geotechnical investigation phase ii final … · sewpp sedimentation basin a & b improvements...

GEOTECHNICAL INVESTIGATION PHASE II FINAL DESIGN FOR ENGINEERING

SEWPP SEDIMENTATION BASIN A & B IMPROVEMENTS (TM1), FLOCCULATORS REHABILITATION BASINS (TM1), REHABILITATION OF

TRANSFER AND DISTRIBUTION PUMPS & MOTORS AND ON-CALL MISCELLANEOUS ENGINEERING SERVICES

WBS NO. S-000012-0028-3

THICKENED SLUDGE PUMP STATION NO. 2 AND SPARE PARTS BUILDING

Report No. 1140187501

Reported to:

CHESTER ENGINEERS, INC.

Houston, Texas

Reported by:

GEOTEST ENGINEERING, INC.

Houston, Texas

Date: October 10, 2013

Key Map Nos. 577 R

TABLE OF CONTENTS Page EXECUTIVE SUMMARY.............................................................................................. 1

1.0 INTRODUCTION

1.1 General ..................................................................................................... 4

1.2 Project Description ................................................................................... 4

1.3 Purpose and Scope ................................................................................... 4

2.0 FIELD EXPLORATION ...................................................................................... 6

3.0 LABORATORY TESTS...................................................................................... 8

4.0 GENERAL SUBSURFACE CONDITIONS ....................................................... 9

4.1 Thickened Sludge Pump Station No. 2 (Boring GB-1) ........................... 9

4.2 Parts Building (Boring GB-2) ................................................................. 10

4.3 Faults ........................................................................................................ 10

5.0 GEOTECHNICAL ENGINEERING RECOMMENDATIONS ......................... 11

5.1 General ..................................................................................................... 11

5.2 Thickened Sludge Pump Station No. 2 ................................................... 11

5.2.1 General Parameters ........................................................................ 11

5.2.2 Excavation Stability....................................................................... 11

5.2.3 Excavation Dewatering ................................................................. 13

5.2.4 Foundation Recommendations ...................................................... 14

5.3 Parts Building ........................................................................................... 15

5.3.1 Foundation Type, Depth and Allowable Bearing Pressure ........... 15

5.3.2 Foundation Settlement ................................................................... 16

5.3.3 Site Preparation and Structural Fill Requirements ........................ 16

5.3.4 Building Pad .................................................................................. 17

5.3.5 Floor Slabs ..................................................................................... 17

5.3.6 Landscaping ................................................................................... 17

5.3.7 Surface Drainage ........................................................................... 17

6.0 CONSTRUCTION CONSIDERATIONS ........................................................... 18

7.0 PROVISIONS ...................................................................................................... 19

ILLUSTRATIONS Figure

Vicinity Map ................................................................................................................. 1

Plan of Borings ............................................................................................................. 2

Boring Log Profile ........................................................................................................ 3

Symbols and Abbreviations Used on Boring Log Profile ............................................. 4

Excavation Support Earth Pressure ............................................................................... 5.1 and 5.2

Stability of Bottom for Braced Cut ............................................................................... 6

Lateral Earth Pressure Diagram for Permanent Wall .................................................... 7.1 and 7.2

Uplift Pressure and Resistance ...................................................................................... 8

TABLE

Table Geotechnical Design Parameter Summary ............................................................ 1

APPENDIX A

Figure

Log of Borings from this study ............................................................................. A-1 and A-2

Symbols and Terms Used on Boring Logs............................................................ A-3

Piezometer Installation Report .............................................................................. A-4

APPENDIX B

Figure

Summary of Laboratory Tests ............................................................................... B-1 and B-2

Grain Size Distribution Curves ............................................................................. B-3

APPENDIX C

Piezometer Abandonment Report

Geotest Engineering, Inc. Report No. 1140187501 City of Houston Southeast Water Purification Plant Improvements October 10, 2013 Thickened Sludge Pump Station No. 2 and Spare Parts Building WBS No. S-000012-0028-3, Houston, Texas

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EXECUTIVE SUMMARY

A geotechnical investigation was conducted in connection with the design and construction

of the Thickened Sludge Pump Station (TSPS) No. 2 and parts building at Southeast Water

Purification Plant in Houston, Texas. The Thickened Sludge Pump Station No. 2 is approximately

27 feet x 27 feet in plan dimension. The bottom of the TSPS #2 will be placed at 22-foot deep (El.

