Performance of Soil-Bearing Spread Footings Supporting Highway Structures

doi:10.21203/rs.3.rs-3130234/v1

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Performance of Soil-Bearing Spread Footings Supporting Highway Structures

https://doi.org/10.21203/rs.3.rs-3130234/v1

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published 23 Feb, 2024

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Soil-bearing spread footings (SBSF) are becoming an appealing alternative to support highway structures. SBSF have many advantages compared to deep foundations, such as cost-saving, expedited construction, simple design, environmentally friendly, and less maintenance. This paper aims to evaluate highway structures' performance supported by SBSF at four sites in Ohio. This evaluation's outcomes are recommendations for future use of spread footings and potential limitations on their use. The project team reviewed the documented performance data, evaluated existing footings' performance, and compared them to the structural performance criteria. The team also examined the calculated settlement, provided an estimate of the tolerable structure settlement, compared the measured settlement to the predicted settlement, and compared performance to soil conditions. The collected data and the field review of all four sites included in this research showed that SBSF performed as designed, and there were no signs of settlements and/or rideability issues. SBSF were constructed following the manuals and procedures by the Ohio Department of Transportation (ODOT) and performed adequately. The paper provides recommendations for the method of settlement monitoring and the Ohio department of transportation plan notes.

Spread footings

Soil Bearing

Highway structures

Settlement

Performance

Spread footings on soil have many benefits compared to deep foundations. The benefits include cost saving, expedited construction, simple design, environmentally friendly, and less maintenance (1–4).

Shallow foundations are typically at least 30% more economical than deep foundations and can support many civil engineering structures (5). Foundations contribute significantly to the construction cost of concrete bridges; their cost ranges from 19 to 27% of the total bridge construction cost, depending on the construction method used and the bridge design system (6).

Spread footings are usually considered in conditions where deep foundation installation is not possible, for example, to a) accommodate the presence of aquifers, underground structures such as utilities, and obstructions beneath foundations; and b) generate less noise, ground vibrations, and movements of nearby structures. Construction of spread footings uses common materials and can be constructed with readily available labor and simple and small equipment. Construction is often easier and faster, and quality control is simpler than deep foundations. Because of these advantages, the construction of spread footing is supposed to provide a safe work environment and fewer claims (7).

A national FHWA survey of the geotechnical practices of the state DOTs was developed and distributed in 2007 (8). Forty-four (44) states responded to this survey. Survey results indicated that the average distribution of bridge foundation types considered by State DOTs across the United States is approximately 24% spread footings (11.5% founded on soils, 12.5% founded on rock) and 76% deep foundations (56.5% driven piles and 19.5% drilled shafts). The FHWA national survey identified the states with significant and moderate use (> 10%) of spread footings on soils to support highway bridges and the states with limited or no use (< 5%). Based on this survey, the FHWA concluded that some State DOTs could save time and cost if spread footings bearing on soils are used when appropriate to support bridges.

Civil engineering professionals must document and share data from case histories to promote spread footings for highway structures and report shortcomings. Understanding settlement and other behavior of spread footing foundations under various loading and environmental conditions associated with highway bridge structures is essential in encouraging their use in construction. Also, further verification of the performance prediction methods through such case histories contributes to the increased use of the spread footing foundation (9).

However, well-documented, comprehensive case histories must be established and made available to the bridge and geotechnical engineers to encourage their utilization. Despite previous shallow foundation studies' success, more research is needed to evaluate the performance of spread footings as a highway bridge foundation.

The use of spread footings may not be suitable or economical under specific design conditions, for example, the presence of deep soft soil near the ground surface or very high lateral loads (e.g., due to a major earthquake), and at sites with large scour or liquefaction depths (8).

This research aims to evaluate the performance of highway structures built on spread footings-bearing soils. The evaluation aims to identify potential limitations and future use of the structures. The research team completed the following tasks to meet the research goals:

Reviewed the documented performance data.

Evaluated the performance of existing footings and compared them to the bridge performance criteria.

Reviewed the calculated settlement.

Provided an estimate of the structure tolerable settlement at the foundation location.

Compared the measured settlement to the predicted long-term settlement.

Compared performance to soil conditions and calculated bearing pressures.

Many performance prediction methods have been proposed to estimate spread footing behavior (bearing capacity and settlement) based on the standard penetration test (SPT) or the cone penetration test (CPT) data. Examples of these include the work by Hough (10), Alpan (11), Meyerhof (12), Terzaghi and Peck (13), D'Appolonia et al. (14), Peck and Bazaraa (15), Schmertmann (16), and Schmertmann et al. (17).

