Chapter 3 List in chronological order the steps involved

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CHAPTER 3

Site Exploration and Characterization

QUESTIONS AND PRACTICE PROBLEMS

Introduction

3.1 Define site exploration and site characterization.

Solution

Site exploration is the exploration of the subsurface using borings and other techniques to

3.2 List in chronological order the steps involved in performing a site exploration and

characterization program.

Solution

1. Project assessment

Section 3.1 Project Assessment

3.3 What information should be gathered for the planning of a site exploration and

characterization program?

Solution

1. The types, locations, and approximate dimensions of the proposed improvements.

Section 3.2 Literature Search

3.4 What information can be obtained from geologic maps?

3-2 Site Exploration and Characterization Chap. 3

Solution

Geologic maps show the extents of various geologic formations, alignments of faults,

Section 3.5 Subsurface Exploration

3.5 Describe the following drilling methods and give advantages and disadvantages of each:

solid stem auger, hollow stem auger, rotary wash, and coring.

Solution

•Solid stem auger: The auger is lowered into the hole and rotated to dig into the

soil. Then, it is removed, the soil is discharged onto the ground, and the process

oAdvantages: cost effective; suitable for all coarse- and fine-grained soils;

allows relatively undisturbed samples to be obtained through the hollow

stem; suitable below the groundwater table.

oDisadvantages: cost and size of the equipment required to operate the

auger.

•Rotary wash: The boring is filled with drilling mud, a mixture of bentonite or

attapulgite clay and water. This drilling mud exerts a hydrostatic pressure on the

walls of the boring, thus preventing caving or squeezing. During drilling, the

Chap. 3 Site Exploration and Characterization 3-3

3.6 Describe the advantages and disadvantages of using an exploratory trench in site

exploration.

Solution

Advantages

•Provides more subsurface information than a boring of comparable depth.

Section 3.6 Soil and Rock Sampling

3.7 Describe the concept of sample disturbance and explain the relationship between sampler

type and disturbance.

Solution

Soil sample disturbance occurs when the soil is not recovered completely intact and its

3.8 Describe the Shelby tube sampler and the heavy-wall sampler and state the advantages

and disadvantages of each.

Solution

Shelby tube

Advantages:

•Provides soil samples.

3-4 Site Exploration and Characterization Chap. 3

Disadvantages

•Difficult to use in hard soils

3.9 A one-story, 50-m wide × 90-m long manufacturing building is to be built on a site

underlain by medium dense to dense silty sand with occasional gravel. This soil probably

has better-than-average engineering properties and average uniformity. There are no

indications of previous grading or fill at this site, and the groundwater table is believed to

be about 30 m below the ground surface. We anticipate supporting this building on

spread footing foundations located about 0.5 m below the ground surface. There are no

accessibility problems at this site.

(a) How many exploratory borings will be required, and to what depths should they

be drilled?

(b) What type of drilling and sampling equipment would you recommend for this

project?

Solution

()()

m 4500m 90m 50 ==A

Chap. 3 Site Exploration and Characterization 3-5

3.10 A one-story, 20-m wide × 50-m long concrete tilt-up office building is to be built at a site

near a wetlands. Previous exploratory borings at nearby sites encountered about 1 m of

moderately stiff clayey fill underlain by about 4 m of very soft organic silts and clays,

then 15 m of progressively stiffer sandy clays and clayey sands. Limestone bedrock is

located about 20 m below the ground surface. The groundwater table is thought to be at a

depth of about 0.5 m. Because of the soft soils, we will probably need to support this

building on deep foundations that extend at least into the stiffer soils, and possibly to

bedrock. There are no accessibility problems at this site.

(a) How many exploratory borings will be required, and to what depth should they be

drilled?

(b) What type of drilling and sampling equipment would you recommend for this

project, and what kinds of problems should the field crew be prepared to solve?

Solution

()()

m 0001m 50m 20 ==A

b. The very soft organic silts and clays will almost certainly have squeezing and caving

problems. In addition, some of the borings need to penetrate into the limestone bedrock.

3.11 A ten-story steel-frame office building with a 200 ft × 200 ft footprint is to be built on a

site underlain by alluvial sands and silts. These soils are fairly uniform and probably

have good engineering properties. The building will have one 12-ft deep basement and

will probably be supported on either a mat foundation located 5 ft below the bottom of

3-6 Site Exploration and Characterization Chap. 3

the basement, or a deep foundation extending about 60 ft below the bottom of the

basement. The groundwater table is about 30 ft below the ground surface and bedrock is

several 100 ft below the ground surface. There are no accessibility problems at this site.

(a) How many exploratory borings will be required, and to what depth should they be

drilled?

(b) What type of drilling and sampling equipment would you recommend for this

project?

Solution

()()

m 40,000ft 200ft 200 ==A

3.12 A small commercial development consisting of a one-story supermarket and a one-story

retail store building is to be built on the site shown in Figure 3.40. The proposed spread

footing foundations will be located at a depth of 2 ft below the ground surface. The site

has never been developed before, but a study of old aerial photographs indicates a fill was

placed in the northeast section. This fill appears to be up to 5-ft thick, probably was not

compacted, and most likely will need to be removed during construction. However, we

may be able to reuse this material as fill, so long as it does not contain trash or other

deleterious substances. The remainder of the soils are probably stiff clayey silts and

sandy silts. The groundwater table is believed to be about 20 ft below the ground surface.

