site and subsurface conditions 4 … project files/1103a/1103 9451 107 st. george...field...

Don Crichlow & Associates - Report of Geotechnical Exploration February 11, 2009 MACTEC Project No. 6734-08-9451

TABLE OF CONTENTS SUBJECT PAGE NUMBER PROJECT INFORMATION AND STRUCTURAL CONDITIONS ............................................... 1 FIELD EXPLORATION AND LABORATORY TESTING ............................................................ 3 SITE AND SUBSURFACE CONDITIONS .................................................................................. 4 RECOMMENDATIONS FOR SHALLOW FOUNDATION DESIGN AND SITE PREPARATION .................................................................................................................. 6 APPENDIX Site Location Map Field Exploration Plan Generalized Subsurface Profile Soil Test Boring Records Summary of Laboratory Test Results Grain Size Distribution Report Field and Laboratory Procedures Key to Symbols and Descriptions


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PROJECT INFORMATION AND STRUCTURAL CONDITIONS

The purpose of this exploration was to develop information concerning the site and subsurface

conditions in order to evaluate site preparation requirements and foundation support alternatives for the

planned renovations to the existing building located at 107 St. George Street in St. Augustine, Florida.

This report briefly describes the field and laboratory testing activities and presents the findings. The

enclosed guideline recommendations for shallow foundation design, site preparation, and foundation

construction represent approaches we feel would be appropriate for the planned construction.

Project information was provided by Mr. Don Crichlow of Don Crichlow & Associates and by Mr. Bill

Simpson of Bill Simpson and Associates, Inc. during the period of June 3, 2008, to February 5, 2009.

We were furnished with the following items:

Seven Plans Prepared by: Don Crichlow & Associates Dated: May, 2008

Twenty One Site Photographs Provided by: Bill Simpson and Associates, Inc. Undated (received via e-mail on June 3, 2008)

Existing 1st Floor Plan with Structural Loads Prepared by: Bill Simpson and Associates, Inc. Undated (received via email on November 20, 2008)

Sketch of Typical Pilaster Footing Provided by: Don Crichlow & Associates Undated (received via fax on January 29, 2009)

Two Photographs of an Excavated Footing Provided by: Don Crichlow & Associates Undated (received via e-mail on January 29, 2009)

As shown on the Site Location Map and Field Exploration Plan in the Appendix, the project site is

located approximately 120 feet south of Hypolita Street, on the east side of St. George Street, in St.

Augustine, Florida. We understand that renovations are planned for the existing three-story structure

constructed of clay masonry tile exterior walls with clay masonry brick pilasters, and steel and wood

structural framing for the floors and roof. We further understand that the building is supported on a


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shallow foundation system. At one of the pilaster locations, part of the concrete floor slab was removed

by others, and soil was excavated so that the footing supporting the wall and pilaster could be exposed.

Based on the provided typical pilaster footing sketch, the structural clay block walls are 12 inches thick.

At the pilasters, the width of the structural clay block is 19 inches. The walls and pilasters are supported

by continuous concrete footings that extend 7 inches laterally beyond the inside faces of the walls and

pilasters. We assume the footings extend 7 inches laterally beyond the outside faces of the walls, such

that the overall footing width would be 33 inches at the pilaster locations, and 26 inches between the

pilasters. We understand the length of the widened portions of the footings at the pilaster locations is

42 inches (in the longitudinal direction of the wall footing). Based on one of the furnished photographs,

it appears that the footing bears about 19 inches below the surface of the floor slab (assuming the slab

is about 4 inches thick). Information on the existing interior wood column foundations was not provided

to us prior to preparation of this report, although we understand that their foundations appear to be

thickened sections of the floor slab.

We understand that exterior brick pilaster and wall loads for the structure after renovations will not

exceed 80 kips and 2.25 klf, respectively, and the interior column load will not exceed 50 kips. We

assume that soil-supported ground floor loads (dead load plus live load) in the building will not

exceed 150 psf. Existing wall and pilaster loads were not furnished.

We understand that the existing footings are considered by Mr. Simpson to be unsatisfactory for

their present loads (based on the visible wall cracking), and will therefore be even more

unsatisfactory for any additional loading. As such, the existing foundations will be repaired (enlarged)

to accommodate the new loading.


