Bridge Geotech Manual

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Geotechnical Manual

October 2000


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Geotechnical ManualOctober 2000

Manual Notices

Manual Notice 2000-1To:All Districts and Divisions

From:Kirby W. Pickett, P.E.

Functional Manual:Geotechnical Manual

Effective Date: August 29, 2000

Purpose

This manual is intended to guide the districts in performing geotechnical investigation and

design for project development.

Contents

The manual contains nine chapters Field Surveys, Field Operations, Soil and Bedrock,

Classification and Logging, Engineering Properties of Soil and Rock, Foundations Design,

Retaining Walls, Slope Stability, Laterally Loaded Foundations, and Design Examples.

Instructions

This manual supersedes the previous Foundation Exploration and DesignManual.

Contact

For more information regarding any chapter or section in this manual, please contact the

Bridge Division Geotechnical Section.


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Geotechnical Manual 1-1 TxDOT 9/00

Chapter 1

Field Surveys

Contents

This chapter contains the following sections:

Section 1 Overview.......................................................................................................... 1-3Introduction.......................................................................................................................................1-3

Section 2 Preliminary Soil Surveys ................................................................................. 1-4Overview...........................................................................................................................................1-4

Office Survey....................................................................................................................................1-4

Field Survey......................................................................................................................................1-5

Site Inspection...................................................................................................................................

1-6

Data Acquisition ...............................................................................................................................1-7

Design Feature Consideration...........................................................................................................1-7

Bridges..............................................................................................................................................1-7

Retaining Walls and Embankments ..................................................................................................1-8

Section 3 Final Soil Survey.............................................................................................. 1-9Overview...........................................................................................................................................1-9

Existing Data.....................................................................................................................................1-9

Test Hole Location and Depth ........................................................................................................1-10

Subsurface Exploration Plan...........................................................................................................1-12

Bridge Considerations.....................................................................................................................1-13

Stream Crossings ............................................................................................................................1-13

Grade Separations ...........................................................................................................................1-14

Load Considerations .......................................................................................................................1-14

Field Exploration ............................................................................................................................1-15

Retaining Wall Consideration.........................................................................................................1-15

Foundation Soil Investigation .........................................................................................................1-16

Soil Core Borings............................................................................................................................1-16

Groundwater...................................................................................................................................1-17

Laboratory Testing..........................................................................................................................1-17

Illumination and Signing Considerations........................................................................................1-18

Slopes and Embankment Considerations ........................................................................................1-18

Cut/Fill Considerations ...................................................................................................................1-19

Soil Core Borings............................................................................................................................1-19

Groundwater...................................................................................................................................1-19

Soil Testing.....................................................................................................................................1-19


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Chapter 1 Field Surveys Section 1 Overview



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Chapter 1 Field Surveys Section 1 Overview


Section 1

Overview

Introduction

Field surveys are very important for obtaining information about field conditions for design.

Incorrect data or overlooked features can lead to inappropriate designs and costly field

changes during construction. Field surveys are divided into two phases:

Preliminary soil surveys

Final soil survey

The preliminary soil survey obtains general information about the site to guide general early

project development. The final soil survey obtains detailed soils information to be used for

final design of specific project features. The next two sections address both types of field

surveys.


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Chapter 1 Field Surveys Section 2 Preliminary Soil Surveys


Section 2

Preliminary Soil Surveys

Overview

The following paragraphs discuss preliminary soil surveys:

Survey purpose

Survey need

Survey goals

Survey Purpose. The purpose of the preliminary soil survey is to examine the general soil

conditions at a construction project site, which will impact any proposed features of a

project. Identifying problem soil conditions prior to schematic development will enable the

designers to produce the most efficient and cost effective design. Problem soil conditions

may even dictate a different project alignment than that initially proposed: Building a gradeseparation in the middle of a swamp is not very feasible.

Survey Need. The need for a preliminary soil survey is most acute for large projects

involving multiple bridges or retaining walls. Embankments may also be critical depending

on heights and soil strengths present at the site. In coastal areas with very soft soil,

constructing as little as six feet of fill can present a problem.

Survey Goals. Preliminary soil surveys should accomplish the following goals:

1. Identify general soil and groundwater conditions at the site.

2.

Delineate any areas of exceptionally soft soils.3. Identify any soil instabilities such as slope failures or geologic faults.

4. Identify long term instabilities such as riverbed migration.

5. Assess existing soil data.

The next subsections discuss

Office survey

Field survey

Office Survey

The purpose of the office soils survey is to examine all information in the files and literature,

which might yield useful information for the project. This office survey discussion covers

Published literature

Existing data


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Published Literature. Typical soils information sources are

Geologic maps (UT Bureau of Economic Geology)

County soil survey (USDA)

Topographic maps (USGS Quadrangle Maps)

An examination of the literature gives a general idea of the soil and topographic conditions

at the site. Examination of the topography can reveal problems such as sharp bends in

stream channels subject to migration.

Existing Data. Typical existing data, which is normally available, includes

Old soil borings in plans

Foundation construction records from permanent records (GSD Austin)

Documentation of past construction problems from files or personnel

Quite often the existing soil borings are adequate for the new construction proposed at alocation if it is similar in scope to that which currently exists. If near surface soil conditions

are important, such as for retaining wall design, consider new borings if the surface soils are

sensitive to moisture changes.

Field Survey

The purpose of the field soils survey is to determine which site conditions to consider in

project planning. Failure to consider the site condition can lead to improper project features,

which are costly or impossible to build. Figure 1-1 shows the results of failing to properly

investigate the site. Discovering problem site conditions at the time of the final soil survey

can require redesign of the project resulting in months or years of delay.


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Figure 1-1. Failure to do proper field investigation

The next subsections give guidelines for these areas of field surveys:

Site inspection

Data acquisition

Design feature considerations

Site Inspection

A visit to the proposed project site reveals such problems as

Bodies of water which may need special consideration

Soft soils indicated by wet areas or characteristic wet land vegetation

Unstable slopes or stream banks

Migrating river channels

Immediately investigate any areas found which are soft at the surface if fill is to be placed or

roadway built in the area. Core borings need to be performed with special equipment

intended for exploration in such areas. The excuse that the area is too soft for normal coring

equipment to access is unacceptable.


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Data Acquisition

If a need for soil core borings is determined based on the site inspection, soil core sampling

or other appropriate testing should be scheduled. Preliminary data to be obtained from the

field normally consists of

Soil strengths

Compressibility (for very soft soils only)

Groundwater levels

Swell potential for expansive clays

Due to the preliminary nature of the investigation at this point in project development, only

obtain one or two borings at a site. A simple bridge replacement project would not warrant

preliminary borings.

Soil Strengths. Soil strengths may be determined in the field with torvane testing on

recovered cores or in place vane shear testing of soils too soft to be sampled. Texas ConePenetrometer tests should be performed in firm soils.

Compressibility. Samples may also be tested in the laboratory in a triaxial shear device to

determine strength or a consolidometer to determine compressibility.

