Building RCC dams on sand foundations

12 November 2008

Although RCC is generally not used for small dams on sand foundations, the design requirements of the Upper and Lower Aetna Lakes project made it the preferred alternative

The Borough of Medford Lakes, New Jersey, has designed, permitted, and constructed two small roller compacted concrete (RCC) dams to impound the Upper and Lower Aetna Lakes. The project began as a response to the flooding events in July 2004 across southern New Jersey, when both existing Upper and Lower Aetna Lakes Dams were overtopped and breached by flood waters. In addition to providing recreational impoundments, Upper Aetna Dam had a public roadway along the crest, and Lower Aetna Dam had a limited access roadway primarily used by children on their way to and from school. The Borough recognized the importance of the dams and the lakes to the community and began the reconstruction process quickly. Design of the dams began in 2005 and construction was completed in the fall of 2007.

Even though the dams are small by typical RCC standards, RCC was selected after an alternative analysis determined that it was the most cost effective solution for replacing the dams. Replacing the dams in-kind with new embankment dams was uneconomical because 1) a new embankment dam would require a much larger footprint to meet current dam safety standards, 2) impervious core material is not available locally, and 3) a new embankment dam would need overtopping protection on both upstream and downstream embankments. A major design requirement was not altering the upstream or downstream flow characteristics and, therefore, not increasing the spillway capacity or storage of the lakes. The hydrologic and hydraulic analyses indicated that an increase in both the spillway capacity and storage would have been necessary to pass the spillway design flood (SDF) without overtopping the embankment. Because the dams had to be designed to overtop, the durability of RCC made it a very attractive option.

Many other factors helped make RCC even more of an attractive option to the Borough and the designers, such as constructibility, minimal footprint, minimal land acquisition, and the aesthetics of using dyed conventional concrete facing elements. However, there were also many factors that made RCC construction difficult such as construction on an overburden foundation, small RCC volume, off-site batching, and various placement issues.

Material properties

The foundations for the Upper and Lower Aetna Lake Dams consist of sands overlying silts and clays (Figure 1). For analyses, the soil was characterised into three main layers as described below. Design parameters were estimated for each subsurface layer based on the results of laboratory testing programs, as well as empirical correlations to standard penetration test N-values.

• Layer 1 – Upper Sand: Typically extended from the existing ground surface to about 10ft (3m) below the lake bed. Grain-size analysis tests indicated relatively low fines content (about 3% to 11%).

• Layer 2 – Lower Sand: Typically encountered about 10-15ft (3-4.6m) below the lake bed and extended as deep as 50ft (15m) below the lake bed. Grain-size analysis tests indicated significant fines content (about 15% to 49%).

• Layer 3 – Lower Silt and Clay: The lower silts and clays typically underlie the lower sands. The lower silts and clays were not encountered in all of the test borings, and none of the test borings fully penetrated the lower silts and clays.

Based on stratigraphy, lab test results, and empirical correlations, the estimated soil parameters shown in Table 1 were used in the design.

Excavation support system design

Conceptual excavation support systems were analysed using WALLAP Version 4.0, a computer program that evaluates staged construction of excavation support systems. This program performs a finite element soil-structure interaction analysis of earth retaining systems using a two-dimensional beam-on-elastic foundation model. Each stage in the construction sequence (excavation, dewatering, brace installation, etc.) is analyzed to develop envelopes of maximum brace loads and wall stresses for design. The analysis also computes wall deflections for use in evaluating potential ground movements. Conventional soil parameters for strength, modulus (stiffness) and earth pressure are used as input.

Design cases

During the design phase, analyses indicated that the excavation support system would require internal bracing where the ground surface was above El. 58.0 at the Upper Dam and El. 54.0 at the Lower Dam. At locations along the alignments with lower ground surface elevations, the excavation support system did not require internal bracing and was designed as a cantilevered system. Two cross-sections were selected for design:

• Braced: Maximum depth of cut, occurring at or near the abutments of the Upper Dam and from the left abutment along approximately two-thirds of the length of the Lower Dam.

• Unbraced: Maximum unbraced depth of cut, occurring near the middle of the Upper Dam and near the right abutment of the Lower Dam.

