Jet Grouting Experience at

Posey Webster Street Tubes Seismic Retrofit Project

Umakant Dash1, Ph.D., P.E., Thomas S. Lee2, P.E., G. E., and Randy Anderson3, P.E.

Abstract This paper summarizes the construction experience of a seismic retrofit project for the Posey and Webster Street Tubes near Oakland, California. The Posey and Webster Street Tubes serve as the main tunnels for the California Route SR 722 along the city streets connecting cities of Alameda and Oakland. The retrofit work consisted, among other things, jet grouting and chemical grouting of the soils supporting the tubes. The primary purpose of the grouting work was to strengthen and densify the soils immediately surrounding the tubes to avoid liquefaction of the soils and loss of support for the tubes during major earthquakes with magnitude of M-7.5 or greater. Some of the criteria used for measurement of success of the work consisted of testing the soils after grouting. The tests included density, unconfined compressive strength field permeability tests and visual inspection of the grouted columns. Several construction difficulties were encountered during jet grouting. This paper summarizes materials, equipment, soil and groundwater conditions, grouting operation, rate of production, costs, and time requirements for jet grouting operation at Posey and Webster Street Tube Joint No.1. The paper provides a summary of the experience gained from the jet grouting operation and presents conclusions and recommendations useful for similar applications. Introduction In 1928, the Posey Immersed Tube, designed by the late George Posey, Alameda County engineer, was constructed between the city of Alameda and Oakland by sinking precast tunnel segments in a dredged trench, aligning them accurately and connecting them underwater. The so called “trench” system, a revolutionary construction method seven decades ago, was used in building the tunnel. It was the first pre-cast reinforced concrete immersed-tunnel with a bituminous coating in the United States and the longest in the world at that time. For 35 years the Posey Tunnel, has served two-way traffic across the Oakland Estuary. During 1962, approximately 30,000 vehicles a day passed through this tunnel.

In 1962, the Webster Street Immersed Tube was designed, financed and constructed by the California Department of Transportation. The tunnel was planned to relieve the Posey Tunnel traffic congestion. The “trench” construction system similar to

1 Transportation Engineer, Caltrans, 801, 12th Street, Sacramento, CA 2 Senior Professional Associate, Parsons Brinckerhoff Quade & Douglas, Inc., 303 2nd Street, Suite 700N, San Francisco, CA 3 Project Manager, Caltrans, 801, 12th Street, Sacramento, CA

Posey Tunnel was also used in building the Webster Street Tube. Figure 1 shows the locations of the two tubes spanning between Oakland and Alameda, California. Figure 2 shows a cross section of the tube with simplified subsurface conditions.

Figure 1 Locations of Posey and Webster Street Tubes, Oakland-Alameda, California

Figure 2 Simplified Subsurface Conditions of the Tubes

The primary seismic retrofit for the two immersed tubes is the ground improvement of the soils immediately surrounding the tubes. The ground improvement work was required to prevent the immersed tubes from floating in the event the backfill soil liquefied during the maximum design M-7.5 earthquake (Lee, el., 2002). The ground improvement work was tested under a demonstration project which was a separate part of the main project. During the Demonstration Project, field and laboratory tests were conducted on the improved soil (Parsons Brinckerhoff, 2001).

In addition to ground improvement, the secondary retrofit work was the releasing of tube joints to reduce the high tensile stresses that could rupture or seriously crack the tubes. During construction, many unexpected soil conditions were uncovered. For example, very soft clayey soils to loose sandy soils were encountered at Joint No.1 of the two tubes, each of which was connected to a pile-supported portal building in Alameda,

California. In addition, there was a blow out on the ground during excavation of the east cofferdam of Webster Street Joint No. 1, where sand was running into the cofferdam after the soil was excavated to a depth of about 35 feet inside the cofferdam. Concern was raised as to potential subsidence of the tube after Joint No. 1 was completely released or cut loose to allow for insertion of flexible membrane at the joint. This prompted to the need of stabilizing the soft and loose foundation soils by means of jet grouting.

