slake (Index) test and the Slake Durability (Index) test. The description and use of these tests are given in Section Selecting Samples for Laboratory Testing Rock samples obtained from stratum that may be used to provide resistance for shallow and deep foundations, and that are potentially susceptible to slaking (e.g., as determined by previously discussed methods) must be tested in the laboratory. A minimum of two rock samples should be tested from each substructure and from each stratum that is potentially slake prone for the Jar Slake Test. Per ASTM D4644, ten samples are required for the Slake Durability Test. Only stratums that are anticipated to provide foundation resistance need to be tested. Figure provides some examples of how to select rock samples for testing based on stratigraphy and foundation type. These are only guidelines, and testing requirements will vary based on numerous factors, including: project size and complexity; proposed construction/foundation type; and rock type(s), stratigraphy, thickness, and consistency. The samples should first be tested using the Jar Slake Test method. Based on the results of the Jar Slake Test, a minimum of two additional samples may need to be tested using the Slake Durability Test method. Guidelines to determine when it is necessary to perform the Slake Durability Test are provided in Section

14 Figure Guidelines for Laboratory Sample Location Selection 15.4 SLAKE TESTING METHODS, RESULTS AND ITERPRETATION Introduction This section provides requirements for slake testing and guidance for the interpretation of laboratory slake testing results. Interpretations include the rock s anticipated geotechnical load bearing performance, and if supplemental laboratory testing is required. As described by Richardson (1990) well over 20 slake classification systems, involving over 50 types of laboratory tests/methods, have been developed. A common deficiency with these methods is the lack of direct correlation between classification or index test values and fundamental material properties used for foundation design (Chapman, 1976). The type and magnitude of slake stressors applied by the laboratory testing (short-term) are likely different than the actual field conditions (long-term) Despite this deficiency, empirical correlations of laboratory slake index values remains the current standard of practice for foundation design Jar Slake Test The Jar Slake Test is a relatively simple, inexpensive test that is intended to quickly assess the overall slake potential of rock. This test can conclusively identify rocks that are significantly prone to slaking (i.e., low slake durability). This test is also used to determine if more detailed testing, specifically Slake Durability Testing discussed in Section , is needed. The Jar Slake Test is not effective for estimating the slake potential of medium to high durability rock. The Jar Slake Test method that is to be used on Department projects is detailed in PTM 122. This PTM closely follows the jar slake test method developed by Deo (1972). The test is done by placing a 100 to 200-gram oven-dried sample in water and observing the sample at 15-14

15 specified intervals over a 24-hour period. It is important that oven-dried samples be used because it has been reported that material tested at its natural moisture content is notably less sensitive to degradation compared with like samples that have been tested after being ovendried (Caltrans, 2007). Table provides guidelines for interpreting Jar Slake Test results. The table correlates the observations made during the Jar Slake Test to a Jar Slake Index (IJ) value ranging from 1 to 6. The table further provides interpretation of the slake potential of the sample tested, and if more rigorous testing (i.e., Slake Durability testing per ASTM D4644) is required

16 Table Jar Slake Test Interpretation Guideline v Observations of Rock Sample During Jar Slake Test Example Photograph I J Material Interpretations Further Slake Durability Testing Required? Rapid breakdown (several minutes or hours) of entire sample into fine flakes and/or fine sediment (mud). Breakdown of majority of sample into fine sediment and flakes. 1 2 Slaking Material Rock is most certainly slake prone. If rock/stratum is used to provide foundation resistance mitigation is required. Material interpretation the same for I J values of 1 and 2. No. Breakdown of sample into separate rock fragments (fracturing) with significant surface slaking (softening). 3 Same as material interpretation for I J values of 1 and 2. No. Breakdown into rock fragments (fracturing) with minor surface slaking (softening). Soak water will be cloudy when jar is rotated. 4 Likely Slaking Material Rock is likely slake prone. May not slake under short-term exposure. May require mitigation if used to provide foundation resistance. Yes. Proceed to

17 Observation of Rock Sample During Jar Slake Test Example Photograph I J Material Interpretations Further Slake Durability Testing Required? Very minor fracturing of sample is evident with no surface slaking (softening) after 24-hour soak. Soak water remains clear or nearly clear. 5 Possibly Slaking Material: No change to condition of the rock sample discernable after 24-hour soak. Soak water remains clear. Rock is possibly slake prone. May require mitigation if used to provide foundation resistance. Yes. Proceed to Slake Durability Test If the results of the Jar Slake Test do not conclusively indicate that the rock is slake prone (i.e., IJ value of 4 to 6), more rigorous testing using the Slake Durability Test method per ASTM D4644 is required. Similar to the Jar Slake Test, the Slake Durability Test is an empirical test that is used to estimate the long-term performance of the rock based on results of a short-term laboratory test. The main difference between the two test methods is the rock sample is subjected to more and longer disturbance during the Slake Durability Test. Although the Slake Durability Test was originally developed for testing shales, any potentially slaking rock can be tested

