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Additional Information ● MCHRP 71-9, Moisture, Density, and Slope requirements in High Fills ● MCHRP 74-1, Design Criteria for Cut slopes in Loess ● MCHRP 75-1, Strength and Drainage of a Rocky Residual Soil ● MCHRP 76-1, Evaluation of a One-Point Compaction Test for Missouri Soils ● MCHRP 79-1, Engineering Properties of Three Problem Earth Materials ● Physical Studies of Pennsylvanian Shale in a High Fill ● Procedures for Design of Earth Slopes Using LRFD

Commentary Boxes These boxes will provide supplemental information or commentary on application of specific provisions, supporting documentation for the provisions, or explanations for how the provisions should be applied.

This article provides requirements for design of earth slopes commonly encountered in transportation rights of way. Such slopes commonly include embankment slopes for fills and bridge approaches and excavated slopes in cut sections of roadway. Earth slopes should be designed to maintain stability for conditions and loads that can reasonably be expected to be encountered throughout the life of the slope based on available information regarding site conditions and anticipated loadings. Earth slopes should also be designed so that excessive deformations are not experienced throughout the life of the structure or so that deformations that do occur do not adversely affect the travel way.

In the context of these guidelines, “design” of earth slopes generally involves selection of some combination of the following to produce a final slope that will satisfy performance requirements:

1) Slope geometry to include slope inclination, slope width, and slope height,

2) Slope materials to include selection of fill materials for embankments,

3) Materials and methods to provide for appropriate drainage of surface and/or groundwater,

4) Materials and methods for reinforcing a slope to provide necessary stability, and

5) Loading conditions

For any given slope, some of these parameters will be constrained to satisfy site or project specific requirements so the specific parameters that can be varied to produce acceptable performance will be case dependent.

The remainder of these guidelines is organized into three different sections. EPG 321.1.1 describes general requirements and limits for earth slopes that can be considered routine. The majority of slope designs will be established based on the provisions in this section. EPG 321.1.2 describes provisions that apply for special slope stability problems to include design of remedial measures for sites where slides have occurred, evaluation and design of relatively large embankments on soft soils, and other complex slope stability problems that generally involve considerable risk and potential expense.

321.1.1 Slope Inclination for Preliminary Geotechnical Report

The provisions of EPG 321.1.2 and EPG 321.1.3 shall be followed for site/location specific stability analyses and for overall stability evaluations for retaining walls and spread footings.

The majority of soil and rock slopes designed for transportation right of way will be designed based on the provisions of this section. Complex slope stability cases (e.g. cases where there is uncertainty regarding likely loading or ground conditions), cases where the consequences of failure are great (e.g. slopes supporting foundations for bridges or retaining walls, unusually large fills, etc.), or cases where a slide has already occurred shall be designed according to EPG 321.1.2 and EPG 321.1.3.

321.1.1.1 Soil Slopes

For design of routine slopes without notable complications, the guidelines provided in Table 321.1 shall be used to select an appropriate maximum slope inclination based on the soil/rock types present at the specific site. These recommendations should be considered along with other factors that may influence the stability and performance of slopes in establishing final design recommendations. Factors such as presence or absence of structural foundations, adverse seepage conditions, susceptibility to inundation, or presence of notably poor soil/rock, etc. may dictate use of flatter slopes. MCHRP Report 79-1 provides recommendations for handling of notoriously problematic soils including gley, Cheltenham claystone, and Maquoketa clay shale. Soils classified as OH, OL and MH in the ASTM classification are rare and, if encountered, will require special design and/or handling.

Table 321.1 Guide for Selection of Slope Inclination for Routine Design

Geologic Origin	Glacial, Alluvial and Loessial Soils (Rock-free Residual Soils Derived from Shale, Claystone and Siltstone)					Residual Soil with Admixed Chert or Rock Fragments 4	Class C 5
General Description	Sand1		Silt/Loess2	Clay of Low Plasticity	Clay of High Plasticity3
ASTM Classification	SP,SM	SW,SC	ML, ML-CL	CL	CH	CL,CH,GC
Backslope	2.5H:1V	2H:1V	2.5H:1V	2.5H:1V	3H:1V	2H:1V	(Standard)
Fill Side Slope	2.5H:1V	2H:1V	2.5H:1V	2.5H:1V	3H:1V	2H:1V	2H:1V
Fill Spill Slope6:
H ≤ 20 ft.	2.5H:1V	2H:1V	2H:1V	2H:1V	2.5H:1V	2H:1V	2H:1V
H > 20 ft.	2.5H:1V	2H:1V	2.5H:1V	2.5H:1V	3H:1V	2H:1V	2H:1V
1 Soil caps to control erosion may be required for sandy soils other than SC soils.
2 Essentially vertical cut slopes may be used in loess when indicated to be practical by criteria outlined in MCHRP Report 74-1
3 Soils with extremely high PI (>50) should be used with extreme caution. Consideration should be given to wasting such materials or to use of even flatter slopes than those listed.
4 Consider flatter slopes where height of fill exceeds 40 feet and percentage of admixed granular material is less than 40 percent. Refer to MCHRP Report 75-1 for additional information.
5 Locally steeper slopes for Class C fills are practical only with special handling in excavation and placement.
6 Steeper slopes for low spill slopes assume that some form of slope protection be used to control erosion and/or seasonal moisture changes.

In most cases, a single value for slope inclination shall be selected for an entire project. See commentary for additional explanation and description.