Design considerations

• In the braced section, sufficient vertical clearance (about 10ft [3m]) was maintained between the bottom of the bracing and the top of the RCC (Lift 4) to allow for the operation of heavy equipment.

• The construction surcharge was applied to the braced section following installation of the bracing, as the excavation was completed using equipment located outside the cofferdam. In the unbraced section, the surcharge was applied after the placement of the first four lifts of RCC, as work can be conducted inside the cofferdam without interference from any bracing.

• In modeling the stiffness of the RCC and the mud slab, the concrete was assumed to have attained a compressive strength of 1000 psi prior to the subsequent construction stage.

• The cofferdam was not modeled under flood conditions prior to the placement and curing of the fourth lift of RCC.

• If field conditions required variations from the assumed construction sequence, the excavation support system could easily be reanalyzed to reflect the field conditions.

• The sheeting serves as both an excavation support system and a seepage control system. During the analyses, it was determined that a deeper sheeting toe was required for seepage control than for stability of the excavation support system. The deeper toe depth was shown on the drawings, and the excavation support design was therefore conservative.

Excavation sequences were analysed to evaluate loadings on the excavation support systems and the freshly placed RCC to determine if overstresses in the system would occur. A conceptual design was provided with minimum section properties on the design drawings and the Contractor was required to perform the final design for the temporary excavation support. The Contractor was also required to submit the excavation support design for both dams for review.

The conceptual design allowed the Contractor the option of pre-excavating the existing ground surface adjacent to the sheeting to a maximum elevation of El. 58.0 at the Upper Dam and El. 54.0 for the Lower Dam. Pre-excavation to these elevations reduced the horizontal soil pressure acting on the excavation support system and allowed an unbraced excavation support system to be installed along the entire alignment.

Permanent sheeting considerations

At the completion of construction, the sheeting was cutoff at the mudline. The remaining sheeting along the upstream face of the dams was left in place to provide a seepage cutoff, and the remaining sheeting along the downstream toe and abutments was left in place to provide scour protection.

The loads on the sheeting were estimated as the worst of three independent load cases:

• Settlement of the foundation under the dam with the sheeting supporting the weight of a localized section of the dam.

• Liquefaction of material under the dam resulting in pressures on the sheeting.

• 10ft (3m) of downstream scour.

Stability analyses

Limit-equilibrium-based sliding stability analyses for the dam were performed using a simplified version of the maximum section (Figure 2). Typical driving forces included forces calculated from hydrostatic pressures and horizontal soil pressures acting against the upstream face of the dam, uplift pressures acting on the base of the dam, and earthquake forces. Typical resisting forces included self weight, silt, water, and traffic surcharge of 250 psf/ft on top of the dam, and forces from the soil and tailwater acting against the dam toe.

Forces were developed using widely accepted earth pressure theories. A resisting horizontal friction force was developed by applying a reduction factor (tand) to the sum of the weights of the dam, silt, and water on top of the dam to account for frictional interaction between the concrete base of the dam and the native alluvium. Earthquake soil forces were developed using the Mononobe-Okabe method (AASHTO, 2002; [1]). The Mononobe-Okabe method is a pseudo-static approach that uses ground acceleration factors to develop supplemental inertia loads developed by earthquake-induced movement of the soil mass acting on the heel of the dam. For the flood cases where the dam overtops, the weight of the water on the top and downstream side of the dam was ignored because it is moving and the tailwater was reduced to 60% of its full value. The full tailwater was used to compute the uplift pressures. The reduction in tailwater is taken to account for the fact that the water flowing over the dam tends to push the tailwater downstream and away from the dam.

Dewatering occurred inside the temporary steel sheet cofferdam built around the perimeter of the dam. The sheeting was connected to the base of the dam to provide some additional sliding and overturning resistance. This technical approach was conservative and did not incorporate the additional resistance benefits provided by connecting the steel sheets to the dam.

Headwater and tailwater elevations

Headwater and tailwater elevations calculated in the hydraulic analyses, and the elevations used for the stability analyses, are summarised in Table 3.