9.50). The parts building is a one-story masonry or precast concrete building with an approximate

plan dimension of 50 feet x 30 feet. It is understood that a 10-ton bridge crane will be configured in

the building with an eave height of about 25 to 30 feet.

This study included drilling and sampling one (1) 45-foot deep boring (GB-1) for TSPS #2

and one (1) 25-foot deep boring (GB-2) for the parts building, performing laboratory tests,

performing engineering analyses and preparing a geotechnical report.

The principal findings and conclusions developed from this investigation are summarized

as follows:

• Based on the available information, the nearest known surface fault is Library Fault and

is located approximately 2.5 miles south of the project site.

• The boring GB-2 was drilled in the existing concrete pavement. The existing pavement

as encountered at boring GB-2 consists of 9.375 inches of concrete over 2 inches of lime

stabilized clay and gravel mix. The boring GB-1 was drilled in the grass area.

• The subsurface conditions are given below.

Thickened Sludge Pump Station No. 2 (Boring GB-1) : The subsurface soils below the

existing grade, as encountered in boring GB-1, consist of medium stiff to hard gray,

yellowish brown and reddish brown and gray Fat Clay, Lean Clay and Lean Clay with

sand to a depth of 45 feet, the termination depth of the boring. A layer of medium dense

silty sand was encountered between the depths of 16 and 18 feet. Fill material


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consisting of hard dark gray lean clay with sand and grass roots was encountered to a

depth of 2 feet.

Parts Building (Boring GB-2): The subsurface soils below the existing pavement, as

encountered in boring GB-2, consist of soft to very stiff gray and yellowish brown

Sandy Lean Clay to a depth of 6 feet. The sandy lean clay is underlain by loose to

medium dense silty sand to a depth of 25 feet, the termination depth of the boring.

• Free water was first encountered at depth of 16.0 feet in boring GB-1 and 6 feet in

boring GB-2. The water level, measured 15 minutes after water was first encountered,

was at a depth of about 9.7 feet in boring GB-1 and 4 feet in boring GB-2. The

groundwater was measured at a depth of about 6.4 feet in piezometer GB-1P on August

13, 2013.

• All excavation operations should be carried out in accordance with OSHA standards and

the City of Houston Standard Specifications.

• In general, excavation and backfill for utilities should be designed and constructed in

accordance with City of Houston Standard Specification No. 02317.

• Based on the soil conditions revealed by the boring GB-1, the mat foundation for

supporting the thickened sludge pump station placed at a depth of 22 feet (El. 9.5 feet) (into

very stiff fat clay) may be designed for an allowable (net) bearing pressure of 6,000 psf for

total loads. These allowable bearing pressures include a safety factor of 2.0. The above

recommendations assume that the final bearing surfaces consist of undisturbed natural soils

and that underlying semi-transmissive zones are properly pressure-relieved and stable

undisturbed bearing surfaces are attained.

In view of soft soils encountered at shallow depth (4 to 6 feet) in boring GB-2, shallow

foundations are not recommended for supporting the parts building. The proposed parts


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building foundation can be supported on (straight) drilled footings (shafts). Drilled

footings should be placed at a depth of at least 8 feet and proportioned for an allowable

bearing pressure of 3,000 psf for total dead and sustained loads or 2,000 psf for dead and

sustained live loads, whichever results in a larger footing. These allowable bearing

pressures contain safety factors of 2 for total load and 3 for sustained load conditions.

• Construction considerations are included in Section 6.0 of this report.


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1.0 INTRODUCTION

1.1 General A geotechnical investigation was conducted in connection with the design and construction

of the Thickened Sludge Pump Station (TSPS) No. 2 and parts building at Southeast Water

Purification Plant in Houston, Texas.