Jet Grouting For an immersed tube tunnel to move upward in a liquefied soil mass, the liquid must be able to flow under the tube as it moves up (Figure 3). If this mechanism is prevented, the tube would not float, even if the soil under the tube were to liquefy. Thus was born the isolation principle (Schmidt, et. al, 1998). The space under the tubes can be isolated from the surrounding material by building “isolation” walls, consisting of material that would not liquefy and that would withstand differential pressure from the liquefied mass. The isolation walls can be sheet piles or any impermeable systems such as jet grouted columns. Due to close proximity to the tubes, jet grout columns were selected to serve as the “isolation” walls adjacent to the tubes at Joint No. 1 (Figure 4).

Two methods of jet grouting were employed as the ground improvement at Joint No.1 of both tubes. The jet grout columns at Joint No.1 of the Webster Street Tube were constructed by means of a single-fluid system using grout as a single fluid to cut and mix the in situ soil to form a jet grout column. Those at Joint No. 1 of the Posey Tube were installed using a double-fluid system using air and grout to construct the jet grout columns. The jet grout columns at Webster Street Tube Joint No. 1 were installed first and then followed by those at the Posey Tube. The reason two different jet grouting systems were used was because of the slow progress of the jet grout operation using a single fluid system, which will be discussed in more details in a subsequent section.

Figure 3 Flotation of the Tube under Uplift Pressure (Schmidt, et al., 1998)

Webster Street Tube Joint No. 1

The isolation walls at Joint No. 1 needed to have a minimum width of 6 feet. With the use of a single-fluid system, the maximum diameter of a jet grout column was found to be 2.5 feet based upon experience at the adjacent cofferdam construction where an impermeable barrier slab was formed at the base of the cofferdam 35 feet below the ground surface. Based on the as-built drawing at Joint No. 1, the tube segment immediately north of the Alameda portal building was founded on a pile-supported tremie concrete platform. However, a 15-ft section between the center of the collar of Joint No. 1 and the tube segment was founded on loose to very loose silty fine sand over a tremie concrete unsupported by any piles. The blowout on the ground surface was believed to have created voids within the loose sand layer immediately beneath the tube. For this reason, two areas, Zone 1 and Zone 2 were developed with different rows of jet grout columns (Figure 5). At Zone 2 area, because of the presence of the pile-supported tremie concrete platform below the tube, the jet grout columns at Zone 2 area were driven 1-ft into the tremie concrete at an average depth of 42 feet. In Zone 1, four rows of jet grout columns were constructed adjacent to the tube (Figure 5). Two of the four rows were vertical columns and the other two were inclined columns at 65° to 75° with the horizontal; all four columns were extended to a depth of 85 to 90 feet (Figure 6). Additionally, four chemical grout holes on each side of the tube in Zone 1 were also constructed. The loose sand immediately below Joint 1 was to be densified by two rows of chemical grout columns constructed inside the tube. As of the time of writing this paper, all jet grout columns were completed and the eight chemical grout holes with pvc pipes were installed in place and were ready for chemical grout injection.

Backfill Jet Grout

Loose Sand Figure 4 Jet Grout Cut-off Walls (Schematic Diagram)

Construction of the jet grout columns was carried out by means of a single fluid

system to depths varying from 38 ft to 95 ft below ground surface. This system is the simplest due to the sole dependence of the high velocity grout jet stream to cut, remove, and mix the in situ soil in place. It also gives the smallest diameter grout column ranging from 18 inches to 36 inches. For the same amount of cement injected per volume treated, this system produces the strongest soilcrete (a hybrid of grout and soil).

Table 1 below shows the injection parameters of the single fluid system. At least one column spacing was allowed for constructing two adjoining columns. A total of 71 vertical columns and 31 inclined columns were installed. On the average, the contractor constructed the jet grout column at a rate of approximately 90 feet per day. The pace of construction was very slow due to the following reasons:

1. The rig did not have a long mast to hold the string of rods up to 90 feet long, which had to be assembled section by section during drilling and broken down section by section during grouting operation;

2. The presence of obstructions slowed down the construction operation; such obstructions were not disclosed in the as-built drawings. Obstructions consisted of tremie concrete, steel bars, wood chips, boulders, bricks, etc;

3. Wearing down of the drilling bit due to extremely hard obstructions slowed down the installation pace;

4. Breakdown of equipment; and 5. The work was done under a “force” account. There were neither stimuli nor

incentives for the contractor to speed up the pace of construction by altering methods of drilling/grouting or changing drilling equipment and tools.

