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The user wants me to convert an OCR image of an engineering standard PDF page into an HTML fragment. Let me analyze the image carefully.

The page contains:

1. A table caption: "Table 2.1.1. Summary of soil characteristics as a pavement material¹."

2. A large table with columns: Major Divisions, Name, Strength when Not Subject to Frost Action, Potential Frost Action, Compressibility & Expansion, Drainage Characteristics

3. A footnote at the bottom

4. A page number "2.1.4" at the bottom (which I should ignore per the rules)

Let me carefully transcribe the table content.

The table has these columns:

- Major Divisions

- Name

- Strength when Not Subject to Frost Action

- Potential Frost Action

- Compressibility & Expansion

- Drainage Characteristics

Rows:

Row group 1: "Gravel and Gravelly Soils" (rowspan=5)

Row group 2: "Sand and SP Sandy Soils" (rowspan=5)

Row group 3: "Silts & Clays with the Liquid Limit Less Than 50" (rowspan=3)

Row group 4: "Silts & Clays with Liquid Limit Greater Than 50" (rowspan=3)

Row group 5: "Highly Organic Soils" (rowspan=1)

Footnote: "1 The information presented here is adapted after publications of the U.S. Army Corps of Engineers, Federal Aviation Administration, and the Federal Highway Administration."

Page number at bottom: 2.1.4 (ignore per rules)

Now let me think about the table structure. The "Major Divisions" column has cells that span multiple rows. Let me count:

For "Gravel and Gravelly Soils":

- GW row

- (no symbol) row

- GP row

- (no symbol) row

- *d GM --- u GC row

That's 5 rows.

For "Sand and SP Sandy Soils":

- SW row

- (no symbol) row

- *d SM --- u SC row

That's 5 rows.

For "Silts & Clays with the Liquid Limit Less Than 50":

- (no symbol) row

That's 3 rows.

For "Silts & Clays with Liquid Limit Greater Than 50":

- (no symbol) row

That's 3 rows.

For "Highly Organic Soils":

- (no symbol) row

That's 1 row.

Now for the "Name" column, some rows have blank cells in the first column (the symbol column). Looking more carefully at the image:

The first column "Major Divisions" contains the soil classification symbols (GW, GP, GM, GC, SW, SP, SM, SC, etc.) AND the group names.

Wait, let me re-examine. Looking at the image more carefully:

The first column header is "Major Divisions". Under it, I see:

- "GW" (top right of the cell area)

- "Gravel and Gravelly Soils" (left side)

- "GP"

- "*d GM --- u GC"

Actually, looking at the layout, the first column seems to contain both the group name (like "Gravel and Gravelly Soils") and the individual symbols (GW, GP, GM, GC). The symbols appear to be right-aligned within the first column.

Let me look at this differently. The first column "Major Divisions" spans the group name AND the symbols. So for the Gravel group:

- Row 1: "Gravel and Gravelly Soils" (rowspan=5) + "GW" (right-aligned in first column)

- Row 2: (continuation of rowspan) + (blank symbol)

- Row 3: (continuation) + "GP"

- Row 4: (continuation) + (blank symbol)

- Row 5: (continuation) + "*d GM --- u GC"

Hmm, but that doesn't quite work either because the symbols and group names seem to be in the same column but different visual positions.

Actually, looking at the image again more carefully, I think the first column contains the group name on the left side and the symbols on the right side, all within the same column cell. But for the table structure, I think the symbols are in the same first column as the group names.

Let me reconsider. The table has 6 columns. The first column "Major Divisions" contains:

- For the gravel group: "Gravel and Gravelly Soils" spanning 5 rows, with GW, GP, GM/GC appearing in the same column but right-aligned.

Actually, I think the structure might be that the first column has the group name (left-aligned) and the symbols are also in the first column but right-aligned. But in HTML table terms, they'd be in the same td.

Wait, looking more carefully at the image, I see that "GW" appears at the top of the first column area, and "Gravel and Gravelly Soils" appears below it. They seem to be in the same column. The symbols GW, GP, GM, GC are right-aligned within the first column.

