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Geotechnical Engineering Report

South Laredo WWTP Wet-Well Addition

Laredo, Texas

October 14, 2011

Terracon Project No.: 89115043

Prepared for:

CDM

San Antonio, Texas

Prepared by:

Terracon Consultants, Inc.

Laredo, Texas

October 14, 2011

COM

1777 NE Loop 410, Suite 500 San Antonio, Texas 7821 7

Attention : Mr. Don Vandertulip , P.E. P: [210] 826.3200 F: (210] 826.8876 M: [210] 426.5843

E: [email protected]

Re: Geotechnical Engineering Report

South Laredo WWTP Wet-Well Addition River Front Street Laredo, Texas Terracon Project No. : 8911 5043

Dear Mr. Vandertulip:

1rerracan

Terracon Consultants, Inc. (Terracon) is pleased to submit our Geotechnical Engineering Report for the South Laredo WWTP Wet-Well Addition in Laredo, Texas. We trust that th is report is responsive to your project needs. Please contact us if you have any questions or if we can be of further assistance.

We appreciate the opportunity to work with you on this project and look forward to provid ing additional Geotechnical Engineering and Construction Materials Testing services in the future.

Sincerely, Terracon Consulta (Firm Registration: T

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Staff Ge _echnica_l Eng_ineer_ . . l~o~);·}:cENSI=-~-;.~/·'. Department Manager Geotechnical Eng1neenng D1v1s1on -•:f-s,0~:~·;·~ '.:J

APR reviewby JosephA.Waxse. P.E. - 8911 5043 ' tt>[ ltt{-z.o\\

Copies To: Addressee: (2) Bound & (1) Electronic

Terracon Cons ultants, Inc. 615 Gale Street, Bldg B. Laredo, Texas 78041 P [9561 729-1100 F [956) 791 -1071 Firm Registration No. F-3272 www.terracon.co m

Geotechnical a Environmental a Construction Materials a Facilities

Geotechnical Engineering Report South Laredo WWTP Wet-Well Addition ■ Laredo, Texas October 14, 2011 ■ Terracon Project No.: 89115043

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TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY ............................................................................................................. i

1.0 INTRODUCTION ............................................................................................................ 1

2.0 PROJECT INFORMATION ............................................................................................. 1

2.1 Project Description ................................................................................................. 1

2.2 Site Location and Description ................................................................................. 2

3.0 SUBSURFACE CONDITIONS ........................................................................................ 2

3.1 Geology ................................................................................................................. 2

3.2 Typical Profile ........................................................................................................ 2

3.3 Groundwater .......................................................................................................... 2

3.3.1 Dewatering ................................................................................................. 3

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION ........................................ 4

4.1 General Foundation Considerations ....................................................................... 4

4.2 Shoring Methods .................................................................................................... 5

4.3 Earthwork ............................................................................................................... 6

4.3.1 General Site Preparation ............................................................................ 6

4.3.2 Mat Subgrade Preparation .......................................................................... 7

4.3.3 Fill Materials and Placement ....................................................................... 8

4.3.4 Compaction Requirements ......................................................................... 8

4.3.5 Grading and Drainage ................................................................................ 8

4.3.6 Construction Considerations ....................................................................... 9

4.4 Foundations ........................................................................................................... 9

4.5 Mat Foundation ...................................................................................................... 9

4.5.1 Buoyant Uplift ........................................................................................... 10

4.6 Lateral Earth Pressures for Below Grade Structures ............................................ 10

4.7 Seismic Considerations ........................................................................................ 11

4.8 Excavations ......................................................................................................... 11

4.8.1 Trench Shoring ......................................................................................... 11

4.8.2 Trenches .................................................................................................. 12

4.8.3 Occupational Safety and Health Administration (OSHA) Guidelines ......... 12

4.9 Other Design/Construction Considerations .......................................................... 13

4.9.1 Concrete Considerations .......................................................................... 13

5.0 GENERAL COMMENTS ............................................................................................... 14 TABLES Table 1 Soil Properties APPENDIX A – FIELD EXPLORATION

Exhibit A-1 Field Exploration Description

Exhibit A-2 Site Location Plan

Exhibit A-3 Bore Location Plan

Exhibit A-4 Boring Log



APPENDIX B – LABORATORY TESTING

Exhibit B-1 Laboratory Testing

APPENDIX C – SUPPORTING DOCUMENTS

Exhibit C-1 General Notes

Exhibit C-2 Unified Soil Classification System


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EXECUTIVE SUMMARY

This summary should be used in conjunction with the entire report for design purposes. It should

be recognized that details were not included or fully developed in this section, and the report must

be read in its entirety for a comprehensive understanding of the items contained herein. The

section titled GENERAL COMMENTS should be read for an understanding of the report

limitations.

A geotechnical investigation has been performed for the proposed Wet-Well Addition to be located

at South Laredo Waste Water Treatment Plant (WWTP) on River Front Street in Laredo, Texas.

One (1) boring was drilled to a depth of approximately 80 feet below the existing grade within the

proposed structure area. Groundwater was encountered during and after completion of the

drilling operations. Groundwater conditions are discussed in the report.

Based on the information obtained from our subsurface exploration, the subsurface soil conditions

appear to be suitable to support the proposed structure provided the structure excavation is

properly shored and the foundation is properly designed. The following geotechnical

considerations were identified:

Multiple options are feasible for shoring of the deep excavation required for the wet well.

The shoring methods selected are critical with respect to the costs and performance of

the project.

A mat foundation system would be appropriate to support the structural loads of the

proposed wet-well structure.

A mat foundation system may be designed for a net allowable bearing pressure of 7,500

psf based on total load or 5,000 psf based on dead load plus long-term live load,

whichever results in a larger bearing surface.

Based on the 2009 International Building Code, Tables 1613.5.6 (1) and 1613.5.6 (2),

IBC seismic site classification for this site is D.

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GEOTECHNICAL ENGINEERING REPORT

SOUTH LAREDO WWTP WET-WELL ADDITION

RIVER FRONT STREET

LAREDO, TEXAS TERRACON PROJECT NO.: 89115043

OCTOBER 14, 2011

1.0 INTRODUCTION

Terracon is pleased to submit our Geotechnical Engineering Report for the South Laredo

WWTP Wet-Well Addition in Laredo, Texas. This project was authorized by Ms. Mari Garza Bird

of CDM, through signature of our “Agreement for Services” on August 8, 2011. The project

scope was performed in general accordance with Terracon Proposal No. P89110057 dated

June 9, 2011.