Not only was the operation slow, but also construction of the jet grout columns was

expensive with the single fluid system method. For example, a column constructed to a depth of 80 feet consumed 24 tons of cement (with withdrawal rate of 10 inches/minute or 29,000 liters of grout (cement:water ratio = 1:1). The spoil return was observed to be very viscous and thick, indicating a substantial volume of grout was wasted. Accordingly, a total of 1,770 tons of cement were used for constructing 104 columns. Instrumentation Program A pair of ground settlement devices were installed over Joint No. 1 to measure movements of the tube during construction of the production columns. The readings were taken daily by the contractor and reported to the field inspector for reference.

Figure 5 Layout Plan of Jet Grout Columns at Webster Street Tube Joint No. 1

Figure 6 Cross Section of Jet Grout Columns in Zone 1 Adjacent to the Webster Street Tube Joint No. 1

Table 1 – Jet Grout Parameters for Webster Street Tube Joint No.1 Injection Parameter Parameter Value Column Withdrawal Rate 10-20 inches/minute Nozzles 2 @3.5 mm Water:Cement Ratio 1:1 by weight Cement Grout Pressure 6,860 psi (470bars) Cement Grout Flow Rate 78 gal/minute Grout Volume 94 gal/ft Rotational Speed 14-20 rpm

Zone 1

Zone 2

Figure 7 Cross Section of Jet Grout Columns in Zone 2 Adjacent to the

Webster Street Tube Joint No. 1

Ground Heave Two longitudinal cracks developed at the joint between the Portal Building and the first tube segment next to the Portal Building. A major longitudinal crack opened up as a hair-line crack since the blowout occurred in July 2001, growing bigger with time during

grouting operations. The cracks opened and closed during the day possibly due to the fluctuations of the tide and ebb during the day. The total cumulative crack opening was measured to be about 0.75 inches. The crack opening was found to be mainly related to the jet grouting operations which caused ground heave at Joint No. 1. Despite the crack opening at the joint, no cracks of major concern appeared on the ground surface. Posey Tube Joint No. 1 Due to the slow construction pace of the production jet grout columns using a single fluid system, a double fluid system was used for the jet grout columns at Posey Tube Joint No. 1. The double fluid system encapsulates simultaneous injection of high velocity grout within a cone of compressed air, allowing better cutting efficiency of the slurry grout. The system is capable of creating columns at least twice as large as those made by the single fluid system. The increased size of the column was ascribed to:

1. Compressed air acted as a buffer between the jet stream and any ground water present. Therefore, the jet stream could cut at least twice as far in water as compared to when no compressed air was used.

2. Soil cut by the jet is prevented from falling back into the jet stream , thus cutting down on energy lost through the turbulent action of the cut soil.

3. Cut soil was more efficiently removed from the area of cutting by the “bubbling action” of the compressed air lifting soil debris to the surface.

Just like any systems, there are three drawbacks of the double fluid system. The first one is that the soilcrete ( a hybrid of soil and grout) has a high air content, rendering a lower unit weight and strength compared to the single fluid system. Secondly, the addition of compressed air requires pathway, which consists of an inner and outer set of rods with an annulus of about 5 mm. This pathway may become clogged during operation and cannot be used under limited overhead room where the drill rods have to be uncoupled. Thirdly, the amount of air used is much that it could easily lift overlying soils, thus causing substantial ground heave as compared to the single fluid system.

Because of a bigger column that could be created, a total of 18 jet grout columns were needed to form the cut off impermeable walls at Posey Tube Joint No. 1, as shown on Figure 7. To minimize uncoupling and re-coupling drilling rods, the drilling rig mast was lengthened to 50 feet. This saved tremendous of time in breaking and assembling the rods during construction. Each jet grout column was drilled to a depth of about 95 feet and jet grouted to the design elevations as shown in Figure 7.

Monitoring of the jetting parameters during the performance of jet grouting works was accomplished by means of a real-time instrumentation recorder attached to the drill rig. The jet grouting injection parameters used are presented in Table 2 below. Return flow of the injection was collected via an 8-in plastic pipe to a dumper. Once the injection monitor reached the proposed top of the jet grout column, the high pressure jet was deactivated and the outer casing was removed to the surface.

