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CITY OF HOUSTON
Department
of Public Works & Engineering
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DESIGN
MANUAL
Geotechnical Requirements
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Chapter
11
GEOTECHNICAL REQUIREMENTS
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11.01
INTRODUCTION
A.
This chapter includes minimum geotechnical investigation requirements
for design of utilities and streets. Utilities include water lines, wastewater
systems, and storm water drainage conduits and open channels. Streets
include design of street pavements.
B.
This chapter is not intended to address tunnel projects.
11.02
REFERENCES
A.
AASHTO - American Association of State Highway and Transportation Officials.
B.
AREA - American Railroad Engineering Association.
C.
ASTM - American Society for Testing and Materials.
D.
OSHA - Occupational Safety and Health Administration.
E.TxDOT
- Texas Department of Transportation, Foundation Exploration and Design
Manual.
F.
TNRCC - Texas Natural Resource Conservation Commission.
11.03
INVESTIGATION REQUIREMENTS
A.
A geotechnical investigation is required prior to design under the conditions
listed below. Data from earlier project design activities can be incorporated
if sufficient and reliable for the current project. A geotechnical investigation
by borings is required for design of:
1.
Underground utilities using open cut methods.
2. Underground utilities to be augered beneath existing streets, pipelines,
or other obstructions or structures.
3.
Street paving.
4.
Construction which could affect the integrity of adjacent structures,
with the exception of utilities at a depth less than 5 feet, interconnections
such as service connections, and meter vault installations.
B.
Install piezometer(s) in a water bearing layer.
1.
Read water levels 24 hours after initial installation and at 30 days
just prior to removal and grouting.
2. Space piezometers no greater than 2500 feet apart.
C.
The minimum geotechnical recommendations shall consist of the following:
1.
Open cut Trenches: Bedding, backfill, excavation wall and bottom stability,
thrust restraint, ground water control requirements at boring locations,
dewatering method, and flexible pipe design parameters.
2. Auger Installation: Soil design parameters, ground stability, auger
pit excavation stability, dewatering recommendation
3. Appurtenance: Bearing capacity, lateral earth pressures, excavation
stability, dewatering
4. Open Channel: Slope angle or slope ratio, setback distance, and erosion
protection.
D.
All projects requiring a geotechnical investigation shall have a reconnaissance
fault study to determine the potential for known active faults to impact
the project. If the project is part of a larger tract for which a reconnaissance
fault study is available, the results of the study on the larger tract
will satisfy this requirement.
E.
For privately funded subdivision development in the City or subdivision
development in a utility district, when borings have been conducted for
a master study of the development and development of an adjacent section
has been conducted, subsequent design within sections of the master planned
area may not require individual borings or a geotechnical report. The
borings and report may not be needed if the project design provides for
field verification of encountered conditions by direct supervision of
a registered. geotechnical engineer and provides for response to unexpected
soil conditions encountered during construction and those responses are
in accordance with City design standards.
11.04
INVESTIGATION OF GRANULAR AND COHESIVE SOILS
A.
The borehole sampling and testing procedure shall include obtaining undisturbed
Shelby Tube samples in cohesive soils and Standard Penetration Test Split
Barrel samples in granular soils, if not otherwise required (see paragraph
11.05). Continuous sampling shall be performed to a minimum depth of 10
feet, and at 5 foot intervals below that depth. Additional samples shall
be obtained at strata changes encountered within the standard 5-foot sampling
intervals.
11.05
INVESTIGATION CRITERIA
A.
Utility Line Projects. The frequency and depth of borings for utility
line projects shall not be less than described below unless waived by
the City Engineer. Bore an additional 5 feet if the last planned sample
is in water bearing sand. Make borings within an offset distance of no
more than 20 feet from the centerline alignment of the utility line or
at the location of the proposed structure. Soil borings shall be made
at a spacing not greater than 500 feet with additional boring at closer
spacing to better define areas of inconsistent stratigraphy.
1.
Open Cut Construction and Auger Pits:
|
Table
1.1
REQUIREMENTS FOR BORING DEPTHS
|
| TRENCH
WIDTH |
TRENCH
DEPTH |
BORING
DEPTH |
| Up
to 6 feet |
All
|
Trench
Depth + 5 feet |
| Greater
than 6 feet and up to 10 feet |
Up
to 10 feet |
Trench
Depth + 5 feet |
| Greater
than 6 feet and up to 10 feet |
10
to 15 feet |
Trench
Depth + 10 feet |
| Greater
than 6 feet and up to 10 feet |
Greater
than 15 feet |
Trench
Depth + 1.5 times trench width |
| Greater
than 10 feet |
All
|
Trench
Depth + 1.5 times trench width |
2.
Augered Sections: Make boring depth to line depth plus 5 feet for lines
up to 10 feet deep. For lines over 10 feet deep, make boring depth to
line depth plus 10 feet.
B.
Water Line Projects.
1.
The frequency and depth of borings for water line projects shall conform
to those listed in paragraph 1 1.05A, Utility Line Projects.
2.
A geotechnical investigation is required for any waterline with nominal
bedding depth greater than 5 feet or water line diameter greater than
20 inches.
C.
Wastewater Line Projects.
I.
The frequency and depth of borings for wastewater line projects shall
conform to those listed in paragraph I 1.05A Utility Line Projects.
2.
In addition, at least one boring must be made within 20 feet of the
proposed center of a lift station. For lift stations 30 feet in diameter
or larger, make one boring at the center; add borings around the periphery
at maximum 50 feet spacing.
a.