18 A brief outline of the Slake Durability Test (ASTM D4644) is as follows. Refer to the actual ASTM test document for the detailed requirements of the test. 1. Oven-dry a rock sample weighing approximately 500 grams 2. Place sample in a testing drum, and rotate the drum in water for 200 revolutions. 3. Remove the sample from the drum and oven-dry. 4. Repeat Steps 2 and 3. The results of the test are expressed as the percentage of dry weight of rock retained following second cycle, ID(2). Dry Weight retained (after two cycles) I D(2) = x 100% Dry Weight (before test) As discussed in Section (a), rock composition (type) is to be considered as part of the assessment of slake durability. Table presents some anticipated slake durability index ranges for various slake-prone rock types that are commonly encountered in Pennsylvania. The information presented is to be used to help assess the validity of the actual laboratory test results conducted for the project. This information is not to be used as a substitute for laboratory testing. Table Typical Range of Slake Durability ID(2 ) Based on Rock Type* Rock Type Typical Dry Unit Weight, pcf Range Typical Slake Durability Index, I D(2), % Range Siltstone Shale Claystone * After Masada, 2013, Geyer 1982, Underwood 1967, et al.. Values can differ significantly for a given rock type. The durability classification proposed by Gamble (1971) and recommended by the International Society for Rock Mechanics in 1979 is based on the ID(2) value. This classification has six classes of durability, which include: very high (100-98%), high (98-95%), medium high (95-85%), medium (85-60%), low (60-30%), and very low (30-0%). Dick (1994) proposed a durability classification which looked at ID(2) values in conjunction with lithologic characteristics such as clay content, dry density, and microfracturing. This classification categorizes all claystone as low-durability (ID(2) <50%) whereas, siltstones, and shales can occur in one of the three classes: low (ID(2) <50%), medium (ID(2) 50% - 85%), and high (ID(2) >85%). The classification proposed in the Gautam (2012) study is based on a calculated disintegration ratio in an effort to account for the variation in size of fragments produced during the slake durability test

19 Figure shows a comparison of the slake durability classifications given by Gamble, Dick, and Gautam to demonstrate the relative consistency of the conclusions that have been presented by various researchers over a period of several decades. In general, rock having a slake durability index above approximately 85% is considered durable, while material having a value below approximately 60% is considered non-durable. Figure Comparison of Published Slake Durability Classifications 15-19

20 Table provides the Department s general interpretations of the Slake Durability Test results. These general interpretations simply indicate the anticipated durability of the samples tested. Table Slake Durability Test General Interpretation I D(2) Material Interpretations, ASTM D4644 Material Loss <60% Low slake durability. The highest suspect rock types (typically claystone) will be in this range. Material can be expected to revert to soil. >40% 60%-85% Medium slake durability. When sufficiently exposed to drying and cyclical moisture conditions, material can be expected to revert to an intermediate mix of rock and soil. Subsequent decomposition yields a matrix of soil particles, gravel particles, and intact pieces of rock. Likely maintains a sufficient rock fraction to provide higher shear strength over a soil alone. I D(2) test results having significant rock fragments remaining suggest a joint or fracture softened condition % >85% High slake durability. Material exposed to drying and cyclical moisture conditions can be anticipated to maintain rock structure and performance in long-term service life. In this range, no additional requirements or restrictions are needed relative to slaking potential for materials. <15% These general interpretations are anticipated to be applicable to a majority of Department projects based on typical rocks encountered in Pennsylvania. It is recognized and emphasized that atypical and unique conditions do exist and will be encountered on some projects. Engineering judgement must always be used to identify and address uncommon conditions when they occur

21 15.5 FOUNDATION DESIGN GUIDELINES FOR SLAKING ROCK Mitigation Strategies and Foundation Design Recommendations for Slaking Rock There are no industry standard foundation design recommendations based on laboratory slake test results. Therefore, designers must consider the slake test results in combination with the other rock properties discussed in this chapter, engineering judgment, and past experience when designing a foundation in slake-prone rock. District Geotechnical Engineers are encouraged to consult with the Chief Geotechnical Engineer when projects involve foundations in or in proximity to low slake durability rock. Tables and provide the Department s general foundation design recommendations based on the Jar Slake Test and the Slake Durability Test. Table Foundation Design Recommendations Based on Jar Slake Test Results Jar Slake Index, I J Spread Footings Driven Piles Micropiles and Drilled Shafts 1 to 3 Do not place spread footings directly on low-durability rock. Place a 6-inch layer of Class C concrete over bedrock foundation immediately upon excavation / foundation approval. Design per DM-4 based on results of unconfined compression tests on intact rock cores, or as an equivalent soil mass. Do not terminate driven piles in low-durability rock. If piles cannot be driven through slake prone rock, predrilling will be needed. DM-4 D P indicates predrilling is recommended for penetrating claystone layers 2 feet or more in thickness. Do not terminate drilled shafts or micropiles in low-durability rock. Intervals of slaking rock are to be ignored when calculating shaft resistance. 4 to 6 See Table for foundation design recommendations