1.2 Project Description

The Thickened Sludge Pump Station No. 2 is approximately 27 feet x 27 feet in plan

dimension. The bottom of the TSPS #2 will be placed at a depth of 22-foot deep (El. 9.50). The

parts building is a one-story masonry or precast concrete building with an approximate plan

dimension of 50 feet x 30 feet. It is understood that a 10-ton bridge crane will be configured in the

building with an eave height of about 25 to 30 feet. A Vicinity Map is shown on Figure 1.

1.3 Purpose and Scope

The purposes of this investigation were to explore the subsurface conditions at the proposed

site and to develop geotechnical recommendations pertinent to the design and construction of the

proposed thickened Sludge Pump Station No. 2 and parts building. The scope of work consists of

the following tasks:

1. drilling and sampling one (1) 45-foot boring for the TSPS location and one (1) 25-foot

boring for the parts building;

2. performing laboratory tests to determine the physical and engineering properties of the

subsurface soils;

3. performing engineering analyses to develop geotechnical recommendations for the

proposed TSPS #2 and parts building foundation, groundwater control, uplift


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resistance, pavement section and subgrade stabilization and construction

considerations; and

4. preparation of a geotechnical report including field and laboratory data and engineering

recommendations.


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2.0 FIELD EXPLORATION

Subsurface conditions were explored by drilling 2 soil borings, designated as GB-1 and

GB-2. Boring GB-1 was drilled for the Proposed Thickened Sludge Pump Station No. 2 to a depth

of 45 feet and boring GB-2 was drilled for the new parts building to a depth of 25 feet. All the

borings were drilled with a truck-mounted rotary drilling rig. The approximate locations of borings

are shown on Figure 2, Plan of Borings.

Samples were obtained continuously to 45 feet for boring GB-1 and 20-foot depth and at 5-

foot intervals thereafter for boring GB-2. In general, samples of cohesive soils were obtained with a

3-inch diameter thin-walled tube sampler in accordance with ASTM D1587 and samples of granular

soils were obtained with a 2-inch diameter split-barrel sampler in general accordance with ASTM

Method D 1586. Each sample was removed from the sampler in the field, carefully examined and

logged by an experienced soils technician. Suitable portions of each sample were sealed and

packaged for transportation to Geotest's laboratory. The shear strength of cohesive soil samples was

estimated by use of a calibrated pocket penetrometer in the field. Driving resistances for the split-

barrel sampler were recorded as "Blows per Foot" on the boring logs. All the borings, except the

ones converted to piezometers, were grouted with cement-bentonite grout after completion of drilling

and obtaining water level measurements.

Detailed descriptions of the soils encountered in the borings are given on the boring logs

presented on Figures A-1 and A-2 in Appendix A. A key to the symbols and terms used on the

boring logs is given on Figure A-3 in Appendix A.

During the field investigation, a piezometer was installed in the open borehole of boring

GB-1. The location of the piezometer, designated as GB-1P is shown on Figure 2, Plan of Borings.

The piezometer installation report showing the details of the construction of the piezometer is

provided on Figure A-4 in Appendix A.


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Groundwater level observations were made at each boring location during field investigation.

The results of these observations are noted on the boring logs.


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3.0 LABORATORY TESTS

The laboratory testing program was designed to evaluate the pertinent physical properties and

shear strength characteristics of the subsurface soils. Classification tests were performed on selected

samples to aid in soil classification. All geotechnical tests were performed in accordance with

appropriate ASTM procedures.

The undrained shear strengths of selected cohesive samples were estimated by unconsolidated

undrained (UU) triaxial compression (ASTM D2850) tests. The results of UU triaxial compression

tests are plotted on the boring logs as solid squares. Estimated shear strengths of the cohesive

samples were also evaluated in the field with a calibrated hand penetrometer and also in the

laboratory with a Torvane. The shear strength values obtained from the penetrometer and Torvane

are plotted on the boring logs as open circles and triangles, respectively.

Water content and dry unit weight of soil samples were determined as a part of the UU

triaxial compression tests. Water content determinations (ASTM D2216) were also made on all

other samples to define the moisture profile at the boring locations. Liquid and plastic limit (ASTM

D4318) tests were performed on appropriate samples. The percent passing sieve No. 200 (ASTM

D1140) were performed for selected samples. The results of all the tests are plotted or summarized

on the boring logs presented on Figures A-1 and A-2.