For City projects, the boring shall extend to:
(1)
A depth of B below the bottom of the lift station, or
(2)
A depth of 0.75 D below the bottom of the lift station, whichever
is greater,
where:
B is the width of diameter of the lift station, and
D
is the depth of the lift station or excavation.
b.
For projects within the City's extra territorial jurisdiction (ETJ),
the boring shall be to a minimum depth of 10 feet below the base of
the structure.
3.
For City projects, install a piezometer within 20 feet of the center
of the lift station. For projects within the ETJ, install a piezometer
within 20 feet of the center of each lift station location at which
the groundwater level in the boring, measured at least 24 hours after
drilling, is above or within 3 feet below the bottom elevation of the
lift station. Read the water level again at 30 days after initial installation,
then remove the piezometer and gout the hole.
4.
Other Structures: A boring program shall be established by the owner
and structural engineer in consultation with the geotechnical engineer.
D.
Storm Drainage, Box Culverts, and Open Channels.
1.
The frequency and depth of borings for storm drainage pipes and box
culverts shall conform to those listed in paragraph 11.05A, Utility
Line Projects.
2.Open
Channels.
a.Soil
borings shall be made at a spacing not greater than 500 feet with
additional boring at closer spacing to better define areas of inconsistent
stratigraphy.
b.Channel
depth (D) less than or equal to 10 feet, extend boring D feet below
the ditch bottom.
c.For
channel depth greater than 10 feet and less than or equal to 20 feet,
extend boring 10 feet below the ditch bottom.
d.For
channel depth greater than 20 feet establish boring depth to provide
sufficient geotechnical information for design.
3.
Soils information for culverts in roadside ditches shall be obtained
from soil borings made for the paving design, as described in paragraph
E.
Street Paving.
1.
Soil borings shall be made at a spacing not greater than 500 feet.
2. The depth of borings shall be at least 5 feet below the top of the
curb for curb and gutter sections and 5 feet below the crown of the
road for open ditch sections, or 3 feet below ditch invert, whichever
is greater.
3. Rigid paving.
a.
The minimum geotechnical recommendations shall consist of the following:
Pavement thickness, including minimum reinforcing and minimum subgrade
treatment, as required in Chapter 10, Street Paving Design Requirements.
4.
Flexible paving.
a.
Design Structural Number (SN), pavement thickness, subgrade treatment,
as required in Chapter 10, Street Paving Design Requirements.
5.
Rehabilitation rehabilitation recommendations.
11.06
LABORATORY TESTING PROGRAM
A.
Laboratory Tests may include but not be limited to the following:
1.
ASTM D 4318 Liquid Limit, Plastic Limits and Plasticity Index of Soils.
2.
ASTM D 1140 Amount of Material in Soils Finer Than the No. 200 Sieve.
3.
ASTM D 2216 Laboratory Determination of Water Content of Soil, Rock
and Soil Aggregate Mixture.
4.
ASTM D 422 Particle Size Analysis of Soils.
5.
ASTM D 2487 Classification of Soils for Engineering Purposes.
6.
ASTM D 2166 Unconfined Compressive Strength of Cohesive Soils.
7.
ASTM D 2850 Unconsolidated Undrained Compressive Strength of Cohesive
Soils in Triaxial Compression.
8.
ASTM D 2435 One Dimensional Consolidation Properties of Soils.
9.
ASTM D 698 Laboratory Compaction Characteristics of Soil Using Standard
Effort.
10.
ASTM D 1883 CBR (California Bearing Ratio) of Laboratory Compacted Soils.
11.07
SURVEY REQUIREMENTS
A.
Within the public domain, the locations and elevations of boreholes and
piezometers shall be determined by surveying. Elevation and coordinates
shall be shown on boring logs. Station and offset shall be shown on boring
logs for street projects and utility line work.
11.08
SITE RESTORATION
A.
Site Cleanup along developed right of way. Clean the boring site by removing
cuttings and mud and other debris. Fill ruts or pits in the ground to
original conditions and elevation.
B.
Abandonment of Borings and Piezometers.
1.Piezometer
abandon in accordance with TNRCC Rules, Chapter 238 Well Drillers and
Water Well Pump Installers..
2.
Borehole: Backfill with cement gout using tremie method. For non paved
areas and depths less than or equal to 10 feet, soil backfill tamped
into the borehole is acceptable provided that the conditions and methods
are in agreement with TNRCC Rules. Boreholes or piezometers installed
in known contaminated areas, or in which contamination otherwise has
been detected, shall be abandoned in accordance with the applicable
provisions of TNRCC Rules, Chapter 238 Well Drillers and Water Well
Pump Installers.
C.
Restoration of Cores Through Pavement. Boreholes or other cored penetrations
of pavements shall be restored with the same or equivalent materials as
the existing pavement. Larger penetrations shall be repaired following
City of Houston Standard Detail No. 02902 01, Pavement Repair Details.
Do not restore the pavement until the borehole grout has taken initial
set to allow for any settlement or shrinkage of the grout.
10.09
GEOTECHNICAL REPORT
A.
A sample Table of Contents for a typical geotechnical report follows:
Summary
Field Investigation and Laboratory Testing
1. Introduction
2. Field Investigation
3. Laboratory Testing
Interpretive Report
4. Site Characterization
5. Design Criteria and Recommendations
6. Ground Faults
7. Construction Considerations
8. Monitoring
9. Earthwork Specification and Recommendations
Attachments
B.
Geotechnical report for trench safety system shall be provided for all
City of Houston projects with excavation over 5 feet deep. Report shall
satisfy statutory requirements for contracting for trench safety construction.
END
OF CHAPTER
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