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FIELD EXPLORATION AND LABORATORY TESTING

Field Exploration

In order to explore the subsurface conditions in the area of the existing building, two Standard

Penetration Test (SPT) borings were drilled to a depth of 55 feet each below the existing ground

surface, one near the northeast corner of the building and the other just east of the loading dock and

just north of the dumpster area. The approximate boring locations are shown on the Field Exploration

Plan in the Appendix. The boring locations were selected by a geotechnical engineer from this office

and were established in the field by our personnel using measurements from existing site features and

should be considered approximate. We note that access was extremely limited, and the selected boring

locations were the only accessible locations. Ground surface elevations at the boring locations were

neither furnished to us nor determined by us.

The Soil Test Boring Records, in the Appendix, graphically show the penetration resistances and

groundwater levels, and present the soil descriptions for each SPT boring. The stratification lines and

depth designations on the boring records represent the approximate boundaries between soil types. In

some instances, the transition between soil types may be gradual. Brief descriptions of the exploratory

drilling, testing and sampling techniques used are presented in the Field and Laboratory Procedures

section of the Appendix.

Laboratory Testing

In order to aid in classifying the soils and to help quantify and correlate engineering properties,

laboratory index property and classification tests were performed on representative soil sample

obtained from the borings. The laboratory testing included the following:

Four water content tests

Three fines content (percentage of soil particles finer than the No. 200 mesh sieve) tests

One grain size distribution test

The results of these tests are presented on the Summary of Laboratory Test Results sheet and the

Grain Size Distribution Report sheet in the Appendix. Brief descriptions of the laboratory test

procedures used are presented in the Field and Laboratory Procedures section in the Appendix.


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SITE AND SUBSURFACE CONDITIONS

Site Conditions

The existing site conditions were observed by representatives from this office on June 16 and

September 9, 2008. The existing building was generally bounded by St. George Street to the west

(front of the building), existing structures to the north and south, and a paved driveway to the east.

Noticeable cracks were observed on the exterior walls. The overall site topography appeared to be

relatively flat and level.

Subsurface Conditions

General – An illustrated representation of the subsurface conditions encountered in the proposed

renovation area is shown on the Generalized Subsurface Profile presented in the Appendix. The profile

and the soil conditions outlined below highlight the major subsurface stratification. The Soil Test Boring

Records in the Appendix should be consulted for detailed descriptions of the subsurface conditions

encountered at each boring location. When reviewing the boring records and the subsurface profile, it

should be understood that soil conditions may vary between and away from the boring locations.

Soils – In general, the borings encountered very loose to firm light gray-brown, gray-brown, and brown

fine sands (Unified Soil Classification System symbol of SP) and slightly silty fine sands (SP-SM) from

the existing ground surface to a depth of about 27 feet below grade. We note that a 10-inch thick

pavement section (1 inch of asphalt overlying 9 inches (limerock) was present at the surface at Boring

B-1. The upper fine sand layer was underlain by loose to dense brown, gray and tan slightly silty fine

sands (SP-SM) with a few to many shell fragments to depths ranging from approximately 37 to 42 feet.

Below this shelly sand layer, loose to very loose gray slightly silty to very silty fine sands (SP-SM to

SM) were encountered to a depth of about 47 feet. Firm gray clayey to very clayey fine sands (SC)

were then penetrated to a depth of about 52 feet, which were underlain by loose gray-tan slightly silty

shelly fine sands (SP-SM) to the boring termination depth of 55 feet.

Groundwater – The depth to groundwater was measured at the boring locations at the time of drilling.

The groundwater table was encountered at depths of about 4¾ to 5 feet below the existing ground

surface. Fluctuation in groundwater levels should be expected due to seasonal climatic changes,

construction activity, rainfall variations, surface water runoff, and other site-specific factors. Since

groundwater level variations are anticipated, design drawings and specifications should accommodate


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such possibilities and construction planning should be based on the assumption that variations will

occur.


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RECOMMENDATIONS FOR SHALLOW FOUNDATION DESIGN AND SITE PREPARATION

Basis for Recommendations

The following recommendations are based upon the previously presented project information and the

furnished structural geometry and loading conditions along with the data obtained in this exploration.

The field and laboratory test data have been compared with previous performances of similar structures

bearing on soils similar to those encountered at this site. If the structural information is incorrect,

please contact us so that our recommendations may be reviewed. Our evaluations assume that the

subsurface conditions that exist throughout the building area are similar to those encountered by our

borings. The discovery of any site and/or subsurface condition during construction which deviates from

the data obtained in this exploration should also be reported to us for our evaluation. The assessment

of site environmental conditions or the presence of pollutants in the soil, rock or groundwater of the site

is beyond the proposed scope of this geotechnical exploration.