The purpose of this report is to describe the subsurface conditions observed at boring locations

drilled for this study, analyze and evaluate the test data, and provide recommendations with

respect to:

subsurface soil conditions groundwater conditions

foundation design and construction earthwork

seismic considerations

2.0 PROJECT INFORMATION

2.1 Project Description

Item Description

Site Layout See Exhibits A-2 & A-3, Site Location Plan & Bore Location Plan.

Structure

Reinforced concrete wet-well to house up to 6 large submersible

pumps. Water levels inside the wet-well will range between 2 and

15 feet.

Structure Construction

Based on previous communication with CDM personnel, the wet-

well structure will be reinforced concrete with a diameter of about

30 feet, a depth of about 55 feet below grade, and about 24-inch

thick walls supported by a mat foundation system. The alternative

of building several smaller diameter wet wells may also be

considered

Finished Floor Elevation Information not provided at this time.



2.2 Site Location and Description

Item Description

Location This project will be located within South Laredo Waste Water

Treatment Plant facilities at River Front Street in Laredo, Texas.

Existing Improvements Existing WWTP facilities surround the proposed wet-well structure.

Current Ground Cover Bare soils.

Existing Topography Relatively flat and level.

3.0 SUBSURFACE CONDITIONS

3.1 Geology

The Laredo Sheet (1976) of the Geologic Atlas of Texas published by the Bureau of Economic

Geology at the University of Texas at Austin has mapped the Fluviatile Terrace Deposits in the

area of this project. The Fluviatile Terrace Deposits is of the Pleistocene period and consists of

gravel, sand, silt and clay; composed of materials similar to those present in contiguous

alluvium.

3.2 Typical Profile

Based on the results of the boring, subsurface conditions on the project site can be generalized as

follows:

Description Approximate Depth

of Stratum (feet) Material Encountered Consistency/Density

Stratum I 0 to 10 SANDY LEAN CLAY 1 ;

brown Stiff to hard

Stratum IA 10 to 48 LEAN CLAY with SAND 1 ; brown Stiff to hard

Stratum II 48 to 80 CLAYSTONE 2 ; dark bluish gray Hard, cemented

1 The SANDY LEAN CLAY (CL) and LEAN CLAY with SAND (CL) materials could undergo low

volumetric changes (shrink/swell) should they experience changes in their in-place moisture

content.

2 The CLAYSTONE is expected to undergo moderate to high volumetric changes (shrink/swell) with

fluctuations in its moisture content. However, the depth at which this stratum was encountered will

lessen its potential to undergo significant volumetric changes.

Conditions encountered at the boring location are indicated on the individual boring log.

Stratification boundaries on the boring log represent the approximate location of changes in soil

types; in-situ, the transition between materials may be gradual. Details for the boring can be found

on the boring log in Appendix A of this report.

3.3 Groundwater

The boring was drilled using a combination of dry (from 0 to 50 feet depth) and wet (from 50 to

80 feet depth) rotary drilling techniques. Drilling fluid within the boring inhibited further



groundwater observations. The borehole was backfilled with cutting upon completion. The short-

term field observations simply do not permit an accurate evaluation of the groundwater levels at

this location. Groundwater levels are influenced by seasonal and climatic conditions, which can

and will change. If an accurate determination of the groundwater table is necessary, long term

observations in piezometers or observation wells sealed from the influence of surface water are

often required to define groundwater levels in materials of this type. The foundation contractor

should check the subsurface water conditions just prior to foundation excavation activities.

It is recommended that piezometers be installed since it appears the structure will extend below

the groundwater level noted when drilling. Piezometer installation or long term groundwater

observations were not included in our scope of work for this project.

The groundwater measurements obtained for this study are considered approximate and are

short-term, since the borings are open for a short time period. On a long-term basis, the

groundwater levels will vary and may be present at shallower depths. The water levels observed

in the borehole are noted on the attached boring log presented in Appendix A of this report, and

are summarized below:

Boring No. Depth to Groundwater

While Drilling, ft

Depth to Groundwater

While Drilling (after 15 minutes), ft

B-1 46.0 43.5

Note: Depths measured from existing ground surface at time of measurement. Groundwater

observations were measured during dry drilling operations only. Groundwater levels have been

rounded to the nearest one-half (1/2) foot.

3.3.1 Dewatering

Dewatering operations will likely be required during foundation excavation and wet-well

construction. Dewatering requirements will be affected by the type of excavation and shoring

methods selected for the project. The samples obtained in the boring below the observed water

level consisted of lean clay with sand and claystone materials that would not generally be

expected to be highly porous or cause rapid seepage rates. However, sandier and more porous

layers capable of producing higher seepage rates and piping instability could be present.

If porous sandy zones are not encountered, or if a relatively impervious shoring barrier is

installed, a perimeter trench sump system installed at the bottom of the excavation may provide

an effective means of dewatering. However, the dewatering operations will be dependent upon

the actual groundwater conditions present, and the shoring system used, during construction.

Dewatering operations could vary from open-pumping at graded low point(s) in the excavations

to the installation of pump wells or a well point system.



Based on the planned bearing depth of 50 to 55 feet (B-1), the proposed foundation system for

the new lift station is expected to be situated in the Stratum II CLAYSTONE. Depending upon

the possible presence of porous layers in the claystone and the magnitude of groundwater

encountered, buoyant forces or pressures might create a base instability of the excavation.

Furthermore, dewatering may be more difficult to control if significant water seepage is

occurring up through the base of the excavation. The potential for the presence of a porous

layer in the claystone that could pose base instability or seepage concerns could be evaluated

by isolating a piezometer screen interval extended from the base elevation to a depth of about

15 feet below the base. The rate at which the water level in the piezometer recovers after

pumping the level down would help determine whether such problems could be of concern.

We recommend that the pump station structure be designed to resist buoyant forces equivalent

to groundwater levels at the ground surface unless positive measures are taken to prevent

water infiltration and accumulation in the backfill around the structure.

