Cone Penetrating Testing (2007)

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Some obstacles to advancing CPTs in certain geologic forma- tions and working in problematic soils are discussed in this chapter, with brief overviews given regarding novel approaches to solving these situations and special systems developed to cope with such difficulties. A common response to the DOT survey question regarding the limited use of CPT in exploration in their state indicated that the ground conditions were often too hard for penetration or that a dense impenetrable shallow layer prohibited advance of the CPT. Toward this purpose, a section is devoted herein to describing special systems that have been developed toward overcoming cone penetration in hard ground. REMOTE ACCESS CONE PENETRATION TESTS For remote access CPTs, innovations include the construc- tion of special deployment vehicles for cone penetrometer technology, particularly in urban areas; small limited access locations; and remote arctic weather, as shown by the selec- tions presented in Figure 96. In areas of high water table, spe- cial deployment of CPTs can be accomplished by airboats, barges, and/or swamp buggy (Figure 97). Of particular interest is the completely automated PROD (portable remotely operated drill) that was developed for offshore use with capabilities to drill, sample, push CPTs, vane shear testing, and obtain rock coring to depths of up to 100 m (330 ft) below mudline (Randolph et al. 2005). Several PROD components are shown in Figure 98. CONE PENETRATION TESTS IN HARD GROUND From the survey questionnaire (Appendix A, Question 55), one of the biggest obstacles to the use of CPTs by the DOTs is that the ground is too hard for static penetration, as shown by Figure 99. The second highest reported obstacle was the presence of gravels or stones. In this section, available means to overcome these obstacles are discussed. Various creative and novel means of deploying CPTs have been designed to achieve depths of penetration in very dense sands, weak rocks (chalks, mudstones, tuff), and tran- sitional zones of residuum to saprolite, as well as cemented layers and caprocks. An excellent overview on conducting CPTs in very hard soils and weak rocks is provided by Peuchen (1998), based in large part on the long experience 76 of the Dutch, and efforts advanced in the offshore site explo- ration industry. With the proper techniques, electric cone tip stresses of more than 100 MPa (1000 atms) and mechanical CPT resistances up to 150 MPa (1500 atms) have been recorded. Table 14 provides a general overview on methods developed to overcome CPT in hard ground. For increased penetration in dense ground, large dead- weight vehicles on the order of 180 kN (20 tons) are available, having considerable more pushing reaction compared with drill rigs. Trucks with weights as high as 360 kN (40 tons) have been built to facilitate CPTs in very dense sands and gravels [Figure 100 (left)] for routine application at the Hanford nuclear site in Washington State (Bratton 2000). These vehicles are too heavy to meet roadway load requirements at full capacity; therefore, they are mobilized to the site at acceptable weight limits (say 180 kN) and the additional 180 kN deadweight are added at the testing location. Another means to increase the reaction capacity is to employ earth anchors. The anchors can be installed with variable size plates and depths of 1, 2, or 3 m, depending on local conditions [Figure 100 (right)]. Anchoring permits small lightweight CPT rigs (60 kN) to achieve depths of 30 to 40 m and successful penetration in fairly dense sands (N  30 bpf). An illustrative example of a CPTu conducted in hard saprolite and partially weathered rock of the Piedmont in north Atlanta is shown in Figure 101 (Finke and Mayne 1999). The very high resistances measured by the SPT N-values in an adjacent soil boring clearly shows the dense ground conditions. Nevertheless, the piezocone sounding was successfully advanced into these hard residual soils. Note the characteristic negative porewater pressures in the Piedmont upon reaching the groundwater table. When cemented layers, caprock, or hard concretions are encountered in the profile, the CPT sounding can be halted and the penetrometer can be withdrawn. Then, a rotary drill rig can be set up and used to bore through the cemented zone. The prebored hole can be filled with a backfilled sand or pea gravel and the CPT sounding can be resumed. The backfill helps to stabilize the cone rods and prevent buckling. On completion, the results of part A of the sounding can be CHAPTER TWELVE CONE PENETRATION TESTING MODIFICATIONS FOR DIFFICULT GROUND CONDITIONS

77 FIGURE 96 Special CPT deployment systems: (left) single personnel track vehicle (Sweden), (center) cherry-picker for urban access (New Zealand), and (right) portable unit for arctic work (Canada). FIGURE 97 Special CPT deployment by (left) airboat, (center) barge, and (right) New Orleans marsh buggy. FIGURE 98 Components of portable remotely operated drill (PROD): (left) 3000-m-long umbilical cable, (center) remote control panel and data acquisition, (right) CPT and rotary drill platform.

