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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 12 Partially Ground Riprap at Bridge Piers
Partially grouted riprap, when properly designed and used for erosion protection, has an advantage over rigid structures because it is flexible when under attack by river currents, it can remain functional even if some individual stones may be lost, and it can be repaired relatively easily. Properly constructed, partially grouted riprap can provide long-term protection if it is inspected and maintained on a periodic basis as well as after flood events. Partially grouted riprap may be used for bank protection as well as a scour countermeasure at piers and abutments.
Partially grouted riprap consists of specifically sized rocks that are placed and grouted together, with the grout filling only 1/3 to 1/2 of the total void space (Figure 12.1). In contrast to fully grouted riprap, partial grouting increases the overall stability of the riprap installation unit without sacrificing flexibility or permeability. The voids of the riprap matrix are partially filled with a Portland cement based grout by hose or tremie, or by automated mechanical means. Hydraulic stability of the armor is increased significantly over that of loose riprap by virtue of the much larger mass and high degree of interlocking of the "conglomerate" particles created by the grouting process.
Various degrees of grouting are possible, but the optimal performance is achieved when the grout is effective at "gluing" individual stones to neighboring stones at their contact points, but leaves relatively large voids between the stones. Since riprap is a natural material and is readily available in many areas, it has been used extensively in erosion protection works.
Designing partially grouted riprap installations requires knowledge of: river bed and bank material; flow conditions including velocity, depth and orientation; pier size, shape, and skew with respect to flow direction; riprap characteristics of size, density, durability, and availability; and the type of interface material between the partially grouted riprap and underlying foundation. The system typically includes a filter layer, either a geotextile fabric or a filter of sand and/or gravel, specifically selected for compatibility with the subsoil. The filter allows infiltration and exfiltration to occur while providing particle retention.
The guidance for partially grouted riprap applications provided in this document has been developed primarily from the results of National Cooperative Highway Research Program (NCHRP) Report 593 (Lagasse et al. 2007) and publications from the German Federal Waterway Engineering and Research Institute (BAW) in Karlsruhe, Germany. Although partially grouted riprap has been used successfully for many applications in Europe, this Design Guideline has been developed specifically for bridge piers.
Riprap design methods typically yield a required size of stone that will result in stable performance under the design loadings. Because stone is produced and delivered in a range of sizes and shapes, the required size of stone is often stated in terms of a minimum allowable representative size. Typically, the designer specifies a minimum allowable d50 for the rock comprising the riprap, thus indicating the size for which 50% (by weight) of the particles are smaller. Stone sizes can also be specified in terms of weight (e.g., W50) using an accepted relationship between size and volume, and the known (or assumed) density of the particle.
Shape: The shape of a stone can be generally described by designating three axes of measurement: Major, intermediate, and minor, also known as the "A, B, and C" axes, as shown in Figure 12.2. Riprap stones should not be thin and platy, nor should they be long and needle-like. Therefore, specifying a maximum allowable value for the ratio A/C, also known as the shape factor, provides a suitable measure of particle shape, since the B axis is intermediate between the two extremes of length A and thickness C. A maximum allowable value of 3.0 is recommended:
For riprap applications, stones tending toward subangular to angular are preferred, due to the higher degree of interlocking, hence greater stability, compared to rounded particles of the same weight.
Density: A measure of density of natural rock is the specific gravity Sg, which is the ratio of the density of a single (solid) rock particle γs to the density of water γw:
Usually, a minimum allowable specific gravity of 2.5 is required for riprap applications. Where quarry sources uniformly produce rock with a specific gravity significantly greater than 2.5 (such as dolomite, Sg = 2.7 to 2.8), the equivalent stone size can be substantially reduced and still achieve the same particle weight gradation.
Size and weight: Based on field studies, the recommended relationship between size and weight is given by:
Table 12.1 provides recommended gradations for ten standard classes of riprap based on the median particle diameter d50 as determined by the dimension of the intermediate ("B") axis. These gradations were developed under NCHRP Report 568, "Riprap Design Criteria, Specifications, and Quality Control" (Lagasse et al. 2006). The proposedgradation criteria are based on a nominal or "target" d50 and a uniformity ratio d85/d15 that results in riprap that is well graded. The target uniformity ratio is 2.0 and the allowable range is from 1.5 to 2.5.
Note: Only Classes II, III, and IV are suitable for use in partial grouting applications.
The intent of partial grouting is to "glue" stones together to create a conglomerate of particles (see Section 12.6, "Construction"). Each conglomerate is therefore significantly greater than the d50 stone size, and typically is larger than the d100 size of the individual stones in the riprap matrix. Only three standard classes may be used with the partial grouting technique: Classes II, III, and IV. Riprap smaller than Class II exhibits voids that are too small for grout to effectively penetrate to the required depth within the rock matrix, while riprap that is larger than Class IV has voids that are too large to retain the grout, and does not have enough contact area between stones to effectively glue them together.