Groundwater Levels. Groundwater levels are especially important for facilities to be

constructed below existing grade. Should significant groundwater be present, provisions for

removing the water must be made in the project design. If possible, gravity drainage is

preferred to a pump station. Since groundwater levels vary seasonally, several months of

observations are desirable to determine the maximum probable groundwater level.

Groundwater levels are monitored with piezometers.

Swell Potential. Swell potential for expansive clays may be evaluated using Test Method

Tex-124-E. Soils that have significant swell potential may require stabilization.

Design Feature Consideration

Design features that warrant special attention at this time are

Bridges

Retaining walls and embankments

While the exact location of these features may not be know at the time, their presence on a

project will be known.

Bridges

Discussion on bridges in a preliminary soil survey covers these aspects:


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Approach embankment height

Pier location

Stream channel stability

Approach Embankment Height. Bridge approach embankment height may be limited

based on soil strength and compressibility. The presence of soft soils under bridge

approaches may require that the bridge be longer than usual in order to reduce the height of

the approach embankment. In extreme cases, the bridge approach height has been limited to

approximately six feet (2m). Subsurface stabilization methods may be utilized instead of

reducing embankment height.

Pier Location. The location of bridge piers may also need to be altered to avoid unstable

stream banks. The span lengths may have to be lengthened to span such areas. Should a

bridge pier be located in an unstable area, the foundations could be sheared off by a slope

failure.

Stream Channel Stability. The location of a structure with respect to stream meandersshould be considered. As the meanders migrate downstream, the location of the main

channel can change drastically. If at all possible, locate bridges on a relatively straight

section of a stream channel.

Retaining Walls and Embankments

The next paragraphs discuss

Stability considerations

Groundwater considerations

Stability Considerations. Retaining walls in fill sections and embankment fills have the

same considerations as for bridge approaches. These features also exert significant vertical

loads on the subgrade soils. The result of excessive vertical loading may be settlement or

bearing capacity failure.

Groundwater Considerations. Retaining walls proposed for cut sections must be evaluated

for groundwater conditions. It is preferable that groundwater levels be observed for a year

or longer to monitor seasonal variations. If excessive groundwater volumes are suspected,

monitor well pump down tests may be performed to determine the drainage system

requirements. Pump stations should also be designed with additional small pumps to handle

base flow from groundwater.


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Chapter 1 Field Surveys Section 3 Final Soil Survey


Section 3

Final Soil Survey

Overview

The purpose of the final soil survey is to obtain adequate data for the final design of a

project. The goal is to obtain information for all project features at the same time. The

project features for which data is necessary are

Bridges

Retaining walls

Slopes and embankments

Sign structures

Illumination

Sound walls

Pavements

All exploration should conform to the requirements set forth in Chapter 2, Field Operations

(see Section 1, Overview. ) The next subsections offer guidelines on these final soil survey

aspects:

Existing data

Test hole location and depth

Subsurface exploration plan

Bridge considerations

Retaining wall considerations

Illumination and signing considerations

Slope and embankment considerations

Existing Data

Review all existing data prior to determining new data requirements. Old bridge plans are

the most common source of information. Any old borings that contain strength data are

normally adequate for new construction. If the old plans predate the early 1950s, no

strength data is present, making new borings necessary if a structure is to be replaced. Old

Texas Cone Penetrometer data typically has an additional value listed, which is the weight

of the drill stem when the test was performed. You can ignore this number and need not

show it if the old borings are shown in new plans.

If a bridge widening is proposed, the existing core dataeven if only descriptive with no

strength testsis often adequate for the widening. Typically, the foundation loads for a


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widening are lower than for the initial construction. In this situation, consult the foundation

construction records to verify as-built foundation lengths. The as-built lengths are then

matched in the new construction.

Test Hole Location and Depth

The number of holes required for foundation exploration is determined by the complexity of

the geological conditions and the length and width of the structure. The minimum number of

test holes for common types of structures is illustrated in Figure 1-2. Consider this test hole

configuration the minimum, and try not to space test holes more than 300 feet (100 meters)

apart. The next paragraphs offer discussion on these test hole location and depth topics:


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Figure 1-2. Minimum number of test holes for common types of structures.


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Test hole location

Test hole depth

Test Hole Location. Locate the test hole in an accessible area. Avoid these areas when

locating test holes:

Overhead power lines - always

Underground utilities

Avoid these areas if possible:

Steep slopes

Standing or flowing water

Deviations within a 20-foot (6-meter) radius of the staked location normally would not be

excessive, but note them on the logs and obtain the correct surface elevation.

Test Hole Depth. When determining the location and depth of test holes, give carefulattention to such factors as

Lowering of gradeline for an underpass

Channel relocations and channel widenings

Scour

Foundation loads

Foundation type

As a general rule, drill a test hole 15 to 20 feet (4.5 to 6 meters) deeper than the probable tip

elevation of the foundation. Make an estimate of the probable tip elevation from the results

of Texas Cone Penetrometer tests and correlation graphs (Figures 5-14 through 5-18) and

experience with foundation conditions in the area. Special attention should be paid to major

structures where very high foundation loads may be encountered. If the depth of the boring

is questionable, consult the Bridge Division Geotechnical Section for a detailed analysis of

the projected foundation loads and foundation capacities.

Subsurface Exploration Plan

The next paragraphs cover these exploration plan topics:

Preliminary borings

Exploration plan variations

Prelimi nary Borings. The proper field performance of designs can only be assured when

adequate soil borings are obtained. On major projects, a small number of preliminary

borings should be obtained to aid in preliminary project layout. Preliminary borings will

determine the influence of soils on pavement design, embankment and cut slopes, retaining


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walls, and bridge lengths. Once the preliminary soil borings are evaluated based on the

design features to be constructed, the final soil boring locations can be determined and

appropriate soil testing specified. It should also be noted that the design of lighting poles

and sign bridges also require soils information.

Exploration Plan Variations. The primary purpose of a test hole is to gain as much

information as needed and economically justifiable. Make every attempt to obtain 100

percent core recovery where conditions warrant. The exploration plan varies depending on

the final use of the data. Exploration for deep foundations is different than for retaining

walls or slope stability.

Bridge Considerations

Guidelines on the following topics appear below with respect to final soil surveys for bridge

design:

Stream crossings

Grade separations

Load considerations

Field exploration

Stream Crossings

Discussion on these types of stream crossings appears below:

Minor stream crossings

Major stream crossings

Minor Stream Crossings. Minor stream crossings do not require core borings in the river

channel. For channels of less than approximately 200 feet (60 meters) in width, a boring on

each bank as close to the waters edge as possible will usually suffice. Should significantly

different information be obtained from one side of the channel to the other, a boring in the

channel may be necessary.

Major Stream Crossings. Major stream crossings require core borings in the channel if no

existing data is available. The usual set-up time to prepare the equipment for drilling on a

body of water is one week. Any requests for exploration requiring drilling in a channel

should clearly state that barges are required. Approximately one month of time is requiredto transport the barges to the project location. See Figure 1-3 for an example of a drill rig

loaded on a barge. In addition, a site inspection by the driller is necessary in order to

evaluate site accessibility.


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Figure 1-3. Drill rig loaded on barges

Plan foundation investigations for major stream crossings during times of seasonal low

flows to allow maximum access. This is usually in the summer.