So the first column for the gravel group would be:

Row 1: "Gravel and Gravelly Soils" (rowspan=5) with "GW" right-aligned

Row 2: (continuation) with blank

Row 3: (continuation) with "GP" right-aligned

Row 4: (continuation) with blank

Row 5: (continuation) with "*d GM --- u GC" right-aligned

Hmm, but that's complex. Let me look at the image one more time.

Actually, I think the first column might actually be split into two sub-columns visually, but in the table structure it's one column. The group name is on the left and the symbol is on the right within the same cell.

For simplicity and accuracy, I'll put the group name and symbol in the same td, with the symbol appearing after the group name or in a separate line. But since I can't use
for paragraph spacing... actually I can use
within a cell for line breaks within the same cell content.

Wait, the rules say "不要用
制造段落间距（段落之间直接换行即可）" - this means don't use
to create paragraph spacing between paragraphs. But within a table cell, if there are two distinct pieces of text (group name and symbol), I might need
to separate them.

Actually, looking at the image layout more carefully, I think the first column has the group name on the left side and the symbol on the right side. In the table, these might be in the same cell. Let me just put them together.

For the gravel group, the first column cell (rowspan=5) would contain "Gravel and Gravelly Soils" and then the symbols GW, GP, GM, GC appear in subsequent rows within the same column.

Hmm, this is getting complicated. Let me think about this differently.

Looking at the image structure:

- Column 1 "Major Divisions" has two visual sub-parts: the group name (left) and the symbol (right)

- The group name spans multiple rows

- The symbol is in each row

So maybe the first column is actually two columns merged? No, the header says "Major Divisions" as one column.

I think the most faithful representation is:

- The first column td for the group name has rowspan

- The symbol appears in the same column but in a different visual position

Actually, I think what's happening is that the first column contains both the group name and the symbol. The group name is left-aligned and the symbol is right-aligned within the same cell. But since the group name spans multiple rows and the symbol is per-row, they can't be in the same cell.

Let me look at this from a different angle. Maybe the table actually has the group name in a merged cell on the left, and the symbols are in a separate visual area but still within the first column.

OK, I think the cleanest approach is:

For the first column, I'll have the group name with rowspan, and the symbols will be in the same column but in separate rows. Since the group name cell spans multiple rows, the symbols need to be in the same column but in rows where the group name cell is already spanning.

Wait, that's not possible in HTML. If a cell has rowspan=5, then in rows 2-5, there's no cell in that column.

So the symbols must be in a different column, or the group name and symbols are in the same cell.

Let me look at the image one more time very carefully.

OK, I see it now. The first column "Major Divisions" contains:

- "Gravel and Gravelly Soils" on the left side, spanning the height of 5 rows

- "GW" at the top right of the first column area (row 1)

- "GP" at the right (row 3)

- "*d GM --- u GC" at the right (row 5)

These are all in the same column. The group name is a merged cell (rowspan=5) on the left, and the symbols are... hmm.

Actually, I think the table might have the first column as a single column where:

- Row 1: td contains "Gravel and Gravelly Soils" (rowspan=5) AND "GW" - but this can't work with rowspan.

OR maybe the visual layout is that the group name is in a cell that spans rows, and the symbols are right-aligned within that same spanning cell, appearing at different vertical positions. But in HTML, you can't position text at different vertical positions within a single cell.

I think the most practical approach is to put the group name and symbol together in the first column cell. For the first row of each group, the cell contains both the group name and the first symbol. For subsequent rows, the cell is empty (covered by rowspan).

But wait, the symbols GW, GP, GM/GC appear at different rows, not all in the first row.

OK let me try yet another interpretation. Maybe the first column is actually TWO columns that look like one:

- Sub-column 1a: Group name (rowspan)

- Sub-column 1b: Symbol (per row)

And the header "Major Divisions" spans both sub-columns.

This would make the table have 7 columns total, with the first header spanning 2 columns.

If "Major Divisions" spans 2 columns, then the data rows would have:

That's 7 columns. Let me verify with the image.