Evaluation

Based on the furnished footing geometry and new wall and pilaster loads, as well as the assumption

that the walls and pilasters are centered on the continuous footings, we estimate that the applied

bearing pressure on the portions of the footings that support the pilasters (at their current size) would

be about 8,300 psf, and that the applied bearing pressure on the portions of the footings that support

the walls (at their current size) would be about 1,025 psf. Based on these bearing pressures, it appears

obvious that the pilaster footings need to be enlarged to reduce the applied bearing pressure to a more

reasonable value. However, the wall footing bearing pressure is lower than would normally be

recommended, so enlarging the wall footings would not likely reap much benefit.

Settlement Potential

We have evaluated the field and laboratory test data obtained in this exploration along with our

experience with similar structures and empirical relationships for bearing and settlement. For the

pilasters and interior column footings, we estimated settlements assuming the existing footings will be

modified. We evaluated settlements at two footing bearing pressures – 2,000 psf and 2,500 psf. For the

wall footings, we assumed there would be no modifications, and evaluated settlements using the

calculated bearing pressure of 1,025 psf. The following table summarizes the results of our settlement

evaluations:


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Summary of Estimated Settlements

Footing Type Design Load Applied Footing

Bearing Pressure (psf)

Estimated Total Settlement (in.)

Pilaster 80 kips 2,000 < ½ 2,500 ½

Interior Column 50 kips 2,000 < ½ 2,500 ½

Wall 2.25 klf 1,025 < ¼

Our settlement estimates do not take into account the settlements that have already occurred under the

current structural loading, and are therefore considered to be somewhat conservative. The actual

settlements experienced by the structure under the planned design loads are likely to be less than the

estimates presented above. We assume that these estimated settlements would be tolerable.

Differential settlements (between adjacent pilasters and columns or along the length of a continuous

wall footing) should be approximately one-half of the total settlement. These settlements will be the

result of elastic compression of the upper loose sands, and should occur almost immediately following

the application of the structural dead load during construction.

Shallow Foundation Design

We consider the subsurface conditions to be acceptable for support of the proposed renovations using

shallow foundations, assuming the recommended site preparation is performed and the estimated

settlements are tolerable. Individual pilaster footings, column footings and continuous (strip) footings

may bear on compacted acceptable existing soils. The enlarged pilaster and column footings may be

designed using an allowable net soil bearing pressure of 2,500 psf. The allowable bearing pressure

may be increased by about one-third for transient edge stress loading considerations. Long-term edge

stresses should not exceed the allowable bearing pressure. A density equivalent to at least 95 percent

of the Modified Proctor maximum dry density (ASTM D 1557) should be achieved in the sandy footing

bearing level soils. It does not appear that it will be necessary to enlarge the wall footings, based on

the relatively low applied bearing pressure and corresponding estimated settlement using the existing

footing width.


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The site preparation recommendations outlined below should be followed in order for the shallow

foundations to perform as discussed in the Settlement Potential section of this report.

Site Preparation Recommendations

Surface Water Control – The need for surface water runoff control (outside of the building) should be

anticipated during the site preparation and shallow foundation construction process. Construction

areas should be graded to drain stormwater runoff away from the immediate area of preparation. This

is particularly important during wetter climatic seasons. Lack of proper controls could result in ponding

of surface water in foundation bearing areas and on compaction surfaces. The ponded water,

combined with machine or foot traffic during construction operations or other activities, could disturb

otherwise acceptable soils or previously compacted existing soils, causing instability, pumping, and

generally unacceptable conditions. The ponded water will also impede or prevent necessary soil

compaction operations and make construction trafficability difficult.

Shallow Groundwater Control – The need for significant groundwater control is not anticipated.

However, if wet climatic conditions exist at the time of construction, some shallow groundwater control

may be necessary. Groundwater can generally be lowered one to three feet for short time periods by

pumping from barrel sumps located in perimeter ditches or pits if gravity drainage cannot be

established. All sump inlets should be located outside of the bearing areas to avoid loosening of the

fine sandy bearing soils due to upward seepage pressures. Groundwater should be maintained at

least one foot below the bottom of any excavations made during construction and two feet below the

surface of any vibratory compaction operations.

Shallow Foundation Bearing Surface Preparation – The upper 12 inches of sandy bearing soils at the

shallow foundation bearing level should be compacted to a density equivalent to at least 95 percent of

the Modified Proctor maximum dry density (ASTM D 1557). Compaction or recompaction of the footing

bearing level soils (if loosened by the excavation process) may be accomplished utilizing multiple

passes of a relatively lightweight, walk-behind vibratory sled or roller over the compaction surface. In

order to achieve the recommended compaction criterion, it will be necessary for the groundwater table

to be maintained at least two feet below the foundation bearing level.

