Site Investigation. Some unsung heroes of Civil Engineering. buried right under your feet. 4. Need good knowledge of the soil conditions

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1 This is an attempt to create a stand alone self learning module on site investigation. Fasten your seat belts. Sit back, relax and enjoy. 1 2 Site Investigation Some unsung heroes of Civil Engineering Created by: N. Sivakugan & Kate Johnson foundations soil exploration tunneling buried right under your feet. 4 A good site investigation is a prerequisite. Need good knowledge of the soil conditions proposed structure? Problem Soils e.g., reactive clays, soft soils, sink holes, etc. 6

2 Soil data required: Soil profile - layer thickness and soil identification Index properties - water content, Atterberg limits, etc. Strength & compressibility characteristics -c, c u,, C c, C r, OCR, Others (e.g., water table depth) Desk Study First stage of site investigation. Negligible cost. Look for any freebies (i.e., info available currently) Aerial photographs Topographical maps Existing site investigation reports (for nearby sites) Other info. from local councils, literature 7 8 Site Reconnaissance A site visit and chat with locals. back hoe drill rig Site access Topography Site geology Conditions of adjacent structures Any obvious problems foreseen? CLAY Trial Pit 1-2 m width 2-4 m depth Bore hole 7 mm dia -30 m depth 9 Trial Pit Enables visual inspection, locating strata boundaries, and access for undisturbed block samples. In clay layers Clay collect undisturbed clay samples in thin walled sampler (e.g. shelby tube) A Very Large Trial Pit 11 bore hole Consolidation, triaxial tests in lab

3 Undisturbed Clay Samples Required for triaxial, consolidation tests in the lab. Good quality samples necessary. A R <% soil 2 2 O. D. I. D. A R 0 (%) 2 I. D. area ratio sampling tube In Granular Soils Very difficult to get undisturbed samples. Go for in situ tests. e.g., penetration tests Thicker the wall, greater the disturbance. Take good care in transport and handling % of foundation designs are based on penetration tests Penetration Tests How many bore holes? Measuring the soil resistance to penetration by a probe., E Proposed site for a multi-storey shopping complex GL 1 m bore hole drill rod Not enough bore holes; soil profile and properties not well defined.. 16 How many bore holes? How many bore holes? trial pit 1 m 1 m Too many bore holes and blows the budget. About right? 17 18

4 How many bore holes? The number of bore holes depends on: type and size of the project How deep to explore? Explore the soil to a depth where the stress changes become insignificant budget for site investigation soil variability Typically spaced at -40 m for non-residential buildings. Locate the bore holes where the loads are expected. proposed building 19 Standard Penetration Test (SPT) Count the number of blows required for 300 mm penetration Blow count or N-Value drill rod 63. kg hammer 760 mm drop anvil Standard Penetration Test still has some value mainly for granular soils; unreliable in clays N-value correlated to, E done within bore holes at 1. m depth intervals samples (disturbed) collected in split-spoon sampler A R = 112%; use for classification split spoon sampler 21 I.D. = 3 mm O.D.= 1 mm soil 22 Standard Penetration Test SPT Hammer 23 24

5 SPT Corrections Corrected blow count (N 1 ) 60 = C ER C N N Energy correction Overburden correction Measured blow count not corrected for overburden SPT Correlations in Clays N 60 c u (kpa) consistency visual identification very soft Thumb can penetrate > mm soft Thumb can penetrate mm medium Thumb penetrates with moderate effort stiff Thumb will indent 8 mm very stiff Can indent with thumb nail; not thumb >30 >0 hard Cannot indent even with thumb nail Use with caution; unreliable. 26 SPT Correlations in Granular Soils Cone Penetration Test (CPT) (N) 60 D r (%) consistency not corrected for overburden very loose loose medium dense >0 8-0 very dense Dynamic cone penetration test (DCP or DCPT) similar to SPT; hammer driven using cone instead of split spoon Static cone penetration test (CPT or SCPT) pushed into the 2 cm/s gives continuous measurements closed end; no samples 27 gives blow 1. m depth intervals 28 Dynamic Cone Penetration Test Static Cone Penetration Test Simple and rugged. Better than SPT or CPT in hard soils such as dense gravels As crude as SPT; relies on correlations based on blow counts Siva cm 2 cross section sleeve friction (f s ) friction ratio, f R = f s q c 0 % Typically 0 %. Hollow (split spoon) granular cohesive SPT DCP Solid ( no samples) 29 cone resistance (q c ) or tip resistance (q T ) 30