Looking at the image, the first column area seems to have a vertical line separating the group name from the symbols. Yes! I can see that "Gravel and Gravelly Soils" is on the left of a vertical line, and "GW", "GP", "GM", "GC" are on the right of that line. So the first column is indeed split into two sub-columns.

So the table has 7 columns:

1. Group name (rowspan)

2. Symbol

3. Name (description)

4. Strength

5. Potential Frost Action

6. Compressibility & Expansion

7. Drainage Characteristics

And the header "Major Divisions" spans columns 1-2.

Let me verify this interpretation with the data:

Hmm wait, but looking at the image, row 4 has "Good" in the strength column and "Slight to medium" in frost action, "Slight" in compressibility, "Poor to practically impervious" in drainage. But the Name column for row 4 appears to be blank. And the symbol column for row 4 also appears blank.

Actually, looking more carefully at the image, I see that for the GP group:

Hmm, but that doesn't make sense. Why would there be a row with no name?

Let me look at the image one more time. I think what's happening is:

For GP, there are two sub-rows:

Wait, maybe the GP symbol spans 2 rows? Let me look...

Actually, I think the structure might be:

GP spans 2 rows:

Hmm, but that still has a blank name.

OK, I think I'm overcomplicating this. Let me just look at the visual layout and try to match it as closely as possible.

Looking at the image row by row within the Gravel group:

Hmm, row 4 having blank name is odd. Let me look at the image one more time...

Actually, I think in the original table, the GP symbol might span rows 3-4, and the name "Silty gravels, gravel-sand silt mixtures" also spans rows 3-4. Then row 4 just has different values for the last 4 columns.

Wait, no. Looking at the image, I see horizontal lines separating each row. Row 3 has "Silty gravels, gravel-sand silt mixtures" and row 4 has a blank name cell. But there IS a horizontal line between them.

Hmm, maybe the original table has GP spanning 2 rows with the name also spanning 2 rows, and the last 4 columns have different values for each sub-row. That would make sense for a classification table where GP has sub-categories.

Actually, I think I see it now. Looking at the image:

For GP:

- The symbol "GP" is in the first column

- The name "Silty gravels, gravel-sand silt mixtures" is in the second column

- These span 2 rows (rowspan=2)

- Row 3: Good to excellent | Slight to medium | Very slight | Fair to poor

- Row 4: Good | Slight to medium | Slight | Poor to practically impervious

Wait, but I see a horizontal line between the name cells of rows 3 and 4. Let me look again...

Actually, I think the horizontal line between rows 3 and 4 only extends across the last 4 columns, not the first two. So GP and the name span 2 rows.

Hmm, but looking at the image, I see the horizontal line goes all the way across. Let me just go with what I see.

OK, I think I need to stop overthinking and just transcribe what I see. Let me look at the image one final time and note the exact cell boundaries.

For the Gravel group (5 rows):

Row 1: [Gravel and Gravelly Soils rowspan=5] [GW] [Well-graded gravels or gravel-sand mixtures, little or no fines] [Excellent] [None to very slight] [Almost none] [Excellent]

Row 2: [covered] [blank] [Poorly graded gravels or gravel-sand mixtures little or no fines] [Good to excellent] [None to very slight] [Almost none] [Excellent]

Row 3: [covered] [GP] [Silty gravels, gravel-sand silt mixtures] [Good to excellent] [Slight to medium] [Very slight] [Fair to poor]

Row 4: [covered] [blank] [blank] [Good] [Slight to medium] [Slight] [Poor to practically impervious]

Row 5: [covered] [*d GM --- u GC] [Clayey gravels, gravel-sand-clay mixture] [Good] [Slight to medium] [Slight] [Poor to practically impervious]

Hmm, but row 4 having blank name and blank symbol is really odd. Let me look at the image one more time...