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4.0 GENERAL SUBSURFACE CONDITIONS

As revealed by the boring logs, the subsurface soils at each proposed improvement are

described below.

4.1 Thickened Sludge Pump Station No. 2 (Boring GB-1)

Depth Below Ground Surface, ft Description

0 – 2 Fill: hard dark gray lean clay with sand calcareous

and ferrous nodules

2 – 12 Very stiff to hard gray Lean Clay (CL) with sand,

calcareous and ferrous nodules

12 – 16 Stiff brown Lean Clay (CL) with silt seams

16 – 18 Medium dense brown Silty Sand (SM)

18 – 45 Very stiff to hard reddish brown and gray Fat Clay

(CH) with ferrous nodules

The Fat Clay soils are of very high plasticity with liquid limits ranging from 74 to 98 and

plasticity indices ranging from 64 to 47. The Lean Clay and Lean Clay with sand is of medium

plasticity with liquid limits ranging from 28 to 38 and plasticity indices ranging from 10 and 20. The

fines content (passing No. 200 sieve) of Fat Clay ranged from 93 to 100 percent. The fines content

of Lean Clay and Lean Clay with sand ranged from 76 to 91 percent. The fines content of Silty Sand

is about 47 percent.

Free water was first encountered at 16 feet during drilling and measured at 7.3 feet at 15

minutes after water was first encountered. The water level measured in piezometer was measured at

depth of about 6.4 feet on August 13, 2013. However, groundwater levels can be expected to vary

seasonally with changes in precipitation, nearby subsurface construction activities, and change in

area drainage.


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Piezometer was abandoned in place after taking the final water level measurements. The

piezometer abandonment report is presented in Appendix C.

4.2 Parts Building (Boring GB-2)

Depth Below Ground Surface, ft Description

0 – 6 Soft to very stiff gray Sandy Lean Clay (CL) with sand seams

6 – 25 Loose to medium dense gray Silty Sand (SM)

The Sandy Lean Clay soil is of high plasticity with liquid limits ranging from 41 to 43 and

plasticity indices ranging from 22 to 23. The fines content (percent passing No. 200 sieve) of Sandy

Lean Clay is about 61 percent. The fines content of Silty Sand ranges from 20 to 24 percent.

Free water was first encountered at 6 feet during drilling and measured at 4 feet at 15 minutes

after water was first encountered. However, groundwater levels can be expected to vary seasonally

with changes in precipitation, nearby subsurface construction activities, and change in area drainage.

4.3 Faults

The available library information consisted of U.S. Geological Survey and NASA maps, open

file reports, and information contained in our files relating to geologic faults in this area. Based on

this available information no known faults exist within the project site and the nearest known surface

fault is Library Fault and is located approximately 2.5 miles south of the project site.


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5.0 GEOTECHNICAL ENGINEERING RECOMMENDATIONS

5.1 General


dimension. The bottom of the TSPS #2 will be placed at a depth of 22 feet (El. 9.50). The parts

building is a one-story masonry or precast concrete building with an approximate plan dimension of

50 feet x 30 feet. It is understood that a 10-ton bridge crane will be configured in the building with

an eave height of about 25 to 30 feet.

5.2 Thickened Sludge Pump Station No. 2


dimension. The bottom of the TSPS #2 will be placed at 22-foot deep (El. 9.50).

5.2.1 Geotechnical Parameters. Based on the soil conditions revealed by the boring GB-1,

geotechnical parameters were developed for the design of open excavation for the thickened sludge

pump station. The design parameters are provided in Table 2. For design, the groundwater level

should be assumed to exist at the ground surface.

5.2.2 Excavation Stability. The open excavation may be shored or laid back to a stable slope

or supported by some other equivalent means used to provide safety for workers and adjacent

structures, if any. The excavating operations should be in accordance with OSHA Standards, OSHA

2207, Subpart P, latest revision and the City of Houston Standard Specification.