Sargand and Masada (18) instrumented and monitored the settlements of four bridges supported on spread footings in Ohio. Settlements were collected during the stages of construction and after. The spread footing design methods presented in the AASHTO LRFD Bridge Design Specifications (2004) were validated based on the field performance data collected during the research. Twelve SPT-based settlement prediction methods for footings resting on cohesionless or slightly cohesive soils were evaluated considering the field performance data. A general performance analysis of spread footing foundations at bridge construction sites was made to draw some guidelines concerning the use of spread footings for supporting highway bridge structures. Detailed cost comparisons were made between spread footing and pile foundation options. Overall, the research project results indicated that: a) spread footing can be a viable option as a bridge foundation, and b) the design methods presented in the AASHTO LRFD Bridge Design Specifications (19) appear satisfactory.

Sargand et al. (20) instrumented and monitored over fifty spread footings at five highway bridge construction sites in Ohio. The spread footings' overall settlement among all the footings ranged from 0.19 to 36 mm, with an average of 20 mm. About 70% of the total settlement occurred before the deck construction. None of the footings experienced any significant differential movement problems. Limited data collected at the sites within six months after the bridge opened showed that the additional settlement induced by the live load application ranged between 1.27 and 12.7 mm, with an average of 4.32 mm. the acceptable settlement is 25mm.

Baus (21) monitored the settlement of 12 spread footings at three highway bridge sites in South Carolina. Total settlement varied from 10.16 to 55.88 mm. He compared the maximum settlement measured in the field to estimates made by six prediction methods (Alpan (11), Hough (10), Meyerhof (12), Peck-Bazaraa (15), Buisman-De Beer (22), and Schmertmann methods (16, 17)). He concluded that Peck and Bazaraa's (15) and Hough's (10) methods provided better settlement predictions.

DiMillio (23) surveyed the conditions of 148 bridges supported by spread footings on compacted fill in Washington. All bridges were in good condition, and none exhibited any safety or functional problems. He found that the bridges could easily tolerate differential settlement of 25.4 mm to 76.2 mm without severe distress. He estimated those spread footings were 50–60% less expensive than pile foundations.

Moulton et al. (24) examined the tolerable movement of bridges. He reviewed movements and damage data for 204 bridges on spread footings and piles. His study revealed that the average vertical movement of abutments was, on average, 69 mm for abutments supported on piles and 61 for abutments supported on spread footings. The average horizontal movement of abutments was, on average, 142 mm for abutments supported on piles and 155 mm for abutments supported on spread footings. It was concluded that in terms of foundation type, more abutments on spread footings were reported to have moved than abutments found on piles. However, abutments founded on piles experienced larger ranges and slightly larger average vertical and horizontal movements than did those founded on spread footings. These findings, coupled with the relatively larger number of pile-supported abutments that did move, tend to suggest that the mere use of pile foundations does not necessarily guarantee that abutment movements will be within tolerable limits, particularly for the case of rested abutments on fills.

The research sites information and analysis are collected off information from Report No. FHWA/OH-2021-04 Division of Engineering Research on Call Agreement 31796 Task 7 – Service Evaluation of Highway Structures with Soil-Bearing Spread Footings [25]. This section presents background information/data for four sites (MAH-680-2.83, CUY-77-14.35, FAI-33-13.09, and CUY/SUM-271-00.00/14.87), where spread footings were monitored during different phases of construction and surveyed between October 13 and 14, 2020.

MAH-680-2.83 Bridge

This is a four-span bridge (MAH-680-0283) carrying Vestal Road (Rd) over Interstate 680 (IR-680) in Mahoning County in northwestern Youngstown, Ohio. In 2016 rehabilitation work was performed for the bridge. The work removed the existing superstructure and three piers and raised the existing abutment seats. Therefore, new spread footings were constructed for the piers only. Existing spread footings at the abutments were used. The bridge is four-span continuous painted steel girders with reinforced concrete deck on new semi-integral abutments, new bearings, and new cap and column piers founded on spread footings. The footing width for Piers 1 and 3 is 4.4 m, while the footing width is 2.74 m for Pier 2.

Site stratigraphy consists of hard silt and clay (A-4) near the ground surface to an elevation of about 284 m, where a dense sand and gravel or sand layer (A-1-b, A-3) 1.5 to 4.5 m thick was encountered. Below was another hard layer of silt/ sand mixture (A-4) and a hard silt layer beneath it. Groundwater was not encountered. The bottom footing elevations for piers 1,2, and 3 are 286.5 m, 286.2 m, and 285.6 m, respectively. The average SPT N value stayed relatively constant, around 50 below the foundation depth for all substructures.

