Lecture 11

Subsurface Soil Explorations

Selection of Triaxial Test

Soil type Type of construction Type of tests and shear strength

Cohesive Short term (end of

construction time)

Triaxial UU or CU for Undrained Strength with appropriate

level of insitu strength

Stage Construction Triaxial CU for Undrained Strength with appropriate level of

insitu strength

Long term Triaxial CU with pore water pressure measurement or Triaxial

CD for effective shear strength parameter

Granular All Strength parameter which is got from field investigation or direct shear test

Material c- Long Term Triaxial CU with pore water pressure measurement or Triaxial

CD for effective shear strength parameter

Subsurface Soil Exploration

The process of identifying the layers of deposits that underlie a proposed structure and their physical characteristics is generally referred to as subsurface exploration.

Objectives:

1. Selecting the type and depth of foundation suitable for a given structure.

2. Evaluating the load bearing capacity of the foundation.

3. Estimating the probable settlement of a structure.

4. Determining potential foundation problems such as expansive soils.

5. Determining the location of the water table.

6. Predicting the lateral earth pressure for structures such as retaining walls.

7. Establishing construction methods for changing subsoil conditions.

Steps in Soil Explorations

Collection of preliminary information using public domain resources: a) United States Geological Survey maps. b) State government geological survey maps. c) United States Department of Agricultures Soil Conservation Service county soil reports. d) Agronomy maps published by the agriculture departments of various states. e) Hydrological information published by the United States Corps of Engineers, including

records of stream flow, information on high flood levels, tidal records, and etc. f) Highway department soil manuals and Long Term Pavement Performance (LTPP) database.

Visual Investigation

a) The general topography of the site. b) Soil stratification from deep cuts. c) High-water marks on nearby buildings and bridge abutments. d) Groundwater levels, which can be determined by checking nearby wells. e) Possible existence of any cracks in walls or settlements in nearby structure.

Site Investigation

Planning, making test boreholes, and collecting soil samples at desired intervals for subsequent laboratory tests and analysis of the mechanical properties of soils.

Depth of Boring in Soils

1. Determine the effective stress due to geostatic stresses (s0).

2. Determine the stresses due to external loads (Ds).

3. Determine the depth D=D1, at which the effective stress

increase due to external loads is equal to ten percent of the

contact stress (Ds=0.1q).

4. Determine D=D2 , at which (Ds/ s0 ) = 0.05

5. Choose the smaller of D1 and D2 as the minimum required

depth of boring.

ASCE suggests using the following process for the determination of the depth of boring for site investigation:

Borehole Spacing

There are no hard-and-fast rules for borehole

spacing. Its often suggested in the specifications.

Spacing can be increased or decreased, depending

on the uniformity of the subsoil.

Oftentimes, the economics of the project influence

the number and borehole spacing.

The exploration cost can be as high as 10% of the

total project cost.

Standard Penetration Test (SPT)

Vane Shear Test (VST)

Cone Penetration Test (CPT)

Pressuremeter Test

Dilatometer Test

Determination of the Stiffness Properties of Soils in the Field

Standard Penetration Test (SPT)

The test uses a thick-walled sample tube, with an outside diameter of 50 mm. and an inside diameter of 35 mm, and a length of around 650 mm.

The sample tube is driven into the ground at the bottom of a borehole by blows from a slide hammer with a mass of 140 lb falling through a distance of 30 in. The number of blows required for driving the sampler through three 6 in. intervals is recorded.

The sum of the number of blows required for driving the last two 6 in. intervals is referred to as the standard penetration number, N or the blow count.

Factors Influencing the N-Value

SPT hammer efficiency The SPT hammer efficiency can be calculated as:

Similar to calculation of compaction energy in lecture 4,

the compaction energy for the SPT operation can be calculated as:

E (Theoretical Input Energy)=W (Weight of Hammer).H (Drop Height) =140 (lb.). 30 (in.)=4200 in.-lb=0.474 kN-m Borehole diameter Sampling method Rod length factor

100(%) EnergyInput

SamplertoEnergyHammerActualEr

Calculation of SPT Number corrected for Field Conditions (N60)

In the field, the magnitude of Er can vary from 30% to 90%. The standard practice now in the U.S. is to express the N-value to an average energy ratio of 60% (N60). N60 is used as a way to standardize the field penetration number as a function of the input driving energy and its dissipation around the sampler into the surrounding soil.

N60: Corrected SPT number hH: Hammer Efficiency hB: Correction for borehole diameter hS: Sampler correction hR: Correction for rod length

6060

RSBHNNhhhh

Correlations between Liquidity Index, N60, and Soil Strength

Hara (1971) suggested the following relationship between undrained shear strength of cohesive soils (clays) and N60:

Pa: atmospheric pressure, 100 kN/m2; 2000 lb/in2

72.0

6029.0 NPa

Cu

Relationship between N60 and Relative Density of Sand

Cubrinovski and Ishihara (1999) proposed a correlation between N60 and the relative density of granular soils, Dr:

Meyerhof (1957) developed a correlation between Dr and N60 as:

s0: effective overburden pressure D50: Sieve size through which 50% of the soil particles will pass.