Seismic acceleration coefficients

Seismic parameters were obtained from the soil properties summarised in Table 1. Based on the location of the dam, a peak ground acceleration (PGA), expressed as a fraction of gravity (g), of 0.13g was selected from a United States Geological Service (USGS) website [2]. The PGA of 0.13g corresponds to a probability of exceedence of 2% in 50 years (approximately an earthquake return period of 2500 years).

Material properties

Native sand properties were developed from laboratory testing results. Properties for silt were taken from recommendations in EC 1110-2-6058 [3]. The unit weight for concrete was taken from recommendations in Ref. 4. The friction factor (tand) for the concrete base and the native sand subgrade (mass concrete against clean sand) was obtained from NAVFAC DM-7.2 [5].

Safety factors

The results of the sliding analyses were expressed as Factors of Safety (FS). A FS that is greater than 1.0 indicates that the sum of resisting forces is greater than the sum of the driving forces and a FS that is less than 1.0 indicates that driving forces are greater than resisting forces. The resisting factor is the net vertical force multiplied by the friction factor, tand. The driving factor is the net horizontal force.

The location of the resultant of all the forces was checked. If the resultant acts within the middle third of the base of the dam, the entire base remains in contact with the foundation soils. If the resultant falls outside the middle third, there is tension at the heel and the base may crack or lift away from the foundation. Base cracking results in increased uplift forces because full headwater pressure can enter the crack. If this was the case, the uplift was increased and the analysis was iterated until the crack length stabilized.

Required FS for the analyses were obtained from [3] and are summarized in Table 5.

The results of the stability analyses are presented in tables 6 and 7.

The analytical models provided satisfactory factors of safety for all load cases. As indicated previously, the model did not incorporate the additional stability resistance benefits provided by connecting the sheeting used for the temporary cofferdam and permanent seepage cutoff to the RCC foundation.

Seepage analyses

Seepage under the dam and around both abutments was evaluated. For each seepage evaluation, a cross-section was estimated based on the existing topography, proposed dam geometry, subsurface conditions observed in the test borings, and design water surfaces developed from hydrologic analyses. Soil permeability values were estimated from field permeability tests and empirical correlations to soil properties.

SEEP/W models

Seepage calculations were completed using the finite element program SEEP/W, and the analyses were completed by defining two-dimensional planar models using steady-state analysis conditions.

Boundary conditions were set using the following conditions:

• Vertical elements along the left and right side of the model have been defined as infinite elements to minimise any influence of the model boundaries.

• Nodes along the bottom of the model were defined with no-flow boundary conditions.

• Nodes along the left side and the upstream ground surface were defined with a fixed-head boundary condition set to the upstream water surface elevation.

• Nodes along the right side and the downstream ground surface of the model were defined with a fixed-head boundary condition set to the downstream surface elevation.

• At the abutment models, nodes along the downstream surface of the embankment were defined as a potential seepage-surface. Nodes along the upstream surface of the embankment above the upstream water surface elevation were also defined as potential seepage-faces. The node is reviewed and modified in response to the computers calculations.

The design for the Upper and Lower Aetna Lake dams included outlining each dam footprint with sheet piles extending to a depth of at least 20ft (6m) below the bottom of the dam. Based on the analyses, a factor of safety was estimated with respect to uplift gradient, and the uplift pressures at the base the dam were calculated. The results of the analyses are provided in Table 8.

At the Lower Dam, the sheeting extended 20ft (6m) into the left abutment to control seepage. Sheeting was installed about 115ft (35m) upstream along the right abutment to control grades and lake elevations along the public roadway.

At the Upper Dam, sheeting extended 20ft (6m) into both abutments to control seepage through and around the abutments. For all abutment design cases, factors of safety calculated with regard to exit gradients exceeded 3.0.

Settlement analysis

The dams are founded on deposits of sands, silty and clayey sands, with silt and clay lenses, which are also known as the Cohansey Sand formation.

The dams are founded approximately 10ft (3m) below the lowest existing grade along the dam alignment. Therefore the base of the dam is approximately 10-20ft (3-6m) below the existing ground surface. Sheeting was installed for support of excavation around the dam perimeter and was structurally tied to the RCC foundation. The sheeting was cutoff at the mudline at the completion of construction, and acts as a partial groundwater cutoff system that helps control seepage under the dams. Due to the fact that the amount of overexcavation and subsequent dam construction resulted in variable loading along the dam alignment, some amount of differential settlement was anticipated.