There are no accessibility problems at this site.

Chap. 3 Site Exploration and Characterization 3-7

Develop a subsurface exploration program and present it as a 250–350 word

memo to your field crew instructing them what to do. Be sure to include a copy of the

site plan marked with the proposed location of each activity.

Solution

Market

()()

ft ,00003ft 150ft 200 ==A

3-8 Site Exploration and Characterization Chap. 3

Memorandum to Field Crew

Please schedule a backhoe and a hollow stem drill rig to trench and drill at the

proposed shopping center site. The approximate locations and depths of these trenches

and borings are shown on the attached drawing.

An old fill is located in the northeast corner of the site, as shown on the map. The

lateral limits shown are approximate, and the depth is probably no more than 5 ft. The

backhoe trenches need to penetrate through this fill, so we will know its exact depth. We

Section 3.7 Groundwater Exploration and Monitoring

3.13 Describe how to install an observation well and what it measures.

Solution

The installation of an observation well consists of drilling a borehole, inserting a slotted

Chap. 3 Site Exploration and Characterization 3-9

Section 3.9 In Situ Testing

3.14 The vertical effective stress at Sample 1 in Figure 3.14 is 405 lb/ft2. Compute N1,60.

Solution

3.15 The vertical effective stress at Sample 3 in Figure 3.14 is 1270 lb/ft2. Compute N1,60.

Solution

3.16 A standard penetration test has been performed at a depth of 6.5 m in a medium sand

using a standard sampler and a USA-style donut hammer. The N-value recorded in the

field was 16. The boring diameter was about 100 mm, and the vertical effective stress at

the test location was 85 kPa. Compute N1,60.

Solution

3.17 Using Figure 3.29, classify the soils between depths of 23 and 48 ft in the CPT results

presented in Figure 3.28. Why are there spikes in the qc, fsc, and Rf curves between these

depths?

Solution

Per figure 3.28, most of the soil between 23 and 48 ft has the following characteristics:

3.18 Using Figure 3.29, classify the soils between depths of 66 and 80 ft in the CPT results

presented in Figure 3.29. Why are there spikes in the qc, fsc, and Rf curves between

depths of 76 and 78 ft?

3-10 Site Exploration and Characterization Chap. 3

Solution

Per figure 3.29, most of the soil between 66 and 80 ft has the following characteristics:

3.19 A cone penetration test on a sandy soil with mean particle size of 0.5 mm produced a qc

of 80 kg/cm2. Estimate the equivalent SPT N60-value.

Solution

3.20 Compare the standard penetration test and the Becker penetration test.

Solution

The Standard Penetration Test (SPT) is used mostly for medium dense and fine-grained

3.21 Describe how to perform a pressuremeter test and what it measures.

Solution

The pressuremeter test (PMT) produces direct measurements of soil compressibility and

3.22 Describe how to perform a dilatometer test and what it measures.

Solution

The dilatometer test (DMT) entails measuring the pressure applied by the surrounding

Chap. 3 Site Exploration and Characterization 3-11

Comprehensive

3.23 An engineer is planning to use a 24-inch diameter bucket auger similar to the one in

Figure 3.7 to drill several exploratory borings at a site adjacent to a lake. The underlying

soils are probably soft clays and silts with N-values of less than 5. Is this a wise choice?

Why or why not?

Solution

This would be very poor choice, and is almost certainly doomed to failure. Soft clays and

3.24 What type of soil sampling equipment would be most appropriate for the soils described

in Problem 3.23? Why?

Solution

3.25 A large compacted fill is to be placed on a site underlain by a 15 m thick layer of

saturated clay. The weight of this fill will cause the clay layer to consolidate, which will

result in large settlements at the ground surface. Since these settlements would have an

adverse effect on buildings and other improvements planned for this site, a settlement rate

analysis, similar to those we will discuss in Chapter 11, is to be performed to estimate the

time required for a certain percentage of the settlement to be completed.

A series of exploratory borings have already been drilled at this site, samples have

been recovered, and laboratory tests have been performed to evaluate the consolidation

properties of the clay. However, to complete the settlement rate analysis, we need to

know if thin horizontal sand seams are present in the clay, and the approximate spacing

between these seams. If they exist at all, these seams are probably less than 100 mm

thick. Although some of the undisturbed samples contained sand seams, more

information is needed.

What kind of additional exploration would you do to determine whether or not

more sand seams are present? Be sure to consider both technical feasibility and cost, and

explain the reason for your choice.

Solution

One method would be to perform a series of cone penetration tests. Sand seams would

3-12 Site Exploration and Characterization Chap. 3

3.26 A level building pad is to be built at the site shown in Figure 3.41 by cutting and filling as

shown. The final pad elevation is to be 215 ft. Then, a three-story steel-frame office

building is to be built.

Five exploratory borings have been drilled to determine the subsurface conditions.

The logs from these borings were summarized as follows:

Boring 1

Groundwater table depth = 44 ft