Table 2 – Jet Grout Parameters for Posey Tube Joint No.1 Injection Parameter Parameter Value Column Withdrawal Rate 7.90 in/minute Nozzles 2 @3.5 mm Water:Cement Ratio 0.8:1 by weight Cement Grout Pressure 6,100 psi (420 bars) Cement Grout Flow Rate 76 gal/minute Grout Volume 115 gal/ft Air Pressure 145 psi (10 bars) Air Flow Rate 60 liter/min Rotational Speed 9 rpm

Figure 7 Layout of Jet Grout Columns at Posey Tube Joint No. 1 Instrumentation Program An instrumentation program was implemented to monitor the ground heave and the tube differential movement during the jet grouting operation. The program consisted of installing two ground settlement devices over Joint No. 1, 6 settlement points at locations in the vicinity of the production jet grout columns, and 6 tiltmeters installed inside the tube at Joint No. 1. The instrumentation points were monitored daily during construction of the jet grout columns by the contractor and site inspector.

Ground Heave A longitudinal crack of 0.1 inch developed at the joint between the Portal Building and the tube segment during construction of the first jet grout column (B-3) at the east side of the Posey tube. This crack grew bigger with time during grouting operations. The cracks appeared to open and close during the day possibly due to the fluctuations of the tide and ebb. The total cumulative crack opening was measured to be about 0.3 inches. The crack opening was found to be mainly related to the jet grouting operations, which caused ground heave at Joint No. 1.

In addition to the crack at Joint No. 1, cracks of width up to 0.1 inch were observed on the unpaved ground and pavement outside the jet grouting area. This was believed to be due to a large volume of air pumped into the ground, thereby lifting the overlying soil. No damage of existing utility lines was noted or reported. The ground heave as measured by the settlement points averaged at 0.02 inch. Quality Control Testing A significant factor in the successful application of jet grouting on the project was the incorporation of an adequate quality control (QC) and verification testing program. A well planned and executed program would result in early identification of potential problems and allowed the contractor to make proper modifications to deal with these problems. The QC program was implemented in two phases: 1). Pre-production phase, and 2). Production phase. The Owner, engineer and contractor were involved in implementing these procedures. Webster Street Tube Joint No. 1 Pre-Production QC Work Prior to the production column installation, the Owner had a contract with the contractor to construct a base jet grout slab at the bottom of the two cofferdams at Joint No. 1 using the single fluid system. There was no specific requirement for constructing any test columns for subsequent exposure for their overall structural integrity. Excavation of the cofferdam to its proposed bottom exposed the jet grout columns, which were of diameter about 2.5 feet. Production QC Work A manual check of the injection parameters, e.g., rate of rod withdrawal, grouting pressure, air pressure, cement water ratio, grout volume, was periodically made by the field inspector. This permitted the engineer and the contractor to have a complete quantitative documentation of each column to be constructed in the prescribed manner. The verticality of the hole was checked manually by a level during construction of the jet grout column.

Post-Production QC Work The post-production QC work included laboratory strength and unit weight testing of core samples taken from the hardened columns and in situ packer permeability tests performed within the cored columns.

In addition, a test pit was excavated to a depth of about 10 feet below ground surface to expose the production jet grout columns. The exposed columns were observed to be composed of a row of at least 6-ft wide continuous, very tight, strong jet grout columns immediately next to the tube. This is in compliance with the design objective. Remediation Work In areas where the QC results showed deviation from the design requirements, remediation works were implemented. Remediation in these cases consisted of chemical grout and cement grout injections into areas depleted of soilcrete. Posey Tube Joint No. 1 Pre-Production QC Work As part of the construction contract, a pre-production test program was implemented prior to installing the production columns using the double-fluid system. The test program consisted of two test columns constructed to a depth of 15 feet below ground surface. The test columns were installed using actual production parameters and equipment, and then exposed for visual inspection to determine the suitability of the jet grout columns and establish the required injection parameters and procedures. Production QC Work The production QC work consisted of real time data collection and monitoring of the entire grouting operation, plus monitoring of the verticality of the jet grouted column. A real time data collection system monitored the mixing, pumping, drilling and grouting works for each completed column. The collected data was downloaded to an on-site mini-computer and condensed into a series of graphs, which plotted the injection parameters in relation to time and depth. This permitted the engineer and the contractor to have a complete quantitative documentation of each column, and allowed verification that each jet grout column was constructed in the prescribed manner. In addition, manual check of the injection parameters, e.g., rate of rod withdrawal, grouting pressure, air pressure was periodically made by the field inspector.