In order to assess the potential settlements, work proceeded as follows:

• It was first evaluated whether the sheeting will be ‘hung-up’ or settle upon construction completion. The following assumptions were made: the sheeting is structurally tied to the RCC dam; the net load of the RCC dam is acting on the sheeting upon completion of the dam; the groundwater level upon dam completion is within 2ft (0.6m) below the downstream pool elevation – the analysis indicated that the dam load will exceed the ultimate frictional resistance provided by the surrounding soils. Therefore the sheeting will likely move downward and the net load of the dam will be transferred to the subgrade soils.

• The amount of total settlement anticipated at the end-of-construction and at Normal Pool operating conditions at three representative cross-sections along the dam alignment was then evaluated. By inspection, the geometry, stratigraphy and groundwater conditions of two of the sections were very similar. Therefore, detailed settlement analyses were performed on the two different sections. It is the authors opinion that these two sections are representative of the extreme conditions along the dam alignment.

The foundation soils were initially treated as cohesionless soils and the Schmertmann approximation [6] was used to estimate the total settlement along the cross-sections. The Schmertmann procedure assumes that the distribution of vertical strain is compatible with a linear elastic half space subjected to a uniform pressure. The elastic half space is divided into sublayers, and settlement is calculated at the center of each sublayer. It should be noted that all the settlement analyses assume that loose soils below the base of the dam (N < 5) have been removed or improved.

The results of the settlement analyses were checked using the computer program WinSafi, which can perform both elastic and consolidation settlement analyses.

Elastic analyses were performed for the conditions analysed by the Schmertmann approximation. In order to use this program it was necessary to convert the elastic modulus to an equivalent coefficient of volume change, mv, using the following equation and a Poisson’s ratio, n, of 0.25:

mv = 1 /{ [ E (1-n)]/[(1+n) (1-2n)]}

Since some of the soils encountered below the proposed base of the dam are sandy silts and clays, they may exhibit cohesive behaviour – WinSafi was used to calculate potential consolidation settlements of these soils using the following assumptions:

• The Lower Sands immediately below the dam are considered incompressible for this analysis. Elastic settlements of these soils were determined in previous WinSafi analysis and were added to the consolidation settlements.

• The Lower sandy silts and clays extend to a depth of twice the foundation width below the dam.

• The lower silts and clays have been slightly overconsolidated, based on the results of Atterberg limits, empirical correlations, and on the fact that a dam previously existed at this location. An OCR of 1.2 was used for these analyses (normally consolidated soils would result in greater computed settlements).

As shown in Table 9, there was fairly good agreement between the two methods of analysis. It was estimated that under Normal Pool conditions, the dams would experience approximately 2 to 2.5 inches of settlement at Section 1 and 1-1.5 inches (2.5-3.8cm) at Section 2, regardless of whether cohesionless or cohesive behaviour is assumed. These total settlements will result in differential settlement of approximately 1-1.5 inches (2.5-3.8cm) under the Normal Pool operating conditions. The angular distortion calculated for the worst condition between Sections 1 and 2 is approximately 1/500. As indicated in Table 9, settlements and angular distortions at the end-of construction are estimated to be slightly higher than those calculated for the Normal Pool.

Liquefaction analysis

The method described in Youd, T. L. et. al. (2001), [7] was used to evaluate the liquefaction potential of soils at the Upper and Lower Aetna Lake Dam sites. This method presents a summary of the 1996 NCEER and 1998 NCEER/NSF workshops on the evaluation of liquefaction resistance of soils. At these workshops, the lead researchers in the US presented state-of-art information and their recommendations to update current practice and methodologies to correct and interpret field penetration data.

The liquefaction analyses followed the procedures outlined in your et. al. and were based on an earthquake magnitude of 6.0 and peak ground surface acceleration of 0.13g. Based on the findings of the liquefaction analyses, it was concluded that the majority of the soils along the alignment of the dams will not liquefy during the design earthquake and that occurrences of liquefaction will be limited to local areas. Soils that are most susceptible to liquefaction are the Upper Sands soil layer and, to a lesser degree, the Lower Sands.