Relationship between SPT Blow Count and Friction Angle ()

Vane Shear Test (VST)

The vane shear test (ASTM D-2573) may be used during the drilling operation to determine the in situ undrained shear strength of clay soils, particularly soft clays.

The vane shear apparatus consists of four blades on the end of a rod. The height, H, of the vane is twice the diameter, D. The vane can be either rectangular or tapered.

The vanes of the apparatus are pushed into the soil at the bottom of a borehole without disturbing the soil appreciably. Torque is applied at the top of the rod to rotate the vanes at a standard rate of (0.1o/sec.) This rotation will induce failure in a soil of cylindrical shape surrounding the vanes.

The maximum torque, T, applied to cause failure is measured.

The measure torque at failure is a function of soil strength, and the geometry of the vane.

ASTM Recommended Dimensions of Field Vanes

Calculation of the Undrained Shear Strength of Clays with VST

Undrained shear strength of clays (cu) is a function of the geometry and shape of the vane and the torque at failure (T).

K

TCu

H

DHDK

31

210

2

6

If H/D=2, then in SI units we have:

3810366 DK

If H/D=2, then in English units we have:

30021.0 DK

Design Correction Factors for Cu Determined from VST

Field vane shear tests are moderately rapid and economical and are used extensively in field soil-exploration programs. The test gives good results in soft and medium-stiff clays and gives excellent results in determining the properties of sensitive clays. For design purposes, the undrained shear strength values obtained from field vane shear tests are too high, and it is recommended that they be corrected according to:

Bjerrum correction factor (1972):

Morris and Williams correction factor (1994):

)()( VSTuCorrectedu CC

(%)log54.07.1 PI

)5(57.001.7

)5(57.018.1

)(08.0

)(08.0

PIFore

PIFore

LL

PI

Cone Penetration Test (CPT)

The Dutch cone penetrometer is a device by which a 60 cone with a base area of 1.54 in2 is pushed into the soil, and the cone end resistance, qc, to penetration is measured.

Most cone penetrometers that are used commonly have friction sleeves that follow the point. This allows independent determination of the cone resistance (qc) and the frictional resistance (fc) of the soil above it.

One of the major advantages of the cone penetration test is that boreholes are not necessary to conduct the test.

Unlike the standard penetration test, soil samples cannot be recovered for visual observation and laboratory tests.

Components of an Electric Friction Cone Penetrometer

Cone and Frictional Resistance

Two parameters are typically measured during the CPT test: (a) Cone resistance (qu)to penetration developed by the

cone, which is equal to the vertical force applied to the cone, divided by its horizontally projected area;

(b) Frictional resistance (fc) which is the resistance measured by a sleeve located above the cone with the local soil surrounding it. The frictional resistance is equal to the vertical force applied to the sleeve, divided by its surface areaactually, the sum of friction and adhesion.

Correlations between Relative Density (Dr) and Cone Resistance (qu)

Lancellotta (1983) and Jamiolkowski et al. (1985) relationship for normally consolidated sands:

Relationship developed by Kulhawy and Mayne, (1990)

Pa: Atmospheric pressure (100 kN/m2) so= Effective overburden pressure

Correlations between Relative Density (Dr) and Cone Resistance (qu), Cont.

(Based on Lancellotta, 1983, and Jamiolski et al., 1985)

Empirical Correlations for CPT Test

Baldi et al. (1982), and Robertson and Campanella (1983) recommended the empirical relationship shown in the plot to describe the relationship between Dr, qc and vertical effective stress so.

Kulhawy and Mayne (1990) proposed the following relationship to correlate Dr, qc and the effective stress so.

OCR: Overconsolidation ratio Pa: Atmospheric Pressure Qc: Compressibility Factor Qc=0.92 for highly compressible sand Qc=1 for moderately compressible sand Qc=1.1 for sand with low compressibility

Correlations between the qu from CPT test and Drained Friction Angle ()

Based on experimental data, Robertson and Campanella (1983) suggested the following relationship to describe the variation of Dr, so , qc and for normally consolidated quartz sand:

Based on the cone penetration tests on the soils in the Venice Lagoon (Italy), Ricceri et al. (2002) proposed a similar relationship for soil with classifications of ML and SP-SM as:

Lee et al. (2004) developed a correlation between , and the horizontal effective stress (sh ) in the form of:

Empirical Relationships between Modulus of Elasticity and qu

The cone penetration resistance has been correlated with the equivalent modulus of elasticity, Es, of soils by various investigators. Schmertmann (1970) gave a simple correlation for sand as:

cs qE 2

Trofimenkov (1974) also gave the following correlations for the modulus of elasticity in sand and clay:

ClaysforqE

SandsforqE

cs

cs

7

3

The elastic modulus of the soil is an input for the calculation of immediate (elastic) settlements under the foundations.

Relationship between CPT, SPT and Soil Texture

26.0

50

60

6429.7 DN

Pa

qc