Ground engineering specialist Keller said half year profit fell by a fourth as the construction market, particularly in the US, continued to deteriorate.The group, which laid the foundations of the London Olympic stadium and worked on Dubai's Palm Jumeirah Island, said full year expectations for the group remain within the current range of market expectations.The group, which issed a profit warning in May, said pre-tax profit slumped to £11.3m the six months to 30 June from £41.0m the same time a year before. Revenue for the period fell to £496.9m from £552.6m."Since our May 2010 interim management statement, we have not seen any real change in our key construction markets," the group said in a statement. Its US order book at the end of June was 13% higher than at the same time last year, fuelling hopes that activity levels in its US foundation businesses will gather momentum through the second half of the year. "However, we remain of the view that margins will not improve materially until there is confidence in a sustained recovery in US construction," it said in a statement.In Australia market conditions remain very strong while the UK market remains very challenging. As expected, the French and Spanish construction markets contracted further.The group's order book is currently 1% ahead of the same time last year. Net debt increased to £121.5m from £95.3m at 30 June 2009.