Permeability of the completed installation is maintained because less than 50% of the void space is filled with grout. Flexibility of the installation occurs because the matrix will fracture into the conglomerate-sized pieces under hydraulic loading and/or differential settlement. The surface of each conglomerate particle is highly rough and irregular, and so maintains excellent interlocking between particles after fracturing occurs.
Based on Equation 12.3, which assumes the volume of the stone is 85% of a cube, Table 12.2 provides the equivalent particle weights for the same ten classes, using a specific gravity of 2.65 for the particle density.
Note: Only Classes II, III, and IV are suitable for use in partial grouting applications.
Standard test methods relating to material type, characteristics, and testing of rock and aggregates typically associated with riprap installations (e.g., filter stone and bedding layers) are provided in this section and are recommended for specifying the quality of the riprap stone. In general, the test methods recommended in this section are intended to ensure that the stone is dense and durable, and will not degrade significantly over time.
Rocks used for riprap should only break with difficulty, have no earthy odor, no closely spaced discontinuities (joints or bedding planes), and should not absorb water easily. Rocks comprised of appreciable amounts of clay, such as shales, mudstones, and claystones, are never acceptable for use as fill for gabion mattresses. Table 12.3 summarizes the recommended tests and allowable values for rock and aggregate.
(1) Testing frequency for acceptance of riprap from certified quarries, unless otherwise noted. Project-specific tests exceeding quarry certification requirements, either in performance value or frequency of testing, must be specified by the Engineer.
For partially grouted riprap applications, only Portland cement based grout is appropriate. General requirements for grouting materials are based on guidance developed by the Federal Waterway Engineering and Research Institute (BAW) in Germany (MAV 1990). The following provides guidance on the basic grout mix for one cubic yard (0.76 m3) of grout:
The mix should result in a wet grout density ranging from 120 to 140lb/ft3 (2.0 to 2.3 kg/dm3). Wet densities outside this range should be rejected and the mix re-evaluated for material properties of the individual constituents.
A variety of tests have been developed by the BAW in Germany. The two most relevant tests are described below. The full document entitled, "Guidelines for Testing of Cement and Bitumen Bonded Materials for the Grouting of Armor Stones on Waterways" has been translated into English as part of NCHRP Project 24-07(2) and can be found in Volume 2 of the final report for that project (Lagasse et al. 2007).
Consistency Test: The consistency of Portland cement based grouting material is determined using a slump test. A standardized slump cone and portable test table has been developed for this purpose. Figure 12.3 provides photographs illustrating the method. The diameter of the slumped grout is measured after pulling the cone without tapping, and then again after 15 taps of the test table. Target values for the measurement are as follows:
Washout Test: The washout test provides an indication of resistance to erosion by measuring the loss of grout material when immersed in water. A screened basket 13 cm in diameter with a 3 mm mesh size is filled with 2.0 kg of fresh grout. The grout is lightly tamped and the grout filled basket is weighed. The basket is then dropped three times into a water tank of 1 m height. Afterwards the grout and basket are weighed again, and the loss of mass is determined. The maximum permissible loss of mass is 6.0%.
With partially grouted riprap, there are no relationships per se for selecting the size of rock, other than the practical considerations of proper void size and adequate stone-to-stone contact area as discussed in Section 12.2.
Prototype-scale tests of partially grouted riprap at a pier were performed for NCHRP Project 24-07(2) by Colorado State University (CSU) in 2005 (see Section 12.6). The CSU tests were conducted in a 20-foot (6m) wide outdoor flume. In the laboratory setting, Class I riprap with a d50 of 6 in. (15 cm) was partially grouted on one side of the pier and standard (loose) rock having the same gradation was placed on the other side. Discharge was steadily increased until an approach velocity of 6.6 ft/s (2.0 m/s) was achieved upstream of the pier, at which point the maximum discharge capacity of the flume was reached. Using a velocity multiplier of 1.7 to account for the square-nose pier shape, local velocity at the pier was estimated to be approximately 11 ft/s (3.4 m/s). The partially grouted riprap was undamaged after several hours of testing, whereas the loose riprap experienced damage by particle displacement.
Tests of partially grouted riprap at Braunschwieg University, Germany demonstrated the ability of partially grouted riprap to remain stable and undamaged in high velocity flow of 26 ft/s (8 m/s). (Heibaum 2000). However, those tests were not conducted at a pier.
While Class I riprap was used in the laboratory setting, it is recommended that for field applications, the class of riprap (II, III, or IV) used for a partially grouted pier scour countermeasure be selected based on the economics of locally available riprap material that satisfies the gradation requirements of Section 12.2.
In general, the layout dimensions for partially grouted riprap follow those for loose riprap in applications involving bank protection and for armoring bridge abutments (See Design Guidelines 4 and 14, respectively). At bridge piers, however, the recommended guidance for partially grouted riprap provides for a reduced lateral extent compared to loose riprap, as explained in this section.
