Risk Assessment

Projects with class C classification (challenging to extremely challenging) must be carefully examined in terms of required forces and ground displacements. The risks associated with pipe bursting include damage to nearby utilities and structures, failure to complete the project using pipe bursting, and time and/or budget overrun. There is risk of damage to nearby utilities, buried structures, and pavement if there are adverse soil conditions, improper construction techniques, design mistakes, inappropriate toning of utilities, etc. There are also many risks associated with flow bypass, dewatering, shoring, etc if the appropriate procedures were compromised. A list of additional risks that may stop the bursting operation and/or create problems include:

  • Settlement at insertion/pulling pits if the density of the backfill exceeds that of native soil.
  • Bursting through sharp curves.
  • Concrete encasement or steel point repair inside existing pipe.
  • Excessive bursting lengths.
  • Damage to new pipe from sharp edge or fragments of existing pipe being burst/split.
  • Damage to laterals from bursting of main line.
  • The presence of rock under the existing pipe may create a "bump" in the replacement pipe.
  • Collapsed pipe.

Projects with class C classification-challenging to extremely challenging must be carefully examined in terms of required forces and ground displacements. Additionally, the depth of the old pipe affects the expansion of surrounding soil and consequently the extent of ground displacement around the pipe. If the pipe is shallow, damage to the pavement may take place. Saw cutting the pavement prior to bursting might be advisable. If the existing pipe is below the GWT, the difficulties increase. Insertion and pulling shafts grow larger and more complex as the depth increases.

If there are unacceptable sags in the existing sewer line, these sags need to be corrected before bursting. The sags can be corrected by local excavation, surface grouting, or grouting from within the pipe. Some reduction of sag magnitude may be expected (without corrective measures) from the bursting operation, but the extent to which the problem is corrected depends on the relative stiffness of the soil below the sagging section.

If there is erosion of the soil around the pipe, the bursting head and the following PE pipe will tend to deviate toward the void or lower density region. If there is a hard soil layer or rock close to the pipe, the bursting head will tend to displace towards the softer soil. In shallow conditions, the ground will deviate mostly upwards towards the ground surface. If the conditions change substantially along the length of the burst, this may cause some change in the grade and/or alignment of the pipe. When the grade is critical, these possibilities should be considered.

Most pipe bursting operations can be done safely if site and project conditions are known before bursting and appropriate measure are taken to address these conditions. There are well known solutions to all of the above mentioned risks and problems, and successful project engineers or construction managers identify these risks and develop a risk management plan to address these specific risks for this project. This plan includes quantification of the occurrence probability of the identified events and their associated impact or damage; it also includes measures to eliminate, mitigate, transfer, or undertake these risks. One of the general measures to mitigate the project risks is building and maintaining cooperative relationships among owners, engineers, contractors, equipment manufacturers, and pipe suppliers. Identifying and developing a realistic plan to manage and share risks appropriately is an important part of effectively communicating responsibilities, defining roles, and building a strong team. It is important to pay close attention to the project surroundings (surface and subsurface conditions) for unfavorable conditions and risks. These conditions require extra attention in order to ensure the safety of all involved people as well as surrounding facilities and infrastructure.