Chapter 4: Geotechnical Considerations
4.3 Geotechnical Design
4.3.2 Soil Classification & Testing
4.3.2.1 Soil Classification
The geotechnical component of the design for rural bridges involves an estimate of
the resistance to pull-out of an anchor. The parameters governing the mechanical
response of the soil to such loadings as well as the recommended testing approaches
are dependent on the rate of loading and the drainage characteristics of the soil. The
main parameters needed are the shear strength, usually represented by the Mohr-
Coulomb failure envelope where the strength is sensitive to the water content and
density:
π = π + π tan (π)
For the case of short-span pedestrian footbridge design, the anchorage systems have
been proven in Sections 4.1 and 4.2 to be relatively insensitive to input soil
parameters. As such, it is recommended by the author that the soil at a minimum be
56 further testing is possible, use of the conservative soil parameters are suggested for
these groups.
The Unified Soil Classification System (USCS) groups soils using their grain-size
distribution and plasticity characteristics, in order to separate them by their expected
engineering behavior (Appendix 6). The USCS assigns a group symbol to the soil,
along with standardized descriptions appropriate for that group name which is useful
for selection of design strategies. The USCS begins by separating the soil into either
coarse-grained or fine-grained, depending if greater that 50 percent of the material is
larger or smaller than a 200 sieve, with the exception of highly organic soil. Highly
organic soils often will smell have fibers and are typically dark in color. If found on
site, organic soil should be excavated and discarded due to their poor properties and
thus will not be discussed herein.
57 USCS further differentiates between the coarse-grain into βgravels and sandsβ and
fine-grain into βsilts and claysβ. This second classification step requires further
sieving for coarse-grained soils and laboratory work including the Atterberg limit
tests for fine-grained soils.
For on-site feasibility, the use of a 0.074 mm screen, equivalent to sieve size #200
should be used. If the in-situ soil is clumped, the soil must be washed prior to using
the sieve. To collect the soil sample, the site investigator shall dig a small trench and
sieve one 5-gallon bucket of material onto a standard 75 micrometer mesh
(Wovenwire, 2009). The action of digging a test-pit also gives one a better
understanding of soil variability and an increased awareness of drainage issues to
better identify where the soil may present excavation difficulties.
A second required classification step is to administer the dilatancy test detailed in
Section 4.4.2.2. Given the results of the sieve and dilatancy test, respective field
testing approaches should be completed for soils classified with greater than 50
percent passing the 0.074 mm sieve. The test requires a sample with a soft putty
consistency. Observe the reaction during shaking, followed by squeezing the soil in
ones hand with vigorous tapping. During the test, if the soil behaves as a fine-grained
soil, the vibration would densify the soil and water would appear on the surface. In a
clay sample, no change occurs and thus may be classified as fine-grained (Field,
58 surface. In such case, this soil behaves similarly to a coarse-grained soil and should
be tested and modeled as such.
4.3.2.2 Shear Strength of Fine-Grained or Cohesive Soils
For a designer interested in optimizing the size of the anchor, soil testing would
reduce conservative assumptions.
For fine-grained soils, it is relatively straight forward to obtain an undisturbed
sample, and test it in the laboratory. In a triaxial test a cylindrical soil specimen is
confined within a flexible membrane which permits the application of isotropic
stresses while permitting the specimen to deform under axial loads. The stress-strain
curve can then be obtained for different confining pressures (Saada & Townsend,
1981).
Strength values can be defined on the stress-strain curve plotted on a Mohr-Coulomb
diagram to get c and Ο. As drainage does not occur quickly in the field, excess pore Figure 20 Typical Triaxial Testing Apparatus
59 water pressure does not dissipate quickly. Therefore, the shear strength corresponds
to short-term or undrained conditions. With ideal testing and laboratory accessibility,
an unconsolidated, undrained (UU) triaxial test would simulate a similar loading
(Coduto, 2001). The UU test is performed in the triaxial cell with the drain valves
closed throughout the test.
For the bridge described in Chapter 2, the structural loading condition would
correspond to a sudden, large volume of bridge traffic. Sudden bridge loadings are
common in the case of festivals, post-school departures. During the rainy season it
would not be expected to have nearly saturated soil along the banks of a river. Figure
21 shows representative data expected from UU triaxial tests in a laboratory.