Based on laboratory studies performed for NCHRP Project 24-07(2) (published as NCHRP Report 593, Lagasse 2007), the optimum performance of partially-grouted riprap as a pier scour countermeasure was obtained when the armor extended a distance of at least 1.5 times the pier width in all directions around the pier. In contrast, with loose (ungrouted) riprap, the recommended extent is 2.0 times the pier width.
In the case of wall piers or pile bents consisting of multiple columns where the axis of the structure is skewed to the flow direction,the lateral extent of the protection should be increased in proportion to the additional scour potential caused by the skew. Therefore, in the absence of definitive guidance for pier scour countermeasures, it is recommended that the extent of the armor layer should be multiplied by a factor Kα, which is a function of the width (a) and length (L) of the pier (or pile bents) and the skew angle (α) as given below (after Richardson and Davis 2001):
Riprap should be placed in a pre-excavated hole around the pier so that the top of the riprap layer is level with the ambient channel bed elevation. Placing the top of the partially grouted riprap flush with the bed is ideal for inspection purposes, and does not create any added obstruction to the flow. Mounding riprap around a pier is not acceptable for design in most cases, because it obstructs flow, captures debris, and increases scour at the periphery of the installation. The riprap layer should have a thickness of at least 2 times the d50 size of the rock, as shown in Figure 12.4. When placement must occur under water, the thickness of the riprap layer should be increased by 50% to account for uncertainties in placement; however, in this case the recommended grout application quantity should not be increased in kind .
When contraction scour through the bridge opening exceeds 2d50, the thickness of the armor must be increased to the full depth of the contraction scour plus any long-term degradation. In river systems where dune bedforms are present during flood flows, the depth of the trough below the ambient bed elevation should be estimated using the methods of Karim (1999) and/or van Rijn (1984). In general, an upper limit on the crest-to-trough height Δ is provided by Bennett (1997) as Δ < 0.4y where y is the depth of flow. This suggests that the maximum depth of the bedform trough below ambient bed elevation will not exceed 0.2 times the depth of flow. Additional armor thickness due to any of these conditions may warrant an increase in the extent of the partially grouted riprap away from the pier faces.
A filter layer is typically required for partially grouted riprap at bridge piers. The filter should not be extended fully beneath the armor; instead, it should be terminated 2/3 of the distance from the pier to the edge of the armor layer (Figure 12.4). When using a granular stone filter, the layer should have a minimum thickness of 4 times the d50 of the filter stone or 6 in., whichever is greater. As with riprap, the layer thickness should be increased by 50% when placing under water. Sand-filled geotextile containers made of properly-selected materials provide a convenient method for controlled placement of a filter in flowing water. This method can also be used to partially fill an existing scour hole when placement must occur under water, as illustrated in Figure 12.5. For more detail, see Lagasse et al. (2006). Design Guideline 11 describes prototype scale laboratory testing of constructability issues related to placing geotextile sand containers in a pier scour hole.
The importance of the filter component of a partially grouted riprap installation should not be underestimated. There are two kinds of filters used in conjunction with partially grouted riprap; granular filters and geotextile filters. Some situations call for a composite filter consisting of both a granular layer and a geotextile. The specific characteristics of the base soil determine the need for, and design considerations of the filter layer. In cases where dune-type bedforms may be present, it is strongly recommended that only a geotextile filter be considered.
The filter must retain the coarser particles of the subgrade while remaining permeable enough to allow infiltration and exfiltration to occur freely. It is not necessary to retain all the particle sizes in the subgrade; in fact, it is beneficial to allow the smaller particles to pass through the filter, leaving a coarser substrate behind. Detailed aspects of filter design are presented in Design Guideline 16 of this document.
For NCHRP Project 24-07(2) the use of sand filled geotextile containers as a filter under partially grouted riprap was tested at a prototype scale pier (Lagasse et al. 2007). A test section was created that was 30.7 ft (9 m) long and spanned the width of the flume. It was filled with sand level with the approach section. Upstream and downstream of the test section the flume bed consists of smooth concrete floors. A rectangular pier measuring 1.5 ft (0.5 m) by 4.5 ft (1.5 m)was installed in the center of the test section. Figure 12.6 is a layout diagram for the prototype testing program. Surrounding the pier, a scour hole measuring 12 ft by 16 ft (4m x 5 m) was pre-formed into the sand bed to a maximum depth of 3 ft (0.9 m).
An approach flow 1 ft (0.3 m) deep at approximately 1.5 ft/s (0.5 m/s) was established. A total of 32 geotextile containers were placed around the pier by dropping from a height of about 5 ft (1.5 m) above the water surface. For the geotextile containers the test at prototype scale was, primarily, to demonstrate constructability and performance in high velocity flow conditions. Details on the specifications, fabrication, and placement of the geotextile containers can be found in Design Guideline 11 (Section 11.5).