On October 6, 2020, a site visit was performed to the MAH-680-0283 bridge. The bridge deck and sidewalk were inspected for cracking and settlements; however, no indications of settlements or cracking were found. Piers were found to be in good condition; no settlement was observed. One of the PVC pipes over the settlement pins was observed to be broken off at the groundline. The existing bridge foundation was founded on spread footing functions as designed, and no signs of any settlements. Figure 1. shows a picture of the MAH-680-0283 bridge.

Recently, multiple pier footings were surveyed. Piers spread footings were monitored after pouring the footings' concrete, before and after beam placement, after pouring the deck's concrete, and after project completion. Table 1 presents the recorded monitoring data for the right and left monuments of each pier.

Table 1

MAH-680-2.83 Footings Settlement Monitoring Data
Stage	Elevation of Left Monument (m)			Elevation of Right Monument (m)			Date
Stage	Pier 1	Pier 2	Pier 3	Pier 1	Pier 2	Pier 3	Date
After Footings Poured	287.722	287.497	286.808	287.707	287.466	286.762	2016-05-31
Before Beams	287.710	287.487	286.808	287.704	287.466	286.762	2016-06-28
After Beams	287.707	287.484	286.805	287.707	287.463	286.759	2016-07-28
After Deck Pour	287.710	287.487	286.808	287.713	287.466	286.765	2016-10-05
Project Completion	287.710	287.484	286.805	287.707	287.466	286.765	2016-11-12
This Task	287.710	287.478	286.811	287.713	287.469	286.768	2020-10-13
Measured Sett. at Project Completion (mm)	12.00	13.00	3.00	0.000	0.000	-3.00
Measured Settlements under this Task (mm)	12.00	19.00	-3.00	-6.00	-3.00	-6.00

At project completion, the maximum measured settlement is about 13 mm. Recently, the final maximum measured settlement is about 19 mm. It should be noted that the minus values appear due to elevation reading tolerable error and refer to no settlement. After approximately four years, settlements did not change significantly. The settlement was within tolerable limits for these span lengths. The measured settlement to girder length ratio was 0.00067. This is well below the acceptable limit of 0.004, as documented by Felix Yokel (26).

The measured settlement was negative for some of the substructure units. The negative settlement can result from either settlement of the benchmark used or measurement error, as the surveying accuracy is to the nearest 3.175 mm.

CUY-77-14.35 Walls

The project consists of four cast-in-place (CIP) concrete cantilever walls (Walls 1,2,3 and 4) and a bridge replacement for Bridge No. CUY-1433 L&R over I.R. 490 and Ramps, south of Cleveland, Ohio. The bridge is a three-span continuous steel hybrid girder composite with a reinforced concrete deck on reinforced concrete piers and semi-integral abutments. The bridge is supported on 406 mm of cast-in-place reinforced concrete piles. Settlements were monitored for walls 1 and 4, the left rear abutment wing wall (Wall 2), and the right forward abutment wing wall (Wall 3).

The footing width for walls 1 and 4 is 3.8 m and 3 m, respectively. The footing width for walls 2 and 3 is 4.26 m.

On October 6, 2020, a site visit was performed. The cantilevered cast-in-place concrete wing walls were inspected and found to be plumb vertically; no leaning or sliding of the walls was observed. The existing bridge retaining walls foundation founded on spread footing is functioning as designed. Figure 2 shows the Right Forward Abutment Wing Wall (Wall 3).

CUY-77-14.35 Soil Profile

Wall 1 (I.R. 77 Sta. 72 + 19.25 to Sta. 74 + 20.54)

Fill was encountered below the pavement to a depth of 7.77 m and consisted of medium-dense to hard sandy silt (A-4) and dense coarse and fine sand (A-3). Natural soils were encountered below the fill to the termination depth of 12.2 m and consisted of medium dense, fine sand (A-3). Groundwater seepage was not encountered. The average SPT N value ranged from 23 to 46 under the wall footing.

Wall 4 (I.R. 77 Sta. 80 + 34.56 to Sta. 82 + 71.00)

Fill was encountered below the pavement to a depth of 7 m and consisted of medium-dense sandy silt (A-4) and dense gravel with sand (A-1-b). Natural soils were encountered below the fill to the termination depth of 12.2 m and consisted of soft to medium-stiff silt and clay (A-6), medium-dense to dense coarse and fine sand (A-3), and dense, fine sand (A-3). No groundwater seepage was encountered. The average SPT N value ranged from 15 to 45 under the wall footing.