• Excavation Shallower Than 5 Feet - Excavations that are less than 5 feet deep (critical

height) should be effectively protected when an indication of dangerous ground movement is

anticipated.


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• Excavations Deeper Than 5 Feet. For the excavation deeper than 5 feet, such as 22 feet in

this case, the excavation should be shored, sheeted, braced, or supported by some other equivalent

means of protection such that workers are not exposed to moving ground or cave-ins. The shoring

should be in accordance with OSHA requirements.

A soil retention system is recommended and essential for this project. Whichever system is

used should remain in place until backfilling is within 5 feet of the ground surface. Based on the soil

conditions and proposed excavation 22-foot deep, the following alternatives can be considered for

soil retention.

1. Temporary sheet piles

2. H-piles with wooden lagging

Sheet piles may be driven or vibrated in place.

The following items provide design criteria for excavation stability.

(i) OSHA Soil Type. Based on the soil conditions revealed by the geotechnical borings,

OSHA’s soil type “C” should be used for the design of a shoring system. For shoring

deeper than 20 feet, an engineering evaluation is required.

(ii) Excavation Support Earth Pressure. Lateral earth pressure diagram was developed based

on the subsurface conditions indicated by our field and laboratory investigations. The

earth pressure diagrams developed for excavation support are presented on Figures 5.1

and 5.2. The pressure diagram can be used for the design of temporary excavation

bracing. The effects of any surcharge loads at the ground surface should be added to the

computed lateral earth pressures. A surcharge load, q, will typically result in a lateral

load equal to 0.5q. The computation of the equivalent fluid pressure assumes that

groundwater level is at ground surface, since these conditions may exist after a heavy rain


13

or flooding. For the design of the shoring or bracing, for resisting lateral earth pressures

can be based on an equivalent fluid pressure of 96 psf.

If H-piles with wooden lagging are planned, the piles should penetrate at least 10 feet below

the bottom of the excavation with a bracing at about 6 feet from the ground surface.

(iii) Bottom Stability. In braced cuts, if tight sheeting is terminated at the base of the cut, the

bottom of the excavation can become unstable under certain conditions. The stability of the bottom

of the excavation is governed by the shear strength of the soils and by the differential hydrostatic

head. For cuts in cohesive soils (such as lean clay and lean clay with sand), stability of the bottom

may be evaluated in accordance with the procedure outlined in Figure 6. In cohesionless soils (such

as silty sand encountered in borings between depths of 16 and 18 feet), the excavation should be

done after dewatering or installing sheet pile cut off wall to avoid bottom stability problems.

5.2.3 Excavation Dewatering. Excavations for the proposed TSPS #2 structure will

encounter groundwater seepage to varying degrees depending upon groundwater conditions at the

time of construction. In cohesive soils (such as lean clay, and sandy lean clay), groundwater may be

managed by collection in excavation bottom sumps for pumped disposal. The trench excavation up

to 22 feet will be formed in cohesive soils with water bearing sands (approximately 2-foot thick) near

the bottom of the excavation. The groundwater may be controlled by using eductor well system if

can be successfully lowered 5 feet below the excavation bottom otherwise or alternatively installing

continuous interlock (water tight) sheet piling with trench bottom sumps for pumped disposal.

It is recommended that the actual groundwater conditions be verified at the time of

construction and that groundwater control be performed in general accordance with City of Houston

Standard Specifications, Section 01578, “Control of Ground Water and Surface Water.”


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5.2.4 Foundation Recommendations

5.2.4.1 Foundation Conditions. Foundation conditions were explored by boring GB-1.

Based on the soil conditions revealed by this boring, the bottom of the TSPS #2 will be in very stiff

Fat Clay.

5.2.4.2 Foundation Design Recommendations. The following items provide

recommendations and design criteria for construction of the TSPS #2 structure.

• Allowable Bearing Pressures. Based on the soil conditions, the mat foundation for

supporting the thickened sludge pump station placed at a depth of 22 feet (El. 9.5 feet)

(into very stiff fat clay) may be designed for an allowable (net) bearing pressure of

6,000 psf for total loads. These allowable bearing pressures include a safety factor of

2.0. The above recommendations assume that the final bearing surfaces consist of

undisturbed natural soils and that underlying semi-transmissive zones are properly

pressure-relieved and stable undisturbed bearing surfaces are attained

Based on mat foundation depths of 22 feet (bottom of footing into very stiff clay) the

modulus of subgrade reaction, K will be 50 pci.