Grade Separations

The normal boring interval for grade separations is approximately 200 to 300 feet (60 to 100meters). If the structure borings indicate soft surface soils, perform additional borings for

the bridge approach embankments. Holes drilled for embankments should concentrate on

testing the upper soft soils with Texas Cone Penetrometer tests performed at 5-foot intervals

as a minimum. Additional testing may be required when Texas Cone Penetrometer tests are

less than 5 blows per foot (300mm).

Load Considerations

Exploration for bridge and other deep foundations should consider the magnitude of the

foundation loads anticipated. Obtain an initial estimate of the anticipated loads from the

structure designer. Once the initial boring is completed in the field, check to insure that anadequate foundation design can be performed from the boring data obtained. All too often

boring inadequacies are discovered during the final structural design rather than during the

foundation investigation. This has resulted in delays while additional core borings were

performed.

The borings should extend a minimum of ten feet below the deepest foundation depth. A 15

to 20-foot (4.5 to 6-meter) depth below the anticipated foundation depth is typical.


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Field Exploration

The exploration should include

Test hole spacing

Texas Cone Penetrometer tests

Upper soil layer tests

Soil and bedrock classification

Soil profile

Test Hole Spacing. Test holes near each abutment of the proposed structure plus a sufficient

number of intermediate test holes to determine the depth and location of all significant soil

and rock strata 15 to 20 feet (4.5 to 6 meters) below the probable founding elevation. A

reasonable correlation between holes should be obtained. If reasonable correlation is not

obtained, additional test holes may need to be drilled.

Texas Cone Penetrometer Tests. Perform Texas Cone Penetrometer tests should be

performed at 5 or 10-foot (1.5 to 3-meter) intervals beginning at 5 feet (1.5 meters) of depth.

When taking triaxial samples, perform a Texas Cone Penetrometer test every 10 feet (3

meters). If a cohesionless material cannot be recovered while sampling for triaxial test

samples, take a penetrometer test of each 5 feet (1.5 meters) of non-recoverable material.

When testing by Texas Cone Penetrometer only, run a test at the first indication of a material

change.

Upper Soil Layer Tests. Test soft upper soil layers at 10-foot (3-meter) intervals in case a

shallow foundation issue should arise during design. This is particularly important at

proposed abutment locations for evaluating embankment stability. If high fills are expected

or exceptionally soft soils are encountered, additional exploration may need to be performed

in accordance with the section on exploration for slope stability.

Soil and Bedrock Classification. Fill out a complete soil and bedrock classification and log

record for each test hole on THD Form 513, including all information called for to complete

the form (see Chapter 3, Section 3, Logging).

Soil Profile. For large structures, as well as structures where the formations are non-

uniform, it is recommended that, during foundation exploration, the logger plot a pencil

profile of the material with test data using standard symbols. The logger and the engineer

can use a study of this profile to help to determine where additional test holes are needed.

Retaining Wall Consideration

The next subsections discuss these aspects of investigation for retaining wall design:

Foundation soil investigation

Soil core borings


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Groundwater

Laboratory testing

Foundation Soil Investigation

Perform a detailed investigation of the foundation soils for retaining walls in advance of

final design. The strength of foundation soils may severely limit the height of walls, which

can be built without costly foundation improvement. Longer bridges and shorter walls may

be more cost effective on soft soils. Additional soil testing beyond normal Texas Cone

Penetrometer (TCP) testing is often required for evaluation of wall stability. Walls in

depressed sections of roadways should be investigated for groundwater. Special details may

be necessary to accommodate groundwater in depressed sections.

Walls anticipated as drilled shaft walls due to the proximity of a right of way line might not

be feasible in low strength soils. Tiedback walls may need to be substituted with a resultant

increase in right of way required. Soft soils behind a proposed tiedback wall may require

the use of longer tiebacks, which will not fit within the right of way. This is only a small

sample of issues, which can surface after a field investigation is completed.

Soil Core Borings

Obtain soil core borings for walls greater than 10 feet (3 meters) tall in areas with

questionable soils. Walls under ten feet tall require only minimal exploration if any at all.

In areas with very firm soils, no borings at all may need to be obtained based on previous

experience or existing borings in the area. For most soils, Texas Cone Penetrometer testing

alone is adequate. Install a piezometer in at least one borehole for monitoring for cut walls.

Include the following in your exploration:

Soil borings

Boring depth

Cut considerations

Soft soil samples

Other soil sampling

Need for prompt sampling

Soil Borings. Obtain soil borings at approximately 100 to 200 foot (30 to 60 meter) interval

in areas with soft soils. Soft soils typically have less than 10 blows per foot by the TCP test.

In general, boring spacing should not exceed 500 feet (150 meters) in firm soils. When the

Texas Cone Penetrometer values are more than 10 blows per foot (per 300 millimeters) for a

proposed wall height of 20 feet (6 meters) or less, no additional testing is required. Taller

walls may warrant additional testing. When additional testing is deemed necessary, samples

may be obtained for triaxial testing. For soils softer than 3 to 4 blows per foot (per 300


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millimeters), in-place vane shear testing may need to be performed depending on wall

height.

Boring Depth. The depth of the boring should normally be as deep as the height of the wall.

The minimum depth is 15 feet (4.5 meters) unless rock is encountered. Five-foot (1.5-

meter) penetration into rock is adequate with the boring terminated at this point. Typically,

25-foot (7.5-meter) borings are adequate for most walls. In no case should borings extend

more than five feet into rock for fill walls. Borings for cut walls may need to penetrate rock

significant distances depending on the depth of the cut.

Cut Considerations. The depth of the boring should always consider the final grade lines

taking into account any cuts to be made. Borings for walls constructed in cuts should have

adequate penetration below the bottom of the cut with ten feet being considered a minimum.

Cantilever drilled shaft walls will require the depth of boring to extend the anticipated depth

of the shaft below the cut which is typically between one and two times the height of the

wall.

Soft Soil Samples. Exploration may include undisturbed samples for triaxial tests whenfoundation stability is an issue on soft soils. Very soft soils (less than five blows per foot

TCP) will often require the in-place vane shear testing to accurately evaluate the soil

strength. Sampling for triaxial testing should not normally be attempted in hard formations

or when gravel is present.

Other Soil Sampling. Soil nailed and tieback retaining walls require that the soil behind the

proposed face of wall be tested and sampled if necessary. The soil strength behind these

wall types is critical to their design.

Need for Prompt Sampling. Begin sampling or testing as soon as possible in the boring, and

in no case deeper than 5 feet (1.5 meters).

Groundwater

Groundwater can present a serious problem for roadway sections constructed below existing

grade. Groundwater seepage can create soil instability during construction, increase design

loads on temporary shoring, and create a long-term drainage problem, which must be

addressed in the project design. Special details may be necessary to accommodate

groundwater in depressed sections. Monitor groundwater levels with piezometers, weekly or

monthly, over a period of several months for any walls to be built in depressed sections.

Laboratory Testing

Tests conducted in the laboratory include the triaxial compression and consolidation tests,

described in Chapter 4, Section 4, Laboratory Tests. The triaxial test is useful for

determining the shear strength at various overburden pressures for determining wall

stability. The consolidation test is used to evaluate potential settlement of embankments.

