STRUCTURAL SHELL THICKNESS DESIGN GUIDE¹

FOR CALCULATING THE APPROPRIATE
THICKNESS TO REINFORCE, SEAL AND PROTECT
EXISTING UNDERGROUND STRUCTURES

This method is selected when the existing manhole is structurally
stable but degraded with actual or potential points of infiltration

1. MORTAR THICKNESS DESIGN
Factors to be considered for calculating the thickness of a cementitious liner placed against the existing interior wall of a cylindrical structure are:
• strength
• density
• elasticity of the mortar
• static and dynamic loading
• soil type
• ground water pressures
• existing structural conditions
• anticipated changes in conditions
• diameter and depth

The following minimum design parameters of the mortar material were used for these calculations:
• minimum 3,000 psi compressive strength in first 24 hours
• minimum 150,000 psi modulus of elasticity in first 24 hours
• extremely dense to prevent water migration (levels less than 1,000 coulombs)
Allowances can be made for materials of lesser physical properties by proportionately increasing the liner thickness.

When a measurable thickness of the original wall has been lost, the new structural liner should be replaced with a thickness equal to that cross sectional value that was lost. For example, a pre-cast concrete manhole with an original cross sectional thickness of 5 inches (130 mm) which has lost 1.5

inches (40 mm) of its original thickness would need to receive 1.5 inches (40 mm) of new liner material to restore it to its original diameter if it were of the same C495 Portland cement concrete as the original pre-cast manhole. If material meeting the minimum parameters described above were used, then the liner thickness would be as shown in Table 2.

Manholes are most adversely affected by traffic loads passing directly over or near the structure and hydrostatic loads from external ground water pressures. Since these conditions are the most critical factors for design calculations, design strengths which are able to overcome each of these factors, are sufficient to overcome all other factors. Vehicular loading moments affect the top 2 feet (600 mm) only and can be categorized as light or heavy. Ground water pressure increases with depth. The greater the depth and pressure, the thicker the liner must be. For equivalent strength levels in diameters larger than 48 inches (1200 mm), the thickness of the liner would increase proportionately. Larger diameter manholes require a greater thickness to provide a correspondingly acceptable structural reinforcement value (much like SDR thickness of pipe).

Therefore, shell thickness as a function of the moment load capacity can be computed. Results of these computations are summarized in Figure 1. These relationships included a bedding factor of 3.

An evaluation, comparing external symmetric pressures causing compressive hoop stresses and external pressures causing elastic instability, showed that buckling is the critical mode of failure when shell radius to thickness ratio is greater than ten.

¹This information was determined from field observations and tests conducted at the Spangler Geotechnical Laboratory by the Civil & Construction Engineering Department of Iowa State University in Ames, Iowa under a research grant from the National Science Foundation.