• Lateral Earth Pressure. The pressure diagrams presented on Figures 5.1 and 5.2 can

be used for the design of braced excavation. The lateral earth pressure diagrams

presented on Figures 7.1 and 7.2 are applicable for the design of the permanent walls.

• Hydrostatic Uplift Resistance. Structures extending below the groundwater level

should be designed to resist uplift pressure resulting from excess piezometric head.

Design uplift pressures should be computed based on the assumption that the water

table is at ground surface. To resist the hydrostatic uplift at the bottom of the

structure, one of the following sources of resistance can be utilized in each of the

designs.


15

a. Dead weight of structure,

b. Weight of soil above base extensions plus weight of structure, or

c. Soil-wall friction plus dead weight of structure.

The uplift force and resistance to uplift should be computed as detailed on Figure 8.

In determining the configuration and dimensions of the structure using one of the

approaches presented on Figure 8, the following factors of safety are recommended.

a. Dead weight of concrete structure, Sf1 = 1.10,

b. Weight of soil (backfill) above base extension, Sf2 = 1.5, and

c. Soil-wall friction, Sf3 = 3.0.

Friction resistance should be discounted for the upper 5 feet, since this zone is

affected by seasonal moisture changes.

5.3 Parts Building

The parts building is a one-story masonry or precast concrete building with an approximate

plan dimension of 50 feet x 30 feet. It is understood that a 10-ton bridge crane will be configured in

the building with an eave height of about 25 to 30 feet.

5.3.1 Foundation Type, Depth and Allowable Bearing Pressure. As revealed by boring GB-2,

the surficial soils consist of soft to medium stiff Sandy Lean Clay. Hence, the slab-on-grade or

shallow foundations are not recommended for the crane and building foundation. The proposed

building can be supported on (straight) drilled footings (shaft). Drilled footings should be placed at a

depth of at least 8 feet and proportioned for an allowable bearing pressure of 3,000 psf for total dead

and sustained loads or 2,000 psf for dead and sustained live loads, whichever results in a larger

footing. These allowable bearing pressures contain safety factors of 2 for total load and 3 for

sustained load conditions.


16

Pier reinforcement should be checked to ensure that design provides sufficient resistance to

tensile loads imposed due to swell of near surface soils. Reinforcement should be sufficient to resist

a tensile load of 19d kips, where “d” is the diameter of the shaft, in feet.

Lateral Resistance

The lateral capacity for designing the drilled shafts (straight shafts) for a shaft depth of 8 feet are

given below.

Shaft Diameter, ft Lateral Capacity, kips

3 9

5 15

The above mentioned lateral capacity includes a factor of safety of 2. The upper 3 feet of soil is

neglected due to moisture variation zone.

5.3.2 Foundation Settlement. Depending upon the footing size and magnitude of the

sustained footing pressure, some total and differential settlements should be anticipated due to

consolidation of the foundation soils. Although detailed settlement analysis was not within the scope

of this study, it is believed that the footings designed in accordance with the above recommendations

should experience small acceptable settlements. Small differential settlement may also result from

variation in subsurface conditions across the site, loading conditions and construction procedures.

5.3.3 Site Preparation and Structural Fill Requirements. The sites should be cleared,

grubbed, and stripped of all organic material, soft soils and foreign material from the building and

paved areas.

Structural fill required to raise the grade or backfill grub holes should consist of lean clay or

sandy lean clay with a liquid limit less than 40 and a plasticity index between 10 and 20. The


17

structural fill material shall be placed in loose lifts not exceeding 8 inches and should be compacted

to 95 percent of the maximum dry density as determined by ASTM D 698. The moisture content of

structural fill should be one percent dry and two percent wet of the optimum moisture content as

determined by the Standard Proctor Test. The on-site sandy lean clays after removal of all foreign

material, if it meets the structural fill requirement can be used as structural fill.