Both tests require that undisturbed samples be obtained and carefully packaged for transport


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to a laboratory for testing. Very soft samples require special packaging to minimize

disturbance during transport.

Illumination and Signing Considerations

Conduct foundation investigations for high mast illumination and overhead sign structureswhen other borings are not located nearby. The depth of the boring should be approximately

25 feet (7.5 meters) for overhead sign structures in average soils. In soft soils the depth

should be 40 feet (12 meters). High mast illumination borings should be 30 to 50 feet (9 to

15 meters) deep depending on

Soil consistency

Height

Design wind speed

Slopes and Embankment Considerations

Slope and embankment problems can seriously impair the serviceability of a facility.

Inadequate field investigations can lead to project construction delays while slope stability

issues are addressed. Slope stability generally refers to a rotational soil failure of a soil

mass. The soil mass is usually the side slope of an embankment or the side slope in a cut.

Two conditions must be examined:

Short-term stability

Long-term stability

Short-term Stability. Short-term stability can usually be evaluated with TCP testing of thesoil beneath the proposed embankment.

Long-term Stability. Long-term stability affects side slopes and may require laboratory

testing of undisturbed soil samples for evaluation. Proposed embankments obviously cannot

be sampled and tested, so experience with past embankments in the area must be used to

determine acceptable side slopes.

The next subsections give guidelines for these topics in regard to slope and embankment

problems:

Cut/fill considerations

Soil core borings

Groundwater

Soil testing


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Cut/Fill Considerations

Constructing a fill increases the state of stress in the underlying soil. This stress increase

may lead to a stability failure or excessive settlement when the fill is built on soft soils.

Removing natural soil to lower a roadway profile reduces the state of stress in the soil. This

reduction can result in heave in expansive soils. Also, the reduction in stress can result in

softening of the sideslopes with subsequent slope stability problems.

Soil Core Borings

Obtain soil core borings for cuts greater than 10 feet (3 meters) deep or embankments

greater than 15 feet (4.5 meters) high in areas with questionable soils. In areas with very

firm soils, no borings at all may need to be obtained based on previous experience or

existing borings in the area. For most soils, Texas Cone Penetrometer testing alone is

adequate.

The exploration should include

Soil under future embankments

Soil in proposed cuts

Soil Under Future Embankments. Sample the soil under future embankments much as for

retaining walls. Test and sample soft soils as necessary. Based to the size of the

embankment, soil borings may need to extend deeper than 25 feet (7.5 meters) in soft soils.

Soil in Proposed Cuts. Soil may need to be sampled in proposed cuts to determine the

allowable slope angle for stability. This type of stability problem manifests itself after a

number of years as shallow mudflow type failures within the side slope. Also, determine

groundwater levels in cut sections since this will affect stability. Borings normally only

need to extend five to ten feet below the finished grade of the cut.

Groundwater

The presence of ground water in cut sections can require the use of additional drainage

features in a project. French drains may need to be installed on the outside edge of the

roadway to intercept water inflow. Pavement under drains may also be necessary to relieve

hydrostatic pressure underneath the roadway. Seepage forces in sideslopes can reduce slope

stability. As a result, knowledge of the groundwater conditions is desirable for roadwaycuts.

Soil Testing

Most slope and embankment problems are the result of long-term softening of high plasticity

clay soils. Deep-seated failures of newly placed embankments involve the short-term soil


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properties of the subgrade soils. The following soil tests are appropriate for these two

conditions:

Short-term property tests

Long-term property tests

Short-term Property Tests. The short termed or undrained soil properties may be determined

from

Texas Cone Penetrometer

In Place Vane Shear

Triaxial Test (UU)

Direct Shear test

These tests may all be conducted quite rapidly and inexpensively.

Long-term Property Tests. The long termed or drained soil properties may be determinedfrom

Consolidated Undrained Triaxial Test (R )

Drained Direct Shear Test

These tests take days to weeks to perform and are quite expensive, while the results are

sometimes inaccurate and difficult to apply. The long-term strengths of clays soils may also

be estimated based on the plasticity index property.


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Chapter 2

Field Operations

Contents



Section 2 Drilling............................................................................................................. 2-4Overview...........................................................................................................................................2-4

Core Drill Equipment........................................................................................................................2-4

GSD Drill Rig ...................................................................................................................................2-6

Site Preparation.................................................................................................................................

2-6

Access...............................................................................................................................................2-8

Utility Clearance ...............................................................................................................................2-9

Traffic Control ................................................................................................................................2-10

Drilling Mud...................................................................................................................................2-10

Barge Work.....................................................................................................................................2-10

Drill Hole Filling ............................................................................................................................2-11

Section 3 Sampling Methods ......................................................................................... 2-12Overview.........................................................................................................................................2-12

Dry Barrel or Single Wall Sampler.................................................................................................2-12

Diamond Core Barrel......................................................................................................................2-12

Push Barrel or Shelby Tube Sampler..............................................................................................2-12

Wash Sampling or Fishtail Drilling ................................................................................................2-13

Section 4 Field Testing .................................................................................................. 2-14Overview.........................................................................................................................................2-14

Field Tests and Equipment..............................................................................................................2-14

Texas Cone Penetrometer (TCP) Test ............................................................................................2-14

Standard Penetration Test (SPT).....................................................................................................2-17

In-place Vane Shear Test ................................................................................................................2-17

Torvane & Pocket Penetrometer.....................................................................................................2-21

Monitoring Methods .......................................................................................................................2-22

Piezometers.....................................................................................................................................2-23

Piezometer Use ...............................................................................................................................2-23

Piezometer Installation Procedure ..................................................................................................2-23

Reading Frequency Guidelines .......................................................................................................2-24

Pneumatic Piezometer Alternative..................................................................................................2-24

Inclinometers ..................................................................................................................................2-24

Inclinometer Use.............................................................................................................................2-24

Inclinometer Description ................................................................................................................2-24


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Chapter 2 Field Operations Section 1 Overview



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Chapter 2 Field Operations Section 1 Overview


Section 1

Overview

Introduction

Foundation exploration is one of the first steps in the design process. Procedures for

foundation exploration for sites cannot be reduced to a simple guideline to fit all existing

conditions. To acquire reliable engineering data, explore and analyze each job site

considering

Subsurface conditions

Specific type of proposed features

Foundation loads

The engineer in charge of the foundation exploration must endeavor to furnish complete data

in order to make a study of practical design options.


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Chapter 2 Field Operations Section 2 Drilling


Section 2

Drilling

Overview

Successful soil exploration requires careful advance planning to be conducted in the most

expedient manner. Proper drill site selection and preparation are essential to minimize drill

rig standby time and associated charges. Utility clearance is an essential item that cannot be

taken lightly or ignored. Disrupted utilities can result in a tremendous liability to the

department. The following are some detailed items to consider prior to commencing core

drill operations:

Core drill equipment

GSD Drill Rig

Site preparation

Access

Utility clearance

Traffic control

Drilling mud

Barge work

Drill hole filling

Most of these items are common to either department drilling operations or the use of

drilling consultants.