Slope inclinations for excavated (cut) slopes shall be based on Table 321.1 and are to be carried uninterrupted beneath structures regardless of the height of cut. No steepening or warping of cut slopes shall be allowed, including cut slopes less than 20 ft. high.

321.1.1.2 Rock Slopes

Rock slopes in limestones, dolomites and sandstones are normally cut vertically or with a slight batter. Benches shall be provided at a vertical spacing not to exceed 30 ft. for cut slopes greater than 30 ft. high. Benches shall be a minimum of 10 ft. wide. Benches may be provided at the contacts of different formations (not necessarily at 30 ft.) and may vary in width.

Cut slopes in shale, siltstone, and other soft rocks shall be inclined at 2H:1V.

A flat bottom ditch with a minimum width of 10 ft. is required for all rock cut slopes.

Commentary on EPG 321.1 Slope Inclination for Preliminary Geotechnical Report

It is not intended that a slope’s inclination be varied with different soil horizons or for each soil type encountered. Rather, the slope inclination selected for a project shall be determined from an overall evaluation of the predominant soils encountered on a project. In the event of uncertainty, a conservative selection of inclination should be made, or site specific analyses according to EPG 321.1.2 shall be required. For example, A and B horizons will normally be relatively thin and less plastic compared to the C horizon. In such cases, the slope selection would logically be based on the C horizon as both the worst and predominant material to be encountered.

In most cases, a constant slope inclination shall be used throughout a given project. Slope inclinations should only be varied horizontally within a project if the alignment traverses two or more distinct soil types and only if it is known where material from any cut in the transition zone will be placed in fill. In the event of uncertainty, the more conservative slope should be extended to the point where the uncertainty is minimal.

A somewhat common source of confusion with Table 321.1 has been in selecting slopes for some CH residual soils. Note that the top column of the chart deals with geologic origin and this is the first division before looking at ASTM classification. A CH soil residual from rock with admixed chert or other rock fragments may be constructed with an inclination of 2H:1V. However, a slope inclination of 3H:1V is required for a CH residual soil derived from shale and claystone - and without admixed granular material.

A soil series that has been especially confusing is the Union. The lower part of the profile is typically a cherty, residual clay and the upper part is of wind-blow origin, usually CL. Soil survey recommendations have ranged from 2H:1V through 3H:1V. The 3H:1V has been based on the CH classification, which is a misinterpretation since it is a CH residual from carbonates and is cherty. The slope selected should be based on the predominant phase. If it is mostly CL loess with only a few feet of residual soil, the 2.5H:1V should probably be used. If it is almost all residual soil with only a few feet of windblown soil then 2H:1V should be adequate.

It should be emphasized that this chart is a guide. It is based on some theory and it is tempered by experience. It fits most situations, but there are exceptions. Some of the exceptions have been addressed in research reports. For the most part, this chart is based on stability considerations, but in one area it is been shaded a bit for erosion control purposes. This is for the ML loesses. When dealing with a very tight right of way situation, it may be practical to steepen slopes in this material to 2H:1V at the expense of some increased erosion problems or erosion control measures. If in doubt, shear tests can be done. To repeat, this chart is a guide - it is not carved in stone.

For grade separations, consideration should be given to selective grading of fill materials so that better materials are placed in fill spill slopes (i.e. beneath bridge abutments) so that the spill slopes can be steepened. While such handling will likely increase costs for fill placement, some additional handling can be justified if it results in reducing bridge length. Selective fill placement is not generally practical on stream crossings -- only on grade separations.

Table 321.1 permits spill slopes to be 0.5:1V steeper than side slopes for several soil types, but no steeper than 2H:1V, where the elevation differential between the toe of slope and grade at the bridge end is less than 20 feet. This recognizes the fact that the effective height of the spill slope will be reduced by at least 6 to 8 ft. because of the abutment headwall. This concept becomes more complicated at stream channel crossings where it is necessary to consider the depth and condition of the stream channel and their effect on bridge end location. For example, one might have CL glacial soils and a height differential between grade and toe of slope of some 15 feet. Table 321.1 requires 2.5H:1V side slopes and 2H:1V spill slopes for CL fill soils. However, if the stream channel is entrenched in CL soil another 15 feet deep so that the total height differential is greater than 20 feet, the bridge ends should be stepped back to or beyond a point determined by projecting a 2.5H:1V upward from the toe of the channel slope to intersection with grade. For typically steep channel banks, this will generally leave a substantial bench at natural ground level, which provides some room for bank sloughing without affecting the integrity of the spill slope. The spill slope would remain at 2H:1V but the bridge end would be located as if it were at least 2.5H:1V.

Now things get even more complicated. To this point, we have not really considered some of the possible complications to the stability of stream and channel slopes. There is a caution in the text beneath the slope selection chart that, "Factors such as foundations, seepage, susceptibility to inundation, etc. may dictate flatter slopes." Even ignoring foundations, which call for a special investigation, there is no simple way of considering the effects of water that will fit every case. Determining proper slopes in such circumstances involves consideration of a complex intermingling of factors such as flooding rate, height and duration, rate of recession, water velocity, scour potential, soil strength, weight, permeability, swell potential, and seepage rates, all further complicated by considerations of costs and the risks and consequences of failure.

The following general comments and guidelines are offered, however, to supplement the Guide for Slope Recommendations. First, use the chart to determine the slope (spill or side) you would use if water were not a factor. This is the slope to which you will make adjustments based on the following considerations.