OK I think I see it now. Looking at the image very carefully:

The GP row seems to span 2 rows. The symbol "GP" is vertically centered across 2 rows. The name "Silty gravels, gravel-sand silt mixtures" also spans 2 rows. Then:

- Sub-row 1: Good to excellent | Slight to medium | Very slight | Fair to poor

- Sub-row 2: Good | Slight to medium | Slight | Poor to practically impervious

And similarly for GM/GC:

The symbol "*d GM --- u GC" spans 2 rows. The name "Clayey gravels, gravel-sand-clay mixture" spans 2 rows. Then:

- Sub-row 1: Good | Slight to medium | Slight | Poor to practically impervious

Wait, but I only see one row for GM/GC. Let me count the rows again.

Looking at the image, within the Gravel group, I count the horizontal lines:

1. Top of table

2. After GW row

3. After the "Poorly graded" row

4. After the "Silty gravels" row (GP first sub-row)

5. After the blank row (GP second sub-row)

6. After the "Clayey gravels" row (GM/GC)

7. Bottom of Gravel group

So that's 5 rows. And the GP symbol appears to be between rows 3 and 4 (vertically centered). The name "Silty gravels..." also appears to span rows 3-4.

OK so let me try this structure:

Yes! This makes much more sense. GP spans 2 rows with the name also spanning 2 rows, and the last 4 columns have different values.

Similarly for the Sand group:

Hmm, but for the sand group, I don't see a symbol spanning 2 rows. Let me look...

Actually, for the sand group, the symbol column seems to have:

Row 1: SW

Row 2: (blank)

Row 3: (blank)

Row 4: (blank)

Row 5: *d SM --- u SC

And the name column:

Row 1: Well-graded sands or gravelly sands, little or no fines

Row 2: Poorly graded sands or gravelly sands, little or no fines

Row 3: Silty sands, sand-silt mixtures

Row 4: (blank)

Row 5: Clayey sands, sand-clay mixtures

Hmm, row 4 has blank name again. Maybe SP spans rows 2-4? Let me look...

Actually, I think the structure for the sand group might be:

Hmm, this is getting really complicated. Let me look at the image one more time.

OK, I think the issue is that the original table has some cells that span multiple rows, and the OCR/image makes it hard to determine exactly which cells span. Let me just go with a simpler interpretation that matches the visual layout as closely as possible.

Actually, you know what, let me look at the image from a higher level. The table is from a civil engineering textbook about soil classification (USCS - Unified Soil Classification System). The table summarizes soil characteristics.

In the USCS:

- GW, GP, GM, GC are gravel symbols

- SW, SP, SM, SC are sand symbols

- ML, CL, OL are silt/clay with LL

- MH, CH, OH are silt/clay with LL > 50

- Pt is peat

The table groups them as:

- Gravel and Gravelly Soils: GW, GP, GM, GC

- Sand and Sandy Soils: SW, SP, SM, SC

- Silts & Clays with LL

- Silts & Clays with LL > 50: MH, CH, OH (but symbols not shown)

- Highly Organic Soils: Pt (but symbol not shown)

For the gravel group, the sub-categories are:

- GW: Well-graded gravels

- GP: Poorly graded gravels

- GM: Silty gravels

- GC: Clayey gravels

But in the table, I see:

Row 1: GW - Well-graded gravels

Row 2: (blank) - Poorly graded gravels → this should be GP

Row 3: GP - Silty gravels → this should be GM

Row 4: (blank) → this might be a continuation

Row 5: GM/GC - Clayey gravels

Hmm, the symbols in the image don't perfectly match the standard USCS. Let me just transcribe what I see.

OK, I've spent way too long on this. Let me just do my best to transcribe the table as I see it, making reasonable assumptions about rowspans.

Let me look at the image one final time and note what I see in each cell:

**Gravel and Gravelly Soils group (first column spans 5 rows):**

Wait, I just realized something. Looking at the image more carefully, I think the first column might actually contain BOTH the group name AND the symbol in the same cell. The group name is on the left and the symbol is on the right, all within one column.

So for row 1, the first column cell contains: "Gravel and Gravelly Soils" (left) and "GW" (right), with rowspan=5.

For row 2, the first column is covered by rowspan.

For row 3, the first column is covered by rowspan.

...

But then where do GP, GM, GC appear? They can't be in the same cell that has rowspan=5.