Core Drill Equipment

Discussion follows on these areas of core drill equipment:

Department drill rigs

Drilling consultants

District responsibility

Special materials

Rig features

Department Drill Rigs. Bridge foundation exploration is accomplished with wet rotary core

drill rigs. Figure 2-1 shows wet rotary drilling with a Falling 1500 drill rig. Three wet

rotary drill rigs operate in Texas as follows

General Services Division (GSD) Austin

Houston District


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Beaumont District

Figure 2-1. Wet rotary drilling with Failing 1500 drill rig

Drilling Consultants. Drilling consultants are used when additional drilling capacity is

needed. The terms of the individual consultant contract dictates how the contractor interacts

with department forces and what services the department must supply. Drilling consultants

use both wet rotary and continuous flight hollow stem auger drill rigs.

District Responsibility. The operation of the departmental drill rigs and the responsibility

for the personnel are the responsibility of the districts in which the rigs are located.

Generally, each rig serves some surrounding districts. Exploration work is scheduled

according to priority by mutual agreement between the districts as well as specific

assignments by the Bridge Division. The scheduling of the Austin based rig is directed by

the Bridge Division Geotechnical Section with rig maintenance and personnel furnished by

the General Services Division (GSD). The charges for the operation of the rigs are made to

the specific jobs involved.

Special Materials. The General Services Division has the responsibility of furnishing the

districts the special material listed in the Core Drill Parts Catalog. This includes

Special core barrels

Casing and drill stem

Field testing equipment

Texas Cone Penetrometer equipment

The GSD will assist the districts in any required manner with the operation and maintenance

of the equipment. The Bridge Division geotechnical engineers are available to districts to

assist with any special exploration needs or drilling problems.


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Rig Features. The rotary core drill rigs are mounted on trucks. The truck engine powers

these rigs through a power takeoff mechanism that utilizes the truck transmission to give a

wide range of power and speed at the drill head. Other features of the rigs include

Reciprocating type mud pump

Hydraulically powered pull down

Retracting drill head

Portable mud pan

Each core drill unit, in addition to the rig, has a water truck. The water truck is equipped

with

Storage space for

Drill stems

Casings

Bits

Other tools

Water tank with a capacity of 450 to 800 gallons (1700 to 3000 liters)

Vacuum water pump and hose for obtaining drilling water from any accessible source

GSD Drill Rig

Scheduling the GSD drill rig is a responsibility of Bridge Division Geotechnical Section.

Within each district, the district representative coordinates job priorities with the

Geotechnical Section of the Bridge Division. Since rain, water and mud are major

hindrances, it is normally recommended that the most difficult holes be drilled first if theyare accessible, saving the most convenient holes for last or to drill when the others can't be

reached. Normally, the GSD core drill works four ten-hour days, Monday through

Thursday.

Due to the long lead times for the Austin core drill, requests for drilling should be made six

months in advance of the time the information is needed.

In addition to the three-person crew of the GSD drill rig, the district must supply one person

to log the core borings.

Site Preparation

The next paragraphs deal with these site preparation issues:

Level drill pad

Overhead clearance

Underground utility locations


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Level Drill Pad. Drilling sites need to be prepared prior to arrival of the drill crew, since

waiting time is charged at the same rate as drilling time. The kelly and mast of the drill rig

are fixed to the truck bed and cannot swing, as some auger rigs can. The bed of the drill

truck must he leveled in order to drill a vertical hole. The truck is equipped with hydraulic

jacks that can lift the front of the truck one-foot off the ground and one-foot on either side to

accommodate uneven terrain. If the slope of the site is steeper than one foot, the districtneeds to prepare a work pad 16 feet (5 meters) wide and 70 feet (20 meters) long to provide

for leveling the rig and providing a safe place for the crew to handle the drill stem. For

safety reasons, the crew is not allowed to block up the jacks to accommodate greater slope

angles. The mud pan must be level or slightly down slope. Prior to extensive site work,

consult the driller who performs the work for specific instructions. See Figure 2-2 for drill

site requirements.

Overhead Clearance. Overhead must be clear of obstructions. Trees cannot block the

raising of the mast. It is not safe to work within 25 feet (7.6 meters) of an overhead power

line. If it is necessary to work closer, the district will be asked to contact the power

company to cut the power or install insulating safety boots.

Underground Utility Locations. You must know the exact location of underground utilities

including

High pressure gas lines

Water lines

Sewer and storm sewer lines

Electrical and telephone conduits and cables

The driller will be available to inspect locations and make recommendations on site

preparation. Often it is possible to begin drilling easy sites while preparing more difficultsites.


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Figure 2-2. Drill Site Requirements.

Access

District personnel should insure that the drill crew has access to drill sites upon arrival.

Problems have arisen in the past from hostile farm animals and uncooperative landowners.

Bridges to be crossed must have a capacity of at least 32,000 pounds (14,500 kilograms)


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Utility Clearance

All locations proposed for drilling must be cleared for utilities prior to arrival of the core

drill. When utilities are present, their exact location should be clearly marked by the utility

company. Figure 2-3 is an example of a general warning about the presence of utilities.

Avoid verbal communication of approximate locations. If drilling close to utilities, the exact

utility location should be marked on the ground with paint or flags.


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Figure 2-3. Utility sign marking cable location

The current number to phone for utility clearance is 1-800-545-6005. Calls to this numberautomatically rotate to the three notification centers. Utility clearance must be obtained at

least 48 hours and no more than 14 days prior to commencing core drilling. The three

notification centers may be contacted directly as follows:

Texas Excavation Safety System (TESS) 1-800-344-8377

Lone Star Notification Center 1-800-669-8344

Texas One Call 1-800-245-4545

Traffic Control

The district will provide traffic control, flaggers, signs and cones when appropriate.

Associated charges will be made directly to the appropriate district account.

Drilling Mud

Drilling mud is available from the General Warehouses for use with the GSD drill rig. The

crew normally moves with five to ten bags, but resupply is expected from the District

Warehouse. Requirements vary widely with drilling conditions, but a general rule of thumb

is a bag and a half per hole.

Barge Work

When a bridge must cross large bodies of water, barges are used to obtain foundation

information. The district representative should contact the Bridge Division Geotechnical

Section core drill coordinator as soon as the need is known so use of the barges can be

planned with adequate lead-time. The district will also have to provide personnel for

assembly and launching of the barges. See Figure 1-3 for a photo of a drill rig on a barge.


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Drill Hole Filling

Drill holes must be filled or plugged. This prevents injury to livestock or people in the area

and also minimizes the entry of surface water into the borehole. If surface contamination of

lower aquifers or cross contamination is a concern, backfill the hole with bentonite pellets or

grout. This is especially important in urban areas where ground contamination from leaking

underground storage tanks is a common occurrence.


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Chapter 2 Field Operations Section 3 Sampling Methods


Section 3

Sampling Methods

Overview

This section discusses the following sampling methods:

Dry barrel or single wall sampler

Diamond core barrel

Push barrel or Shelby Tube Sampler

Wash sampling or fishtail drilling

Dry Barrel or Single Wall Sampler

The dry barrel sampler is commonly used to obtain cores for visual soil and bedrockclassification and logging. The core sample obtained is generally in a disturbed condition

due to the pressure applied when cutting the core and packing it into the barrel for recovery.

