Retaining wall determination demonstrates importance of surcharge calculations

A Ministry of Business, Innovation and Employment (MBIE) determination highlighted the importance of correctly calculating retaining wall surcharges at the design stage 

A recent MBIE determination found that a new masonry block retaining wall on a property in Tauranga could not establish compliance with Building Code Clause B1 Structure. 

The design for the retaining wall formed part of the building work detailed in a broader project to build a new two-storey dwelling on the owner’s property. 

The determination was applied for by the property owner after Tauranga City Council raised concerns that the new retaining wall may affect the existing masonry blockwork retaining walls on a neighbouring property. It considers the part of the wall that is directly opposite, and runs parallel to, the neighbours’ existing retaining wall. 

New 1.2m retaining wall required 

Cut-and-fill earthworks were required to form a building platform for the owner’s new development, which is at a higher level than the neighbours’ existing dwelling. The new 1.2m high retaining wall retains the fill material from the earthworks – and is built incorporating reinforced 20 series concrete blockwork supported on reinforced concrete foundations. The Producer Statement – Design stated that it was “understood the proposed retaining walls are not surcharged by any adjacent structures”. 

The determination explained that the neighbours’ retaining wall is constructed from 190mm-thick reinforced concrete masonry blockwork on reinforced concrete foundations and provides structural support to the upper floor level, external wall and roof construction. The wall retains a maximum depth of 1.4m of backfill material and soil. 

On 15 November 2023, the authority inspected the building work and issued a failed inspection report. The authority was concerned the retaining wall was surcharging the neighbour’s retaining walls. 

“Based on a review of the structural design and drawings, no allowance appears to be made to avoid the passive resistance on the new masonry wall applying a surcharge load to the existing neighbour’s walls,” said the authority. 

Passive load questioned 

In response, the owner submitted that the retaining wall did not apply a passive load on the neighbour’s existing wall. The authority countered that, saying: “the original design plans and structural calculations for the new retaining wall do not consider its impact on the existing retaining walls.” 

In the determination, Andrew Eames, MBIE Principal Advisor Determinations, wrote: “I must consider whether the original design of the new retaining wall relied on the existing retaining walls on other property to achieve Building Code compliance.” 

He continued, stating the engineering design for the new retaining wall did not consider or allow for the neighbours’ existing retaining walls. 

“I have received no plans and specifications to suggest the original design for the new retaining wall relied on Building Code compliance being demonstrated using any element of the neighbour’s existing retaining walls,” said Eames. 

Critical of the design 

The design for the new retaining wall featured several elements that Eames criticised, including: 

• An assumption there were no changes in topography in the vicinity of the new wall. 

• The building consent for failing to quantify or address the passive (horizontal) load imposed on the lower-level ground on the neighbours’ side of the property boundary. 

• The lack of accounting for the varying horizontal distances between the new retaining wall and the existing retaining walls. 

• A back-calculation that demonstrated a theoretical key depth of 55mm that would not impose a passive (horizontal) load, when the actual design was for a key depth of 300mm. 

“Based on the lack of design information provided in the original building consent regarding the stability of the new retaining wall, taking into consideration the issues described above, I have insufficient information for me to reach a decision on whether the new retaining wall complied with clause B1,” said Eames. 

Regarding the requirement of section 17, clause B1, to take into consideration protecting other property from physical damage caused by structural failure, Eames stated he had been presented with no structural calculations or other similar data by the parties that discussed the probability or likelihood of physical damage to other property as a result of the proposed building work to construct a new retaining wall. 

“Therefore, I can only conclude I have received insufficient information that demonstrates compliance for the protection of other property based on the original design of the proposed retaining wall.”