Cone Penetrating Testing (2007)

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5Site-specific soil investigations are required for the analysis and design of all highway bridge foundations, embankments, retaining walls, slopes, excavations, and pavements. Toward the optimal design, the state engineer will want to consider safety, reliability, long-term maintenance, and economy in deliberations of various solutions. To collect geotechnical in- formation, most state departments of transportation (DOTs) either maintain their own in-house drill rigs with field crews or else subcontract soil drilling and sampling services from outside consultant companies. Rotary drilling methods have been around for two millenia and are well-established in geotechnical practice as a means to study soil and rock conditions (Broms and Flodin 1988). Although drilling and sampling practices can be adequate, the work is manual and time-consuming, with follow-up laboratory testing often adding two to four weeks for completion of results. For soil exploration, a modern and expedient approach is offered by cone penetration testing (CPT), which involves pushing an instrumented electronic penetrometer into the soil and recording multiple measurements continuously with depth (e.g., Schmertmann 1978a; Campanella and Robertson 1988; Briaud and Miran 1992). By using ASTM and interna- tional standards, three separate measurements of tip resistance (qc), sleeve friction (fs), and porewater pressure (u) are ob- tained with depth, as depicted in Figure 1. Under certain cir- cumstances, the tip and sleeve readings alone can suffice to produce a basic cone sounding that serves well for delineat- ing soil stratigraphy and testing natural sands, sandy fills, and soils with deep water tables. Generally, this is accomplished using an electric cone penetration test (ECPT), with readings taken at 2 cm (0.8 in.) or 5 cm (2.0 in.), although a system for mechanical cone penetration testing (MCPT) is also available that is less prone to damage, but that is advanced slower and provides coarser resolutions using an incremental vertical step of 20-cm (8-in.) intervals. With piezocone penetration testing (CPTu), transducers obtain readings of penetration porewater pressures that are paramount when conditions con- tain shallow groundwater conditions and fine-grained soils consisting of clays, silts, and sands with fines. The porewater pressures at the shoulder position are required for correcting the measured qc to the total cone tip resistance, designated qt. This is especially important in the post-processing phase when determining soil engineering parameters; for example, preconsolidation stress (Pc), undrained shear strength (su), lateral stress ratio (K0), and pile side friction (fp). Additional sensors can be provided to increase the numbers and types of measurements taken, with Table 1 providing a quick sum- mary of the various types of CPTs commonly available. With CPT, results are immediately available on the com- puter for assessment in real time by the field engineer or geologist. A 10-m (30-ft) sounding can be completed in approximately 15 to 20 min, in comparison with a conven- tional soil boring that may take between 60 and 90 min. No spoil is generated during the CPT; thus, the method is less disruptive than drilling operations. Therefore, CPTs are espe- cially advantageous when investigating environmentally sen- sitive areas and/or potentially contaminated sites, because the workers are exposed to a minimal amount of hazardous ma- terial. CPTs can be advanced into most soil types, ranging from soft clays and firm silts to dense sands and hard over- consolidated clays, but are not well suited to gravels, cobbles, or hard rock terrain. Soil samples are not normally obtained during routine CPT and therefore may be a disadvantage to those who rely strictly on laboratory testing for specifications and state code requirements. Nevertheless, a large amount of high-quality in situ digital data can be recorded directly by CPT in a relatively short time in the field. These data can sub- sequently be post-processed to provide quick delineations of the subsurface conditions, including layering, soil types, and geotechnical engineering parameters, as well as both direct and indirect evaluations of foundation systems, including shallow footings, driven pilings, drilled shafts, and ground modification. A number of difficulties are now recognized with routine drilling practices in obtaining field test values, drive samples, and undisturbed samples (e.g., Schmertmann 1978b; Tanaka and Tanaka 1999). During the advance of the soil boring, the normal practice is to secure small diameter drive samples (termed “split-spoons” or “split-barrel” samples) at 1.5-m (5-ft) vertical intervals, often in general accordance with ASTM D 1586 or AASHTO T-206 procedures for the “Stan- dard Penetration Test” (SPT). The recorded number of blows to drive the sampler 0.3 m (12 in.) is termed the “N-value,” “blow counts,” or SPT resistance. It is well known that this N-value can be severely affected by energy inefficiencies in the drop hammer system, as well as additional influences such as borehole diameter, hammer system, sample liner, rod length, and other factors (e.g., Fletcher 1965; Ireland et al. 1970). Thus, these recorded N-values require significant cor- rections to the field measurements before they can be used in engineering analysis (e.g., Robertson et al. 1983; Skempton CHAPTER ONE INTRODUCTION