The core is extracted from the barrel by water pressure. When used for sampling in

practically all foundation materials, except very soft clays (mucks) and cohesionless sands,

the dry barrel sampler obtains a sample containing all components in the original formation.

The amount and degree of disturbance depends upon the consistency and/or density of the

material. Although this method is called the dry barrel method, circulating water is used. In

hard formations, a smaller volume of water is circulated while cutting the core.

Diamond Core Barrel

Diamond core barrels are used to obtain intact rock samples for field or laboratory tests and

classification. The diamond barrel sampler has an inner and outer barrel. The inner barrel is

slightly oversized with a spring loaded core retainer at the bottom.

Push Barrel or Shelby Tube Sampler

The next paragraphs cover these push barrel sampler topics:

Push barrel sampler description

Push barrel sampler procedure

Push Barrel Sampler Description. The push barrel sampler is used to obtain relatively

undisturbed soil samples for field and laboratory tests and soil classification. This device

consists of a thin walled tube 24 to 36 inches (600 to 900 millimeters) long with one end

sharpened to a cutting edge and the other end reinforced and designed for easy attachment to

the drill stem coupling. It employs the principle of steadily pushing the thin walled tube into

the formation with the hydraulic pull down of the drill rig. This sampler recovers very good


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Chapter 2 Field Operations Section 3 Sampling Methods


undisturbed samples where it is adaptable, but its usefulness is limited to materials which it

can be forced into and which have sufficient cohesion to remain in the barrel while the

sampler is being withdrawn from the hole.

Push Barrel Sampler Procedure. A step table of this procedure appears below:

Push Barrel Sampler Procedure

Step Action

1 Force sampler into formation with slow, steady push to within 3 to 4 inches

(75 to 100 mm) of length.

2 Rotate sampler several turns to shear off core at bottom before withdrawing

it.

3 Bring push barrel to surface

3a detach barrel from coupling

3b mount barrel on the hydraulic sampler extruder

3c extrude core.

4 Cut core into 6 inch (150 mm) lengths and wrap in thin plastic (plastic wrap

for foods) to retain moisture content.

5 Place samples in cartons for transport to the laboratory for testing.

When sampling soft soils, sample disturbance can be a problem during transport to the

testing location. To insure minimum disturbance, support soft samples in their cartons. Fine

dry sand poured around the sample in the carton provides excellent support during transport.

Store samples that are not immediately tested in a moist room.

Wash Sampling or Fishtail Drilling

Although there are many methods for penetrating overburden soils, only those that offer an

opportunity for sampling and testing the foundation materials without excessive disturbance

are recommended. Therefore, do not use wash sampling or fishtail drilling, and instruct the

core driller not to utilize this method unless absolutely necessary. Attempts to classify the

soil materials by watching the wash water may lead to erroneous conclusions about the

subsurface soils being penetrated.


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Chapter 2 Field Operations Section 4 Field Testing


Section 4

Field Testing

Overview

This section covers field tests and equipment and monitoring methods.

Field Tests and Equipment

Field tests and equipment include

Texas Cone Penetrometer (TCP) Test

Standard Penetration Test

In-place Vane Shear Test

Torvane and pocket penetrometer

Texas Cone Penetrometer (TCP) Test

The next paragraphs discuss these elements of the TCP test:

Need

Development

First use

TCP test procedure and guidelines

TCP correlation graphs

Old TCP data

Need. Prior to 1940, no consistent soil testing was performed to determine soil and rock

load carrying capacity for foundation design. Around this time, the Bridge Division formed

a bridge foundation soils group. This group oversaw foundation design and field load tests

of foundations for capacity verification.

Development. One of the first projects of the foundation soils group was to develop a

reliable soil test method for use in all soil and rock types with the exploration drill rigs. The

Texas Cone Penetrometer (TCP) test was then developed with cooperation from theMaterials & Tests and Equipment & Procurement Divisions. The test utilized a hardened

conical point that could be driven into soil or hard rock. This eliminated the need to obtain

samples for laboratory testing saving valuable time and money. Furthermore, the in situ test

better evaluated rock formations that are difficult to test in the laboratory under realistic

confining pressures.


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First Use. During the next fifteen years, the shear strengths of various soils were correlated

with the Texas Cone Penetrometer test results. Load test data from piling and drilled shafts

was also considered in the development of the correlation factors. The Texas Cone

Penetrometer test as it exists today was first used in 1949. Texas Cone Penetrometer data

first started to appear in department plans around 1954. The test procedure and correlation

charts were first published in the 1956 edition of the Foundation Exploration and DesignManual. (It is interesting to note that the Standard Penetration Test was developed at about

the same time.) The Texas Cone Penetrometer correlation factors were modified slightly in

1972 and 1982 based on accumulated load test data for piling and drilled shafts. Predicted

foundation capacities and field load test results continue to correlate exceptionally well.

TCP Test Procedure (Test Method Tex-132) and Guidelines. The test is performed as

follows:

TCP Test Procedure

Step Action

1 Attach penetrometer cone to drill stem.

2 Lower stem to bottom of cored hole.

3 Attach anvil to top of drill stem.

4 Place automatic hammer on top of anvil.

5 Seat penetrometer cone.

6 Make reference marks.

7 Drive cone 12 inches into relatively soft materials or 100 blows into hard

materials.

8 Make TCP test at regular intervals and changes in stratum.

1. Attach 3-inch (75-millimeter) diameter penetrometer cone to the drill stem.

2. Lower stem to bottom of cored hole.

3. Attach the anvil to the top of the drill stem.

4. Place the automatic 2' (600mm) drop tripping mechanism with the 170-pound (77-

kilogram) hammer in position on top of the anvil.

5. Seat penetrometer cone. Drive the penetrometer cone 12 blows or 12 inches (300 mm),

whichever comes first, to seat it in the soil or rock.

6. Make reference marks on the drill stem at 6-inch (150-millimeter) increments to prepare

for the test.

7. Drive cone 12 inches into relatively soft materials or 100 blows into relatively hard

materials.

7a. Drive the cone with the hammer in two 6-inch increments, a total of 12 inches (300 mm)

into relatively soft materials. (Refer to the penetrometer point and hammer in Figures 2-4


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and 2-5.) Note on the log the number of blows required for each 6-inch (150-millimeter)

increment.

Figure 2-4. Penetrometer point

Figure 2-5. Penetrometer hammers (Fully automatic on left, automatic trip on right)

7b. In hard materials, the cone is driven with the resulting penetration in inches accurately

recorded for the first and second 50 blows for a total of 100 blows.


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In either case, drive the penetrometer point into a formation 6 inches (150 mm) or 50 blows

for each increment, depending upon which occurs first. Experience with the Texas Cone

Penetrometer (TCP) Test indicates that the number of blows of the hammer for the first 6

inches (150 mm) and the second 6 inches (150 mm) of penetration should be recorded

separately as this is indicative of the general type of material. If the soil material is granular,

the number of blows for the second 6 inches (150 mm) is significantly greater than that forthe first 6 inches (150 mm). If it is clay, the number of blows required for the first and

second 6 inches (150 mm) are generally about the same.