Structural Backfilling – Structural backfill, as required, may be placed in lifts not exceeding 6 inches in

loose thickness. Each lift should be thoroughly compacted with the compaction equipment mentioned

above until densities equivalent to at least 95 percent of the Modified Proctor maximum dry density


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(ASTM D 1557) are uniformly obtained. Ideally, structural fill should consist of an inorganic, non-

plastic, granular soil containing less than 10 percent material passing the No. 200 mesh sieve

(relatively clean sand with a Unified Soil Classification of SP, SP-SM, or SP-SC).

Preconstruction Survey

We recommend that a preconstruction crack and damage survey be performed on the adjacent

structures to document (with photographs) their existing condition. All existing cracks and indications of

damage should be documented.

General Construction Monitoring and Testing Guidelines

Prior to initiating compaction operations, we recommend that representative samples of the structural

backfill material to be used and acceptable exposed in-place soils be collected and tested to determine

their compaction and classification characteristics. The maximum dry density, optimum moisture

content, gradation and plasticity characteristics should be determined. These tests are needed for

compaction quality control of the structural fill/backfill and existing soils and to determine if the

fill/backfill material is acceptable.

In-place density tests should be performed at representative locations in the bearing level soils in the

footing foundation excavation bottoms to confirm that the required degree of compaction has been

obtained. We recommend that one field density test be conducted at each column and pilaster location.

Construction Plans and Specifications Review

We recommend that this office be provided the opportunity to make a general review of the foundation

and earthwork plans and specifications prepared from the recommendations presented in this report.

We would then suggest any modifications such that our recommendations are properly interpreted and

implemented. Our report has been written in a guideline recommendation format and is not appropriate

for use as a specification without in-part being reworded into a specification-type format. We

recommend that this report not be made a part of the contract documents; however, it should be made

available to prospective contractors for information purposes.

The evaluation of conditions which may be encountered in construction requires engineering judgment

and interpretation. For this reason, we recommend that MACTEC remain involved with this project

during the construction process, particularly during foundation construction and general earthwork

operations. If we are not retained during construction, we cannot assume responsibility for


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misinterpretation of our recommendations or for unfavorable foundation performance as a result of

judgments rendered by others.

APPENDIX

BORING TERMINATED

SPT-5

SPT-4

SPT-3

SPT-2SPT-1

SPT-7

Asphalt (1" thick) over Limerock Base (9" thick)

LOOSE light gray-brown slightly silty fine SAND (SP-SM)with many fine to coarse sand-sized shell fragments

FIRM gray very clayey fine SAND (SC) with a trace of fine tomedium sand-sized shell fragments

VERY LOOSE gray slightly clayey silty fine SAND (SM)with a trace of fine sand-sized shell fragments

LOOSE gray slightly silty fine SAND (SP-SM) with a few fineto medium sand-sized shell fragments

LOOSE gray slightly silty fine SAND (SP-SM) with many fineto coarse sand-sized shell fragments

DENSE brown slightly silty fine SAND (SP-SM) with a fewfine to medium sand-sized shell fragments

FIRM brown fine SAND (SP) with a trace of fine sand-sizedshell fragments

LOOSE brown to light gray-brown fine SAND (SP)

SPT-6

Gray-brown fine SAND (SP) with a few fine to coarselimerock fragments (Fill)

SPT-14

SPT-13

SPT-12

SPT-11

SOIL

945

1-01

LOG

S.G

PJ L

AW

_GIB

B.G

DT

2/1

1/09

SPT-9

VERY LOOSE brown fine SAND (SP)

13-18-15

6-5-5

3-5-7

(WOH=12",1)WOH-0-1

5-3-5

4-5-8

1-3-4

3-4-5

1-2-3

1-2-3

1-1-2

1-1-12

4-6-4

SPT-10

SPT-8

Building Evaluation - 107 St. George Street

September 9, 20086734-08-9451

SOIL TEST BORING RECORD

NM (%)

Project:Coord N:Coord E:

Proj. No.:

PL (%)

0

5

10

15

20

25

30

35

40

45

50

55

60

LL (%)SOIL CLASSIFICATIONAND REMARKS

0 10 20 30 40 50 60 70 80 90 100

SEE KEY SYMBOL SHEET FOR EXPLANATION OFSYMBOLS AND ABBREVIATIONS BELOW.