Bridge No. CUY-1433 L&R over I.R. 490 and Ramps Wing Walls

Left Rear Abutment wing wall-Boring BB-106 (Wall 2)

Material visually identified as fill was encountered below the pavement to a depth of 9.75 m and consisted of medium-dense to very-dense gravel with sand (A-1-b) and dense coarse and fine sand (A-3). Natural soils were encountered below the fill to the termination depth of 27.4 m and consisted of medium-dense fine sand (A-3), medium-dense to very-dense coarse and fine sand (A-3), stiff to very-stiff silt (A-4), and medium-stiff to stiff silty-clay (A-6). Groundwater seepage was encountered at a depth of 16.3 m, and groundwater was encountered at a depth of 17.8 m. The average SPT N value ranged from 15 to 44 under the wall footing.

Right forward abutment wing wall (Wall 3)

Material visually identified as fill was encountered below the pavement to a depth of 9.75 m and consisted of dense sandy silt (A-4), dense silt (A-4), and medium-dense fine sand (A-3), and dense to very-dense coarse and fine sand (A-3). Natural soils were encountered below the fill to the termination depth of 27.4 m and consisted of medium-dense to very-dense fine sand (A-3), medium-stiff to dense sandy silt (A-4), and dense silt (A-4). Groundwater seepage was encountered at a depth of 17.8 m, and water was measured at a depth of 20.9 m at the completion of the drilling. The average SPT N value ranged from 14 to 41 under the wall footing.

CUY-77-14.35 Settlement Monitoring Data

Spread footings were monitored after pouring the footings' concrete and after project completion. Recently, footings were surveyed. Table 2 presents the recorded monitoring data for the monuments of the left rear abutment wing wall (Wall 2) and the right forward abutment wing wall (Wall 3). At project completion, the measured settlements are about 6.0 mm for Wall 2 and Wall 3. Recently, the maximum measured settlement is about 37 mm and 18 mm for Wall 2 and Wall 3, respectively. It should be noted that settlements did not change significantly except for Wall 2, monument 2.

Table 2

Bridge No. CUY-77-1433 Wing Walls Settlement Monitoring Data
Stage	Elevation (m)				Date
	Left Rear Abutment Wing Wall (Wall 2)		Right Forward Abutment Wing Wall (Wall 3)
	Monument-1	Monument-2	Monument-1	Monument-2
	22 + 91.63,19 m LT	22 + 74.98,19 m LT	24 + 19.2, 19 m RT	24 + 34.64, 19 m RT
After Footing Concrete Placed	205.325	205.338	207.380	207.383	N. A
Project Completion	205.319	205.332	207.374	207.377	N. A
Recent, this Task	205.319	205.301	207.362	207.368	2020-10-13
Measured Sett. at Project Completion (mm)	6.00	6.00	6.00	6.00
Measured Settlements under this Task (mm)	6.00	37.00	18.00	15.00

Table 3 presents the recorded monitoring data for the monuments of Walls 1 and 4. At project completion, the maximum measured settlement is about 3 mm for Wall 1 and Wall 4. Recently, the measured settlements ranged from 3 to 6 mm for Wall 1 and from 125 to 195 mm for Wall 4. Settlements did not change significantly for Wall 1. However, Wall 4 experienced excessive settlements due to the existing 0.5 to 1.0 m layer of soft to medium-stiff silt and clay (A-6), which was encountered immediately beneath the fill material at the elevation of 203 m. This layer is located 3.25 m below the bottom of Wall 4 footing. The wall was inspected and found to be plumb vertically; no leaning or sliding of the walls was observed. Therefore, the benchmark may have a problem.

Table 3

Walls 1 and 4 Settlement Monitoring Data
Stage	Elevation (m)				Date
	Wall 1		Wall 4
	Monument-1	Monument-2	Monument-1	Monument-2
	22 + 32.81, 19.5 m RT	22 + 60.96, 19.8 m RT	24 + 49.72, 18.9 m LT	24 + 94.64, 18.9 m LT
After Footing Concrete Placed	205.502	205.490	207.121	207.651	N. A
Project Completion	205.505	205.490	207.118	207.651	N. A
Recent, this Task	205.499	205.484	206.996	207.456	2020-10-13
Measured Sett. at Project Completion (mm)	-3.00	0.000	3.00	0.000
Measured Settlements under this Task (mm)	3.00	6.00	125.00	195.00

FAI-33-13.09 Bridge

This is a four-span bridge (FAI-33-1309) carrying Delmont Road over U.S. Route 33 (Lancaster Bypass) west of Lancaster in Fairfield County, Ohio. The bridge is a four-Span continuous composite steel girder bridge with semi-integral type abutments and cap and columns type piers on spread footings. The footing width is 2.4 m for the rear and forward abutment. The footing width is 4.25 m for all piers.