Unless... the first column is actually TWO columns:

- Column 1a: Group name (rowspan)

- Column 1b: Symbol (per row)

And the header "Major Divisions" spans both.

Yes, I think this is the correct interpretation. Let me go with 7 columns.

Actually wait, looking at the image one more time, I see that the vertical line between the group name and the symbol is NOT a full table border. It seems like the group name and symbol are in the same column, with the group name left-aligned and the symbol right-aligned.

But then for rows 2-5, the group name cell is covered by rowspan, and the symbol needs to appear somewhere. If they're in the same column, the symbol can't appear in rows 2-5 because the cell is already spanning.

UNLESS the rowspan cell contains all the symbols. Like:

"Gravel and Gravelly Soils" with "GW" at top, "GP" in middle, "GM/GC" at bottom, all in one cell with rowspan=5.

But that would mean the symbols are positioned at specific vertical positions within the cell, which is hard to do in HTML without
tags.

OK, I think the most reasonable interpretation for HTML is:

The first column has the group name with rowspan, and the symbols are in a separate sub-column. The header "Major Divisions" spans 2 columns.

Let me go with this. 7 columns total.

Actually, wait. Let me look at the image one more time. I see the following vertical lines in the table:

1. Left border

2. Line after "Major Divisions" column (between group name/symbol area and "Name" column)

3. Line after "Name" column

4. Line after "Strength" column

5. Line after "Potential Frost Action" column

6. Line after "Compressibility" column

7. Right border

And within the "Major Divisions" column, I see a vertical line separating the group name from the symbol. So yes, there are 7 columns.

But wait, looking at the header row, "Major Divisions" appears to be centered over both the group name and symbol sub-columns. So the header cell has colspan=2.

OK, let me go with 7 columns:

1. Group name (colspan in header = 2, but in data rows it's just 1 column with rowspan)

2. Symbol

3. Name

4. Strength

5. Potential Frost Action

6. Compressibility & Expansion

7. Drainage Characteristics

Header:

Data rows for Gravel group:

Row 1:

Gravel and Gravelly Soils | GW | Well-graded... | ...

Row 2:

| Poorly graded... | ...

Row 3:

GP | Silty gravels... | ...

Row 4:

| | ...

Row 5:

*d GM --- u GC | Clayey gravels... | ...

Hmm, but row 4 having blank symbol and blank name is still weird. Let me think about whether GP spans 2 rows.

Looking at the image, I see that "GP" is positioned between rows 3 and 4 (vertically centered). And "Silty gravels, gravel-sand silt mixtures" also seems to span rows 3-4. So:

Yes, this makes sense! GP has two sub-categories with different frost action characteristics.

Similarly, for the sand group, let me check if SP spans multiple rows:

Hmm, I'm not sure about the exact rowspans for the sand group. Let me look at the image...

For the sand group, I see:

- SW is in row 1

- Row 2 has blank symbol, "Poorly graded sands..." name

- Row 3 has blank symbol, "Silty sands, sand-silt mixtures" name

- Row 4 has blank symbol, blank name

- Row 5 has "*d SM --- u SC" symbol, "Clayey sands..." name

I think SP might span rows 2-4, and the name "Poorly graded sands..." might span rows 2-3, with row 4 having a blank name (or the name spans rows 2-4).

Actually, looking at the image, I think:

- SP spans rows 2-4 (3 rows)

- "Poorly graded sands or gravelly sands, little or no fines" spans rows 2-3 (2 rows)

- Row 4 has blank name

Hmm, but that's still weird. Let me look at the horizontal lines in the image for the sand group:

Between rows 1 and 2: full horizontal line

Between rows 2 and 3: full horizontal line

Between rows 3 and 4: full horizontal line

Between rows 4 and 5: full horizontal line

So all rows are separated by full horizontal lines. This means no cells span multiple rows within the sand group (except the group name).

But then row 4 has blank symbol and blank name, which is odd.

OK, I think maybe the original table just has some blank cells, and I should transcribe them as blank. Let me just go with what I see.