Next, riprap was positioned on top of the geotextile containers using a backhoe (see Design Guideline 11, Section 11.5). The final step in the testing procedure was to use partial grouting techniques to demonstrate the enhanced stability of partially grouted riprap as a pier scour countermeasure. The following sections describe the placing and testing of the partial grouting procedure in the laboratory flume.
Prior to underwater application of the grout in the flume, a preliminary grout application was performed in the dry on a pile of riprap about 1.5 ft (0.5 m) thick. The trial application was performed to determine if the equipment could supply and control the grout pumping rate as needed for the underwater installation conditions. Grout was dispensed from a flexible hose attached to a boom on a concrete pump truck. Grout was supplied to the pump truck from a standard concrete mixer truck, as shown in Figure 12.7. Figure 12.8 shows the preliminary trial grout application in the dry. Figure 12.9 shows the surface of the riprap after partial grouting, and Figure 12.10 shows the interior of the dry riprap pile after several exterior stones had been removed to display penetration of the grout. Note in Figures 12.9 and 12.10 how the grout bridges between riprap stones forming larger conglomerate particles. In Figure 12.10, note that less than 50% of the total void space has been filled with grout. The preliminary application confirmed that the equipment planned for the underwater partial grout application was satisfactory.
Grout placement in the flume was performed by an experienced underwater grout installation specialist from Germany. The specialist was located in the flume and placed the grout directly on the riprap in 1 ft (0.3 m) of water with a velocity 1 ft/s (0.3 m/s), as illustrated inFigure 12.11.
Application of grout on the riprap lasted approximately 20 minutes. Approximately 1.4 yd3 (1.1 m3) of grout was placed, resulting in an application of 1.6 ft3/yd2 (56 liters/m2). Typical grout application rates in German practice are 60 liters/m2, so this test was representative of standard practice for this countermeasure type. These tests confirmed that geotextile containers can be fabricated locally, that the containers and riprap can be placed with standard equipment, and that the grout mix can be batched, transported, and placed with commercially available equipment.
High Velocity Performance Test. After placing the grout in a zigzag pattern (see Figure 12.4) the flume was drained and prepared for high velocity performance testing. Loose riprap around the surface perimeter of the installation that was not firmly secured during the grouting process was removed and replaced with sand. In order to prevent degradation of the sand bed during high velocity testing, the upper 4 in. (100 mm) was stabilized by tilling 4% Portland cement by dry weight (of the sand) into the sand bed. The material was compressed with a vibrating plate compactor after addition of the Portland cement.
The high velocity test ran for two hours and was terminated when the soil cement bed began to visibly fail. Approach velocities at 60% of depth during the high velocity test ranged from 4.2 to 5.6 ft/s (1.3 to 1.7 m/s). After draining the flume, several scour holes were observed in the soil cement bed, and a significant scour hole was observed downstream of the riprap installation. The soil cement in these areas had been destabilized and the underlying sand scoured to a depth of about 2.5 ft (0.8 m). The partially grouted riprap and underlying geotextile containers remained intact.
High Velocity Comparison Test. To facilitate a comparison of the performance of loose riprap to partially grouted riprap, all riprap and grout were removed from the left side of the pier and replaced with loose riprap of the same gradation and d50. Because the soil cement proved to be inadequate to stabilize the area around the partially grouted riprap, it was completely removed from the bed, exposing the underlying sand bed 4 in. (100 mm) lower than the surrounding flume floor and top surface of the riprap. A geotextile fabric was installed over the exposed sand portion of the test section. Four-inch (100 mm) thick articulating concrete blocks (ACBs) were installed on the geotextile fabric adjacent to the riprap. The ACBs were intended to prevent degradation of the bed in the test section as well as facilitate a smooth transition from the flume floor to the test section.
Temporary walls were installed to reduce cross sectional area of the flow and increase velocity in the test section. Walls were installed 2.5 ft (0.76 m) from the existing flume walls, transitioning the section from 20 ft (6 m) to 15 ft (4.6 m). Figure 12.12 shows the test section after the modifications were completed.
The high velocity comparison test ran for 4 hours, during which time the discharge was steadily increased to the full flow capacity. At maximum discharge, the approach velocity upstream of the pier reached a maximum of 6.4 ft/s (2 m/s). At the higher flows, the loose riprap began to displace. Figure 12.13 shows the loose riprap side of the installation after completion of the second half of the high velocity comparison test. Note the scour hole on the near side of the pier and the displaced riprap behind and downstream of the pier compared to the previous figure. The partially grouted side of the riprap installation can be seen in this figure, and remained essentially undisturbed. Figure 12.14 shows the partially grouted side of the installation after the end of this test.