On October 5, 2020, a site visit was performed to the FAI-33-1309 bridge. The team inspected the embankment slopes, roadway settlement at abutments, concrete deck, and relative substructure orientation. The team found no evidence of settlements. The soils around the piers were found to be soft, but this was due to drainage. Some of the PVC caps were missing or damaged due to vandalism. The existing bridge foundations founded on spread footing are functioning as designed. Figure 3. shows the FAI-33-1390 bridge.

Field exploration was performed using five boreholes per the original geotechnical report. Each one of the five borings first encountered between 0.1 and 0.3 m of topsoil. Underlying the topsoil, the five borings typically encountered cohesive soils consisting of stiff to hard silt and clay (A-6) and silty clay (A-6) to depths of between 3.2 and 6.25 m. Some of these soils were organic in nature. Underlying these cohesive soils, each of the five borings generally encountered medium dense to very dense non-cohesive soils, including gravel with sand (A-1-b), gravel with sand and silt (A-2-4), fine sand (A-3), and coarse and fine sand (A-3). These soils were encountered to the completion depths of the borings in boring B-33 and B-34 and were encountered to depths of between 15 and 19 m in borings B-30, B-31, and B-32, where bedrock was encountered. It should be noted that material classified as silt (A-4) was encountered in boring B-34. However, this material was encountered at depths greater than 15 m. Bedrock was encountered in borings B-30, B-31, and B-32 at depths of between 15 and 19 m. The bedrock consisted of medium-hard broken

sandstone with RQDs of between 30% and 50%. Water seepage was encountered at depths of between 2 and 5 m. The average SPT N value ranged from 12 to 50 below the foundation depth for all substructures.

Spread footings were monitored after pouring the footings' concrete, before and after beams placement, after pouring the deck's concrete, and after project completion. Recently, footings were surveyed. Table 4 presents the recorded monitoring data for the right and left monuments of each pier. At project completion, the measured settlements ranged from 9 mm to 30 mm. Recently, the final measured settlements ranged from 3 to 64 mm. After approximately 19 years, settlements did not change significantly except for the rear abutment. It should be noted that monuments could not be located at the right rear abutment, left and right pier 3. As at these locations, survey monument information was not available; the survey crew established temporary benchmarks, which were tied into two ODOT benchmarks via GPS observations.

Table 4

FAI-33-13.09 Footings Settlement Monitoring Data
Stage	Elevation of Left Monument (m)					Elevation of Right Monument (m)					Date
Stage	Rear Abut.	Pier 1	Pier 2	Pier 3	F.R. Abut.	Rear Abut.	Pier 1	Pier 2	Pier 3	F.R. Abut.	Date
After Footings Poured	278.929	274.143	274.738	273.601	279.145	278.956	274.165	274.744	273.656	279.163	Between 06 − 05 and 07-12-2001
Before Beams	278.929	274.143	274.735	273.601	279.139	278.956	274.165	274.744	273.656	279.157	2001-10-19
After Beams	278.901	274.137	274.735	273.598	279.130	278.938	274.158	274.741	273.649	279.148	2002-03-21
After Deck Pour	278.910	274.131	274.731	273.595	279.136	278.932	274.152	274.735	273.646	279.154	2002-04-26
Project Completion	278.904	274.131	274.728	273.592	279.133	278.926	N. A^a	274.735	273.646	279.151	2002-08-05
Recent, this Task	278.865	274.146	274.735	273.741	279.121	279.063	274.155	274.738	273.835	279.145	2020-10-14
Measured Sett. at Project Completion (mm)	25.00	12.00	10.00	9.00	12.00	30.00	N. A^a	9.00	10.00	12.00
Measured Settlements under this Task (mm)	64.00	-3.00	3.00	-140^b	24.00	-107^b	10.00	6.00	-179^b	18.00

^a Not Available, ^b Monuments could not be located.

CUY/SUM-271-00.00/14.87

The CUY/SUM-271-00.00/14.87 and CUY/SUM-480-29.58/00.00 project calls for the design and construction of three (3) new retaining walls identified as RW-1 (WS1), RW-2 (SW1), and RW-3 (WS2) in Summit/Cuyahoga Counties, Ohio. These retaining walls were constructed in association with constructing two additional Lanes identified as S-W and W-S located along the outside shoulders of IR-271 SB and N.B. between the Summit County Line and Alexander Road, south of Cleveland, Ohio. Settlements were monitored for RW-1 (WS1).