Base stability concerns and dewatering methods selected should be further discussed and

addressed by Terracon prior to construction. A pre-construction meeting with the general

contractor is also recommended to discuss possible excavation and dewatering methods as well

as shoring, bracing, sheeting, or sloping of the excavation sidewalls for safe construction.

Dewatering, shoring, bracing, sheeting or sloping of the excavation sidewalls are means and

methods selected by the contractor.

As a result of the groundwater and the potential for granular soil layers, the contractor should

check and be prepared to deal with the groundwater conditions prior to pump station structure

excavation activities.

Specific information concerning groundwater is noted on the boring log presented in Appendix A of this report.

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

The following recommendations are based upon the data obtained from our field and laboratory

programs, project information provided to us and on our experience with similar subsurface and

site conditions.

4.1 General Foundation Considerations

Final grading around for the proposed structure was not available at the time of this report.

However, we anticipate that the structure will be mostly below grade and only a portion of it,

about 2 to 3 feet, will be above grade. About 55 to 60 feet of cut will be necessary to achieve

the finished wet well invert elevation, assuming a 5-foot thick mat foundation. If this information

changes, we should be contacted to review and revise our recommendation as appropriate. We

understand that a mat foundation system is planned to support the structure at this site.



The foundation type being considered to provide support for the planned structure must satisfy

several completely independent engineering criteria with respect to the subsurface conditions

encountered at the site. One criterion is the foundation system must be designed with an

appropriate factor of safety to reduce the possibility of a bearing capacity failure of the soils

underlying the foundation when subjected to axial and lateral load conditions. The other

criterion is movement of the foundation system due to compression (consolidation) or expansion

(swell) of the underlying soils must be within tolerable limits for the structure. In addition to the

criteria affecting the performance of the foundations, consideration must also be given to the

effects of lateral earth pressures and hydrostatic pressures that may develop on the buried

portion of the structure.

The suitability and performance of a soil supported foundation for a structure depends on many

factors including the magnitude of soil movement expected, the type of structure, the intended

use of the structure, the construction methods available to stabilize the soils, and our

understanding of the owner’s expectations of the completed structure's performance. Based on

the soil data a mat foundation is considered appropriate for support the anticipated wet-well

structure. Recommendations for a mat foundation are provided in this report. A mat foundation

system may be used at this site provided the bearing surface and foundation are designed and

constructed as recommended in this report.

4.2 Shoring Methods

The primary issue in design and construction of the wet well structure at this site is the selected

method of excavation and shoring. A sloped, open-cut excavation would not be considered

feasible at this site due to the resulting size of the excavation and the presence of adjacent

structures. A vertical shoring system is therefore expected to be required and may consist of a

variety of methods, including, but not limited to:

1. Circular secant or tangent pile wall system.

2. Designing the wet well wall structure with a beveled bottom “cutting edge”, constructing

segments at grade and then internally excavating so that the structure is caused to sink

progressively, in a controlled manner, into the ground as a self-braced caisson.

3. Opting to install multiple wet wells of a diameter that can be drilled with a large diameter

auger (up to ~ 14 feet) into the claystone stratum and stabilizing the excavation with

slurry drilling techniques. This would require lowering pre-fabricated wet well structures

into slurry-filled holes followed by tremmie placement of a wet well bottom/rock socket

concrete mass that is adequate for foundation support and uplift resistance.

4. Tied-back or internally braced sheet pile or soldier pile and lagging system.

5. Progressively installed soil-nailed wall with a reinforced grouted face.

For option No.1, the secant piles could be advanced to the full structure depth and the internal

soils and claystone excavated with a clam shell. The internal secant pile wall surface could be

cleaned and used as the external form for the wet well wall. The secant pile wall would be

designed to carry full lateral soil loads, removing that load from the wet well wall design. The



secant pile system would also develop large uplift resistance equal to the weight of the secant

piles plus an allowable uplift average skin friction of 1,000 psf times the exterior secant pile wall

surface area. This should minimize the bottom slab thickness required to resist buoyant uplift

forces.

For option No.2, enabling penetration of the caisson into the claystone is expected to pose

potential problems. It would technically be feasible to pre-drill the claystone with a circle of

secant pile excavations backfilled with sand. However, the cost of the drilling would likely

approach the cost of installing an actual secant pile shoring system. An average side friction of

750 psf could be assumed for uplift resistance of a caisson installed in non-pre-drilled soils. If

the perimeter soils are pre-drilled and backfilled with sand, the average side friction assumed for

uplift resistance should be reduced to 300 psf.

Options No.4 and 5 may or may not engage uplift resistance against the shoring or allow use of

the shoring as an exterior wall form.

4.3 Earthwork

The following presents recommendations for site preparation, structure pad preparation and

placement of engineered fills on the project. The recommendations presented for design and

construction of earth supported elements including foundations and slabs are contingent upon

following the recommendations outlined in this section.

Earthwork on the project should be observed and evaluated by Terracon. The evaluation of

earthwork should include observation and testing of engineered fill, mat subgrade preparation,

foundation bearing soils, and other geotechnical conditions exposed during the construction of

the project.

4.3.1 General Site Preparation

If formation of a stabilized working pad is desired, construction areas around the wet well should

be stripped of vegetation and any deleterious material. After site stripping, the exposed

subgrade should be proofrolled with appropriate construction equipment weighing at least 20

tons. The purpose of this recommendation is to check the subgrade for weak, loose, or soft

areas prior to fill placement and compaction of a stabilized working surface, such as crushed

limestone. This operation should be observed and evaluated by Terracon’s qualified

geotechnical personnel experienced in earthwork operations.

If weak, loose, or soft areas are evidenced during proofrolling operations, the soil in the subject

area should be removed to expose competent subgrade soils in both horizontal and vertical

limits. The excavated soils provided they are not contaminated with deleterious materials, or

clean imported fill soils can be used to restore grade at these isolated areas. Any imported fill

should meet the requirements for select fill as presented in the “Fill Materials and Placement”

section of this report.



The resulting subgrade in cut areas should be scarified to a depth of 8 inches; moisture

conditioned, and compacted to the requirements given in this report. Fill and backfill soils

should be placed; moisture conditioned, and compacted as noted in this report. Soils within

structure area should not be allowed to dry out or become excessively wet after compaction is

completed. Placement and compaction of soil is discussed below:

Structure Area: Refer to the “Fill Materials and Placement” section of this

report.