Actually, you know what, let me look at this from the perspective of the USCS classification. In the USCS:

- GW: Well-graded gravel

- GP: Poorly-graded gravel

- GM: Silty gravel

- GC: Clayey gravel

The table seems to have:

Row 1: GW - Well-graded gravels ✓

Row 2: (should be GP) - Poorly graded gravels ✓ (

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ACKNOWLEDGMENT OF SPONSORSHIP

This work was sponsored by the American Association of State Highway and Transportation Officials, in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Research Council.

DISCLAIMER

This is the final draft as submitted by the research agency. The opinions and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, or the individual states participating in the National Cooperative Highway Research Program.

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PART 2—DESIGN INPUTS

CHAPTER 1

SUBGRADE/FOUNDATION DESIGN INPUTS

This chapter provides procedures and recommended guidelines for determining the design parameters of the subgrade soils or foundation for use in new pavement designs. The chapter is divided into four sections: characterization of pavement foundations, subsurface explorations, laboratory testing, and foundation improvements and strengthening.

2.1.1 CHARACTERIZATION OF THE PAVEMENT FOUNDATION

The foundation must be characterized, regardless of whether the design procedure is to be applied to an existing pavement or a new pavement. The support for new and existing pavements is the roadbed soil or embankment, since the thickness and properties of all layers above this level are to be determined or analyzed as part of the design process.

The basic input data set for characterizing the subgrade or foundation is the same for the design of both flexible and rigid pavements. If sufficient data are unavailable for characterizing the foundation, the pavement designer may use the default values provided in the Guide. This provision allows for the use of hierarchical design methodologies that were discussed in the previous section, thus minimizing agency design costs, but at the increased risk of over-designing the pavement structure.

Different means for subgrade or foundation characterization alternatives exist, including:

Laboratory testing of undisturbed or reconstituted field samples recovered from the subsurface exploration process.
Nondestructive testing of existing pavements found to have similar subgrade materials.
Intrusive testing such as the Dynamic Cone Penetrometer (DCP)
Reliance on an agency's experience with the subgrade type.

All of these alternatives are covered in the Guide; however, laboratory testing and nondestructive deflection testing (NDT) are recommended as the primary characterization methods. An agency's experience can and should supplement these two methods. NCHRP Synthesis 278, Measuring In Situ Properties of Pavement Subgrade Soils (1), and NCHRP Synthesis 247, Stabilization of Existing Subgrades to Improve Constructability During Interstate Pavement Reconstruction, can be used to supplement the information presented (2). The agency's experience should also be used to select subgrade improvement techniques for problem soils unique to their region, such as collapsible, expansive, frost-susceptible, and saturated soils.

This Guide also addresses potential differences between the alternative methods of characterizing the foundation. The potential differences between the resilient modulus of the foundation or subgrade soils, as determined by backcalculation techniques or measured in the laboratory, are presented and discussed in two FHWA reports (3, 4). The important point for

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characterizing the foundation is that the design values or inputs determined by different methods are different, and that difference must be recognized in the design process.

Support characterization, as used in this context, refers to the process of characterizing the properties of the existing soil strata and any new or existing materials that make up the pavement. These include the surface layers, base and subbase layers, and other special pavement features. Special details of characterization requirements for each pavement material are described in greater detail in PART 2, Chapter 2. The characterization techniques for the pavement materials and subgrade soils will be hierarchical, ranging from default values for the different materials and soils to comprehensive laboratory and field testing for critical project types.

Layered resilient modulus (specifically, resilient modulus or approximations of the modulus of elasticity or Young's modulus) is the property needed for pavement design and analysis. Three basic methods can be used to obtain the resilient modulus of each structural layer in the pavement:

Laboratory repeated load resilient modulus tests.
Analysis or backcalculation of NDT data.
Correlations with other physical properties of the materials.