On October 6, 2020, a site visit was performed by E.L. Robinson Engineering. The cantilevered cast-in-place concrete wall was inspected and found to be plumb vertically; no leaning or sliding of the walls was observed. The existing retaining wall foundation founded on spread footing is functioning as designed. Figure 4 shows RW-1 (WS1).

Field exploration was performed using two boreholes per the original geotechnical report. The subsurface soils encountered in both test borings were predominantly cohesive in nature and consisted of both fill materials and natural soils. The fill materials located above the natural soils consisted of silt and clay (A-6) and silty clay (A-6). The fill material's approximate thickness was 2.5 m in boring test B-007-1-13 and one meter in boring test B-007-4-13. Natural soils encountered above bedrock in boring test B-007-4-13 and to the termination depth in boring test B-007-1-13 consisted of sandy silt (A-4), silt, and clay (A-6), non-plastic sandy silt (A-4), and coarse and fine sand (A-3). Bedrock consisting of gray, severely to highly weathered shale was encountered at an approximate depth of 18 m in boring test B-007-4-13. The consistency of the cohesive soils ranged from "medium stiff" to "hard" but was generally "very stiff.” The relative density of the non-cohesive soils ranged from "dense" to "very dense."

Table 5 presents the recorded monitoring data for RW-1 (WS1). Settlements were not collected for this recently constructed wall. At project completion, the wall did not experience any settlements based on the provided data.

Table 5

RW-1 (WS1) Settlement Monitoring Data
Stage	Elevation (m)						Date
Stage	Sta. 988 + 69.80	Sta. 988 + 91.3	Sta. 989 + 16.69	Sta. 989 + 33.60	Sta. 989 + 50.51	Sta. 989 + 75.88	Date
After Footing Concrete is Placed	316.550	316.527	316.550	316.536	316.530	316.379	1/8/2019
After Wall Concrete is Placed and backfilled	316.550	316.527	316.550	316.536	316.530	316.379	7/26/2019
Project Completion	316.550	316.527	316.550	316.536	316.530	316.379	10/30/2019
Recent, this Task	N. A^a	N. A^a	N. A^a	N. A^a	N. A^a	N. A^a
Measured Sett. at Project Completion (mm)	N.A.	N.A.	N.A.	N.A.	N.A.	N.A.
Measured Settlements under this Task (mm)	N. A^a	N. A^a	N. A^a	N. A^a	N. A^a	N. A^a
^a Not Available, settlements data were not collected under this task

It should be noted that consolidation settlement calculations require soil parameters that are not included in the provided soil reports. The only soil report that provided such information is CUY/SUM 271-00.00/14.87 RW-1. Therefore, the estimated settlement for this wall is the total settlement.

Comparison Between Estimated and Measured Settlements

In this section, a comparison is made between the estimated and measured settlements. Measured settlements are presented in the previous sections of this report. Hough (10) is used to estimate the settlements of spread footing on cohesionless soil as specified in section 10.6.2.4 of the AASHTO LRFD Bridge Design Specifications.

Spread footing settlements should be estimated using computational methods based on laboratory or in-situ testing results or both. The soil parameters used in the computations should be chosen to reflect the loading history of the ground, the construction sequence, and the effects of soil layering.

Settlements of cohesionless soil can be estimated using Hough’s (10) method as explained in Equations 1 and 2. Generally, conservative settlement estimates may be obtained using the elastic half-space procedure or the empirical method by Hough (10). The Hough method has several advantages over other methods used to estimate settlement in cohesionless soil deposits, including express consideration of soil layering and the zone of stress influence beneath a footing of finite size. The subsurface soil profile should be subdivided into layers based on stratigraphy to a depth of about three times the footing width. The maximum layer thickness should be about 3 m.

$${\text{S}}_{e}={\sum }_{i=1}^{n}\varDelta {\text{H}}_{i}$$

$$\varDelta {\text{H}}_{i}={\text{H}}_{c}\frac{1}{{C}^{{\prime }}}\text{l}\text{o}\text{g}\left(\frac{{\sigma }_{o}^{{\prime }}+\varDelta {\sigma }_{v}}{{\sigma }_{o }^{{\prime }}}\right)$$

Where:

$n$ = Number of soil layers within the zone of stress influence of the footing

$\varDelta {\text{H}}_{i}$ = Elastic settlement of layer i

${\text{H}}_{c}$ = Initial height of layer i

${C}^{{\prime }}$ = Bearing Capacity Index, from Fig. 10.6.2.4.2-1 of the AASHTO LRFD Bridge Specifications.