Figure 21 Expected UU Triaxial Test Results for Cohesive Soil
The UU tests on saturated fine-grained soils may be carried out either on undisturbed
or remolded samples. With the Ο3 acting on the entire sample, the axial pressure is increased until failure occurs at the deviator stress (Ο1- Ο3), from which the major
60 principle stress is determined. Several tests should be completed to create a similar
plot to that detailed in Figure 21. In this case, (Ο1- Ο3) is not sensitive to Ο3 as the increase in total stress is carried completely by the pore water. The input parameter
from the test to use in the design models is su which is related to the maximum
principal stress difference (Ο1- Ο3), by the following relationship. π = π1β π3
2
The VST is often used in-situ to obtain approximations of shear strength of saturated
cohesive soils, specifically where undisturbed samples of acceptable quality are
difficult to obtain (Terzaghi, Peck & Mesri, 1996). The VST consists of a metal
vane which is inserted into the ground and torque is applied until the soil fails in
shear, when the test is completed according to ASTM D2573. It is pertinent to note
that the rate of vane rotation is intended to ensure undrained conditions at failure. As
such, it is very beneficial to sample the soil either before or after testing, to
understand the drainage conditions of the soil tested because the presence of a silt or
coarse-grained soil will not produce usable results (ASTM D2573, 2008).
Furthermore, as the soil must be saturated prior to testing, it is advised to take a
sample near the stream bed rather than in the intended area of excavation, assuming
homogeneity between the two sites.
The undrained shear strength of a fine grained soil is correlated to the torque required
61 π π’ = π7ππ6ππ3
Where:
su = undrained shear strength
Tf = torque at failure
d = diameter of vane
π= Bjerrum correction factor
To properly identify the Bjerrum correction factor (Appendix 7), the plasticity index,
Ip, must be found. The Plasticity Index of a soil is the numerical difference between
the liquid limit and the plastic limit, (LL-PL) (Coduto, 2001). The water content is
one of the parameters which is very difficult to ascertain in the field without access to
an oven.
The pocket vane shear tester is a more portable and inexpensive version of the VST.
The pocket VST test should be completed according to ASTM D 4648 which
designates the rotation of a 12.7-mm high x 12.7-mm diameter vane at approximately
90 degrees per minute (Geotest, 2009). The vane may be advanced to depths of
interest by first excavating a small pit, to 1.5 to 2.0 meters in depth, or to a depth
62 Figure 22 Pocket Vane Shear Test
The pocket penetrometer is another method to obtain the undrained shear strength of
a saturated soil. By pushing the small probe into a fine-grained soil, the measured
unconfined compressive strength measured can be converted to shear strength by
diving by 2 (Coduto, 2006). Figure 24 shows a picture of a typical pocket
penetrometer.
Figure 23 Pocket Penetrometer
The spring operated pocket penetrometer is a small and transportable device that
measures the undrained compressive strength by pushing a 0.25β (6.4 mm) diameter
loading piston into the material of interest, to the depth of a calibration groove
machined on the piston 0.25 cm from the end. The strength in kN per square cm is
63 until reset (Professional, 2009). Both of these testing devices are highly mobile and
inexpensive thus providing a viable testing solution for rural applications.
4.3.2.3 Strength of Coarse-Grained or Cohesionless Soils
If the soil is classified as coarse-grained, obtaining undisturbed samples is nearly
impossible, especially in rural areas. Accordingly, it is difficult to quantify strength
without field tests like the Standard Penetration Test (SPT) or the Cone Penetration
Test (CPT). However, these tests require specialized equipment unavailable in the
field. Accordingly, it is recommended to use correlations. Correlations involve an
estimate of the soil density. Efforts should be made to estimate the density in the
field and use correlations such as those presented in Table 6.
Table 6 Correlations for Coarse Grained Soils (Terzaghi, Peck & Mesri, 1996)
If advanced testing is not available, conservative soil strength parameters are given in
Figure 24. These were developed based on the findings of the analyses in Sections
4.1 and 4.2. For every soil type, the first step is to sieve with a 0.074 screen. The
second step is the dilatancy test, outlined in Section 4.3.2. If the soil shows properties
64 Based on the classification of the soil, either tests or correlations should be used to
identify soil strength properties. If adequate testing is devices are not available, the
analyses suggest that conservative values can be used.
65