22 Table Foundation Design Recommendations Based on Slake Durability Test Slake Durability Index, 2 nd Cycle, I D(2) Spread Footings Driven Piles Micropiles and Drilled Shafts <60% Do not place spread footings directly on lowdurability rock. Place a 6- inch layer of Class C concrete over bedrock foundation immediately upon approval of the footing excavation. Design per DM-4 based on results of unconfined compression tests on intact rock cores or as an equivalent soil mass. Do not terminate driven piles in low-durability rock. If piles cannot be driven through low-durability rock, predrilling will be needed. DM-4 indicates piles can typically be driven through up to 2 feet of low-durability rock. DM-4 D P indicates predrilling is recommended for penetrating claystone layers 2 feet or more in thickness. Do not terminate drilled shafts or micropiles in lowdurability rock. Intervals of slaking rock are to be ignored when calculating shaft resistance % Do not place spread footings directly on medium-durability rock. Place a 6-inch layer of Class C concrete over bedrock foundation immediately upon approval of the footing excavation. Design per DM-4 based on results of unconfined compression tests on intact rock cores or as an equivalent soil mass. Specify a restrike and the use of dynamic monitoring for driven piles with potential to terminate in mediumdurability rock in order to assess the effect of relaxation or setup and confirm pile capacity. Do not terminate drilled shafts or micropiles in medium-durability rock unless they are supporting lightly-loaded structures, such as sound barriers, high mast lights, certain ITS equipment, etc. >85% No consideration for slake is needed for design or construction on high-durability rock

23 15.6 REFERENCES Bryson, L.S., 2011, Correlation between Durability and Geotechnical Properties of Compacted Shales, ASCE Geo-Frontiers. Caltrans, California Department of Transportation, 2007, Soil and Rock Logging, Classification, and Presentation Manual. Section Rock Identification Procedures for Borehole Cores. Chapman, D. R., L. E. Wood, C. W. Lovell, and W. J. Sisiliano, 1976, A Comparative Study of Shale Classification Tests and Systems: Technical Paper, Publication FHWA/IN/JHRP- 76/10, Joint Highway Research Project, Indiana DOT and Purdue University. Dick, J.C., Shakoor, A., and Wells, N.A., 1994, A Geological approach toward developing a mudrock durability classification: Canadian Geotechnical. Journal, v. 31. Gamble, J. C., 1971, Durability-Plasticity Classification of Shales and Other Argillaceous Rocks, Th. D. Thesis, University of Illinois at Urbana-Champaign. Gautam, T.P., 2012, An Investigation of Disintegration Behavior of Mudrocks Based on Laboratory and Field Tests, PhD Thesis, Kent State University Geyer, A. R., et al, 1982, Engineering characteristics of the rocks of Pennsylvania (2nd ed.): Pennsylvania Geological Survey, 4th ser., Environmental Geology Report 1 Hudec, P.P., 1983, Statistical Analysis of Shale Durability Factors. Overconsolidated Clays: Shales. TRB Transportation Research Record 873. Masada T., Han X., 2013, Rock Mass Classification System: Transition from RMR to GSI, Ohio Department of Transportation. Noble, D.F, 1977, Accelerated Weathering of Tough Shales, Virginia Highway and Transportation Research Council ODOT, Ohio Department of Transportation, 2011, Rock Slope Design Guide. Pennsylvania Department of Transportation, Publication 15, 2015, Design Manual Part-4 (DM- 4), Structures, Sections 7.2 and Pennsylvania Department of Transportation, Publication 222, 2015, Geotechnical Investigation Manual, Section Standard Descriptors for Rock Core. Pennsylvania Department of Transportation, Publication 408, 2016, Specifications, Section Richardson, D.N, 1990, Shale Durability Rating System Based on Loss of Strength, Journal of Geotechnical Engineering, Vol. 16 No. 12. Schaefer V.R., Birchmier M.A., 2013, Mechanisms of Strength Loss During Wetting and Drying of Pierre Shale, Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris Underwood, L.B., 1967, Classification and Identification of Shales. Journal of the Soil Mechanics and Foundations Division, Proceedings of the American Society of Civil Engineers