As part of the prototype scale laboratory testing, water quality was monitored before, during, and after the grout placement. Water quality parameters monitored continuously were pH, conductivity, temperature, and turbidity. Based on research performed by the Virginia DOT, pH is the only water quality parameter that is expected to change significantly during grout placement (Fitch 2003). In the VDOT study, permit conditions required that pH levels remain below a value of 9.0, otherwise grouting activities were to be stopped, and mitigation measures such as silt curtains were to be employed. VDOT did not monitor turbidity during their study.
Water quality was monitored with a series of In-Situ Troll 9000 Profilers placed in stream at the seven locations depicted in Figure 12.15. The Troll 9000 Profilers continually recorded measurements of pH, conductivity, turbidity, and temperature. Baseline conditions were established prior to initiation of the grout placement 12 ft (3.7 m) upstream of the pier along the centerline of the flume (Station "A" in Figure 12.15).
During the test, the water discharge was 20 ft3/s (0.6 m3/s) and the average rate of grout placement was 0.032 ft3/s (0.001 m3/s); therefore, the water:grout dilution ratio was 20:0.032, or 625:1. Three grab samples were selected for analysis corresponding to a baseline sample taken at Station A when testing commenced, Station C five minutes after grout application began, and Station F when grout application finished. Grab samples were collected in 250 mL polyethylene bottles that had been washed and rinsed with distilled water. Bottles were filled by dipping the bottle into the water upstream of where the sampling personnel were standing in the flume. The grab samples were analyzed for selected inorganics and metals. The laboratory results for the samples are presented in Lagasse et al. (2007). Continuous water quality data, collected by the Troll 9000 Profilers, was calibrated to background data collected at Station A prior to grout placement.
Background pH was 7.0 at all stations located in the flume itself. Downstream of the flume, Station J (located in the natural channel 150 ft (46 m) downstream of the flume tailgates) exhibited a background pH of 7.4.
A spike in pH was observed at the locations directly downstream of the pier during grout pumping. A maximum pH of 9.9 was recorded by the continuous monitor located 12 ft (3.7 m) directly downstream of the pier three minutes after pumping began. After grout pumping was completed, pH values dropped off quickly and typically returned to baseline conditions within 30 minutes. The one exception was the probe at Station C, which was directly in the wake of the pier and at the downstream edge of the grouted area. At this location, the pH returned to background levels after about 4 hours. Considering its location, this probe was in position to record the cumulative effect of the entire grouted area for the duration required for it to cure. At Station F, located 12 ft (3.7 m) directly downstream of Station C, a much less pronounced pH profile and more rapid decay of concentration was observed. Results of monitoring by the Troll 9000 Profilers are presented in Table 12.4, and Figure 12.16 shows the pH measurements at all stations.
Note: Data at Stations A-G from continuous monitors;
Partially grouted riprap is placed in a riverine or coastal environment to prevent scour or erosion of the bed, banks, shoreline, or near structures such as bridge piers and abutments. Partially grouted riprap construction involves placement of rock and stone in layers on top of a bedding or filter layer composed of sand, gravel and/or geotechnical fabric. The voids of the riprap matrix are then partially filled with a Portland cement based grout by hose or tremie, or by automated mechanical means. The final configuration results in an armor layer that retains approximately 50 to 65% of the void space of the original riprap. Hydraulic stability of the armor is increased significantly over that of loose (ungrouted) riprap by virtue of the much larger mass and high degree of interlocking of the "conglomerate" particles created by the grouting process.
Factors to consider when designing partially grouted riprap countermeasures begin with the source for the rock, the method to obtain or manufacture the rock, competence of the rock, and the methods and equipment to collect, transport, and place the riprap. Rock for riprap may be obtained from quarries, by screening oversized rock from earth borrow pits, by collecting rock from fields, or from talus deposits. Screening borrow pit material and collecting field rocks present different problems such as rocks that are too large, or that have unsatisfactory length to width ratios for riprap. Quarry stones are generally the best source for obtaining rock for riprap. Because the partial grouting process effectively creates larger particles from smaller ones, potential concerns regarding quarrying practices needed to produce large, competent, and unfractured riprap sizes are essentially eliminated.
In most cases, the production of the rock material will occur at a quarry that is relatively remote from the construction area. Therefore, this discussion assumes that the rock is hauled to the site of the installation, where it is dumped either directly, stockpiled, or loaded onto waterborne equipment.
Riprap should be fully grouted along vertical surfaces such as piers, where void space is higher and settling would result in larger gaps. Flowability of the grout should be tested prior to placement. Grout placed underwater requires special additives to prevent segregation of the aggregates and washout of the Portland cement during placement. "Stickiness" of the grout in underwater applications is important, therefore the Sicotan® product is recommended for this reason (see Section 12.2.3) based on extensive testing and field application by the Federal Waterway Engineering and Research Institute in Germany.
