The International Information Center for Geotechnical Engineers

Deep Soil Mixing for Retention of Excavations - Case Study 3: Pennsylvania DOT Excavation Support

 

Case Study: Pennsylvania DOT Excavation Support

Project Overview

The primary goal of the project was to connect a newly constructed bridge crossing the Susquehanna River to State Route 54 through the Danville, business district in Pennsylvania.  This was achieved by creating a highway tunnel underpass that replaced an old street, Factory Street, and extended below another Street, Market Street.  The site location consisted of historic mansions, which were as close as 0.9 to 1.2 m to the proposed construction (McMahon, 2001).  These structures were problematic to many design options because they were three stories high and were constructed from brittle materials such as brick.  Therefore, the difficulty of construction in an urban area was only worsened by the conditions of the structures in the direct vicinity of the project.  Limiting wall movements and vibrations became an absolute necessity for successfully completing the project.  The excavation reached depths of 6.7-9.4 m.  The Project Location relative to the Susquehanna River and the historical buildings is shown below in Figure 3.1.

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Figure 3.1 Connector (Underpass) and Excavation Support Location (McMahon, 2001)

To achieve low wall movements and to minimize potentially damaging vibrations associated with other ground improvement methods, the excavation wall was constructed using a DMM wall.  The method used in-situ soil mixing with a cement stabilizer that was mixed with the soil using rotary energy to create an overlapping, continuous wall type configuration.  The minimum allowable compressive strength for design was specified as 1.38 MPa (McMahon 2001).  Borehole tests for the site showed loose to dense sands, gravels, and silts with some boulders.  Unlike some of the previous case studies mentioned in this paper, the groundwater table was not shallow for this project.  In fact, the groundwater elevation fell below the bottom of the DMM wall. 

Design Considerations of DMM Wall and Soldier Pile Reinforcement

The design was complicated by the difficulty in modeling the wall.  The behavior of the DMM wall in terms of lateral movement, stiffness, and embedment could only be approximated by research on continuous walls, sheet-pile walls, and segmented walls.  The method employed in design was to identify which assumptions could be made in terms of behavior of the wall and how to use theory to predict lateral movements.  For instance, above the subgrade of the excavation, the wall could be approximated by a segmental wall for design purposes due to the modular ratio of the steel soldier beam/soil cement structure.  Since the soldier beams are much stiffer than the soil cement, the soldier beams need to have capacity to carry the surcharge load at the surface (McMahon, 2001).  These loads resulted from the fact that the foundations of the historic building were close enough to the excavation wall to impose additional loads above the geostatic earth pressures assumed in design.  Below the subgrade, the wall was modeled as continuous due to the high stiffness of the DMM wall relative to the soil.  

The design also called for cross lot bracing struts connected using W18X40 soldier piles which were anchored to the DMM wall.  These would further reduce lateral movements and improve the performance of the entire structure.  These horizontal braces were stressed to 100% of the design load to ensure that loads could effectively be transferred between the two walls and that they did not lose contact (McMahon, 2001).  The maximum vertical spacing between horizontal bracing struts was 3.0 meters.  The top two struts are close to the surface to help control surcharge loading.  Figure 3.2 below shows a cross section view of the structural elements mentioned above.Alex C2.2

Figure 3.2. Cross sectional view of excavation and rigid frame DMM Wall (McMahon, 2001)

Depending on the level of the bracing system and the assumptions about design, different earth pressure diagrams could be used to model the forces acting on the wall.  The rectangular pressure diagram presented by Terzaghi and Peck (1967) was used where there were multiple bracing levels. It would be applicable to use active Rankine pressures for single bracing areas.  After summing the areas of the pressure diagrams mentioned above, the force per unit length acting on the wall can be determined.  The surcharge loading must also be included as mentioned earlier.  However, to ensure a conservative design, the lateral earth pressures calculated above were also multiplied by a safety factor (which was 1.4 in this case).

To determine whether or not the design met some allowable standards in terms of settlement, previous case studies and other literature sources were used to create a rough estimate of the expected vertical movements.  Figure 3.3 shows two plots for predicting vertical movement.  The first one is a normalized plot of the distance from the wall vs the expected settlement (Peck, 1969).  The second plot uses the same idea but presents actual deflections from various other case studies (O’Rourke, 1989).

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Figure 3.3 Estimation of  settlements away from DMM wall (Left: Peck, 1967 and Right: O’Rourke, 1989) 

Considering both plots and the simplifying assumptions above, it was found that the left figure predicted settlements of 102 mm (which is clearly unacceptable) and the right figure predicted settlements from 28 mm - 10mm which is about 1% of the excavation height.  Therefore, for this project, deflections of more than 10 mm would not be acceptable.  Anything higher than 10mm settlement could result in cracking and differential settlement of the basement walls of the historical mansions next to the excavation.  

Construction Aspects of the DMM Walls

The construction called for a total of 372 m of soil mixed wall (SMW).  The mixing tools used rotation and a high amount of torque to mechanically agitate and mix the soil with the stabilizing agent.  In this case, the stabilizing agent consisted of a mixture of cement grout in a wet state injected through the end of a hollow shaft with rotary mixing.  The water/cement ratio by weight was 1.5:1. Cutting heads at the end of the rotating rods ensured that the soil came into contact with the mixing blades to be uniformly mixed. Four mixing augers were grouped together and penetrated the soil at the proper locations.  Upon the withdrawal of the tools, cement grout was mixed with the soil.  Consecutive passes would then be made with the rig by spacing less than 4 auger diameters.  Then on later passes, the curing columns would be re-augered and overlapped to create the final product.    

Results and Performance

The unconfined compressive strength (UCS) was measured for the wet soilcrete by taking in-situ samples.  The 7 day compressive strength reached 3.02 MPa which already exceeded the design specified value of 1.38 MPa.  The 28 day strength reached as high as 4.2 MPa (McMahon, 2001).  Coring samples were taken from various locations on the finished wall and UCS on the samples indicated strengths of about 3.8 MPa . The results of the construction were monitored using inclinometers and tape extensometers. A plot of the horizontal movements is shown in figure 3.4 below. 

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Figure 3.4. Lateral deflection vs. depth for the wall (McMahon, 2001) 

Both lateral and vertical displacements were acceptable and within the predicted range above.  As shown in figure 3.4, the maximum lateral movement of 6 mm occurred at around depths of  7 m and the final settlement was 10 mm (the lower bound of O’Rourke).  Although these values are effectively within the ranges of other case studies, the success of this project was in its effective implementation of proper construction practices with DMM, bracing struts, and anchorage using reinforcing soldier beams.  The effects of poor construction would outweigh even the soundest design.  The combination of careful consideration of literature and case studies coupled with good construction practice and knowledge of site conditions leads to the best results in ground improvement, even in the most challenging and limiting situations.

 

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