6 Piezocone (CPTU) A modern static cone; measures pore water pressure also. Pushed mm/s rate Continuous measurements of q c, f s and u. Porous Pushed Piezocone stone into with the for leads ground pore pressure measurement Interpreting CPT Data Interpreting CPT (Piezocone) Data See next slide 33 Soil Behavior Type (Robertson et al., 1986; Robertson & Campanella, 1988) 1 Sensitive fine grained Clayey silt to silty clay 9 sand 2 Organic material 6 Sandy silt to silty sand Gravelly sand to sand 3 Clay 7 Silty sand to sandy silt 11 Very stiff fine grained* 4 Silty clay to clay 8 Sand to silty sand 12 Sand to clayey sand* *Note: Overconsolidated or cemented 34 0 q t (MPa) 0 Sounding - Shelby County, TN (U.S.A) 0 Friction Ratio (%) u 2 (kpa) V s (m/sec) SM Clayey Silt Dense Sand Dense Sand In Clays, c CPT Correlations u qc vo N k cone factor (-); varies with cone Depth BGS (m) Stiff Clay In Sands, E = q c (for young normally consolidated sands) Courtesy: Professor. P.W. Mayne, Georgia Inst. of Technology 3 36

7 q c /N Relation in Granular Soils from SPT/CPT in Granular Soils After Peck et al. (1974) After Meyerhof (1976) q c in kg/cm 2 (1 kg/cm 2 = kpa) Pressuremeter Test Expand a cylindrical probe inside a bore hole. Most rational of all in situ tests Gives strength, modulus, K 0, c v For all soils Siva cylindrical probe guard cell Dilatometer Test mm/s. Test every mm. Nitrogen tank for inflating the membrane. Gives c u, K 0, OCR, c v, k, soil stiffness. Can identify soil (from a chart). Similar to the cone pressuremeter 60 mm dia. flexible steel membrane 40 Cone Pressuremeter Vane Shear Test Combines piezocone and pressuremeter. Uncommon; specialised. bore hole measuring (torque) head For clays, and mainly for soft clays. Measure torque required to quickly shear the vane pushed into soft clay. pressuremeter undrained vane h 2d torque undrained shear strength c u Typical d = -0 mm. piezocone 41 d soft clay vane 42

8 Vane Shear Test Plate Loading Test Load a square plate (300 mm x 300 mm) to failure. Plot pressure vs. settlement. Extrapolate to prototype. Loading arrangement makes it expensive. Good on random fills; indicateses an average behaviour. pressure plate Test in Progress Failure surface settlement Doing it in Sri Lankan style. 43 Common In Situ Testing Devices Pocket penetrometer A simple hand-held device for measuring unconfined compressive strength (q u = 2 c u ) of a clay. Used in trial pits and samples. very rough Must for every practicing geotechnical engineer. SPT VST PMT CPT DMT Push into the clay, and....read the strength In bore holes 4 46 Cost versus Accuracy If only they had proper site investigation Relative Cost per Test Dynamic cone penetration test Pressuremeter test Static cone penetration test Pocket penetrometer test Standard penetration test Relative Test Accuracy Tower of Pisa will not be leaning today! 48

9 Hypertext References: Fugro Internationalwww.ce.gatech.edu Georgia Institute of Technology Pagani Geotechnical Equipment