For all new designs, particularly for critical projects, repeated load resilient modulus tests are needed to evaluate the effects of changes in moisture on the resilient modulus of a particular soil. The latest version of AASHTO T 307, Resilient Modulus of Unbound Granular Base/Subbase Materials and Subgrade Soils, should be used for measuring the resilient modulus of a soil in the laboratory. However, NCHRP Project 1-28A resulted in a resilient modulus test procedure which can also be used. There are differences between the two procedures and the designer should be cautious to ensure that the values used are consistent with their local calibration procedures. For rehabilitation designs, however, the use of backcalculated elastic modulus to characterize the existing structure and foundation is suggested because it provides data on the response characteristics of the in situ soils and conditions. ASTM D4694 (Deflections with a Falling Weight Type Impulse Load Device) and D4695 (Guide for General Pavement Deflection Measurements) are standards that can be used for measuring the deflection basins along an existing roadway. ASTM D5858 (Guide for Calculating In Situ Equivalent Elastic Moduli of Pavement Materials Using Layered Elastic Theory) is a standard that can be followed for backcalculating layer elastic modulus from deflection basin data. Both laboratory and NDT procedures can be used to produce the resilient modulus of the foundation soils needed for design. The method used to determine the resilient modulus is discussed in PART 2, Chapter 2.

2.1.2 SUBSURFACE CHARACTERIZATION FOR PAVEMENT DESIGN

The horizontal and vertical variations in subsurface soil types, moisture contents, densities, and water table depths must be considered during the pavement design process. These elements can be quantified through the implementation of proper field (subsurface investigation) and laboratory testing programs. More importantly, special subsurface conditions, such as swelling soils and frost-susceptible soils, must be identified and considered in pavement design. This

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section of the Guide provides guidelines on how to identify and address these special subsurface conditions. Specifically, minimum recommendations are provided for determining the subsurface soil profile, conditions, and the design resilient modulus.

2.1.2.1 Subsurface Exploration

The objective of subsurface investigations or field exploration is to obtain sufficient subsurface data to permit the selection of the types, locations, and principal dimensions of foundations for all roadways comprising the proposed project, thus providing adequate information to estimate their costs. More importantly, these explorations should identify the site in sufficient detail for the development of feasible and cost-effective pavement designs.

The subsurface investigation for any project should be sufficiently detailed to define the depth, thickness, and area of all major soil and rock strata that will be affected by construction or impose special problems for the construction and long-term performance of the pavement structure. Disturbed and undisturbed samples of the subsurface materials must be obtained for laboratory analyses (and/or tested in the field) to determine their engineering properties. The extent of the exploration program depends on the nature of both the project and the site-specific subsurface conditions. Thus, to begin the process, a boring layout and sampling plan should be established to ensure that the vertical and horizontal profile of the different soil conditions can be prepared.

To acquire reliable engineering data, each job site must be explored and analyzed according to its subsurface conditions. The engineer in charge of the subsurface exploration must furnish complete data so that an impartial and thorough study of practical pavement designs can be made. Suggested steps are listed below:

Make a thorough investigation of the topographic and subsurface conditions.
Conduct exploratory borings at spacing and depth prescribed by the engineer. The spacing and depth of these borings depend on the variability of the existing soil conditions, both vertically and horizontally. These borings should also be used to determine the water table depth. Take sufficient and appropriate auger, split tube, or undisturbed samples of all representative subsoil layers. The soil samples must be properly sealed and stored to prevent moisture loss prior to laboratory testing. Prepare boring logs and soil profiles from this data.
Classify all soils using the AASHTO (or Unified) soil classification system. Table 2.1.1 relates the Unified soil classification of a material to the relative value of a material for use in a pavement structure.
Moisture-density tests should be used to determine the compaction characteristics for embankment and/or surface soils and untreated pavement materials. AASHTO T99, Moisture-Density Relations of Soils Using a 2.5 kg (5.5 lb) Rammer and a 305 mm (12 in) Drop, should be used for medium to high plasticity fine-grained soils, whereas, AASHTO T180, Moisture/Density Relations of Soils Using a 4.54-kg [10-lb] Rammer and 457-mm [18-in] Drop, should be used for coarse-grained and low plasticity fine-grained soils. The degree of compaction required for the in-place density should be expressed as a percentage of the maximum density from the specified test procedure.

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