INDIRECT METHODS SOUNDING OR PENETRATION TESTS. Dr. K. M. Kouzer, Associate Professor in Civil Engineering, GEC Kozhikode

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1 INDIRECT METHODS SOUNDING OR PENETRATION TESTS STANDARD PENETRATION TEST (SPT) Reference can be made to IS for details on SPT. It is a field edtest to estimate e the penetration e resistance s of soil. It consists of a split spoon sampler 50.8 mm OD, 35 mm ID, min 600 mm long and 63.5 kg hammer freely dropped from a height of 750 mm. Test is performed on a clean hole 50 mm to 150 mm diameter. Split spoon sampler is placed vertically in the hole, allowed to freely settle under its own weight or with blows for first 150 mm which is called seating drive. The number of blows required for the next 300 mm penetration ti into the ground is the standard penetration number N N value is termed as Refusal if 50 blows for any 150 mm penetration or 100 blows for 300 mm penetration or 10 successive blows produce no advance.

2 STANDARD PENETRATION TEST (SPT)

3 N Denseness φ 0-4 Very Loose < Loose Medium Dense > 50 Very Dense > 42 CORRELATIONS OF SPT VALUES FOR COHESIONLESS SOILS CORRELATIONS OF SPT VALUES FOR COHESIVE SOILS N q u kn / m 2 N Consistency q u (kn/m 2 ) 0-2 Very soft < Soft Medium Stiff Very Stiff > 30 Hard > 400

4 CORRECTIONS FOR SPT VALUE Correction for Overburden Pressure For Cohesionless soils, 2000 N = 0.77 log10 N q 2N where q is Effective overburden pressure in kn/m 2. Correction for Dilatancy For fine sand and silt below water table, N = (N 15) for N > 15 N = N for N 15 ADVANTAGES Relatively quick & simple to perform Equipment & expertise for testt is widely available Provides representative soil sample N is correlated with most properties of soil such as friction angle, undrained cohesion, density etc. Provides useful index for relative strength & compressibility of soil Able to penetrate dense & stiff layers Results reflect soil density, stress strain behavior Numerous case histories available

5 DISADVANTAGES Requires the preparation of bore hole. Dynamic effort is related to mostly static ti performance Standards regarding g energy are not uniform If hard stone is encountered, difficult to obtain reliable result. Test procedure is tedious and requires heavy equipment. Not possible to obtain properties continuously with depth. PROBLEMS A SPT is conducted in a sand deposit at a depth of 20 m. Water table is at ground level. If N value is 40, find the corrected N value. Saturated Unit weight of sand is 19 KN/m 3.Takeγ w =10kN/m 3. Solution Correction for overburden pressure γ = γ sat - γ w =19 10=9kN/m 3 q = Eff. Overburden pressure = γ D = 9 x 20 = 180 kn/m N = 0.77 log10 N q Correction for dilatancy =0.77xlog 10 (2000/180) x 40 = x 40 = < 2 N N = (N 15) =15+0.5x(32 15) = Corrected N Value = 23

6 A SPT is conducted in a sand deposit at a depth of 20 m. Water table is at 8m below ground level. If N value is 40, find the corrected N value. Unit weight of sand above water table is 18 KN/m 3 and saturated unit weight of sand below water table is 19 KN/m 3.Takeγ w =10kN/m 3. Solution Correction for overburden pressure γ = γ sat - γ w =19 10=9kN/m 3 q = Eff. Overburden pressure = γ 1 D 1 + γ D 2 = 18 x x 12 = 252 kn/m N = 0.77 log 10 q Correction for dilatancy N N = (N 15) =21 Corrected N Value = 21 =0.77xlog 10 (2000/252) x N = N < 2 N = x 40 = =15+0.5x(27 15) A SPT is conducted in a sand deposit at a depth of 20 m. Water table is at 25m below ground level. If N value is 40, find the corrected N value. Unit weight of sand above water table is 18 KN/m 3 and saturated unit weight of sand below water table is 19 KN/m 3. Take γ w =10kN/m 3. Solution Correction for overburden pressure q=eff. Overburden pressure = γ 1 D 1 = 18 x 20 = 360 kn/m N = 0.77 log10 N q =0.77xlog 10 (2000/360) x N = N < 2 N = x 40 = There is no correction for dilatancy as the water table is below SPT depth Corrected N Value = 22

7 CONE PENETRATION TEST (CPT) STATIC CONE PENETRATION TEST (SCPT)

8 CONE PENETROMETERS Advantages and Disadvantages Advantages of SCPT are Continuous resistance with depth is recorded. d Static resistance is more appropriate p to determine static properties of soil. Can be correlated with most properties of soil. Disadvantages of SCPT are Not very popular in India. If a small rock piece is encountered, resistance shown is erratic & incorrect. Involves handling heavy equipment.