Loose granular soils, sands, and silty or clayey sands, with N < 5, would be susceptible to liquefaction if left under the dam footprint. However, since these soils will be fully contained within the sheeting, they are not expected to have adverse effects on the dam performance. Based on a review of the borings, it appears that these soils may extend a maximum of 5ft (1.5m) below the base of the proposed dams. Prior to construction, the subgrade was proof-rolled to aid with identification of loose zones. Proof-rolling the subgrade will also tend to decrease the liquefaction susceptibility of these soils. The subgrade was evaluated during construction and localized zones of loose material were removed.

RCC design

The majority of the RCC details used were based on details used in the successful construction and operation of multiple projects with much larger upstream pools and driving forces. Therefore no analysis is provided for the dimensions of the facing elements, grout beds, and other structural dam details.

The top of dam width is controlled by the minimum dimension to allow typical RCC equipment to place the material. This width is much larger than would be required to resist the driving forces from the lake. The downstream slope was selected as the upper end of slopes that are easily constructed and provide adequate stability. Contact pressures along the bottom of the dam were limited by the strength of the soil, which is significantly lower than the strength of the RCC. By inspection the critical interface within the dam mass is at the mud line where the dam is attached to the foundation block.

Based on forces required for the excavation support system, the minimum compressive strength for the RCC is 1,000 psi before the bracing can be removed from the sheeting.

A mix design and laboratory testing program was conducted on two separate aggregate sources from Lambertville and Moore’s Station Quarry. The mix design and test results are summarized below.

RCC mixes

Moore’s Station Quarry

The first mix design contained material located from Moore’s Station Quarry. Three mixes were prepared with equal cement and flyash contents by dry weight. The cementitious contents of the mixes were 9% and 11%, also by dry weight. Test results for the first two mixes were discarded because of inconsistent strength values. The authors believe the inconsistency was due to the coarseness of the aggregate, which contained material with a maximum nominal size of 1.5 -inches.

Aggregate for the third mix was separated on the ?-inch sieve to reduce the overall coarseness of the gradation and this resulted in more consistent strength values over time.

Lambertville Quarry

A source of aggregate was located from Lambertville Quarry that contained material with a maximum nominal size of 0.5-inch. Two RCC mixes were prepared with varying cement and flyash contents. The ratio of cement to flyash were 50/50 and 75/25 by dry weight. The cementitious contents of the mixes were 9%, also by dry weight. Test results from these mixes indicated higher and more consistent strength values.

Based on strength values, the authors recommended that construction of the RCC dam consist of aggregate located from Lambertville Quarry and contain 9% cementitious material consisting of 75% cement and 25% flyash by dry weight. The design recommendations were based on the schedule for construction and use of the dam.


Both the Upper and Lower Aetna Lake dams are located within residential areas, which significantly limited site access. Two houses are located in close proximity to the Upper Dam on the left abutment, and building lots are near the right abutment. These lots have been developed, and easement difficulties on one lot required modification of the dam, further complicating the construction and limiting access to the Upper Dam site.

A local road also exists immediately adjacent to the Lower Dam at the right abutment. This road was to remain in service throughout the duration of the dam construction, thus restricting access to the Lower Dam to the left abutment only. The left abutment contains houses on either side of a tree-lined drive.

Restricted site access created several issues during the construction of both dams. The limited access required that the RCC material be batched off-site and hauled to the dams. With the batching site located approximately 15 minutes driving time from the dam sites, moisture loss during transport was monitored and was accounted for during the batching process. At the beginning of each day, several trucks were loaded before any material arrived at the site to be placed and compacted. This required a proactive approach involving the Engineer and the Contractor to anticipate the required moisture at the time of batching in order to avoid rejecting significant amounts of material.

Due to limited access for a telebelt delivery system, the RCC material was placed directly onto the working surface by the transport trucks. This required an appropriate approach ramp from the surface streets to the working surface. This began with a crushed stone approach and washing station on each of the abutments. These were used to help prevent foreign material from being tracked onto the working surface by the dump trucks or the conventional concrete trucks.