For moderate stream flows of average flood duration, about 0.5H:1V flatter may suffice. For prolonged flooding followed by drawdown, 1:1 flatter may be appropriate. For intermittent or low-flow streams subject only to flash flooding, no flattening may be needed. Always inspect stream slopes for evidence of slides and sloughs and inspect the condition of adjacent structures over the same stream. Consider the width of the embankment; a 4-lane roadway is more likely to fail into a stream channel than a narrow county road or railroad fill. Consider also the consequences of failure; be more conservative for heavily traveled arterial roadways than for minor or rural supplemental roads for example.

Keep in mind that many stream channel slopes are stable only because of mature tree growth along the banks and the reinforcement provided the banks by the root structure. Remember that trees will be destroyed by construction, the roots will rot, and maintenance will prevent their regrowth. The net result will be less inherent stability where most needed.

Channelization has led to much stream bank instability, particularly in the northwest part of the state. It is especially prevalent in Lafayette, Atchison and Holt Counties. The invariable result is channel deepening, sometimes severe deepening. Careful examination of banks will often reveal massive slides, sometimes so massive as to resemble natural terraces. Always look at your county map; if the stream follows a straight line, it has been channelized. The streambed will have deepened and the banks, if not already failed, will be in precarious condition.

321.1.2 Slope Stability Analyses for Special Foundation Investigations

The provisions of this article shall be used for design and analysis of slopes for non-routine slope design. The most common instances for these “site specific” designs are for cases where slides have already occurred, cases with complex site and/or loading conditions, and cases with substantial consequences of failure (e.g. high fills, embankments of soft foundation soils, and slopes with bridge foundations). The provisions of this section shall also be followed for evaluation of overall stability for retaining walls and spread footings founded within slopes.

Global (overall) stability of non-routine slope designs or permanent MSE walls shall be performed by the Geotechnical Section or their agent and the global stability of temporary construction slopes and temporary MSE walls shall be performed by the contractor’s geotechnical and/or wall subcontractor. Compound stability of an MSE wall shall be performed by the Geotechnical Section or their agent if complex conditions exist such as changes in reinforced soil types or reinforcement lengths, seismic loading, sloping-faced structures, significant slopes at the toe or backslopes, or stacked (tiered) structures. The contractor’s geotechnical and/or wall subcontractor shall perform a compound stability analysis for any design changes, temporary wall and slope conditions or high surcharge loads due to construction processes.

321.1.2.1 General Considerations

Embankment (fill) and excavated (cut) slopes shall be designed to remain stable throughout the anticipated life of the slope without excessive deformations. The provisions of this article address the issue of stability, or the strength limit state.

321.1.2.2 General Procedure for Slope Stability Analysis Using LRFD Approach

The procedure for design of earth slopes following LRFD is quite similar to procedures for conventional ASD analysis, with two important differences. The first difference occurs in No. 4, where factored parameters are used as input for the slope stability analyses for LRFD analyses whereas unfactored parameters are used for the traditional ASD analyses. The second difference occurs in No. 5, where instead of comparing the computed factor of safety to some required or target factor of safety as is done in ASD, the computed factor of safety is compared to a limit value (= 1.0) indicating stability or instability. In this respect, the LRFD procedure is indeed more straightforward than current procedures in that the analysis target or limit is consistent for all stability cases for the LRFD procedure whereas the analysis target for conventional ASD procedures varies from one application to another. The result of these differences is simply that, for LRFD procedures, uncertainties in the analyses are accounted for through factoring of the input parameters whereas for ASD procedures the uncertainty is accounted for through a single factor of safety. By factoring individual input parameters, it is possible to more appropriately apply conservatism to the individual parameters involved in the analysis, and therefore to effect more consistent levels of safety across a broad range of cases. Both load and resistance factors in LRFD and factors of safety in ASD are intended to account for uncertainties involved in the respective analyses. They are simply different methods for accounting for these uncertainties.

Five common parameters are used for slope stability analyses including the soil (total) unit weight, $\mathit{\gamma }$, undrained shear strength, su, Mohr-Coulomb shear strength parameters, c and $\mathit{\varphi }$ (or $\bar{c}$ and $\bar{\varphi }$ in the case of effective stress analyses), and the pore water pressure, u. For the current implementation, neither soil unit weight nor pore water pressures are factored. Soil unit weight is not factored because it generally contributes little towards the reliability of an earth slope so that appropriate values for the load factor would only be slightly greater than 1.0 (generally less than 1.03). The variability and uncertainty in the unit weight is thus accounted for in the resistance factors for soil shear strength. In contrast, pore water pressures, and the variability/uncertainty in pore pressure, has a dramatic influence on the reliability of a given slope. Unfortunately procedures for rationally estimating and factoring pore pressures for LRFD analyses have not yet been established so procedures for handling pore water pressures, or piezometric lines and other constructs used to model pore water pressures, remain unchanged and should be estimated following procedures identical to those used for traditional ASD procedures. Commercial slope stability analysis programs are not readily available for AASHTO LRFD Bridge Design Specifications (LRFD BDS) procedures. Therefore, designs today might be performed by traditional (non-LRFD) methods and with existing slope stability programs. The resistance factors of 0.75 and 0.65 (LRFD BDS 11.6.3.7) are approximately equivalent to non-LRFD factors of safety of 1.30 and 1.50, respectively.

Global stability (i.e., overall and compound stability) of the wall shall be performed in accordance with FHWA GEC 011 and LRFD BDS 11.6.3.7 and 11.10.5.6.