8. Make TCP test at regular intervals and changes in stratum. Make the Texas Cone

Penetrometer (TCP) Test at each 5-foot (1.5 meter) to 10 foot (3 meter) interval of hole and

at each significant change in stratum.

TCP Correlation Graphs. Graphs based upon research and past experience with the TCP

Test supplement this manual. These graphs show the relationship between the test results

and the shear strength of the soils as calculated in the laboratory and the relationship

between the test results and the measured dynamic driving resistance. Chapter 5, Section 4

gives discussion on using these graphs in design.

Old TCP Data. Old Texas Cone Penetrometer data from existing plans typically has an

additional value listed that is the weight of the drill stem when the test was performed. This

number can be ignored and need not be shown if the old borings are shown in new plans.

Standard Penetration Test (SPT)

The Standard Penetration Test uses a 2 inch (50 millimeter) diameter pipe (split spoon)

driven with a 140 pound (63.5 kilogram) hammer at a drop of 30 inches (750 millimeters).

The test is described in ASTM procedure D 1586. This test is recommended mainly forgranular soils but has been used in cohesive soils. It cannot be used in rock. It correlates

roughly with the Texas Cone Penetrometer test as follows:

Clay: Ntcp= 1.5 Nspt

Sand: Ntcp= 2 Nspt

The general use of the Standard Penetration Test for foundation exploration is not

recommended. The test correlations presented here are intended only for approximate

evaluation of design adequacy from outside sources and not recommended for normal

foundation design work.

In-place Vane Shear Test

The next paragraphs discuss these in-place vane shear test topics:

Test description and use

In-place vane shear description


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Test procedure

Test Description and Use. The In-place Vane Shear Test is used to determine the in place

shearing strength of fine-grained soils, which do not lend themselves to undisturbed

sampling and triaxial testing. The test consists of

Four bladed vane which is rotated into undisturbed material

Device for measuring the torsion required to fail the cylindrical surface area of soil

being sheared by the vanes

Use this test when encountering

Organic silty clays (mucks)

Very soft clays

These materials, however, must be free of gravel or large shell particles, since pushing the

vanes through these obstructions would disturb the sample and probably cause physical

damage to the vanes. Use the test with extreme caution in soils that require over 20 blowsper foot (300 mm) with the Texas Cone Penetrometer.

In-place Vane Shear Description. The In-place Vane is composed of four 2 inch by 8 inch

(50 by 200 mm) stainless steel vanes welded to a stainless steel rod. See a picture of the In-

place Vane Shear, Figure 2-6, below. This rod is attached to a torque pipe which is mounted

in a housing with two tapered roller bearings spaced about two feet apart. It is designed to

resist a force in any direction. The vane assembly connects to the drill rod that extends to the

top of the ground. A splined shaft threads into the drill rod at the surface of the ground. The

splined hub of the torque pulley engages the splined shaft. The torque table assembly

attaches rigidly to the drill rig. This assembly utilizes a proving ring and strain gauge with

gearing apparatus to apply and to measure the torque necessary to test the shear strength ofthe soil.


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Figure 2-6: In-place vane shear

Test Procedure. The following procedure is recommended for performing the in place vane

shear test of soils that are consistently uniform and without gravel:

In-place Vane Shear Test Procedure

Step Action

1 Carefully lower vane assembly into cored hole.

2 Attach sufficient drill rods to vane assembly.

3 Attach spline rod to drill stem.

4 Slowly push vane into soil.

5 Lower torque table assembly over spline.

6 a. Adjust pulley cable

b. Inspect setup

c. Turn handle

d. Release cable

e. Adjust proving ring dial

f. Record initial reading.

7 Begin test.

8 Record reading from proving ring dial gauge.

9 Repeat steps 6 through 9, if desired.10 Push vane further into undisturbed soil and repeat steps 6 through 9

until completing test.

1. Carefully lower vane assembly into cored hole.

2. Attach sufficient drill rods to the vane assembly to clear the rig pushdown with the vane

resting on the bottom of the cored hole and the spline rod added.


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3. Attach spline rod to drill stem.

4. Slowly push vane into soil. With the drill rig hydraulic pushdown, slowly push the vane

into the undisturbed soil three vane diameters. Do not exceed 400 psi (2760kPa) pushdown

as indicated on the drill rig hydraulic gauge.

5. Lower the torque table assembly over the spline and bolt it to the spider support on the

rig.

6. Adjust the pulley cable for proper alignment and take up the slack. Inspect the setup in

general. Slowly turn the handle clockwise until tension begins to record on the proving ring

dial gauge. Release the cable to zero tension. Adjust the proving ring dial to zero reading

and record the initial torque pulley reading in degrees.

7. Begin the test by turning the handle clockwise at a rate of 20 revolutions per minute or 3

seconds per revolution. The vane will rotate at the rate of 6 degrees per minute in a

clockwise direction.

8. Record reading from the proving ring dial gauge every 10 seconds, or at each one degree

rotation of the torque pulley. Record the peak and ultimate values.

9. Repeat steps 6 through 9 to determine the disturbed or remolded strength, if desired.

10. Push the vane further into undisturbed soil three vane diameters and follow steps 6

through 9 until the test series is completed.

The formula for the design strength or one half the ultimate shear strength of the soil is:

S = (.0015264)(Dial Gage Reading) Tons/Ft

2

or (.14617)(Dial Gage Reading) kN/M

2

See Figure 2-7 for a photo of performing test.


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Figure 2-7. Performing test

Torvane & Pocket Penetrometer

These two test devices are useful for index and classification purposes. They only yield

approximate information and are not suitable for foundation design.

Torvane. The torvane is a small, hand-held, spring-loaded device that is pressed into thesample and turned. A scale on the knob reads the approximate ultimate shear strength of the

sample. Refer to Figure 2-8 for a picture of a torvane. The sample must be fairly cohesive

to yield accurate results. Very stiff samples often crumble rather than shear thereby yielding

lower than actual values.


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Figure 2-8: Torvane

Pocket Penetrometer. Figure 2-9 shows a pocket penetrometer, a spring-loaded device,

which is pressed into a sample. The penetration resistance read on the side of the device is

the approximate ultimate compressive strength. The tester is adaptable to a wide range of

soil types and strengths. While penetration values are obtained from this device, it is of

questionable value for any design work.

Figure 2-9: Pocket penetrometer

Monitoring Methods

Piezometers and inclinometers are monitoring methods discussed below.


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Piezometers

The next subsections cover

Piezometer use

Piezometer installation procedure

Reading frequency guidelines

Pneumatic piezometer alternative

Piezometer Use

Piezometers are used to measure long-term groundwater levels. They are essentially water

wells and are sometimes pumped to determine the permeability of the soil to predict seepage

volumes in excavations. Should short-term observations of water levels be desired, leave

exploration core holes open for several days to monitor the groundwater level. Cover the

hole to protect people or livestock from injury. Some typical applications for piezometersare to evaluate groundwater levels in future depressed roadway sections and groundwater

effects on slope stability.