The verticality of the hole was checked manually by a level during construction of the jet grout column. Post-Production QC Work As part of the contract, the post-production QC work included laboratory strength and unit weight testing of core samples taken from the hardened columns and in situ packer

permeability tests performed within the cored columns. At the time of preparing this paper, such work has not been performed. Test Results Webster Street Tube Joint No. 1 A total of eleven (11) unconfined compressive strength tests were conducted on the core samples. The test results exceeded the specified minimum strength of 600 psi. The unconfined compressive strength of the jet-grouted soil ranged from 650 to 9000 psi with an average of 3,040 psi. No relationship between strength and soil type was observed due to the heterogeneous nature of the soil conditions at the site. However, where the in situ soil is of gravelly nature, it appears the strength of the soilcrete became much higher (in excess of 3,000 psi).

A total of 6 permeability tests were performed. The permeability ranged from 5 x 10-5 cm/sec to 4 x 10-6 cm/sec. The results of these tests, coupled with the exposure of the production jet grout columns at the site, indicated that the tight overlap of the jet grouted columns is generally effective. Posey Tube Joint No. 1 The field and laboratory testing were not carried out at the time of writing this paper and hence no data can be presented. Evaluation Two different jet grouting systems; single fluid (grout) and double fluid (grout/air) were employed in constructing the cut off walls at Posey and Webster Street Tube Joint No. 1. Both systems achieved the minimum 6-ft width of the cut off walls specified in the contracts. However, jet grout columns using the single fluid system required a lot more rows of columns than the double fluid system to create the specified minimum 6-ft wide cut off wall, as a result of which the associated construction cost with the single fluid system became very expensive. For example, it took approximately 10 weeks to finish the 104 “single fluid” jet grout columns as compared to 4 weeks for the 18 “double fluid” jet grout columns. Because of more columns needed to be constructed, it required a lot more grout to construct the cut off wall for the single fluid system than the double fluid system.

Construction of the jet grout columns at both tubes encountered a lot of obstructions which were not disclosed from the as-built drawings except the tremie concrete. The obstructions retrieved consisted of wood chips, steel bars, red bricks, steel coils, tremie concrete, etc. These obstructions, coupled with the need of breaking and re-assembling strings of rods made the single fluid system a time consuming operation. Subsequent extension of the mast length of the drill rig (to 50 feet) for the double fluid system at the Posey Tube Joint No.1 did increase the production rate, thereby minimizing overall construction costs.

Continuous return of jet grout spoil was generally observed during both single-fluid and double-fluid jet grouting. There were a few occasions where grout spoil did not return to the ground surface such as in the first six inclined columns constructed in Zone 1 at the west side of the Webster Street Tube. No such loss of spoil return was subsequently noted at the second rows of jet grout columns in Zone 1 at the Webster Street Tube. It was believed that the grout could have filled the voids at the localized area in Zone 1. Based on the grout-take records, 44,000 liters of grout were pumped in the first inclined hole at A-1, and much less grout was needed for the subsequent inclined columns in Zone 1 of the Webster Street Tube Joint No.1.

The grout spoil return became more viscous and thicker during construction of the final two rows of the jet grout columns in Zone 2 at the Webster Street Tube Joint No.1. This suggested that the ground did not take more grout during jet grouting operation in these two last rows. It was then inferred that a water-tight, continuous, cut off wall was in place next to the tube, which was confirmed by subsequent testing and inspection of the jet grout columns.

A total of eight cased chemical grout holes were drilled to a depth of about 48 feet at an inclined angle of about 65 degrees in Zone 1 of the Webster Street Tube Joint No. 1. Each of these holes was inserted with a pvc grout tube/tube-a-manchette at the end and each filled with cement grout. The purposed of pumping chemical grout in Zone 1 was to stabilize the localized loose sand immediately below the tube, which could not be stabilized by the jet grout columns. At the time of writing this paper, no chemical grout was pumped into the specified locations.

Control of spoil return was handled by two methods. At the Webster Street Tube Joint No.1, the spoil return was contained in a small trench which was led to slurry pit and pumped to a dumpster. The spoil from the jet grouting at the Posey Tube Joint No. 1 was controlled by suing a diverter at the collar of each hole, which was connected to a section of casing installed to a depth of about 20 feet. The spoil was channeled into a pump (with a small container) from the diverter and pumped to a tank.