9 LOCATION OF WATER TABLE Soil strength is usually reduced below water table. Foundations below the water table will be uplifted by the water pressure. For soils with high permeability, GWT is generally determined by directly measuring to the stabilized water levelel in the bore hole after 24 to 48 hours. For soils with low permeability, install a piezometer with a porous base and a removable top cap in the bore hole. GWT is measured periodically until the water level stabilizes. BORE LOG The data obtained from bore holes, site and observations laboratory results and presented in a chart form known as Bore logs

10 BORE LOGS SOIL PROFILE

11 SOIL INVESTIGATION REPORT Introduction gives the scope Description of the proposed structure, location and geological conditions of the site Details of the field exploration programme Details of the methods of explorations General description of the sub soil conditions Details of the Lab tests Depth of ground water table and changes in water levels Discussion of the results Recommendations about type of foundations, All. Soil pressure or pile capacity, etc. Conclusions OTHER IN-SITU TESTS

12 VANE SHEAR TEST (VST) FIELD VANE SHEAR TEST

13 FIELD VANE SHEAR TEST Best suited for the determination of shear strength of saturated cohesive soils, especially sensitive clays, susceptible for sampling disturbances. Torque required to cause the failure is measured. D/ 2 D T = π DH τ π r dr r τ 2 0 T = π D 2 2 H τ + π D 6 3 τ τ = πd 2 T H D PROBLEM A vane used to test a deposit of soft clay, required a torque 75 N.m. The vane dimensions are D = 100 mm, and H = 200 mm. Find the value of undrained shear strength of soil. Solution τ = πd 2 3 T = H D π = N 02046N / mm 2 = kn/m 2

14 GEOPHYSICAL METHODS Technique of determining subsurface materials or stratification of soils and rocks by measuring some physical property of the materials, viz. magnetism, density, electrical resistivity or combination of these properties, and through correlations. Mechanical Wave Measurements Crosshole Tests (CHT) Downhole Tests (DHT) Spectral Analysis of Surface Waves Seismic i Refraction Suspension Logging Electromagnetic Wave Techniques Ground Penetrating Radar Electromagnetic Conductivity Electrical Resistivity Magnetometer Surveys Advantages Advantages and Disadvantages Non destructive ti Fast and economical testing Theoretical basis for interpretation Applicable tosoils and rocks Disadvantages No samples or direct physical penetration Models assumed for interpretation Affected by cemented layers or inclusions Results influenced by water, clay and depth

15 SEISMIC REFRACTION METHOD General soil types & depth of different strata are judged. Shock or seismic waves are created by detonating small charges of explosives or by mechanical blows on a plate near thesurface. Four basic wave types: Compression (P), Shear (S), Rayleigh (R), and Love (L). Amplitude S R Time Oscilloscope P Source Receiver (Geophone) EQUIPMENT Seismograph Spectrum Analyzer Portable Analyzer Velocity Recorder

16 SEISMIC REFRACTION METHOD Seismic waves have different velocities in different types of soil or rock. The waves are refracted when they cross the boundary between different types of soil. Radiating waves are picked up and time of travel from source recorded by geophones or seismometers. SEISMIC REFRACTION METHOD

17 DETERMINATION OF SOIL LAYERS Limitations of Seismic Refraction Test Cannot be used where a hard layer overlies a soft layer Cannot be used in areacovered by concrete or asphalt pavement which represent hard layer Frozen surface layer also may give results similar to the situation of a hard layer over a soft layer. Discontinuities such as a rock fault or earth cuts, dipping or irregular under ground rock surface and existence of thin layers of varying materials may cause misinterpretation of test data.

18 ELECTRICAL RESISTIVITY METHOD Based on the measurement and recording of changes in the mean resistivity or apparent specific resistance of various soils. Significant variations in resistivity can be detected between different types of soils, above and below water table, between unfissured rocks and soils. Four metal spikes are driving in ground along a straight line at equal distances to serve as electrodes Current I from a battery, flows through the soil between the two outer electrodes, producing an electrical field within the soil. Potential difference E between the two inner electrodes is then measured. ELECTRICAL RESISTIVITY METHOD EQUIPMENTS

19 ELECTRICAL RESISTIVITY METHOD ELECTRICAL RESISTIVITY MEASUREMENTS

20 UNIVERSITY QUESTIONS Briefly explain (i) Objectives of soil exploration; (ii) Reconnaissance survey in soil exploration. Explain in detail Auger and Shell boring methods of site exploration. What are its advantages and limitations? Explain (i) Wash Boring; (ii) Rotary Drilling. What are its advantages and limitations? List out the factors, which affect the spacing between the bore holes. What is meant by significant depth of exploration? Give guidelines, which enable the determination of the depth of exploration. UNIVERSITY QUESTIONS What are the various types of samples? Why undisturbed samples are necessary? Briefly explain the methods of obtaining undisturbed samples. Bring out the role of preservation and handling of samples in sub-surface surface exploration. What is a bore hole? Distinguish between thin-wall and thick-wall samplers. Explain the working of a piston sampler with sketches. Define Inside Clearance and Outside Clearance as applied to sampler. What are its significance?

21 UNIVERSITY QUESTIONS What are penetration and sounding tests? Under what conditions do you recommend these tests? What is Standard Penetration Test? Explain the test setup and the procedure of conducting the test. What are the corrections to the observed SPT (N) value? Discuss the merits and demerits of SPT in the sub-surface investigation. Briefly discuss any two methods of determining the ground water table. Prepare a typical soil exploration report for the construction of a multi-storied commercial building. Explain Seismic Refraction Method of exploration. What are its limitations? Explain Electrical Resistivity method of exploration. What are its limitations? it ti PLATE LOAD TEST