General and Common Areas (not requiring structural support): Compact to at

least 95 percent of the maximum dry density as evaluated by ASTM D 698

(Standard Compaction) at compaction moisture contents between -2 and +3

percentage points of optimum moisture content.

4.3.2 Mat Subgrade Preparation

We understand that the mat subgrade will be about 50 to 55 feet below grade. Various shoring

options were discussed in the “Shoring Methods” section of this report.

For options No.1, 2, 4 and 5, typical downhole excavation methods are likely to leave a

somewhat uneven surface in the claystone bearing surface. The bearing surface should be

cleaned of all loose materials and observed by a Terracon representative. A leveling course of

poured concrete or compacted clean crushed stone should then be placed to design mat

bearing elevation. Compaction of crushed stone should be as described later in this section of

the report. Due to the presence of highly cemented claystone at the site, rock excavation

equipment will be required.

If the wet well foundation is to be placed in the dry, the bearing surface should be formed with a

slight slope to create an internal sump for seepage water collection and removal. If water in

excess of 2 inches accumulates at the bottom of the excavation, it should be pumped out prior

to concrete placement. Under no circumstances should water be allowed to adversely affect the

quality of the bearing surface.

Backfill around the structure, if required, should consist of select fill soils. Backfill soils should

be compacted to at least 100 percent of the maximum dry density as evaluated by ASTM D 698

(Standard Compaction) at moisture contents between -2 and +3 percentage points of optimum

moisture content. The backfill should be placed in thin, loose lifts not to exceed 8 inches, with

compacted thickness not to exceed 6 inches.



4.3.3 Fill Materials and Placement

Select fill and and backfill used should meet the following criteria.

Fill Type 1 USCS Classification Acceptable Location for Placement

Granular select fill 2 Varies

Can be used below the mat foundation or as wet well

backfill.

Select fill CL and/or SC

(7≤PI≤20)

Can be used to construct the structure backfill and for

grade adjustments around the structure area.

On-site soils Varies

The upper on-site soils appear suitable for use as

surficial fill around the structure, provided they are free

of organics and debris. Additionally, some of the

existing soils meet the criteria for select fill soils; they

may be used as select fill within the structure area.

Flowable Fill 3 --- Confined areas.

1 Prior to any filling operations, samples of the proposed borrow and on-site materials should be

obtained for laboratory moisture-density testing. The tests will provide a basis for evaluation of fill

compaction by in-place density testing. A qualified soil technician should perform sufficient in-

place density tests during the filling operations to evaluate that proper levels of compaction,

including dry unit weight and moisture content, are being attained.

2 Granular pit-run material (caliche) meeting the criteria specified in the 2004 TxDOT Item 247, Type

B, Grade 1 thru 3 with a Plasticity Index (PI) between 7 and 20 percent may also be used as

granular select fill.

3 Flowable fill should have a 28 day strength between 80 and 150 psi and meet the requirements for

2004 TXDOT Item 401. Although usually more costly, flowable fill does not require placement in

lifts or mechanical compaction.

4.3.4 Compaction Requirements

Item Description

Fill Lift Thickness

All fill should be placed in thin, loose lifts not to exceed 8

inches, with compacted thickness of about exceed 6

inches.

Compaction of On-site and Imported

Select Fill Materials

95% of the material’s standard Proctor maximum dry

density (ASTM D 698) or as indicated otherwise in this

report.

Moisture Content of On-site Soil and

Imported Select Fill Materials

The materials should be moisture conditioned between -2

and +3 percentage points of the optimum moisture

content.

4.3.5 Grading and Drainage

Positive drainage should be provided during construction and maintained throughout the life of

the development. The potential for development of buoyant uplift forces should be mitigated to

the extent practical. Therefore, we recommend that site drainage be developed so that ponding

of surface runoff near the structure does not occur. Accumulation of water near the structure



may cause significant moisture variations in the soils adjacent to the wet-well walls, thus

increasing the potential for structural distress. Special care should be taken such that

underground utilities do not develop leaks with time.

4.3.6 Construction Considerations

It is anticipated that shallow excavations for the proposed construction can be accomplished

with conventional earthmoving equipment. Rock excavation equipment and methods will be

required for excavations extending into the claystone.

Based upon the subsurface conditions determined from the geotechnical exploration, near

surface subgrade soils exposed during construction are anticipated to be relatively unstable.

The stability of the subgrade may be affected by precipitation, repetitive construction traffic or

other factors. If unstable conditions develop, workability may be improved by scarifying and

drying. Overexcavation of wet zones and replacement with granular materials may be

necessary. Lightweight excavation equipment may be required to reduce subgrade pumping.

The contractor is responsible for designing and constructing stable, temporary excavations as

required to maintain the stability of both the excavation sides and bottom. Excavations should

be sloped or shored as required for safety following local and federal regulations, including

current OSHA excavation and trench safety standards.

4.4 Foundations

We understand that a mat foundation system will support the structural loads of the proposed

wet-well structure. Recommendations for this type of foundation system are provided in the

following sections, along with other geotechnical considerations for this project.

4.5 Mat Foundation

The mat should be analyzed using a soil-structure interaction program to identify areas of high

contact stresses, excessive movements and large moments. If a Winkler-type subgrade

modulus model is utilized to model the mat response to load, a subgrade modulus (k) of 100

pounds per cubic inch (pci) can be utilized. Contact stresses should be distributed so that yield

does not occur. The indicated bearing pressure includes a factor of safety against a bearing

capacity failure of at least 3. A value of 0.50 may be used as the ultimate coefficient of friction

between the mat and underlying soil.

If the structure being supported is subjected to uplift loading due to buoyancy or other induced

loads, an over-sized mat, extending beyond the wet well walls would provide more resistance to

uplift since the weight of the soil overlying the mat would also be included in the uplift resistance

computations. Therefore, in addition to the weight of the foundation and structure, the soil

directly overlying the foundation can be considered. A soil unit weight of 100 pounds per cubic

foot (pcf) may be assumed for the on-site soils placed above the footing, provided the fill is

properly compacted. This value should be limited to an allowable maximum passive upward

bearing pressure of 4,000 psf for properly compacted select fill.