${\sigma }_{o}^{{\prime }}$ = Initial average effective stress of the subdivided soil layer.

$\varDelta {\sigma }_{v}$ = Vertical stress increase in the subdivided soil layer due to applied foundation load.

Table 6 presents a comparison between estimated and measured settlements. A comparison is made between the estimated and measured settlements at the end of construction and the recently measured values. It should be noted that average measured settlements were calculated by taking the average of two monuments readings for each footing. The current measured settlements of FAI-33-13.09 pier 3 were ignored because survey monument information was not available.

It can be noted that Hough's method tends to over-predict the immediate settlements (measured at the end of construction). It also over-predicts the current measured settlements for most of the footings. However, Hough's method significantly underpredicts the current measured settlements for some footings where cohesive soil (A-6) layers exist, such as CUY-77-14.35 Wall 4 and the rear abutment of FAI-33-13.09. Therefore, it is important to calculate the long-term settlements when cohesive soils exist.

Table 6

Comparison Between Estimated and Measured Settlements
Project	Structure	Estimated Settlements (mm)	Avg. Measured Settlements (mm)		(Estimated/Measured) Settlement
Project	Structure	Estimated Settlements (mm)	End of Construction	Current Settlement	End of Construction	Current Settlement
MAH-680-2.83	Pier 1	17.53	6.0	3.0	2.92	5.84
	Pier 2	12.70	6.5	8	1.95	1.59
	Pier 3	11.94	0.0	-4.5^b	N.A^a	N.A^a
CUY-77-14.35	Wall 1	17.53	-1.5^b	4.5	N.A^a	3.90
	Wall 4	13.72	1.5	160	9.14	0.09
	LT. R.A.Wing Wall (Wall 2)	22.61	6.0	21.5	3.77	1.05
	RT. F.A.Wing Wall (Wall 3)	19.30	6	16.5	3.22	1.17
FAI-33-13.09	Rear Abut.	23.11	27.5	64	0.84	0.36
	Pier 1	26.16	12	10	2.18	2.62
	Pier 2	33.02	9.5	4.5	3.48	7.34
	Pier 3	26.16	9.5	N.A^a	2.75	N.A^a
	FR. Abut.	18.54	12	21	1.55	0.88
CUY/SUM-271-00.00/14.87	RW-1 (WS1)	19.81	0.0	N.A^a	N.A^a	N.A^a
^a Not Available values
^b negative settlements values, measurement error

Per the Geotechnical report of CUY/SUM-271-00.00/14.87, the recently constructed Wall RW-1 (WS1) is expected to experience an immediate settlement of 6 mm and consolidation settlements of 13 mm. The wall is expected to experience a total settlement of 20 mm. Consolidation settlements are very important because of the existing cohesive soil below the wall's footing.

Differential Settlements between Substructures

Differential settlements between substructures are presented in Table 7. The table shows differential settlements at the end of construction and the current differential settlements based on a recent survey performed in this study.

MAH-680-0283, at the end of construction, experienced 0.5 mm of differential settlements between piers 1 and 2, also it experienced 6.5 mm between piers 2 and 3. Based on recent settlement monitoring data, the structure experienced 5 mm between piers 1 and 2; and 12.5 mm between piers 2 and 3. It should be noted that those values are within tolerable settlement limits.

Table 7

Differential Settlements between Substructures
Structure No.	Substructure	Ave. Measured Settlements (mm)		Differential Settlements between Substructures (mm)
Structure No.	Substructure	End of Construction	Current Settlement	Two Substructures	Based on End of Construction	Based on Current Settlement
MAH-680-0283	Pier 1	6.0	3	Pier 1 and 2	0.5	5
	Pier 2	6.5	8	Pier 2 and 3	6.5	12.5
	Pier 3	0.0	-4.5	----------	----------	----------
FAI-33-1309	Rear Abut.	27.5	64	Rear Abut. and Pier 1	15.5	54
	Pier 1	12	10	Pier 1 and 2	2.5	5.5
	Pier 2	9.5	4.5	Pier 2 and 3	0.0	N. A
	Pier 3	9.5	N.A	P3 and F.R. Abut.	2.5	N. A
	FR. Abut.	12	21	----------	----------	----------

FAI-33-1309 experienced tolerable differential settlements of (0.0 to 15.5) mm at the end of construction. However, a recent settlement monitoring survey showed differential settlements of (5.5 to 54) mm, the values at the piers less than those at the end of construction. This may be due to survey issues. It should be noted that differential settlements of 54 mm between the rear abutment and pier 1 is recorded recently because of the 64 mm current settlements at the rear abutment. This may be due to the encountered cohesive soils consisting of stiff to hard silt and clay (A-6) and silty clay (A-6) at deeper layers below the rear abutment. There are no signs of this differential settlement at the rear abutment approach.