5.3.4 Building Pad. Based on the plasticity characteristics and the physical state of the

subgrade soils at the sites, the shallow surface soils possess low to moderate potential for swelling and

shrinking. During construction, it is essential that the finished surface be protected from excessive

drying. Any material required to raise the grade should meet the criteria described in Section 5.3.3

“Site Preparation and Structural Fill Requirements.” The structural fill, if needed, should extend at

least 5 feet beyond the slab area.

5.3.5 Floor Slabs. Due to low to moderate potential for swelling and shrinkage of the surficial

soils, the floor slabs should be supported on 18 inches of inactive fill material. This inactive material

should be select structural fill meeting the criteria described in the previous section, “Site Preparation

and Structural Fill Requirements.” A modulus of subgrade reaction of 50 pci can be used for design

of floor slabs.

5.3.6 Landscaping. It is recommended that no large trees exist or be planted within 15 feet of

the structure and preferably within the mature drip line. Any flower beds or open lawn areas, if provided

near the structure areas, should have a good sprinkler system to minimize the moisture variations in the

subsurface soils. It is imperative that the sprinkler systems installed in the proximity of structures be

free from leaks, which could provide a continuous source of moisture and promote differential swelling

of the near surface soils.

5.3.7 Surface Drainage. The following drainage precautions should be observed during

construction and maintained at all times after the structure has been completed.


18

1. Backfill around the structure should be moistened and compacted to at least 90

percent of Standard Proctor Density (ASTM D 698).

2. The ground surface surrounding the exterior of the structure should be sloped to drain

away from the structure in all directions.

3. Roof downspouts and drains should discharge well beyond the limits of the

foundation backfill.


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6.0 CONSTRUCTION CONSIDERATIONS

Thickened Sludge Pump Station No. 2:

It is understood that the preliminary plans call for the thickened sludge pump station to be

constructed by cast-in-place method.

Excavation of Pump Station after Dewatering (Dry Method). Instability of the excavation

bottom can be attenuated by dewatering the transmissive silty sand. An appropriate dewatering

system should be installed outside the perimeter of the excavation area. The dewatering system

should maintain the groundwater level at least 5 feet below the proposed bottom of the TSPS

throughout the period of the excavation and construction of the structural slab.

Parts Building:

It is recommended that the footing excavation be inspected by a geotechnical engineer or

experienced engineering technician or an architect's or owner's representative prior to placing steel

and concrete. The excavation should be checked to verify that (a) the footing has been constructed to

the specified dimensions and is placed at the correct depth and into the appropriate stratum with

adequate bearing capacity as recommended in this report, (b) the loose cuttings, and any soft-

compressible materials have been removed from the bottom of the excavation, and (c) placement of

concrete should be accomplished as soon as possible to prevent changes in the state of stress and

caving of the foundation soils. A seal slab of lean concrete should be placed, if concrete placement

is delayed for more than 6 hours after excavation or sooner if rain is forecasted. No footing concrete

should be placed without the prior approval of the Project's Engineer, Architect or Owner's

Representative.


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7.0 PROVISIONS

The subsurface conditions and the design information contained in this report are based on

the test borings made at the time of drilling at specific locations. However, some variation in soil

conditions may occur between the boring locations. Should any subsurface conditions other than

those described in our boring logs be encountered, Geotest should be immediately notified so that

further investigation and supplemental recommendations can be provided. The depth of the

groundwater level can be expected to vary with environmental variations such as frequency and

magnitude of rainfall.

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

from subsurface explorations made at the time test borings were drilled at specific locations and the

results of laboratory tests on selected soil samples from the test borings. The stratification lines on

the log of borings represent the approximate boundaries between soil types, however, the transition

between soil types may be more gradual than depicted.

This report has been prepared for the exclusive use of Chester Engineers, Inc. specifically for

the Phase II final design and construction of Southeast Water Purification Plant in Houston, Texas.

This report shall not be reproduced in whole or part without written permission of Geotest

Engineering, Inc., or Chester Engineers, Inc. or City of Houston.

geotechnical investigation phase ii final … · sewpp sedimentation basin a & b improvements...

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