Post construction settlements for the mat foundation designed for the indicated contact pressure

should be less than 1 inch, with differential settlements between the center and edge of the mat

foundation on the order of ½ to ¾ of an inch assuming proper construction. All piping leading to

the structure should be desiged with flexible connections to reduce potential damage due to

differential movements between the structure and the piping.

The mat may bear on a claystone subgrade prepared as recommended as shown below:

Bearing Stratum

Net Allowable End

Bearing Pressure, psf 1

Total Load Dead Load

Undisturbed Claystone 7,500 5,000 1

These bearing pressures include factors of safety of approximately 2 and 3 respectively.

4.5.1 Buoyant Uplift

The wet-well structure should be designed to withstand buoyant uplift forces. The level of

groundwater level recorded at the time of our field investigation was about 43.5 to 46 feet deep.

However, if water infiltrates and accumulates in the backfill around the structure, buoyancy

forces can develop to whatever height the water rises. We recommend that the wet-well

structure be designed to resist buoyant forces equivalent to groundwater levels at the ground

surface unless positive measures are taken to prevent water infiltration and accumulation in the

backfill around the structure.

4.6 Lateral Earth Pressures for Below Grade Structures

The design lateral earth pressure for a 30-foot diameter circular structure could be calculated as

an equivalent fluid pressure using unit weights of 80 pcf from the ground surface to a depth of

15 feet (0 to 1,200 psf) and then increasing at 12 pcf from 15 feet to a depth of 60 feet (1200 to

1,750 psf).

Depending on the type of shoring system used, the wet-well may or may not require external

backfill materials. The pressure exerted by a “veneer” layer of select backfill will be dependent

on the section thickness and material type. We should be consulted to evaluate lateral

pressures from external fill sections, if required.

If the ground surface surrounding the well is to remain open to the elements, then a protective

cover of at least 24-inches should be placed over the granular backfill to reduce surface run-off

infiltration into the backfill materials. The protective cover should consist of relatively impervious

compacted clayey soils (CL or CH) and be sloped to drain away from the structure. The on-site

sandy lean clay or lean clay with sand can be used to construct the protective cover. This

clayey soils cover should be moisture conditioned between optimum and +4 percentage points

of the optimum moisture content. The clayey soils should then be compacted to at least 95



percent of the maximum dry density determined in accordance with ASTM D 698. The clayey

soils should be placed in thin, loose lifts not to exceed 8 inches, with compacted thickness not to

exceed 6 inches.

4.7 Seismic Considerations

Description Value

2009 International Building Code Site Classification (IBC) 1 D

2

Site Latitude (wet well center) 27.44483°

Site Longitude (wet well center) -99.49250°

Maximum Considered Earthquake 0.2 second Spectral (SS) Acceleration 0.066g

Maximum Considered Earthquake 1.0 second Spectral Acceleration (S1) 0.017g

1 The site class definition was determined using SPT N-values in conjunction with Table 1613.5.2 in the 2009

IBC. The Spectral Acceleration values were determined using publicly available information provided on the

United States Geological Survey (USGS) website. The above criteria can be used to determine the Seismic

Design Category using Tables 1613.5.6 (1) and 1613.5.6 (2) in the 2006 IBC.

2 The 2009 International Building Code (IBC) requires a site soil profile determination extending to a depth of 100

feet for seismic site classification. The current scope does not include the required 100 foot soil profile

determination. The boring extended to a maximum depth of 80 feet, and this seismic site class definition

considers that hard soil continues below the maximum depth of the subsurface exploration. Additional exploration

to deeper depths would be required to confirm the conditions below the current depth of exploration.

4.8 Excavations

The comments and suggestions in this section are provided for planning and informational

purposes so that project specifications can be prepared, and to indicate conventional methods

that can be used to achieve the intent of our design recommendations. Details regarding

excavation, dewatering, selection of equipment/machinery, trafficability, project site safety,

shoring and other similar construction techniques that require "means and methods" to

accomplish the work are the sole responsibility of the project contractor. Construction means

and methods selected by the contractor may differ from those described in this report. Any

variations may significantly impact the anticipated behavior of the subsurface conditions during

the construction process.

Ancillary excavations less than 20 feet in depth should meet the requirements presented in the

following “OSHA Guidelines” section of this report. All excavation should follow OSHA, state

and federal standards and guidelines.

4.8.1 Trench Shoring

Most of the soil encountered throughout the site generally consisted of Sandy Lean Clay and

Clay with Sand materials. The soils encountered at this site were low in plasticity and moisture

content. We anticipate that caving of these soils may occur during excavation operations. The

contractor should be prepared to shore the excavation walls during construction. Safety,



trenching, and shoring are the responsibility of the contractor. The information presented in this

section provides a general guideline for the contractor.

4.8.2 Trenches

We anticipate that the trenches for the pipelines related to the wet-well will extend to depths of

approximately 50 feet below the ground surface. Groundwater was encountered at depths

shallower than the proposed excavations. Groundwater may be present at a shallower depth

after significant amount of rain fall. We anticipate that it may be difficult to effectively dewater

the Sandy Lean Clay and Lean Clay with Sand soils due to the high content of fine material (i.e.

silt and/or clay). Where excavations extend below the groundwater level in these soils, some

sloughing and/or “running” soil conditions could occur as previously discussed in this report.

Shoring will likely be necessary as discussed in the previous section.

4.8.3 Occupational Safety and Health Administration (OSHA) Guidelines

OSHA Safety and Health Standards (29 CFR Part 1926 Revised, 1989) require that all trenches

in excess of 5 feet deep be shored or appropriately sloped unless the trench sidewalls are

comprised of “solid” rock. State of Texas legislation requires that detailed plans and

specifications for trench retention systems meet OSHA standards for a safe construction

environment during utilities installation. Our recommendations are intended for use in

conjunction with OSHA safety regulations and not as a replacement of those regulations. Based

on the laboratory tests results, the soils encountered at this site should be considered as Type B

soils according to OSHA soil classification guidelines. As stated previously, OSHA requires all

soil trenches in excess of 5 feet be shored or appropriately sloped. Currently available and

practiced methods for achieving slope and/or trench wall stability includes sloping, benching,

combinations of sloping and benching, and installation of shoring systems (hydraulic, timber,

etc.). Trench shields may also be considered for use. However, these shields only provide

protection to workers; they are not a means for providing slope or trench wall stability. OSHA

addresses construction slopes in large excavations that are less than 20 feet deep. The table

shown below is a reproduction of the OSHA. The OSHA regulations define short-term as a

period of 24 hours or less.