The results of the unconfined compressive strength tests indicated that the strength of the soilcrete is at least 600 psi as is required in the specification. Packer testing conducted in the field showed that the jet grout columns are in general water tight.

Good recovery of core runs was obtained in all the four cored holes performed. However, coring through the jet grout column A-1 (an inclined column) west of the Webster Street Tube Joint No. 1 encountered a small void (18” deep) each at depth intervals of 39-42 feet, and 52-57 feet. This is believed that the formation of such voids is due to the presence of obstructions at those intervals. In addition, no recovery was obtained from the inclined depth of 86’ to the bottom of the column at an inclined depth of 95 feet. A closer look at the geologic profile at Column A-1 indicated that the interface between the overlying Bay Mud and the dense sand is at about El. –70, about 78 feet below, which is the same as the inclined depth of 86. The missing of the jet grout column from that depth downward is believed to be ascribed to washing away of the grout by the artesian pressure at the interface of the Bay Mud and the dense sand.

Conclusions

• A cut off wall consisting of jet grout columns can be constructed along the two sides of Posey and Webster Street Tube Joint No. 1 using either a single or double fluid system. The jet grout wall is in general water tight, but localized leakage at the obstructions should be expected.

• A jet grout wall of width at least 6-ft could be constructed by a double-fluid system using grout and air to construct the columns as revealed from exposure of the jet grout test columns. For the single fluid system, a jet grout column of up to about 2.5 feet in diameter can be constructed.

• To create a minimum of 6-ft wide jet grout wall at the Webster Street Tube, multiples of rows were needed for the single fluid system. This resulted in longer construction time and expensive construction costs.

• Encountering obstructions during construction of jet grout columns would slow down the pace of jet grouting operation. It could not guarantee the overall water-tightness of the columns in place.

• The rate of rod withdrawal during jet grouting is the key parameter to control the size of the jet grout columns. For our project, the withdrawal rate for the double fluid system was 7.9 inches per minute for a minimum 6-ft diameter jet grout column. For the single fluid system, a 2.5-ft diameter jet grout column can be constructed using 10 inches per minute rate of withdrawal of the rod.

• Construction of large diameter (greater than 6 feet), deep (over 40 ft) jet grout columns should better be done by means of double fluid system rather than the single fluid system from economics point of view.

• Requirement of a long mast (at least 50 feet) to hold a long string of rods is essential to minimize the need of coupling and uncoupling strings of rod during jet grouting operation. Construction of deep jet grout columns should call for the use of one long string of rods to form the jet grout columns.

• Ground heave could be a concern for shallow jet grout columns (with less a few feet of overburden over the top of the jet grout column) if double fluid system is used because of the substantial amount of air pumped into the ground, lifting the overlying soil mass.

• Quality control is a significant factor in the successful application of jet grouting. Multiple phase QC program is recommended. A real-time data collection and monitoring system can be used to control the injection parameters and evaluate the need for any remedial measures.

• A field trial is essential for each jet grouting project. The pre-production trial assists in evaluating the adequacy of the equipment to be used and selection of the appropriate injection parameters.

• Pre-qualification of jet grouting contractors is necessary. Detailed pre-qualification questionnaires should be included in the bidding documents.

• Close cooperation and partnering between the owner, engineer, and contractors are crucial for the successful implementation of a jet grout project.

Acknowledgement The authors are thankful to the California Department of Transportation for the support of publishing this paper. The conclusions of this paper reflect the views of the authors, but do not necessarily reflect official views of the State of California. This paper does not constitute a standard, specification, or regulation. References Lee, T. S., Jackson, T., and Anderson, R. (2002). “Innovative Designs of Seismic Retrofitting The Posey & Webster Street Tubes, Oakland/Alameda, California.” 3rd National Seismic Conference, Portland, Oregon, April 30-May 2. Parsons Brinckerhoff Quade & Douglas, Inc. (2001). Technical Report on Demonstration Program, Posey Webster Street Tubes Seismic Retrofit Project, January. Schmidt, B., Hashash, Y., and Stimac, T. (1998). “US Immersed Tube Retrofit” Tunnel and Tunnelling International Magazine; November 22-24.