Stability analyses for short-term, undrained conditions (generally associated with conditions during or shortly after construction) and long-term, drained conditions (generally associated with conditions long after construction) should be performed.

Design of earth slopes according to LRFD concepts can be confusing because current methods for stability analysis commonly produce a “factor of safety”, which is an artifact of traditional ASD methods. While potentially confusing, this fact does not preclude use of LRFD for design of earth slopes, but it does necessitate slight changes to current procedures used for analysis and design of earth slopes (Loehr et al., 2006).

The following procedure shall be utilized for design of earth slopes according to these provisions:

1) Establish site geometry and stratigraphy using available geologic information, boring logs, site surveys and plans, and other information available to the designer.

2) Estimate parameters for each respective stratum within the slope using available laboratory test results, empirical correlations, back-calculations, and other available information.

3) Estimate anticipated pore pressure conditions (required only for effective stress analyses) based on available historical records and judgement.

4) Evaluate the factor of safety for the conditions established using appropriate slope stability analysis methods; and

5) Global Stability: Confirm the global stability of the wall system as per FHWA GEC 011 and LRFD BDS 11.6.3.7 to confirm it meets the factor of safety requirements for all applicable loading conditions.

Compare the computed factor of safety to the limit factor of safety (= 1.0):

a) If the computed factor of safety is approximately equal to or exceeds 1.0, the design is considered acceptable. Resistance factors (RF) used in LRFD slope stability analysis and Factor-of-Safety (FS) results from non-LRFD slope stability analysis are presented below.

1) RF=0.75 (FS = 1.30) where the geotechnical parameters and subsurface stratigraphy are well-defined, and the slope or MSE wall DOES NOT support or contain a structural element (i.e., building, bridge abutment, etc. that is located within the critical failure surface) and for any temporary MSE wall.

2) RF=0.65 (FS = 1.50) where the geotechnical parameters and subsurface stratigraphy are highly variable, are based on limited information, or the slope or MSE wall supports or contains a structural element such as bridge abutment fill.

3) RF=0.9 (FS = 1.1) should be used for seismic analysis slopes involving or adjacent to walls and structure foundations.

b) If the factor of safety is significantly greater than 1.0, changes to reduce the computed factor of safety may be considered if significant cost savings can be realized.

c) If the factor of safety is less than 1.0, the designer must consider alternative measures to increase the factor of safety and repeat the procedure until a factor of safety approximately equal to 1.0 is achieved.

Current procedures will factor the undrained shear strength, s_u, or the Mohr-Coulomb shear strength parameters, c and ϕ (or $\bar{c}$ and $\bar{\varphi }$). Resistance factors for factoring of these parameters are provided in EPG 321.1.2.4.

321.1.2.3 Load Factors

Despite the fact that stability is really a strength limit state, load factors associated with the Service I limit state are used for all loads by convention with current AASHTO LRFD procedures. This position makes some sense when considering earth loads that often have little variability and uncertainty but may not make sense when stability is dominated by loading from bridge or other structures.

Loads to be used for stability evaluations shall be factored according to the Service I limit state for non-routine slopes or MSE walls that do NOT support a structural element and Strength I limit state for MSE walls and slopes supporting or containing a structural element (LRFD BDS Section 11.6.3.7).

Loads to be applied to slope stability analysis include:

Dead Loads: DC, DW, and EV

Earth Loads: EH (applies to slopes with walls only), ES, and DR

Live Loads: LL and PL

• LL in LRFD BDS is equated to a live load surcharge where LL (or q)=γ_q*h_eq, γ_q is the unit weight of live (125pcf), and h_eq is the equivalent height of soil (ft) for vehicular load. The variable h_eq is dependent on height, orientation and proximity to traffic; refer to LRFD BDS Tables 3.11.6.4.1-1 and 3.11.6.4.1-2 for h_eq and FHWA GEC 011, Appendix C for an example application.

Accordingly, a load factor of 1.0 shall be used for all applied loads, including “surcharge” loads associated with foundations or other surface loads.

321.1.3 Serviceability

The serviceability check for settlement of embankments is primarily intended for use as a criterion for deciding whether bridge approach slabs are necessary and justified, and potentially for establishing when settlement has occurred to a sufficient extent to complete construction in cases where staged construction is planned, or when final paving of a project is being postponed until embankment settlements will be less than established tolerable limits.

The serviceability limit state requirements of this section are intended to provide for evaluation of the potential for excessive settlement of embankments. The primary application of this section is expected to be for predicting settlement of bridge approach embankments to establish whether bridge approach slabs are justified. The provisions of this section may also be utilized to establish whether the remaining settlement of an embankment is sufficiently low to proceed with final paving for projects where final paving is postponed to allow embankment settlement to occur.

321.1.3.1 General Considerations

In general, settlement at the surface of an embankment can arise from compression of the foundations soil due to the weight of the overlying embankment fill soils and compression of the embankment fill soils (e.g. due to wetting induced compression, etc.). Foundation settlement should be estimated using LRFD BDS 10.6.2.4.

321.1.3.2 Settlement Due to Compression of Fill

The compression of the embankment soils shall be computed as Equation 321.1.3.2

$S_{emb}^{\star }=\frac{0.02\cdot H_{fill}}{{\varphi }_{fill}}$

(consistent units of length)

Equation 321.1.3.2

Where H_fill is the total fill thickness and ϕ_fill is the resistance factor for embankment compression. The value of ϕ_fill shall be taken as 0.62.