OSHA Table

Maximum Allowable Slopes, Short-term (24 Hours or Less)

Soil or Rock Type Maximum Allowable Slopes (H:V)

1

for Excavations Less than 20 Feet Deep2

Stable Rock Vertical 90°

Type A ¾ : 1 53°

Type B 1 : 1 45°

Type C 1½ : 1 34°

1 Numbers shown in parentheses next to maximum allowable slopes are angles expressed in degrees from the horizontal. Angles have been rounded off.

2 A short-term maximum allowable slope of ½H:1V (63°) is allowed in excavations in Type A soil that are 12 feet or less in depth. Short-term maximum allowable slopes for excavations greater than 12



OSHA Table

Maximum Allowable Slopes, Short-term (24 Hours or Less)

Soil or Rock Type Maximum Allowable Slopes (H:V)

1

for Excavations Less than 20 Feet Deep2

feet in depth shall be ¾H:1V (53°).

3 Sloping or benching for excavations greater than 20 feet shall be designed by a licensed professional engineer. The OSHA regulations define short-term as a period of 24 hours or less.

4.9 Other Design/Construction Considerations

If corrosivity is critical to construction, it is recommended that laboratory testing be performed on

soil samples collected from the site.

4.9.1 Concrete Considerations

Additional testing on selected soil samples from the project site should be required to determine

chlorides, PH and sulfate content. The sulfate levels in the soil should be compared to the

criteria presented in ACI 318 Table 4.3.1. Provisions should be made to address the potential

sulfate attack when designing the concrete structures. However, Type II Portland Cement or

any other approved by ACI should be used to resist sulfate attack. Recommendations for

water-cement ratio and minimum design compressive strength are also provided by the above

mentioned ACI table.

Additionally, a source of aggregates that is known to be reactive with alkalis shall not be used in

the concrete. If a reactive aggregate must be used, measures should be taken to mitigate the

chances for alkali silica reaction. One method to achieve this is the use of a lithium nitrate

admixture along with a low alkali cement.



5.0 GENERAL COMMENTS Terracon should be retained to review the final design plans and specifications so comments

can be made regarding interpretation and implementation of our geotechnical recommendations

in the design and specifications. Terracon also should be retained to provide observation and

testing services during grading, excavation, foundation construction and other earth-related

construction phases of the project.

The analysis and recommendations presented in this report are based upon the data obtained

from the boring performed at the indicated location and from other information discussed in this

report. This report does not reflect variations that may occur at the boring, or due to the

modifying effects of weather. The nature and extent of such variations may not become evident

until during or after construction. If variations appear, we should be immediately notified so that

further evaluation and supplemental recommendations can be provided.

The scope of services for this project does not include either specifically or by implication any

environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or

prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the

potential for such contamination or pollution, other studies should be undertaken.

This report has been prepared for the exclusive use of our client for specific application to the

project discussed and has been prepared in accordance with generally accepted geotechnical

engineering practices. No warranties, either express or implied, are intended or made. Site

safety, excavation support, and dewatering requirements are the responsibility of others. In the

event that changes in the nature, design, or location of the project as outlined in this report are

planned, the conclusions and recommendations contained in this report shall not be considered

valid unless Terracon reviews the changes and either verifies or modifies the conclusions of this

report in writing.


Responsive ■ Resourceful ■ Reliable Table 1

TABLE 1

SOIL PROPERTIES SOUTH LAREDO WWTP WET-WELL ADDITION

RIVER FRONT STREET

LAREDO, TEXAS

TERRACON PROJECT NO. 89115043

Layer Depth, ft. Cohesion,

psf

Total Unit

Weigth, pcf

Effective

Unit Weight,

pcf

Friction

Angle,

degrees

In-Situ

Modulus E’n

,psi

1 0 – 5 1,000 115 115 --- 450

2 5 – 10 2,000 115 115 --- 750

3 10 – 20 3,000 120 120 --- 1,500

4 20 – 43 3,000 120 120 --- 1,500

5 43 – 50 4,000 120 58 --- 2,000

6 50 – 80 5,000 125 63 --- 2,000

1. Design depth to groundwater is assumed to be at 43 feet.



APPENDIX A

FIELD EXPLORATION


Responsive ■ Resourceful ■ Reliable Exhibit A-1

Field Exploration Description

Terracon personnel used the site plan provided by the client to identify the bore location in the

field. A copy of the Bore Location Plan indicating the approximate boring location is included in

Appendix A. The boring location was staked by others prior to our arriving at the site.

A truck-mounted, rotary drill rig equipped with continuous flight augers was used to advance the

borehole. Soil samples were obtained by the split-barrel sampling procedure. In the split-barrel

sampling procedure, a standard 2-inch O.D. split-barrel sampling spoon is driven into the

ground with a 140-pound hammer falling a height of 30 inches. The number of blows required to

advance the sampling spoon the last 12 inches of a normal 18-inch penetration is recorded as

the standard penetration resistance value. These values are indicated on the boring log at the

depth of occurrence. The samples were sealed and transported to our laboratory for testing and

classification.

Our field representative prepared the field log as part of the drilling operations. The field log

included visual classifications of the materials encountered during drilling and our field

representative interpretation of the subsurface conditions between samples. The boring log

included with this report represent the engineer’s interpretation of the field log and include

modifications based on visual observations and testing of the samples in the laboratory.

The scope of services for our geotechnical engineering services does not include addressing

any environmental issues pertinent to the site.

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PROJECT SITE l

Project Mngr: MMR

Project No. 89115043

Drawn By: Scale: LS(90) N.T.S.