With the initial working surface below the existing grade, a permanent conventional concrete ramp was constructed within the limits of the dam to provide access to the working surface. This ramp was constructed using steps that coincided with the lift heights of the finished RCC. The steps were then covered with crushed stone to provide a smooth driving surface and to reduce any damage to the corners of the concrete. The ramp at the Upper Dam is shown in Figure 3.

With the ramp in place, dump trucks delivered material directly onto the working surface as shown in Figure 4.

The relatively narrow dams combined with the placement of material directly onto the working surface created the busy job site shown in Figure 5. The simultaneous operations shown consist of the dump truck preparing to place RCC material, the dozer spreading RCC, the compactor preparing to compact a lift of RCC, placement of bedding mix between consecutive lifts of RCC, and placement of facing element conventional concrete.

After several lifts were completed, a conventional concrete leveling pad was placed on the RCC at the location of each of the spillways. The precast concrete spillways were then placed on the leveling pads. With the spillways in place and access limited to one abutment, placing material beyond the spillway would require an additional conventional concrete ramp.

Conventional concrete facing elements were used throughout the project, and are shown in Figure 2. These elements provided the owner with the opportunity to tailor the appearance of the dam to match their needs. Tinted concrete with an exposed aggregate surface was used for facing elements above the mudline. The completed Lower Dam is shown in Figure 6.

Flood during construction

During the course of construction, both of the dam sites experienced a significant flood event while RCC placement was underway and below the existing ground surface. The precipitation began on a Sunday while no work was in progress at the sites. By Monday morning, cofferdams for both dams were filled with flood waters. These are shown in Figures 7 and 8. After the flood waters receded, the cofferdams were dewatered, and equipment was repaired or replaced. There was no apparent harm to the RCC that had been placed to date. The surface of all placed material was cleaned, and construction resumed after approximately one week.


Although RCC is generally not used for small dams on sand foundations, the unique requirements of the Upper and Lower Aetna Lakes project made it the preferred alternative for this case. A major design requirement was not altering the upstream or downstream flow characteristics, therefore not increasing the spillway capacity or storage of the lakes. An increase in both the spillway capacity and storage would be required to pass the spillway design flood without overtopping the embankment. Thus the dam would need to be designed to overtop. Because the dams had to be designed to overtop, RCC became a viable option.

Replacing the failed embankment dams in-kind would have required overtopping protection. Embankment dams would also require impervious core material not locally available, and would have footprints larger than the original dams to meet current dam safety standards.

Constructibility, minimal footprint and therefore minimal land acquisition, and the aesthetic options of using tinted conventional concrete facing elements made RCC an even more attractive option for the owner. After overcoming difficulties such as construction on a sand foundation, small RCC volume, and off-site batching, small RCC dams were the preferred alternative for this application.

Bryan M. Scott, Ph.D., P.E., Project Engineer at GEI Consultants, Inc., one of the US’s leading water resources, geotechnical and environmental firms. He is at GEI’s Boulder, Colorado location.

Philip J. Snyder, Project Engineer at GEI’s Woburn, Massachusetts location.

Michael P. Walker, P.E., Vice President at GEI’s Woburn, Massachusetts location.

Peter H. Black, P.E., C.M.E., Associate at Dewberry’s Parsippany, New Jersey location.

The authors thank the Borough of Medford Lakes, The Alaimo Group, and Dewberry-Goodkind for the opportunity to work on this project and for permission to publish this paper. Charles Grant and Jim Nickerson of GEI Consultants, Inc. (GEI) provided assistance with the analyses presented in this paper, and with information required for these analyses.

This paper was originally presented at Dam Safety 08, organized by the Association of State Dam Safety Officials. For further information visit


Table 1
Table 2
Table 3
Table 4
Table 5
Table 6
Table 7
Table 8
Table 9

Figure 1 Figure 1
Figure 7 Figure 7
Figure 2 Figure 2
Figure 4 Figure 4
Figure 8 Figure 8
Figure 3 Figure 3
Figure 5 Figure 5
Figure 6 Figure 6

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