78 FIGURE 99 Questionnaire responses concerning major obstacles to use of CPT. skrameR/stnemmoC ecnerefeR euqinhceT gnicnavdA Heavy 20-Ton Deadweight CPT Trucks and Track Rigs Mayne et al. (1995) Increased weight reaction over standard drill rig Friction Reducer van de Graaf and Schenk (1988) Effective in frictional soils, but not so in very dense sands Cycling of Rods (up and down) Shinn (1995, personal communication) Local encounter in thin hard zones of soil Large diameter penetrometer (i.e., 44-mm cone; 36-mm rods) van de Graaf and Schenk (1988) Works like friction reducer Guide Casing: Double Set of Rods; Standard 36-mm Rods Supported Inside Larger 44- mm Rods; Prevents Buckling Peuchen (1988) Works well in situations involving soft soils with dense soils at depth Drill Out (downhole CPTs) NNI (1996) Alternate between drilling and pushing Mud Injection Van Staveren (1995) Needs pump system for bentonitic slurry Earth Anchors Pagani Geotechnical Equipment Geoprobe Systems Increases capacity for reaction Static–Dynamic Penetrometer Sanglerat et al. (1995) Switches from static mode to dynamic mode when needed Downhole Thrust System Zuidberg (1974) Single push stroke usually limited to 2 m or 3 m Very Heavy 30- and 40-Ton Rigs Bratton (2000) After large 20-ton rig arrives at site, added mass for reaction ROTAP—Outer Coring Bit Sterkx and Van Calster (1995) Special drilling capabilities through cemented zones elihw tset noitartenep enoC )4002( .la te ottehccaS DWTPC drilling Sonic CPT Bratton (2000) Use of a vibrator to facilitate penetration through gravels and hard zones dna nnihS ;)0002( notgnirraF SPAE Haas (2004); Farrington and Shinn (2006) Wireline systems for enhanced access penetrometer system Adapted and modified after Peuchen 1998. NNI = Nederlands Normalisatie Institute; CPTWD = cone penetration test while drilling; EAPS = enhanced access penetrometer system. TABLE 14 SPECIAL TECHNIQUES FOR INCREASED SUCCESS OF CONE PENETRATION IN HARD GEOMATERIALS added to part B of the sounding to produce a complete depth profile. Although this is somewhat unattractive to routine production type CPTs, it does help obtain the desired results, which include electronic readings of tip stress, sleeve fric- tion, porewater pressures, and shear wave velocities. If the geologic conditions of the region normally encounter an embedded hard cemented layer, perhaps the CPT user would wish to obtain a combine rig (as shown in Figure 102) that has capabilities of both static CPT push and rotary drilling operations. The ROTAP tool is specially designed to advance CPTs through hard cemented zones (Sterckx and Van Calster 1995). Initially, the CPT is advanced in a normal procedure

79 until the hard caprock or concretion is encountered. Then the penetrometer is removed and the ROTating AParatus (i.e., ROTAP) is installed and used to drill through the hard zone. Once through the desired hard layer, the penetrometer is rein- stalled to continue the sounding. A special series of Ateliers Mobiles d’Ausultation par Pénétration des Sols (Mobile Soil Testing Unit by Penetra- tion) static–dynamic penetrometer systems has been devel- oped for testing a range of soft soils to very hard and dense geomaterials with reported qc up to 140 MPa (1400 tsf) and depths up to 100 m (Sanglerat et al. 1995). A heavy-duty van den Berg track truck is used for the hydraulic pushing. The sounding has three distinct phases: (1) static electric CPT push, (2) static mechanical CPT push, and (3) dynamic mechanical CPT. The test begins as a standard CPT with either a 44- or 50-cm2 electrical penetrometer (qc and fs) pushed at 20 mm/s until hard static refusal is met at 30 MPa (300 tsf). The sounding is resumed using a 12-cm2 mechani- cal cone in static push mode until 120 MPa (1200 tsf) is reached. To penetrate very dense sands, gravels, rocks, and other obstacles, a special dynamic fast-action hydraulic ham- mer is used to advance the cone and, if conditions permit, resume again with the static push phase. Example results of static–dynamic penetration in dense sandstone are shown in Figure 103 with all three phases of testing shown. A sonic CPT system is detailed by Bratton (2000), whereby a vibrator can be intervened when the soil resis- tance becomes too great for normal static CPT pushing. The sonic vibrator is installed in the CPT truck and uses two twin 25-hp hydraulic motors with eccentric masses at the top of FIGURE 100 CPT vehicles for hard ground including: (left) 40-ton truck, (right) anchored track rig. FIGURE 101 Piezocone advanced into very hard partially weathered gneiss.