Commentary on EPG 321.1.3.2

The estimate provided is based on observational data suggesting that the compression of embankments constructed following common compaction specifications is between 1 and 3 percent of the embankment height.

The equation will produce a value of settlement that has an approximately 1 in 150 chance of settlements exceeding that value.

321.1.3.3 Settlement Limits

A bridge approach slab should be utilized at bridge abutments if the total settlement approaches 3 inches.

For MSE walls settlement limit, see EPG 321.2.4.2 Walls.

321.1.4 References

Loehr, J.E., C.A. Finley, and D. Huaco (2006), Procedures for Design of Earth Slopes Using LRFD, Final Report to Missouri Department of Transportation, Research Investigation RI03-030.

MCHRP 75-1

MCHRP 79-1

Reese, L.C., W.M. Isenhower, and S-T Wang (2006), Analysis and Design of Shallow and Deep Foundations, John Wiley and Sons, 574 pp.

LRFD BDS

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321.2.3.3 General Procedures

The first step after receipt of a request from the Bridge Unit or district is a file search for soil survey reports, preliminary bridge reports, and foundation reports for adjacent structures. A packet of information, which includes plans, correspondence, and prior reports is assembled for field use. Next, the district is consulted for advice as to field conditions, problems with utilities, crops, landowners, etc. If site access conditions are especially bad, someone from the Geotechnical Section may visit the site to determine what equipment may be needed and how the site can be reached. As noted previously, either the Bridge Unit or the district's boring plan may be modified as appropriate given site conditions and constraints. Some of the considerations here include:

1. General knowledge of conditions in the physiographic or geologic area where the work is to be done. This strongly influences the kind of investigation which should be performed and how detailed it should be. For example, in the Springfield area, residual clay over heavily pinnacled rock is likely and the most important thing is to map the rock surface irregularities with a lot of auger borings to rock so that point bearing pile lengths can be determined. In the bootheel area, auger borings are virtually worthless and standard penetration test borings for design of friction piles are most important.

2. Site access conditions are a very practical consideration. It may just not be feasible to drill a hole in the middle of an urban interstate highway and it may be extremely difficult and/or expensive to drill one in the middle of a river. That is where judgments must be made about how necessary that particular boring is: can conditions be reasonably extrapolated from offset borings, would geophysical methods work as well, etc.? In many cases, the borings can be omitted with little risk. In other situations, considerable expense and trouble may be justified to get on location. This may involve a different type of equipment, hiring a bulldozer, mobilizing the portable barge, or temporarily blocking a lane of roadway. The most extreme access problems involve major river or lake crossings where barges, tugs, and support services must be provided by contract.

3. The third consideration involves foundation conditions actually encountered as the investigation progresses. This is a principal reason why all MoDOT foundation investigations are supervised in the field by trained personnel. If conditions encountered are different than anticipated, it is expected that the scope of the investigation will be adjusted as necessary to fit actual conditions.

4. As previously noted, the Geotechnical Section rarely makes recommendations for specific foundation types. The basic aim is to furnish the Bridge Unit with the information needed to develop designs for foundation types practical for a particular site. Several rules of thumb are helpful in deciding what is practical. For example:

a. Spread footings

Spread footings for bridges will not be considered unless foundation material has an unconfined compressive strength of 3 tsf or more, and such material is within a fairly shallow depth. If firm material is 10 feet or more below final grade line, then piles will be used.

b. Deep Foundations

MoDOT guidelines used to estimate how far to carry standard penetration tests for design of friction piles are 30 continuous feet of bearing strata with an N60 value of 20 or greater. It is normal practice to drill half again as deep, or at least 100 ft. in any case, to check depth to rock for a point-bearing option. Point bearing is usually a feasible option almost everywhere in the state except in the southeast lowlands or "bootheel" area where sands extend to depths of several thousand feet. Even here, however, a careful check must be made for the possible presence of soft clay layers within and just below the range of probable friction pile penetration.

c. Culverts with Floor Slab Omitted

Large box culverts may be built more economically if a floor slab can be omitted. The Bridge Unit feels this is generally feasible if rock is within five feet of flowline. So, where rock may be shallow, an attempt is made to drill auger holes every 25 ft. or so along each proposed wall. An attempt is made to judge the durability of the rock based on inspection of exposures and cores and knowledge of past performance of particular formations. The rock should have an RQD equal to or greater than 75 and should not be thin bedded. If rock is deeper, only a few auger borings to verify this fact may be sufficient.

d. Culverts with Compressible Foundations

Culverts, if built over compressible foundations, may require special investigations based on undisturbed sampling to determine need for camber to compensate for settlement and to assess the danger of joints opening due to spreading caused by settlement. In some areas of the state where this is a particular problem, structural collars are sometimes recommended around joints to control spreading and faulting and piping of silty soils into opened joints.

e. Retaining Structures

Analysis and design of retaining walls should be performed as per guidance in EPG 751.24 and FHWA GEC 011. For retaining structures, information is obtained for determination of nominal bearing resistance, resistance factor and angles of internal friction of the materials to be retained and the foundation material. The latter can be done by correlation to Plasticity Index (PI) for walls of low height (using the average correlation less one standard deviation), EPG 321.2.10.5, and by drained shear testing for higher and more critical structures. In some cases, it may be necessary to obtain undisturbed samples for testing in order to evaluate overall stability of the slope of which the wall will be a component. Evaluation of external stability including bearing resistance, sliding and global or overall stability is a Geotechnical Section responsibility and should be completed as per AASHTO LRFD Section 11 and FHWA GEC 011. If inadequate global stability is likely, possible solutions are evaluated - such as lowering the base of the wall, increasing the width to height ratio and excavation and replacement with rock fill, etc.