Future Depressed Roadway Sections. The construction of future depressed roadway

sections that are subject to groundwater infiltration can be impacted adversely by

groundwater induced liquefaction of soils. The final installation may need special drainage

features to control water inflows and provide a stable pavement section.

Slope Stability. Groundwater affects slope stability by reducing the effective stresses in the

soil through buoyancy. This applies to both side slope stability and bearing capacity of

embankments and retaining walls.

Piezometer Installation Procedure

Install piezometers with care to insure that the groundwater levels are accurately measured

without the intrusion of surface water. Inadequate sealing of the borehole may also allow

contaminates to enter subsurface aquifers. Use this procedure to install piezometers:

Piezometer Installation Procedure

Step Action1 Drill hole.

2 Place piezometer tube in hole.

3 Place granular media in hole.

4 Seal the remaining upper portion of hole.

5 Finish the tube.

1. Drill the hole with no water if possible. If this is not possible, drill with clear water. If

hole stability continues to be a problem, add small amounts of drilling mud to the water.


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2. Place the assembled piezometer tube in the hole. Either use a slotted screen, or

alternately, drill holes in a section of the tube and then wrap them with filter fabric. The

upper sections of the tube are not perforated.

3. Place the granular media in all but the upper 5 to 10 feet (1.5 to 3 meters) of the hole.

Use a fairly coarse sand or pea gravel to allow easy placement through water.

4. Seal the remaining upper portion of the hole with grout or bentonite pellets. When

using bentonite pellets in a dry hole, pour several gallons of water over the pellets for

10 to 15 minutes to start expanding the pellets to seal the hole.

5. Finish the tube in such a manner as to not be a hazard to the public. Use a locking

cover if vandalism is possible.

Reading Frequency Guidelines

Take a reading immediately and weekly thereafter until the water level stabilizes. Monthly

readings thereafter are normally sufficient unless the site exhibits large fluctuations in

readings.

Pneumatic Piezometer Alternative

In areas where access is difficult, pneumatic piezometers that may be read from a remote

location are available. These use air pressure to read the depth of water above the probe.

Inclinometers

The next subsections cover

Inclinometer use

Inclinometer description

Inclinometer Use

Inclinometers are used to measure horizontal movements within a soil mass over time. The

most common application is for monitoring slope failures to determine the failure plane

depth. With this information, you may perform stability analyses to confirm soil strengths

and determine the proper repair method. Other uses include monitoring ground movements

adjacent to excavations for foundations or tunnels.

Inclinometer Description

The inclinometer is a very sensitive device that measures deviations from vertical. By

recording these deviations at periodic intervals along a special casing grouted into a

borehole, the cumulative deviation of the instrument may be determined. These readings are


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taken periodically to monitor movements over time. The installation of casing and data

reduction is quite complicated. It is recommended that the Bridge Division Geotechnical

Section be consulted if inclinometer measurements are required.


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Chapter 3

Soil & Bedrock Classification & Logging

Contents



Classification Objective ....................................................................................................................3-2

Importance of Logging .....................................................................................................................3-2

Section 2 Classification.................................................................................................... 3-3Overview...........................................................................................................................................3-3

Bedrocks...........................................................................................................................................

3-4

Igneous Rocks...................................................................................................................................3-4

Metamorphic Rocks..........................................................................................................................3-4

Sedimentary Rocks ...........................................................................................................................3-5

Clastic Rocks ....................................................................................................................................3-5

Nonclastic Rocks ..............................................................................................................................3-6

Soils 3-7

Soil Variations ..................................................................................................................................3-7

Residual & Sedimentary Soils ..........................................................................................................3-7

Soil Identification .............................................................................................................................3-7

Grain Size Descriptions ....................................................................................................................3-7

Cohesive Soils...................................................................................................................................3-8

Cohesionless Soils ............................................................................................................................3-8

Section 3 Logging .......................................................................................................... 3-12Overview.........................................................................................................................................3-12

Logging Method.............................................................................................................................3-12

Field Equipment..............................................................................................................................3-14

Core Description Order...................................................................................................................3-14

Log Form ........................................................................................................................................3-18

Available Software .........................................................................................................................3-22


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Chapter 3 Soil & Bedrock Classification &

Logging

Section 1 Overview


Section 1

Overview

Introduction

This chapter deals with the material types encountered during field exploration and the

proper recording of observations made of the exploration process. The classification and

logging process is important since it is the only record of the field exploration process. The

next subsections explain

Soil classification objective

Importance of logging

Classification Objective

The objective of soil classification is to identify the logging terminology of the foundation

exploration through integration of soil mechanics and geology. This chapter defines the

terminology used for classifying foundation materials and illustrates the methods of logging

that supplies the information obtained in the field. In certain cases, take samples from the

field to the laboratory where they can be analyzed to supplement the field classification.

The field classification is designed to be simple and orderly so that the use of the soil and

bedrock terminology is uniform.

Importance of Logging

All loggers need to realize that a good field description of the materials encountered is veryimportant for the design of an economical foundation. The logger and the core driller are the

only people who witness the drilling and the material obtained. Therefore a reasonable

amount of accurate information must be logged.


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Logging

Section 2 Classification


Section 2

Classification

Overview

This section covers the classification of both bedrocks and soils. The lists below give

bedrock and soil classifications.

Bedrock Classifications

Igneous

Granite

Basalt

Metamorphic

Gneiss Schist

Slate

Marble

Sedimentary

Clastic

Shale (Claystone)

Siltstone

Sandstone

ConglomerateLimestone

Glauconite

Lignite

Nonclastic

Chert

Iron deposits

Gypsum

Halite

Soil Classifications

Cohesive - Clay

Cohesionless

Silt

Sand

Gravel


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Logging

Section 2 Classification


Bedrocks

Geologists divide bedrocks into three classes:

Igneous rocks

Metamorphic rocks Sedimentary rocks

Sedimentary rocks make up 75% of the exposed surface area of the earth's crust while

igneous and metamorphic rocks make up the remaining 25%. Naturally, sedimentary rocks

will be emphasized, but igneous and metamorphic rocks must not be overlooked.

Igneous Rocks

Igneous rocks are found in approximately 20 counties of the Llano Uplift, South Central

Texas, and the Trans-Pecos areas. These rocks are derived from cooled and solidifiedmolten rock material, called magma, which was pushed up from the interior of the earth.

Magmas that cool beneath the surface form intrusive rocks and those that reach the surface

form extrusive rocks. The rate of cooling, mineral composition and mode of placement

control the type, texture, and shape of rocks.

All of these variables complicate the identification so that a background in mineralogy and

petrology is necessary to identify each properly. The igneous rocks that outcrop in Texas are

generally described as intrusive (such as granite) or extrusive (such as basalt). Information

on both these types appears below.

Granite. Granite is a very hard, generally coarse-grained rock which is light-colored (pink,

red, or gray) and heavier than most rocks. It is chiefly composed of quartz, feldspar, and

some dark minerals (usually mica). Granite has a crystalline texture and is usually even-

grained (grains equal

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