The construction objectives for a properly partially grouted riprap armor layer are:
The guidance in this section has been developed to facilitate the proper installation of partially grouted riprap armor to achieve suitable hydraulic performance and maintain stability against hydraulic loading to protect against scour at bridge piers. The proper installation of partially grouted riprap systems is essential to the adequate functioning and performance of the system during the design hydrologic event. Guidelines are provided herein for maximizing the correspondence between the design intent and the actual field-finished conditions of the project. This document addresses the preparation of the subgrade, geotextile placement, and placement of the riprap and grout.
The contractor is responsible for constructing the project according to the plans and specifications; however, ensuring conformance with the project plans and specifications is the responsibility of the owner. This is typically performed through the engineer and inspectors. Inspectors observe and document the construction progress and performance of the contractor. Prior to construction, the contractor should provide a quality control plan to the owner (for example, see USACE ER 1180-1-6, 1995, "Construction Quality Management") and provide labor and equipment to perform tests as required by the project specifications.
Construction requirements for riprap placement are included in the project plans and specifications. Recommended riprap specifications and layout guidance are found in Sections 12.2 and 12.4 of this document. Recommended requirements for the stone, including the tests necessary to ensure that the physical and mechanical properties meet the requirements of the project specifications are provided. Field tests can be performed at the quarry and/or on the job site, or representative samples can be obtained for laboratory testing.
Gradations are specified and plan sheets show locations, grades, and dimensions of rock layers for the revetment. The stone shape is important and riprap should be blocky rather than elongated, platy or round. In addition, the stone should have sharp, angular, clean edges at the intersections of relatively flat surfaces.
Segregation of rock material during transportation, dumping, or off-loading is not acceptable. Inspection of riprap placement consists of visual inspection of the operation and the finished surface. Inspection must ensure that a dense, rough surface of well-keyed graded rock of the specified quality and sizes is obtained, that the layers are placed such that voids are minimized, and that the layers are the specified thickness.
Inspection and quality assurance must be carefully organized and conducted in case potential problems or questions arise over acceptance of stone material. Acceptance should not be made until measurement for payment has been completed. The engineer and inspectors reserve the right to reject stone at the quarry, at the job site or stockpile, and in place in the structures throughout the duration of the contract. Stone rejected at the job site should be removed from the project site. Stone rejected at the quarry should be disposed or otherwise prevented from mixing with satisfactory stone.
Various degrees of grouting are possible, but the optimal performance is achieved when the grout is effective at "gluing" individual stones to neighboring stones at their contact points, but leaves relatively large voids between the stones.
Construction techniques can vary tremendously due to the following factors:
Competency in construction techniques and management in all their aspects cannot be acquired from a book. Training on a variety of job sites and project types under the guidance of experienced senior personnel is required. The following sections provide some general information regarding construction of partially grouted riprap installations and provide some basic information and description of techniques and processes involved in the construction of partially grouted riprap armor as a pier scour countermeasure.
The best time to control the gradation of the riprap mixture is during the quarrying operation. Generally, sorting and mixing later in stockpiles or at the construction site is not recommended. Inspection of the riprap gradation at the job site is usually carried out visually. Therefore, it is helpful to have a pile of rocks with the required gradation at a convenient location where inspectors can see and develop a reference to judge by eye the suitability of the rock being placed. On-site inspection of riprap is necessary both at the quarry and at the job site to ensure proper gradation and material that does not contain excessive amounts of fines. Breakage during handling and transportation should be taken into account
The Wolman Count method (Wolman 1954) as described in NCHRP Report 568 (Lagasse et al. 2006, see also HEC-23, DG4, Section 4.4.1) may be used as a field test to determine a size distribution based on a random sampling of individual stones within a matrix. This method relies on samples taken from the surface of the matrix to make the method practical for use in the field. The procedure determines frequency by size of a surface material rather than using a bulk sample. The intermediate dimension (B axis) is measured for 100 randomly selected particles on the surface.
The Wolman Count method can be done by stretching a survey tape over the material and measuring each particle located at equal intervals along the tape. The interval should be at least one foot for small riprap and increased for larger riprap. The longer and shorter axes (A and C) can also be measured to determine particle shape. One rule that must be followed is that if a single particle is large enough to fall under two interval points along the tape, then it should be included in the count twice. It is best to select an interval large enough that this does not occur frequently.
The grout should not segregate when being applied to the riprap. When placing grout under water, segregation and dispersion of fine particles is prevented by use of a chemical additive (Sicotan®) as described in Section 12.2.3. The target distribution of grout within the riprap matrix is such that about 2/3 of the grout should reside in the upper half of the riprap layer, with 1/3 of the grout penetrating into the lower half.
The grout must not be allowed to pool on the surface of the riprap, nor puddle onto the filter at the base of the riprap. Therefore, prior to actual placement, rates of grout application should be established on test sections and adjusted based on the size of the grout nozzle and consistency of the grout. Construction methods should be closely monitored to ensure that the appropriate voids and surface openings are achieved.