FINES (%)

SPT (bpf)

LEGEND

Boring No.: B-1

SAMPLES

10 20 30 40 50 60 70 80 90 100

5

10

15

20

25

30

35

40

45

50

55

MACTECDH/JW/JDCME 55 - Automatic HammerAuger/Mud Rotary4"

CONTRACTOR:DRILLER:EQUIPMENT:METHOD:HOLE DIA.:REMARKS:

IDENT

TYPE

1st 6

"2n

d 6"

3rd

6"

Checked By:

DEPTH

(ft)

THIS RECORD IS A REASONABLE INTERPRETATION OFSUBSURFACE CONDITIONS AT THE EXPLORATIONLOCATION. SUBSURFACE CONDITIONS AT OTHERLOCATIONS AND AT OTHER TIMES MAY DIFFER.INTERFACES BEWEEN STRATA ARE APPROXIMATE.TRANSITIONS BETWEEN STRATA MAY BE GRADUAL.

Drilled:

ELEV

(ft)

N-COUNT

LOOSE gray-tan slightly silty fine SAND (SP-SM) with manyfine to coarse sand-sized shell fragments

SPT-5

SPT-4

SPT-3

SPT-2

SPT-1

SPT-7

LOOSE brown fine SAND (SP) with a trace of fine to mediumlimerock fragments

FIRM gray clayey fine SAND (SC) with sand seams

VERY LOOSE gray slightly clayey very silty fine SAND(SM) with a trace of fine to medium sand-sized shell fragments

LOOSE gray slightly silty fine SAND (SP-SM) with a trace offine to medium sand-sized shell fragments

LOOSE tan calcareous slightly silty fine to medium SAND(SP-SM) with many fine to coarse sand-sized shell fragments

VERY FIRM brown slightly silty fine SAND (SP-SM) withsome medium to coarse sand-sized shell fragments

FIRM brown fine SAND (SP)

VERY LOOSE gray-brown fine SAND (SP)

LOOSE gray-brown fine SAND (SP) with a trace of roots

SPT-6

LOOSE brown fine SAND (SP)

LOOSE gray-brown slightly silty fine SAND (SP-SM) with afew fine to coarse shell and limerock fragments

SPT-14

SPT-13

SPT-12

SPT-11

SOIL

945

1-01

LOG

S.G

PJ L

AW

_GIB

B.G

DT

2/1

1/09

SPT-9

VERY LOOSE brown fine SAND (SP) with a trace of fineshell fragments

6-5-5

6-4-6

WOH-5-7

(WOH=18")

7-4-4

7-11-13

6-7-8

1-1-3

2-4-5

1-2-2

1-1-3

2-2-3

1-3-7

17-3-3

WOH-0-0

SPT-10

SPT-8

BORING TERMINATED

LL (%)NM (%)PL (%)SAMPLESIDENT

SOIL CLASSIFICATIONAND REMARKS

0 10 20 30 40 50 60 70 80 90 100

SOIL TEST BORING RECORD

SEE KEY SYMBOL SHEET FOR EXPLANATION OFSYMBOLS AND ABBREVIATIONS BELOW.

LEGEND

Boring No.: B-2

FINES (%)

SPT (bpf)

Project:Coord N:Coord E:

Proj. No.:

10 20 30 40 50 60 70 80 90 100

5

10

15

20

25

30

35

40

45

50

55

MACTECDH/JW/JDCME 55 - Automatic HammerAuger/Mud Rotary4"

CONTRACTOR:DRILLER:EQUIPMENT:METHOD:HOLE DIA.:REMARKS:

TYPE

ELEV

(ft)

Building Evaluation - 107 St. George Street

September 9, 20086734-08-9451

Drilled:

DEPTH

(ft)

THIS RECORD IS A REASONABLE INTERPRETATION OFSUBSURFACE CONDITIONS AT THE EXPLORATIONLOCATION. SUBSURFACE CONDITIONS AT OTHERLOCATIONS AND AT OTHER TIMES MAY DIFFER.INTERFACES BEWEEN STRATA ARE APPROXIMATE.TRANSITIONS BETWEEN STRATA MAY BE GRADUAL.

0

5

10

15

20

25

30

35

40

45

50

55

60

N-COUNT

1st 6

"2n

d 6"

3rd

6"

Checked By:

site and subsurface conditions 4 … project files/1103a/1103 9451 107 st. george...field...

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