f. Spill and Channel Slopes

The Geotechnical Section attempts to furnish overall guidance on prudent slope selection. This was done first by development of criteria, based on soil type and geologic origin, which is used by the district geologist in making project slope recommendations. Secondly, a review is made, often by specific request of the Bridge Unit, of the adequacy of embankment stability in the vicinity of the bridge ends, particularly at stream crossings. Geotechnical recommendations may affect bridge length, the fill end slopes, and erosion control measures for the channel banks. Often, for example, evidence will be found of channel bank failures which reflect a need for bank stabilization or bridge lengthening.

g. Special Investigations

These investigations were discussed in Section I where it was noted that, while normally initiated by the district as part of the soil survey, a problem may not be identified until final bridge soundings are being done. Undisturbed foundation sampling is done or supervised by a geologist, engineer, or senior technician. Large diameter samples are strongly preferred, often of 5 in. diameter, although 3 in. diameter samples are also commonly taken. In very soft soils, piston samplers are used in lieu of the normal Shelby tubes. Continuous undisturbed sampling is preferred, with frequent use of a 5 in. sampler, then a 3 in. sampler, followed by pushing a split spoon for inspection before cleaning the hole and restarting the cycle. MoDOT practice differs from that of many agencies in that soil samples are routinely extruded in the field. This permits thorough inspection and logging, obtaining field moisture and Atterberg Limits Classification samples, and preliminary field testing with the Torvane and Pocket Penetrometer. Most important, it permits the technical supervisor to develop a good feel for the problem as the investigation progresses. Samples are selected and designated at this time for certain types of testing, wrapped in foil, and sealed in wax in cartons for transport back to the lab.

Direct Shear Testing

For a typical problem involving an embankment settlement and stability problem, the lab testing program will include moisture contents, Atterberg limits, consolidation tests, unconfined compression, and drained, direct shear tests, all supplemented by Torvane and Pocket Penetrometer tests. Stability analyses are performed using a computer program, either circle analysis (Bishop), block and wedge (Spenser), or both as may be most appropriate for particular circumstances. Total strengths are used to assess the initial or rapid construction case. Effective stress analyses are used to assess fully consolidated conditions as well as intermediate degrees of consolidation. This data can be interpreted to assess the need for controls on rate of construction.

Amount of settlement estimates have been found to be fairly accurate. Actual rates of settlement are usually, but not always faster, than predicted. If a predicted time of settlement appears critical, office calculations are checked by doing field permeability tests and back figuring coefficients of consolidation. Usually, field perms will indicate much faster drainage, but sometimes agree very well with predictions based on laboratory tests. Before using vertical sand drains or any very expensive solution, field permeability testing should be done.

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321.2.3.4 Division of Responsibility

1. The Bridge Unit or district prepares a sounding layout with a suggested boring plan.

2. It is the geotech's responsibility to adjust or modify that plan as necessary to accomplish the objectives previously outlined, based upon the site conditions and practical access problems which may be encountered.

3. The geotech should identify and investigate any geotechnical problems which may preclude or adversely affect the proposed design and be prepared to offer recommendations for alternative designs or design modification.

4. Retaining walls. The Bridge Unit or District is responsible for checking all aspects of structural (internal) stability and for evaluating external stability with respect to overturning, sliding (at the base of the wall) and bearing failure. The Geotechnical Section is responsible for furnishing the data inputs necessary for the external stability checks and for evaluating overall or global stability included slopes for which the proposed wall may be a component.

5. Bridges. The Bridge Unit will design the foundation units in almost all cases but may ask for design assistance in certain instances. It is the geotech's responsibility to furnish data inputs of the type and quantity required to design those foundation types which are technically and economically feasible at each site. This infers that the geotech must have the capability and knowledge, and must have developed the information necessary to design the foundation if requested to do so. Keep in mind that a foundation cannot be designed in isolation. You can design an individual pile or footing but you must also know column and bent loads, group or cluster effects, embankment "drag loads", etc. and understand the interactions of the resulting stresses.

6. Nominal Bearing resistance. "Nominal bearing resistance" is not an intrinsic soil property but rather is a value based upon intrinsic soil properties as influenced by a specific arrangement of specific types, dimensions, and loadings of foundation units and the resulting distributions of stresses. It is not to be confused with "presumptive bearing values" or any specific measure of soil strength. While in most instances the distinction may seem academic, it can be a critical distinction. Examples: (1) A single square footing will have a different "nominal bearing resistance" than a strip footing or a rectangular footing and that of either type may be adversely affected by the proximity of another bearing unit. (2) Similarly, the capacity of a single friction pile or a single earth anchor may be reduced by the proximity of similar units. In any case, "nominal bearing resistance" is influenced not only by the factor of safety against failure (the usual criterion) but also by considerations of allowable deformations in the structure. The underlying reason why higher factors of safety against bearing failure are utilized than for other failure modes is to limit deformation. Keeping unit loads near the unconfined compressive strength (not in excess of about 1.2 Qu) keeps loads at or below the preconsolidation value of the soil.