Geotextiles: Either woven monofilament or non-woven needle-punched geotextiles may be used. If a non-woven fabric is used, it must have a mass density greater than 12 ounces per square yard (400 grams per square meter). Under no circumstances may spun-bond or slit-film fabrics be allowed. Each roll of geotextile shall be labeled with the manufacturer's name, product identification, roll dimensions, lot number, and date of manufacture. Geotextiles shall not be exposed to sunlight prior to placement (see Design Guide 16 of this document).
Granular filters: Samples of granular filter material shall be tested for grain size distribution to ensure compliance with the gradation specification used in design (see Design Guide 16 of this document). Sampling and testing frequency shall be in accordance with the owner or owner's authorized representative.
When placing in the dry, the riprap and filter shall be placed on undisturbed native soil, on an excavated and prepared subgrade, or on acceptably placed and compacted fill. Unsatisfactory soils shall be considered those soils having excessive in-place moisture content, soils containing roots, sod, brush, or other organic materials, soils containing turf clods or rocks, or frozen soil. These soils shall be removed, backfilled with approved material and compacted prior to placement of the riprap. Unsatisfactory soils may also be defined as soils such as very fine noncohesive soils with uniform particle size, gap-graded soils, laminated soils, and dispersive clays, per the geotechnical engineer's recommendations.
The subgrade soil conditions shall meet or exceed the required material properties described in Section 12.7.3 prior to placement of the riprap. Soils not meeting the requirements shall be removed and replaced with acceptable material.
When placing in the dry, the areas to receive the riprap shall be graded to establish a smooth surface and ensure that intimate contact is achieved between the subgrade surface and the filter, and between the filter and the riprap. Stable and compacted subgrade soil shall be prepared to the lines, grades and cross sections shown on the contract drawings. Termination trenches and transitions between slopes, embankment crests, benches, berms and toes shall be compacted, shaped, and uniformly graded. The subgrade should be uniformly compacted to the geotechnical engineer's site-specific requirements.
When placing under water, divers shall be used to ensure that the bed is free of logs, large rocks, construction materials, or other blocky materials that would create voids beneath the system. Immediately prior to placing the filter and riprap system, the prepared subgrade must be inspected.
Placing the Filter
Whether the filter is comprised of one or more layers of granularmaterial or made of geotextile, its placement should result in a continuous installation that maintains intimate contact with the soil beneath. Voids, gaps, tears, or other holes in the filter must be avoided to the extent practicable, and replaced or repaired when they occur.
Placement of Geotextile: The geotextile shall be placed directly on the prepared area, in intimate contact with the subgrade. When placing a geotextile, it should be rolled or spread out directly on the prepared area and be free of folds or wrinkles. The rolls shall not be dragged, lifted by one end, or dropped. The geotextile should be placed in such a manner that placement of the overlying materials (riprap and/or bedding stone) will not excessively stretch or tear the geotextile.
After geotextile placement, the work area shall not be trafficked or disturbed in a manner that might result in a loss of intimate contact between the riprap stone, the geotextile, and the subgrade. The geotextile shall not be left exposed longer than the manufacturer's recommendation to minimize potential damage due to ultraviolet radiation; therefore,placement of the overlying materials should be conducted as soon as practicable.
The geotextile shall be placed so that upstream strips overlap downstream strips. Overlaps shall be in the direction of flow wherever possible. The longitudinal and transverse joints shall be overlapped at least 1.5 feet (46 cm) for dry installations and at least 3 feet (91 cm) for below-water installations. If a sewn seam is to be used for the seaming of the geotextile, the thread to be used shall consist of high strength polypropylene or polyester and shall be resistant to ultraviolet radiation. If necessary to expedite construction and to maintain the recommended overlaps anchoring pins, "U"-staples or weights such as sandbags shall be used. Figure 12.17 illustrates the placement of a geotextile for a coastal shoreline application.
Placing Geotextiles Under Water: Placing geotextiles under water can be problematic for a number of reasons. Most geotextiles that are used as filters beneath riprap are made of polyethylene or polypropylene. These materials have specific gravities ranging from 0.90 to 0.96, meaning that they will float unless weighted down or otherwise anchored to the subgrade prior to placement of the riprap (Koerner 1998).
Flow velocities greater than about 1.0 ft/s (0.3 m/s) create large forces on the geotextile. These forces cause the geotextile to act like a sail, often resulting in wavelike undulations of the fabric (a condition that contractors refer to as "galloping") that are extremely difficult to control. The preferred method of controlling geotextile placement is to isolate the work area from river currents by a temporary cofferdam. In mild currents, geotextiles precut to length have been placed using a roller assembly, with sandbags to hold the filter temporarily.
For partially grouted riprap at piers, sand-filled geotextile containers made of nonwoven needle punched fabric are particularly effective for placement under water as shown in Figure 12.5. The geotextile fabric and sand fill that comprise the geotextile containers should be selected in accordance with appropriate filter design criteria, and placed such that they overlap to cover the required area. For more information, see Lagasse et al. (2006).