The following summarizes the findings of this study:

Spread footings were monitored after pouring the footings' concrete, before and after beams placement, after pouring the deck's concrete, and after project completion. Recently, footings were surveyed to get the final settlement values. Generally, spread footing performed well except in some locations where cohesive soils exist, or there was an issue with the survey benchmark (either it settled or could not be located).
The empirical settlement prediction method proposed by Hough (10) should be utilized when a single value cannot represent the elastic modulus of the sandy subsoil layers (i.e., when the corrected SPT-N value varied significantly with depth over the depth of influence).
The settlement of spread footings on cohesionless soil can be estimated using the method proposed by Hough (1959) as specified in section 10.6.2.4 of the AASHTO LRFD Bridge Design Specifications. Based on the presented results, this method can be used with some confidence.
It can be concluded that Hough's method tends to over-predict the immediate settlements (measured at the end of construction). It also over-predicts the current measured settlements for most of the footings. However, Hough's method significantly underpredicts the current measured settlements for some footings where cohesive soil (A-6) layers exist. Therefore, it is important to calculate the long-term settlements when cohesive soil exists.
The collected data at the sites illustrated that the spread footings could be used to support the highway structures satisfactorily, given that granular subsurface conditions are adequate (i.e., the corrected SPT-N value is larger than 20 blows/0.3m).
All structures in this study experienced tolerable differential settlements between substructures.

Recommendations

Reference monuments for substructure units founded on spread footing are important to collect and document historical performance data. Also, to assure the foundation bearing material is performing as designed. Having such data is essential to improve the reliability of spread footings and evaluate settlement prediction methods' accuracy.
Reference monuments for substructure units founded on spread footing foundations on soil should be surveyed annually as part of the annual bridge inspection. This way, spread footing design can be verified, performance can be investigated, and data be reviewed to take necessary action. It is recommended to measure footing elevation annually, and the collected data should be stored in a database to track the long-term performance of structures.
Coordinates of monuments and reference benchmarks should be provided in the as-built plans to assure accurate readings of the settlements in the future.
It is recommended to install target points on substructures so that elevation data could be more readily collected. The field review indicated damaged PVC pipes, missing PVC pipes, clogged holes, and PVC pipes vandalism. Targets would be more readily seen, and if placed, vandalism is deterred. Relative settlement measurement could be obtained with simple optical survey equipment.
When proposing spread footings, the designer should pay attention to the subsurface investigation and accurately estimate the consolidation settlement for spread footings resting on saturated cohesive soils.
Total expected settlements (immediate and long-term) are usually calculated and presented in the Geotechnical Report. Total expected settlements should be provided in the Spread Foundation Plan Note to be compared with measured values in the future.

Acknowledgments

This research paper builds off information from Report No. FHWA/OH-2021-04 Division of Engineering Research on Call Agreement 31796 Task 7 – Service Evaluation of Highway Structures with Soil-Bearing Spread Footings. The writers are grateful to the FHWA and the ODOT for sponsoring the investigation described in this paper.

AUTHOR CONTRIBUTIONS

The authors confirm their contribution to the paper to the research.

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No competing interests reported.

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Performance of Soil-Bearing Spread Footings Supporting Highway Structures

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Abstract

Figures

Introduction

Methodology

Literature Review

Research Sites

MAH-680-2.83 Bridge

CUY-77-14.35 Walls

CUY-77-14.35 Soil Profile

Wall 1 (I.R. 77 Sta. 72 + 19.25 to Sta. 74 + 20.54)

Wall 4 (I.R. 77 Sta. 80 + 34.56 to Sta. 82 + 71.00)

Bridge No. CUY-1433 L&R over I.R. 490 and Ramps Wing Walls

Left Rear Abutment wing wall-Boring BB-106 (Wall 2)

Right forward abutment wing wall (Wall 3)

CUY-77-14.35 Settlement Monitoring Data

FAI-33-13.09 Bridge

CUY/SUM-271-00.00/14.87

Comparison Between Estimated and Measured Settlements

Differential Settlements between Substructures

Findings

Recommendations

Declarations

References

Additional Declarations

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