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copy 321.2.3.5

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321.2.3.5 Practical Considerations

1. "Proofing" of Foundations. This touches on how foundations are actually built. If point bearing piles are used, the adequacy of the rock supporting the tip is "proofed" in excess of in-service loads by the dynamic stresses associated with driving the pile so there is relatively little cause for concern about the possibility of a void or cavern beneath the pile tip. This affects the conduct of the foundation investigation. A core may be irrelevant and it may be sufficient to rock bit five feet or so into rock in a couple holes and simply auger to rock (augering deep enough to be sure it's not a boulder) in the rest of the borings - even omitting many holes if rock is deep and of relatively constant elevation. Footings and drilled shafts on the other hand are loaded statically as the bridge is built. "Proofing" must be done by borings, either during the foundation investigation or as a construction requirement after the excavation is completed and prior to placing steel or pouring concrete. Drilled shafts often have very high unit loads so cores and even compression tests of the recovered core may be important, especially with weaker rock types. In hard rock, both cored and rock-bitted holes should be advanced to a significant depth below probable tip elevation to detect possible cavities or soft zones. Of course, if the exact location of the drilled shaft is unknown, only a few deep borings may be sufficient for preliminary design providing the contract is structured to require confirmation borings at each shaft location during construction.

2. Construction Problems. In most cases, investigative techniques are clear cut and the scope of the investigation may be less detailed when only one type of foundation is feasible. However, the scope of investigation should be influenced by considerations of the possible consequences of a change in foundation type during construction. If spread footings on rock are anticipated but no rock is found at one column, then a pile driver must be brought in. If there is no bid item for that type of work, then the price must be negotiated. The contractor will likely ask for an additional working day and claim severe impact costs, etc. For these reasons, more thorough work (at least in numbers of borings) are needed where spread footings are anticipated than for most other foundation types. Of course, being shallow, the borings should be completed more quickly. The reverse circumstance is less critical. If piles are planned and a suitable bearing stratum for footings is found on one bent, it is a simple matter to form and pour the footing while under running the piles at that location. This also has implications with respect to the scope of the foundation investigation. There is often little risk in omitting holes when piles are the logical foundation type and subsurface conditions appear uniform.

3. Footings on Hard Rock. For most simple structures, spread footings on hard rock will be of some practical minimum size so that unit loads will usually be 10 tsf or less and almost never in excess of 20 tsf. This is one reason why strength tests on hard rock are usually rather pointless and judgments on hard rock nominal bearing resistances are subjective, sometimes involving building code tables, RQD, and other empirical means. Keep in mind that the discontinuities of rock (bedding planes, joints, etc.) and, in particular, any loss while coring represent the real bearing limitations of that rock. You must rely on the driller's judgment as to why you didn't recover core. If the drill stem dropped quickly with little or no resistance, you have a void, a clay seam, or other soft material. Footings on soft rocks such as clay shales, claystones and even some sandstones and siltstones are another matter. Here the normal bearing resistances may be within a much lower range, requiring substantial enlargement of footings. In such cases, fairly detailed test data (SPT, Qu, and even pocket penetrometer data) may be needed to make judgments about allowable footing loads.

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copy 321.2.4.2

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321.2.4.2 Walls

1. MSE (Mechanically Stabilized Earth)

a. Sample about every 200' with shelby tubes. Two sample holes per wall minimum. Try to sample where the wall is the highest.

b. Take undisturbed soil samples to at least 10 ft. below footing elevation for Qu, Direct Shear, and Atterberg limits. Need to find internal angle of friction for retained and foundation material. If retained material is fill, get internal angle of friction from soil survey. If sand is encountered, take samples for gradations and atterberg limits as appropriate (seismic).

c. If soil is too rocky to use shelby tube, penetrate every 2 1/2' at least 10 feet below footing elevation. Take Atterberg samples, moisture samples, and pocket penetrometer readings.

d. If foundation material is too soft to use shelby tubes or osterberg sampler, run S.P.T. at 2.5 ft. intervals for at least 10 ft. below footing elevation. If still soft, go to 5 ft. increment. May use cantilever wall on piling. Rock bit or core at least 5 ft. of good rock or shale.

e. If rock is encountered above footing elevation or before you sample 10' below bottom of wall, core a minimum of 5 ft. below bottom of wall for MSE wall and 10' minimum below bottom of wall for cantilever wall.

f. Auger holes are usually laid out at about every 25'. If you are in uniform soil and rock is more than 5 ft. below the footing elevation, you can skip every other hole. Auger about 10' below bottom of wall or a little deeper if you suspect rock is close.

g. Settlement Limits

For Settlement analysis, see EPG 321.1.3 Serviceability, LRFD BDS 11.10.4.1 and FHWA-HIF-24-002 FHWA EC 011; section 4.4.7 and section 2.4.3 table 3.

Total Settlement - If the wall contains or supports a structure such as a bridge abutment, then limit the total settlement to 1-inch. If the wall DOES NOT contain or support a structure (e.g., wall supporting a roadway), then 2 inches of total settlement is allowed.

If total settlement is greater than the maximum limit, recommendations for ground improvements shall be provided.

Differential settlement along the face of the wall shall be evaluated in accordance with LRFD BDS 11.10.4.1. Slip joints in the MSE wall structure may be used to address isolated differential settlement issues. When widespread differential settlements is anticipated, provide ground improvement techniques and other recommendations.

Differential settlement along the length of the wall and from front to back of the wall shall be evaluated. If required, the limits of settlement remediation should be provided (ie how many feet in front of the wall and within the final designed reinforcement strap length or other length in the reinforcement zone).