Checked By: File No. 89115043 EAM

Approved Br. Date:

EAM 09-20-11

n :r: m /./1

if 0 O ~ J~ L RIVEIU RONT ST i

{

llerracon ~ ---Ind 8clenlllla

61 5 GALE STREET, BUILDING B LAREDO, TX 78041

PH. (956) 729,1100 FAX. (956) 791-1071

LJ I

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SITE LOCATION PLAN EXHIBIT

Wet Well Addition South Laredo WWTP

Laredo, Texas A-2

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Project No. 89115043 llerracon

BORE LOCATION PLAN Scale:

N.T.S. Wet Well Addition Fi~No.

89115043 ~ --.111c18clenlllll South Laredo WWTP Date: 61 5 GALE STREET, BUILDING B LAREDO, TX 78041 Laredo, Texas

09-20-11 PH. (956) 729-1100 FAX. (956) 791-1071

D

K

EXHIBIT

A-3

10.0

STRATUM ISANDY LEAN CLAY; brown, stiff to hard,silty sand with gravel to 2 feet.

- with SANDY SILT (ML) layers at 8.5feet.STRATUM IALEAN CLAY with SAND; brown, stiff tohard

- with LEAN CLAY (CL) layers at 33.5feet.

6

9

NP

13

65

78

89

SS

SS

SS

SS

SS

SS

SS

SS

SS

SS

SS

19

17

19

N=41

N=31

N=25

N=14

N=12

N=10

N=10

N=19

N=17

N=26

N=37

3

6

6

6

4

9

11

12

16

18

18

25

26

32

CL

CL

Approx. Surface Elevation: Existing GradeGra

phic

Log

Continued Next Page

DESCRIPTION

CDMSan Antonio,, Texas

46 ft

See Bore Location Plan

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EXHIBIT

A-4PROJECT NUMBER

89115043

LOG OF BORING NO. B-1

BORINGLOCATION:

Remarks - Dry augered to 50 feet, wet rotary techniques thereafter. Free water wasencountered at 46 feet. The borehole was backfilled with cuttings after the groundwaterobservations were completed.NP: Non Plastic140 lbs. SPT manual hammer.

STRATIFICATION LINES REPRESENT APPROXIMATEBOUNDARIES BETWEEN SOIL TYPES. IN SITU, THETRANSITION BETWEEN STRATA MAY BE MOREGRADUAL.

CLIENT:

43.5 ft

WATER LEVEL OBSERVATIONS Page 1 of 2DATE DRILLED

8/29/2011

US

CS

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Laredo, TexasSAMPLES

5

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15

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35

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/FT

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/SQ

FT

T:

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48.5

80.0

STRATUM IALEAN CLAY with SAND; brown, stiff tohard

- with ferrous stains at 43.5 feet.

STRATUM IICLAYSTONE; dark bluish gray, hard,highly cemented

Boring Terminated at about 80 feet.

14

5

10

21

30

SS

SS

SS

SS

SS

SS

SS

SS

20

23

23

16

N=19

N=ref/1"

N=ref/2"

N=ref/2"

N=ref/4"

N=ref/4"

N=ref/3"

N=ref/3"

23

17

31

20

28

31

19

34

28

33

37

Approx. Surface Elevation: Existing GradeGra

phic

Log DESCRIPTION

CDMSan Antonio,, Texas

46 ft

See Bore Location Plan

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EXHIBIT

A-4PROJECT NUMBER

89115043

LOG OF BORING NO. B-1

BORINGLOCATION:

Remarks - Dry augered to 50 feet, wet rotary techniques thereafter. Free water wasencountered at 46 feet. The borehole was backfilled with cuttings after the groundwaterobservations were completed.NP: Non Plastic140 lbs. SPT manual hammer.

STRATIFICATION LINES REPRESENT APPROXIMATEBOUNDARIES BETWEEN SOIL TYPES. IN SITU, THETRANSITION BETWEEN STRATA MAY BE MOREGRADUAL.

CLIENT:

43.5 ft

WATER LEVEL OBSERVATIONS Page 2 of 2DATE DRILLED

8/29/2011

US

CS

SY

MB

OL

PLA

ST

ICIT

Y I

ND

EX

CO

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ST

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SITE:

PROJECT:

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MIN

US

#20

0S

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%

Laredo, TexasSAMPLES

45

50

55

60

65

70

75

80

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/FT

P:

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/SQ

FT

T:

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/SQ

FT



APPENDIX B

LABORATORY TESTING


Responsive ■ Resourceful ■ Reliable Exhibit B-1

Laboratory Testing

Samples retrieved during the field exploration were taken to the laboratory for further

observation by the project geotechnical engineer and were classified in accordance with the

Unified Soil Classification System (USCS) described in this Appendix. At that time, the field

descriptions were confirmed or modified as necessary and an applicable laboratory testing

program was formulated to determine engineering properties of the subsurface materials.

Laboratory tests were conducted on selected soil samples and the test results are presented in

this appendix. The laboratory test results were used for the geotechnical engineering analyses,

and the development of foundation and earthwork recommendations. Laboratory tests were

performed in general accordance with the applicable ASTM, local or other accepted standards.

Selected soil samples obtained from the site were tested for the following engineering

properties:

In-situ Water Content

Atterberg Limits

Amount of Material In-Soil Finer than the No 200 Mesh (75-µm) Sieve

Sample Disposal

All samples were returned to our laboratory. The samples not tested in the laboratory will be

stored for a period of 30 days subsequent to submittal of this report and will be discarded after

this period, unless other arrangements are made prior to the disposal period.



APPENDIX C

SUPPORTING DOCUMENTS

Exhibit C-1

GENERAL NOTES

DRILLING & SAMPLING SYMBOLS:

SS: Split Spoon – 1-3/8" I.D., 2" O.D., unless otherwise noted HS: Hollow Stem Auger

ST: Thin-Walled Tube - 2" O.D., unless otherwise noted PA: Power Auger

RS: Ring Sampler - 2.42" I.D., 3" O.D., unless otherwise noted HA: Hand Auger

DB: Diamond Bit Coring - 4", N, B RB: Rock Bit

BS: Bulk Sample or Auger Sample WB: Wash Boring or Mud Rotary

The number of blows required to advance a standard 2-inch O.D. split-spoon sampler (SS) the last 12 inches of the total 18-inch

penetration with a 140-pound hammer falling 30 inches is considered the “Standard Penetration” or “N-value”.