1986). Moreover, there remains considerable uncertainty in the proper correction of the N-values (Kulhawy and Mayne 1990) and the repeatability of SPTs using different equipment and drillers remains an issue (e.g., Anderson et al. 2004). As a complement to (or in some cases, as a replacement for) soil borings with SPT N-values, the cone can provide similar information on the subsurface stratigraphy, soil lay- ers, and consistency. Figure 2 shows a side-by-side compar- ison of an ECPT point resistance (qc) profile with a boring log derived from two adjacent boreholes with SPT resis- 6 tances (N-values) in downtown Memphis, Tennessee. The continuous nature of the CPT point resistance is evident in the profiling of the various strata and soil types. The CPT resistance complements the discrete values from the SPTs at the site and helps to better define the interface between lay- ers, thicknesses, and relative consistencies of each stratum. If geostratification at a site is the primary purpose of the site investigation, then CPT soundings can be readily advanced to detail the strata across the highway alignment. The variations both vertically and laterally can be quickly determined using TABLE 1 BASIC TYPES OF CONE PENETRATION TESTS AVAILABLE FOR SITE CHARACTERIZATION FIGURE 1 Overview of the cone penetration test per ASTM D 5778 procedures. Type of CPT Acronym Measurements Taken Applications Mechanical Cone Penetration Test MCPT qc (or qc and fs) on 20-cm intervals. Uses inner and outer rods to convey loads uphole Stratigraphic profiling, fill control, natural sands, hard ground Electric Friction Cone ECPT qc and fs (taken at 1- to 5-cm intervals) Fill placement, natural sands, soils above the groundwater table Piezocone Penetration Test CPTu and PCPT qc, fs, and either face u1 or shoulder u2 (taken at 1- to 5- cm intervals) All soil types. Note: Requires u2 for correction of qc to qt Piezocone with Dissipation CPTù Same as CPTu with timed readings of u1 or u2 during decay Normally conducted to 50% dissipation in silts and clays Seismic Piezocone Test SCPTu Same as CPTu with downhole shear waves (Vs) at 1-m intervals Provides fundamental soil stiffness with depth: Gmax = ρt Vs2 Resistivity Piezocone Test RCPTu Same as CPTu with electrical conductivity or resistivity readings Detect freshwater–salt water interface. Index to contaminant plumes Notes: qc = measured point stress or cone tip resistance, fs = measured sleeve friction, u = penetration porewater pressure (u1 at face; u2 at shoulder), qt = total cone resistance, Vs = shear wave velocity.

7FIGURE 3 Subsurface profile developed from an array of CPT qc profiles. FIGURE 2 Companion profile of CPT cone tip resistance and soil boring log with SPTN-values.

the cone tip resistance. Figure 3 is an example subsurface pro- file developed from CPT qc profiles. The thicknesses of soft compressible clay and silt layers can be mapped over the region and this information is useful in determining the settle- ments of embankment fills and shallow foundations, as well as the necessary lengths of driven or drilled piling foundations for the project. Because soils are very complex and diverse materials within a natural geologic environment, sole reliance on SPT can lead to significant oversimplifications in predicting true soil behavior. Nevertheless, a number of geotechnical firms and highway departments rely on SPTs from soil borings as their primary data source for bridge, wall, and roadway design. One clear advantage of the CPT is its ability to pro- vide three independent and simultaneous measurements. Additional sensors are available to produce up to five direct readings with depth to ascertain a more realistic evaluation of soil behavior. During routine drilling operations in North America, it is standard practice to obtain “undisturbed samples” using 8 thin-walled (Shelby-type) tubes (e.g., ASTM D 1587 and AASHTO T-207) that will later be used to provide smaller specimens for “high-quality” laboratory testing, such as tri- axial shear, one-dimensional consolidation, permeability, direct shear, or resonant column tests. However, it is now well-recognized that “undisturbed samples” are very diffi- cult to obtain with this simple tube sampler, especially when compared with high-quality and more expensive methods, such as the Laval, Sherbrooke, NGI, and JPN samplers (e.g., Tanaka and Tanaka 1999). Methods for cor- recting laboratory testing for sample disturbance effects in- clude either a consolidation–unloading phase (e.g., Ladd 1991) or reconsolidation phase (DeGroot and Sandven 2004), both of which add to laboratory testing times and more elaborate procedures. In contrast, CPT obtains mea- surements directly on the soil while still in its natural envi- rons, thus offering a direct assessment of soil behavioral response to loading. Perhaps the best approach is one founded on a combination of quick CPT soundings to scan for weak layers and problematic zones, followed by rotary drilling operations to procure soil samples for examination and laboratory testing.