2. Cantilever Walls or Spread Footings

a. Do similar to MSE wall except if rock is near footing elevation and wall may be set on rock, take 10 ft. of core (depending on wall height, 5' of good rock may be adequate) and if shale, run Qu's.

3. Sound Walls

a. Use S.P.T. and 3" shelby tubes to sample a hole about every 200 ft. of wall length.

b. Push 3" shelby tube 2.5 feet followed by the split spoon sampler.

c. Run S.P.T. and shelby tube on the first 5 ft. interval below bottom of wall. Take Qus (for determination of nominal bearing resistance and resistance factor), Atterberg samples, moisture samples, pocket penetrometer readings and torvane readings.

d. Continue to run S.P.T. at 2.5 intervals for at least 20 ft. below bottom of wall. Take Atterberg samples, moisture samples, and pocket penetrometer readings.

e. Amount of Rock Core.

i. If rock is encountered within 5 to 10' below bottom of wall, core 5'.

ii. If rock is less than 5' from bottom of wall, core 10'.

f. Augering

i. Locations same as MSE walls.

ii. Auger 25' below bottom of wall.

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copy 720.2.1

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720.2.1 MSE Wall Systems (Permanent MSE Wall Systems and Temporary MSE Wall Systems) Specified on Plan Details

The contractor shall be responsible for the internal and external stability of the structure including compound stability. Typically, the wall manufacturer/designer will provide these requirements for the contractor. For permanent MSE walls the owner (the Geotechnical Engineer of record or their agent) is responsible for the in-situ soil design parameters on plan details, foundation bearing capacity/resistance, settlement, and overall global stability. The contractor is responsible for verifying that the applied bearing stress is less than the provided bearing capacity/resistance. For staged bridge construction, see EPG 751.1.2.11 Staged Construction.

Global (overall) stability of a permanent MSE wall shall be performed by the Geotechnical Section or their agent and the global stability of a temporary MSE wall shall be performed by the contractor/wall designer. The compound stability of a permanent MSE wall and temporary MSE wall shall be performed by the contractor/wall designer.

Global stability (i.e., overall and compound stability) of the wall shall be performed in accordance with FHWA GEC 011, and LRFD BDS 11.6.3.7 and 11.10.5.6. For additional information, See EPG 321.1.2 Slope Stability Analyses for Special Foundation Investigations.

For MSE walls settlement limit, see EPG 321.2.4.2 Walls.

For temporary MSE walls the contractor will use the in-situ soil design parameters provided by the owner for the permanent MSE wall, but the contractor is responsible for global stability. Typically, the tallest portion of a temporary MSE wall is constructed at the same foundation elevation as the permanent wall except case 2, option 1 where the temporary wall is required to retain the improved foundation. Temporary MSE wall for staged bridge construction might have much higher height than permanent MSE wall height. It is important to ensure that global stability has been performed by the contractor for the temporary wall. For additional information, see EPG 751.24.2 Mechanically Stabilized Earth (MSE) Walls. Typical examples for temporary MSE walls are given below.

For temporary MSE wall, global stability shall be evaluated for the Strength Limit State. The resistance factor for global stability of the temporary MSE wall should be 0.75 (factor of safety 1.3).

Case 1: Temporary MSE wall (unimproved foundation ground)

Tallest section of temporary MSE wall height for a staged bridge construction is from top of temporary MSE wall (top of RDWY/approach slab or where the ground surface intercepts the temporary MSE wall facing) to bottom of temporary MSE wall (top of leveling pad of permanent MSE wall). By comparison, permanent MSE wall height is from top of coping to top of leveling pad.

Elevation – Temporary MSE Wall (Unimproved Foundation Ground)

Plan – Temporary MSE Wall (Unimproved Foundation Ground)

Case 2: Temporary MSE wall (Improved foundation ground required per Geotech report for permanent MSE wall)

Option 1: Without temporary shoring (Temporary MSE wall to retain improved foundation material)

Temporary MSE wall design height = Temporary MSE wall height in case 1 + leveling pad thickness + height of improved foundation ground.

Elevation – Temporary MSE Wall (Improved Foundation Ground: Option 1)

Note: For plan view, see option 2 minus temporary shoring.

Option 2: Temporary MSE wall with temporary shoring to retain improved foundation material

Contractor shall install temporary shoring prior to installing improved foundation ground for stage 1. Contractor shall trim temporary shoring at top of leveling pad of precast MSE wall prior to excavate for next stage. Temporary shoring will retain improved foundation ground material. Temporary MSE wall design height is same as case 1.

Plan – Temporary MSE Wall (Improved Foundation Ground: Option 2)

Elevation – Temporary MSE Wall with Temporary Shoring (Improved Foundation Ground): Option 2

Option 3: Without temporary shoring (No need to retain improved foundation material by temporary MSE wall)

Geotech shall approve this option and suggest required minimum extension of improved foundation material into next staging area. Generally, 3 feet to 5 feet space needed to extend improved foundation from face of the temporary wall in the next staging area. For next stage construction, contractor shall excavate small area at a time and replace with improved foundation material to minimize shifting of improved foundation material from previous staging since there is no temporary shoring to retain improved foundation material. Contractor may need to provide temporary shoring to support existing structure to construct this option in tight space. Temporary MSE wall design height = Temporary MSE wall height in case 1.

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copy 321.2.4.2

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copy 321.2.4.2

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copy 321.2.4.2

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