WATER LEVEL MEASUREMENT SYMBOLS:

WL: Water Level WS: While Sampling N/E: Not Encountered

WCI: Wet Cave in WD: While Drilling

DCI: Dry Cave in BCR: Before Casing Removal

AB: After Boring ACR: After Casing Removal

Water levels indicated on the boring logs are the levels measured in the borings at the times indicated. Groundwater levels at other times and other locations across the site could vary. In pervious soils, the indicated levels may reflect the location of groundwater. In low permeability soils, the accurate determination of groundwater levels may not be possible with only short-term observations.

DESCRIPTIVE SOIL CLASSIFICATION: Soil classification is based on the Unified Classification System. Coarse Grained Soils

have more than 50% of their dry weight retained on a #200 sieve; their principal descriptors are: boulders, cobbles, gravel or sand. Fine Grained Soils have less than 50% of their dry weight retained on a #200 sieve; they are principally described as clays if they are plastic, and silts if they are slightly plastic or non-plastic. Major constituents may be added as modifiers and minor constituents may be added according to the relative proportions based on grain size. In addition to gradation, coarse-grained soils are defined on the basis of their in-place relative density and fine-grained soils on the basis of their consistency.

CONSISTENCY OF FINE-GRAINED SOILS RELATIVE DENSITY OF COARSE-GRAINED SOILS

Unconfined

Compressive

Strength, Qu, psf

Standard Penetration or N-value (SS)

Blows/Ft. Consistency

Standard Penetration or N-value (SS)

Blows/Ft.

Ring Sampler (RS) Blows/Ft.

Relative Density

< 500 <2 Very Soft 0 – 3 0-6 Very Loose

500 – 1,000 2-3 Soft 4 – 9 7-18 Loose

1,001 – 2,000 4-6 Medium Stiff 10 – 29 19-58 Medium Dense

2,001 – 4,000 7-12 Stiff 30 – 49 59-98 Dense

4,001 – 8,000 13-26 Very Stiff 50+ 99+ Very Dense

8,000+ 26+ Hard

RELATIVE PROPORTIONS OF SAND AND GRAVEL GRAIN SIZE TERMINOLOGY

Descriptive Term(s) of other

Constituents

Percent of

Dry Weight

Major Component

of Sample Particle Size

Trace < 15 Boulders Over 12 in. (300mm)

With 15 – 29 Cobbles 12 in. to 3 in. (300mm to 75 mm)

Modifier > 30 Gravel 3 in. to #4 sieve (75mm to 4.75 mm)

Sand

Silt or Clay

#4 to #200 sieve (4.75mm to 0.075mm)

Passing #200 Sieve (0.075mm)

RELATIVE PROPORTIONS OF FINES PLASTICITY DESCRIPTION

Descriptive Term(s) of other

Constituents

Percent of

Dry Weight Term

Plasticity

Index

Trace < 5 Non-plastic 0

With 5 – 12 Low 1-10

Modifiers > 12 Medium 11-30

High 30+

Exhibit C-2

UNIFIED SOIL CLASSIFICATION SYSTEM

Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests A

Soil Classification

Group

Symbol Group Name

B

Coarse Grained Soils:

More than 50% retained

on No. 200 sieve

Gravels:

More than 50% of

coarse

fraction retained on

No. 4 sieve

Clean Gravels:

Less than 5% fines C

Cu 4 and 1 Cc 3 E

GW Well-graded gravel F

Cu 4 and/or 1 Cc 3 E

GP Poorly graded gravel F

Gravels with Fines:

More than 12% fines C

Fines classify as ML or MH GM Silty gravel F,G, H

Fines classify as CL or CH GC Clayey gravel F,G,H

Sands:

50% or more of coarse

fraction passes

No. 4 sieve

Clean Sands:

Less than 5% fines D

Cu 6 and 1 Cc 3 E

SW Well-graded sand I

Cu 6 and/or 1 Cc 3 E

SP Poorly graded sand I

Sands with Fines:

More than 12% fines D

Fines classify as ML or MH SM Silty sand G,H,I

Fines Classify as CL or CH SC Clayey sand G,H,I

Fine-Grained Soils:

50% or more passes the

No. 200 sieve

Silts and Clays:

Liquid limit less than 50

Inorganic: PI 7 and plots on or above “A” line

J CL Lean clay

K,L,M

PI 4 or plots below “A” line J ML Silt

K,L,M

Organic: Liquid limit - oven dried

0.75 OL Organic clay

K,L,M,N

Liquid limit - not dried Organic silt K,L,M,O

Silts and Clays:

Liquid limit 50 or more

Inorganic: PI plots on or above “A” line CH Fat clay

K,L,M

PI plots below “A” line MH Elastic Silt K,L,M

Organic: Liquid limit - oven dried

0.75 OH Organic clay

K,L,M,P

Liquid limit - not dried Organic silt K,L,M,Q

Highly organic soils: Primarily organic matter, dark in color, and organic odor PT Peat

A Based on the material passing the 3-in. (75-mm) sieve

B If field sample contained cobbles or boulders, or both, add “with cobbles

or boulders, or both” to group name. C

Gravels with 5 to 12% fines require dual symbols: GW-GM well-graded

gravel with silt, GW-GC well-graded gravel with clay, GP-GM poorly

graded gravel with silt, GP-GC poorly graded gravel with clay. D

Sands with 5 to 12% fines require dual symbols: SW-SM well-graded

sand with silt, SW-SC well-graded sand with clay, SP-SM poorly graded

sand with silt, SP-SC poorly graded sand with clay

E Cu = D60/D10 Cc =

6010

2

30

DxD

)(D

F If soil contains 15% sand, add “with sand” to group name.

G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.

H If fines are organic, add “with organic fines” to group name.

I If soil contains 15% gravel, add “with gravel” to group name.

J If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay.

K If soil contains 15 to 29% plus No. 200, add “with sand” or “with

gravel,” whichever is predominant. L

If soil contains 30% plus No. 200 predominantly sand, add “sandy”

to group name. M

If soil contains 30% plus No. 200, predominantly gravel, add

“gravelly” to group name. N

PI 4 and plots on or above “A” line. O

PI 4 or plots below “A” line. P

PI plots on or above “A” line. Q

PI plots below “A” line.

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