Placement of Granular Filter: When placing a granular filter, front-end loaders are the preferred method for dumping and spreading the material on slopes milder than approximately 1V:4H. A typical minimum thickness for granular filters is 0.5 to 1.0 feet (0.15 to 0.3 m), depending on the size of the overlying riprap and whether a layer of bedding stone is to be used between the filter and the riprap. When placing a granular filter under water, the thickness should be increased by 50%. Placing granular media under water around a bridge pier is best accomplished using a large diameter tremie pipe to control the placement location and thickness, while minimizing the potential for segregation. NOTE: For riverine applications where dune-type bedforms may be present, it is strongly recommended that only a geotextile filter be considered.
Placing the Riprap
Riprap may be placed from either land-based or water-based operations and can be placed under water or in the dry. Special-purpose equipment such as clamshells, orange-peel grapples, or hydraulic excavators (often equipped with a "thumb") is preferred for placing riprap. Unless the riprap can be placed to the required thickness in one lift using dump trucks or front-end loaders, tracked or wheeled vehicles are discouraged from use because they can destroy the interlocking integrity of the rocks when driven over previously placed riprap. Water-based operations may require specialized equipment for deep-water placement, or can use land-based equipment loaded onto barges for near-shore placement. In all cases, riprap should be placed from the bottom working toward the top of the slope so that rolling and/or segregation does not occur, as shown in Figure 12.18.
Riprap Placement on Geotextiles: Riprap should be placed over the geotextile by methods that do not stretch, tear, puncture, or reposition the fabric. Equipment should be operated to minimize the drop height of the stone without the equipment contacting and damaging the geotextile. Generally, this will be about 1 foot of drop from the bucket to the placement surface (ASTM Standard D 6825). Further guidance on recommended strength properties of geotextiles as related to the severity of stresses during installation are provided in Part 1 of this document. When the preferred equipment cannot be utilized, a bedding layer of coarse granular material on top of the geotextile can serve as a cushion to protect the geotextile. Material comprising the bedding layer must be more permeable than the geotextile to prevent uplift pressures from developing.
Riprap Placement Under Water: Riprap placed in water requires close observation and increased quality control to ensure a continuous well-graded uniform rock layer of the required thickness (ASTM Standard D6825). A systematic process for placing and continuous monitoring to verify the quantity and layer thickness is important. Typically, riprap thickness is increased by 50% when placement must occur under water.
Excavation, grading, and placement of riprap and filter under water require additional measures. For installations of a relatively small scale, diversion of the stream around the work area can be accomplished during the low flow season. For installations on larger rivers or in deeper water, the area can be temporarily enclosed by a cofferdam, which allows for construction dewatering if necessary. Alternatively, a silt curtain made of plastic sheeting may be suspended by buoys around the work area to minimize potential environmental degradation during construction.
Depending on the depth and velocity of the water, sounding surveys using a sounding pole or sounding basket on a lead line, divers, sonar bottom profiles, and remotely operated vehicles (ROVs) can provide some information about the riprap placement under water.
Placing the Grout
Table 12.5 presents the recommended values for quantity of grouting material as a functionof the class (size) of the riprap. The quantities are valid for medium-dense armor layers with a thickness of 2 times the d50 size of the riprap stones. The application quantities should not be exceeded because too much grout can create an impermeable layer on the surface of the armor layer, or on the filter at the bottom of the riprap. In addition, the flexibility of an installation is reduced when grout is applied at greater than the recommended amount.
Two types of grouting procedures, line-by-line and spot-by-spot, produce the desired conglomerate-like elements in the riprap as shown in Figure 12.19, whileFigure 12.20 shows line-by-line grout placement by hand. With a proper grout mixture and appropriate placement rate, partial grouting can be reliably accomplished underwater as well as in the dry. Grout placement can be done by hand only in water less than 3 ft (1 m). Special devices are required for placement in deeper water. Various countries in Europe have developed special grout mixes and construction methods for underwater installation of partially grouted riprap. Discussions with contractors and researchers in Germany indicate that grout placement can be reliably conducted in flowing water up to about 4 ft/s (1.2 m/s) flow velocity.
Grout application rate and associated penetration characteristics will be different in dry conditions compared to underwater placement. Usually test boxes having a surface area of at least 10 ft2 (1 m2) and a depth equal to the armor layer thickness are placed on the bed when placing partially grouted riprap under water, as shown in Figure 12.21 (Heibaum 2000). The underwater boxes are filled in the water with riprap, and then removed after being grouted to confirm that the proper areal coverage and penetration depths have been achieved.
Detailed guidance for inspecting partially grouted riprap installations is provided in NCHRP Report 593 (Lagasse et al. 2007). The guidance includes inspection during construction, periodic inspection, and inspection after flood events.
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