80 memocone penetrometer is used to store the CPTu data downhole in a memory chip. The system also employs MWD (measurements while drilling) during simultaneous opera- tion of the CPT; therefore, two sets of penetration readings are obtained, including piezocone measurements (qt, fs, and u2), as well as the MWD readings of penetration rate, torque, and fluid pressure. When hard impenetrable layers are encountered (too hard for CPT), then the sounding advances strictly on the basis of wireline coring techniques with MWD data still obtained. Figure 105 shows the basic CPTWD scheme, equipment, and a full set of results of six measure- ments from a site near Parma, Italy. An enhanced access penetrometer system (EAPS) is pre- sented by Shinn and Haas (2004) and Farrington and Shinn (2006). This is based on a wireline system (Farrington 2000) and offers a means to interrupt the CPT steady-state rate of penetration and utilize downhole wireline coring intermit- tently and advance soundings through very dense or cemented zones or dense or hard geomaterials. The EAPS also has the ability to take soil samples as needed. Some aspects are illustrated in Figure 106. Comparative studies of the EAPS deployment and normal direct-push technology for CPTs have been made by Applied Research Associates. Figure 107 shows four sets of super- imposed piezocone soundings at a test site with two CPTUs produced by the EAPS downhole wireline method (Nos. 2A and 2B) and two CPTUs per normal push methods (Nos. 2C and 2D), with very good agreement seen for all cases. FIGURE 102 Combine rig with both CPT hydraulic rams and drilling capabilities. FIGURE 103 AMAP static–dynamic penetration system in dense sandstone (after Sanglerat et al. 1999). the rods. The vibrations are in the range of 25 to 125 Hz and used to facilitate CPT penetration through dense sands and gravels. Figure 104 shows the sonic CPT unit within an ARA truck. A special wireline-based system that combines CPT with drilling capabilities has been termed CPTWD (cone penetra- tion test while drilling) and is detailed by Sacchetto et al. (2004). A special modified wireline-type core barrel has been developed to house the cone penetrometer. An Envi-type

81 FIGURE 104 Sonic CPT system with (left) deadweight truck, and (right) sonic vibrator unit. FIGURE 105 CPTWD wireline-based system (Sacchetto et al. 2004).

82 FIGURE 106 Enhanced access penetration system (EAPS) for penetration of hard geomaterials (after Farrington 2000). FIGURE 107 Comparison of CPTUs from standard push and EAPS wireline deployed systems (Farrington 2000).

83 FIGURE 108 Jackup rigs for nearshore CPT deployment: (left) SeaCore, and (right) The Explorer. FIGURE 109 Vessels for offshore CPTs: (left) Markab, Australia; (right) Bucentaur, Brasil (Courtesy: Fugro Geosciences). NEARSHORE AND OFFSHORE DEPLOYMENT On some highway projects, the highway alignment crosses over a body of water, particularly bridges over rivers or streams or waterway canals. The CPTs can be mobilized to conduct soundings from floating barges. In swampy coastal areas with shallow water, movable pontoons are used that are floated empty to their location, then filled with water to pre- pare a working platform for the CPT truck or rig. In nearshore environments, highways may follow the coastal shoreline or connect small islands and land masses. In these cases, the use of a jackup rig may be warranted. Figure 108 shows two jackup-type platforms in use for drilling, sam- pling, and CPT works. If water depths are greater than 15 m, then a special CPT ship can be deployed to conduct offshore site investigations (Figure 109).